De foute vleugeltheorie de meest gebruikte vleugeltheorie en waarom hij fout is. Zeiltheorie. inleiding

http://zeilplan.net/leren/theorie/inleiding.htm

Zeiltheorie inleiding

De foute vleugeltheorie de meest gebruikte vleugeltheorie en waarom hij fout is.

Werking van de zeilen Hier wil ik de betere theorieën achter het zeilen promoten. Klik hier rechts op een link om meer over dat onderwerp te weten te komen.

Uitleg over de werking van het vleugelprofiel van de zeilen.

Proefjes Enkele proefjes om de theorie te te verduidelijken.

Deze informatie is ontstaan doordat er in de loop der tijd nogal wat onzin theorieën zijn ontstaan waar je ook nog eens niks aan Koppels en Krachten Ook dit 'lastige' onderwerp nog maar hebt. Daar wil ik verandering in brengen. even uitgediept.

Foute theorieën zijn zo wijdverbreid dat ze zelf op scholen en zeilscholen worden onderwezen, waardoor theorie niet meer toepasbaar is. Ik hoop dat jij wat hebt aan de inhoud van deze site, en jij beter zult weten na dit te hebben gelzezen! Groet van Pim Geurts.

Het onderwaterschip De Kiel en het roer.

Weerstand uitleg van hoe de scheepsweerstand werkt.

Stabiliteit Waarom blijft een boot overeind?

trim De basis van de boottrim.

Veelgestelde vragen Enkele veelgestelde vragen en hun antwoord.

Links Links naar de bronnen en achtergronden van deze theorie.

http://zeilplan.net/leren/theorie/inleiding.htm24-1-2004 17:22:16

http://zeilplan.net/leren/theorie/tekst.php?tekst=foutetheorie.txt

Terug naar index Hoe werkt een vleugel niet. Voorbeeld van de meest gehoorde foute theorie: Kijk zo ziet een vleugel eruit:

Zoals je ziet is de bovenkant rond en de onderkant vlak. Om langs de bovenkant te gaan moet je een grotere afstand afleggen dan langs de onderkant. Als er lucht langs de vleugel gaat zal dit zich splitsen in lucht die boven langs gaat en lucht die onderlangs gaat. De lucht die bovenlangs gaat zal tegelijkertijd aankomen bij de achterkant als de lucht die onderlangs gaat, Dat kan niet anders, anders zou er een gat in de lucht ontstaan, en dat kan dus niet. De lucht die bovenlangs gaat zal dus in dezelfde tijd een grotere afstand afleggen, en zal dus sneller moeten gaan. Bernoulli zei dat snellere lucht een lagere druk heeft. Dit kan ik bewijzen met het volgende “trucje”. Als ik bovenlangs een blaadje blaas gaat de lucht daar sneller, en krijg je daar dus een lagere druk en wordt het blaadje omhoog gezogen.

Dus, omdat de lucht bovenlangs de vleugel een langere weg moet afleggen zal de lucht daar sneller gaan, en dus een lagere druk hebben, en zo de vleugel als het ware “omhoogzuigen”

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Een zeil werkt eigenlijk net zo. Een zeil is weliswaar even lang aan loef als aan lij, maar omdat het bolling heeft is de binnenbocht toch korter dan de buitenbocht, waardoor de lucht aan lij sneller zal gaan, en het zeil dus als het ware naar lij wordt getrokken. Wat heb je hier nu aan om te weten: hoe meer bolling hoe meer power (want meer weglengteverschil) en de zeilkracht werkt haaks op je zeil. " Dit verhaal heb ik verschillende malen gehoord, en ook enkele malen zelf verteld als zeilinstructeur. Zelfs toen ik in mijn studie mijn eerste lessen stromingsleer over vleugels kreeg heb ik gedacht dat dit verhaal klopte, en gebruikte het naast andere theorieën. Pas vrij laat ben ik echt gaan nadenken en kwam erachter dat de theorie van geen kanten klopte. Vanaf nu noem ik deze theorie de “gelijk aankomen” theorie. Enkele kleine experimenten om te laten zien dat “gelijk aankomen theorie” gewoon niet klopt:

Super vleugel Volgens de “gelijk aankomen” theorie zou deze vleugel zeer goed moeten werken, want er is een groot weglengteverschil Tocht wordt dit type vleugel niet gebruikt omdat hij niet zo goed werkt.



Vliegtuig ondersteboven Veel kleine vliegtuigen kunnen prima ondersteboven vliegen. Vroeger dacht ik altijd dat men dan als het ware de bolling omdraaide door die platen aan het eind van de vleugel de andere kant op te zetten. Toen ik zag dat die platen nooit groot genoeg waren om dat te kunnen doen, en zag dat men dat ook gewoon niet deed snapte ik niks meer van de “gelijk aankomen” theorie

zo dus

en niet zo Vlakke plaat geeft ook lift Een rechte plaat geeft ook lift, als hij onder een hoek met de luchtstroom wordt geplaatst. Dit kun je al zien als je met een vel papier door de lucht beweegt. Vaak krijgt dat stuk papier dan ook nog eens een bolling de verkeerde kant op, en wil toch omhoog.



Een niet aangetrokken doorgelat zeil geeft geen liftkracht. Als je een doorgelat zeil niet aantrekt geeft het geen lift Toch is er dan nog steeds hetzelfde weglengte verschil, dus je zou nog steeds dezelfde liftkracht verwachten. Ook het kracht verschil tussen een iets te slap aangetrokken grootzeil en een normaal aangetrokken zeil zou er niet moeten zijn aangezien de weglengte verschillen hetzelfde blijven. Lucht komt niet tegelijk aan bij de achterkant van het zeil. Op een gegeven moment stond ik te roken op het voordek en zag dat mijn rookpuf absoluut niet gelijk aankwam bij de achterrand van het zeil. Ook niet met de fok ingerold. Toen ben ik gaan spelen met touwtjes in mijn zeil (telltales). Bij sommige zeilstanden stonden de telltales aan loef slap naar beneden, en aan lij netjes naar achter. Aan loef was er dus bijna geen snelheid, terwijl je zou verwachten dat er maar een redelijk klein verschil zou zijn tussen loef en lij, aangezien de weglengte toch ook niet zo heel veel verschilt. Daar heb ik geen foto’s van. Wel van dit vleugelprofiel met pufjes rook in de stroming. De wokjes rook zitten aan de voorkant gelijk, aan de achterkant duidelijk niet meer.



Als je het weglengte verschil uitrekent is de lift veel minder als gemeten. Ergens kwam ik dit soort plaatje tegen van een vleugelprofiel met de bijbehorende lift coëfficiënt (Cm) en natuurlijk drag (Cd)



Ik ging het weglengteverschil opmeten bij een hoek van 0, daarmee kan je het snelheidsverschil bepalen, en daarmee de liftkracht. Ik kwam uit op een liftwaarde die 5 tot 50 keer zo laag als hier werd opgegeven. Afhankelijk van hoe ik het weglengteverschil precies meette. Toch vreemd?? of niet? Onder blaadje blazen Een heel simpele test om te laten zien dat “als de lucht sneller gaat is daar een lagere druk en wordt de vleugel daar naartoe gezogen” niet waar is vond ik de test dat als je onder een blaadje doorblaast het blaadje juist omhoog gaat, dus juist van de snelbewegende lucht af.



Een kleine variatie hierop: Als je het blaadje eerst opgerold hebt zodat het uiteinde omhoog gaat door de bolling en je er dan voverheen blaast, gaat het wel naar beneden. --Plaatje blazen over blaadje wat omhoogkrultOpvallend niet? Ik hoop dat ik met bovenstaand verhaal duidelijk heb gemaakt dat de “gelijk aankomen theorie” de werkelijkheid wel erg slecht beschrijft. Eigenlijk gewoon fout is. Vergeet deze foute theorie alsjeblieft. In het Werking van de zeilen geef ik een beter werkbare theorie, die ook vrij makkelijk is: “een zeil buigt de wind af, daar is een kracht voor nodig is, en die kracht is nou je zeilkracht.” Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=zeilen.txt

Terug naar index Werking van de Zeilen

De betere vleugeltheorie Een vleugel buigt lucht naar beneden af. Als de vleugel de wind naar beneden duwt, duwt de vleugel zich juist omhoog. Dit is nu de kracht “liftkracht” genoemd die het vliegtuig omhoog duwt. Een vleugel staat onder een kleine hoek. Dit betekent dat de luchtdeeltjes onderlangs als het ware tegen de onderkant van de vleugel botsen en naar beneden ketsen. De lucht aan de bovenkant wordt ook omgebogen. Dit gebeurt doordat constant aan de bovenkant als het ware een “gat” wordt gegraven, wat natuurlijk lucht aanzuigt, en dus de lucht naar de onderliggende vleugel zuigt:

Natuurlijk botsen in de praktijk de luchtdeeltjes niet alleen tegen de vleugel, maar ook tegen elkaar. Het gat wordt natuurlijk continu gegraven, en continu aangevuld, waardoor het er meer uit komt te zien als:

Een zeil werkt net zo: ●

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1 Een zeil buigt de wind om. Hiervoor is een kracht nodig. Dit is nou je zeilkracht. Hoe meer je dus ombuigt hoe groter de kracht. 2 Aan loef buigt je zeil de wind af door een soort van botsing, de wind wordt als het ware gewoon de bocht omgeduwd.

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3 Aan lij wordt de wind omgebogen doordat de wind als het ware het gat wat je zeil “graaft” in de lucht graaft word ingezogen

. Nu meer in detail met de stoere temen erbij (voor de iets verder gevorderde) 1 Een zeil buigt de wind om. Hiervoor is een kracht nodig geweest. Volgens Newton heeft elke kracht een tegengestelde reactiekracht. Deze tegengestelde reactiekracht is nu de zeilkracht. 2 Het zeil buigt de wind aan loef door verdringing. De lucht kan niet rechtdoor doordat het zeil in de weg zit. Lucht iets verder van het zeil kan ook niet rechtdoor doordat de lucht die gehinderd was door het zeil in de weg zat. Lucht die nog verder van het zeil zit wordt ook afgebogen, omdat de lucht die gehinderd was door de lucht die gehinderd was door het zeil in de weg zit. Lucht die nog verder van het zeil zit wordt ook zo beïnvloed. Natuurlijk is het wel zo dat hoe verder van het zeil je komt hoe minder de invloed wordt. De lucht stroomt als het ware steeds meer om de lucht heen die beïnvloed wordt door het zeil. 3 Het zeil buigt de wind aan lij af door het Coanda effect: Coanda kwam erachter dat een stroming een flink eind een gebogen oppervlak blijft volgen, mits het niet te sterk is gebogen. Hoe dit kwam wist hij nog niet helemaal. Nu weten wij dat dit met de grenslaag en viscositeit heeft te maken: In het rechterplaatje zie je de start situatie. Uit het gestippelde gebied wordt de lucht meegesleurd door wrijving tussen de snelle lucht en de stilstaande lucht. Wrijving van lucht onderling noemen we viscositeit. De gestippelde lucht gaat daar dus weg. Dat zou dus betekenen dat daar een grote onderdruk heerst De lucht uit de snelle stroom wil dat gat weer opvullen, waardoor de stroming wordt omgebogen. Sommige mensen zeggen dat ook de lucht aan de bovenkant wordt meegesleurd. Dat klopt ook, maar deze lucht wordt makkelijker aangevuld uit de gewone buitenlucht, er is namelijk meer buitenlucht omheen. Dit is nou de reden dat de lucht om het profiel heen stroomt. Waarom laat de stroming dan toch wel eens los? Dit komt door wrijving van de lucht langs het oppervlak. De lucht vlakbij het gebogen oppervlak wordt door wrijving afgeremd. Wordt deze afgeremde lucht teveel dan komt het gestippelde gebied gewoon vol te staan met deze bijna stilstaande lucht en gaat de hoofdstroom net zo lief rechtdoor. Dit rechtdoor gaan of eigenlijk het niet meer volgen van de ronding noemen we "loslaten van de stroming" en bij een zeil of vleugel "overtrokken" Het luchtlaagje wat afgeremd wordt door de wrijving noemen we "grenslaag" Hoe verder je langs je profiel komt hoe meer grenslaag er is, omdat er meer lucht is afgeremd door de wrijving. Daaruit volgt dat aan het begin van je profiel een grotere bolling kunt hebben dan aan het eind van je profiel. Stroming blijft dus aanliggen door de wrijving lucht-lucht, en laat los door de wrijving wandlucht. Lucht aan lij gaat sneller dan aan loef (voor ver gevorderden) Bernoulli wist van de wet van behoud van energie. Hij zei eigenlijk dat als lucht versneld zonder energie uitwisseling met buiten er meer bewegingsenergie inzat. Die energie moet ergens vandaan komen. Volgens Bernoulli komt die energie bijvoorbeeld van druk energie. Uitwisseling van energie met buiten moet je zien als een pomp of wrijving. Anders gezegd; als er geen wrijving is, is de energie in een luchtstroom op verschillende punten gelijk. Energie in een luchtstroom bestaat uit snelheidsenergie en druk energie: Voorbeeld: lucht door een wrijvingsloze pijp:



Hier komt de veelgehoorde uitspraak “hoe sneller de lucht hoe lager de druk” vandaan. Die uitspraak is eigenlijk wat uit zijn verband getrokken, omdat het eigenlijk om snelheidsverandering gaat, en er niks wordt gezegd over wrijving. Vier voorbeelden van stom toepassen van “hoe sneller de lucht hoe lager de druk”: ●

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1 Een auto rijdt met 180 km/u op de snelweg. De lucht in de auto beweegt dus ook met een snelheid van 180 km/u De druk in de auto moet dus erg laag zijn. 2 Ik heb een afgesloten vat met lucht. Daarin zit een roerder. Als ik de roerder aanzet wordt de lucht in het vat heel hard rondgedraaid. Het vat moet dan sterk zijn anders klapt het in elkaar door de onderdruk in het vat. 3 Een gesloten romp moet altijd een beluchtinggaatje hebben. Anders klapt de romp in elkaar als je hard vaart door de onderdruk in de romp. 4 Als je over een velletje papier blaast gaat de lucht omhoog doordat de snelheid boven het papier hoger is, en daar dus een lagere druk is.

Even een uitleg waarom 4 fout is: De wrijving mag je natuurlijk niet verwaarlozen!

Even onder het blaadje blazen en je weet zeker dat het onzin is. Het blaadje gaat dan namelijk omhoog. Deze theorie toegepast op een zeil: De lucht wordt omgebogen door het zeil, dus is er een kracht. Deze zeilkracht wordt op je zeil overgebracht als een druk. Een overdruk aan loefzijde en een onderdruk aan lijzijde. Overdruk betekent een lagere snelheid, en een lagere druk een hogere snelheid. De lucht zal dus sneller gaan aan lij en http://zeilplan.net/leren/theorie/tekst.php?tekst=zeilen.txt (3 of 10)24-1-2004 17:25:18


langzamer aan loef.

Aan de reefknuttels kun je zien dat de lucht aan lij veel sneller gaat. Wat heb je er aan te weten dat de lucht aan lijzijde sneller gaat: -Je stroming aan lijzijde is belangrijker voor je totale kracht dan de kracht aan loefzijde, want aan lijzijde gaat de lucht sneller, en geeft dus meer kracht als je hem ombuigt. (Het kost meer kracht om hard door een bocht te fietsen als zachtjes door een bocht te fietsen) Als je stroming dus loslaat aan lij van je zeil (en deze lucht dus niet meer ombuigt) heb je al snel beduidend minder kracht. Hoe groot is je onderdruk aan lij nu in verhouding tot je druk aan loef? , dat hangt dus af van je zeilkracht!!. Bij een goed zeil als je netjes zeilt kan hier best een factor 4 inzitten. Tipwervels (leklucht) is verlies. (voor ver gevorderden) Er lekt lucht van de hoge druk aan loef naar de lage druk aan lij onder de giek door. (en ook over de gaffel) Dit is een ombuiging de verkeerde kant op.



Je buigt de totale lucht dus minder af, met als gevolg dus minder kracht. Hoe minder dit lekken onder je giek door hoe beter. Bij vliegtuigvleugels gebeurt dit zelfde effect om de uiteinden van de vleugels, de vleugel tips genoemd Vandaar dat dit effect meestal tipwervel wordt genoemd. Wat heb je hieraan om te weten: Een hoog zeil met een korte giek (=hoge aspect ratio) het efficiëntst qua voortstuwing is, omdat deze relatief de minste "tips" en dus tipwervel heeft. Deze tipwervels zijn ook de reden dat bij wedstrijdschepen men graag de fok over het dek laat schuiven, dan gaat er daar geen lucht van loef naar lij, en heb je aan de onderkant van je zeil dus geen tipwervel. Van deze theorie komt de uitspraak "een gaffelgetuigd schip kan minder hoog aan de wind kan varen dan een torengetuigd schip" vandaan. De meeste gaffelgetuigde schepen hebben namelijk een lagere hoogte/lengte verhouding van de zeilen dan torengetuigde schepen. Helaas zijn er zat uitzonderingen, waardoor deze uitspraak vrij dom is. Toepassen van deze theorie. Uitgangspunt van de theorie is dat als je zoveel mogelijk kracht naar voren wil hebben,je zoveel mogelijk lucht naar achter afbuigt. Als je dit voor elkaar krijgt ben je dus goed bezig:



Let wel dat je hier niet dit van maakt:

Nu buig je de lucht gedeeltelijk af naar loef, en wordt je zeilkracht teveel naar lij gericht, en dus niet naar voren. Ook moet je opletten dat er niet dit gebeurt:

In het voorste gedeelte van je zeil moet de lucht heel scherp de bocht om, Dit kan wel eens een te scherpe bocht zijn, zodat



dat niet lukt. Dan laat de stroming los. Dit noemt men ook wel een overtrokken vleugel. Als je je zeil boller maakt voorin moet de lucht minder scherp de bocht om.

Ook moet je natuurlijk niet je zeil te los hebben, dan valt de lucht in het voorlijk aan de verkeerde kant in. Dan gebruik je voorste gedeelte van je zeil niet. Dit noemt men ook wel killen.

Uit dit bovenste verhaal kun je afleiden dat je met een vlakker zeil hoger kunt varen, alhoewel je de lucht dan minder afbuigt en dus minder zeilkracht hebt.



Zou je het zeil verder aantrekken dan buig je ook lucht af naar loef, als je hoger gaat varen begint het zeil te killen. Hierboven keken we alleen naar de bolling in langsrichting, Het zeil heeft echter ook een kleine bolling in de hoogterichting: Het zeil waait aan de bovenkant iets meer uit dan aan de onderkant. Dit noemt men twist:

Een beetje twist is gunstig, aangezien hoe hoger je komt hoe harder de wind, en dus hoe ruimer de wind inkomt. http://zeilplan.net/leren/theorie/tekst.php?tekst=zeilen.txt (8 of 10)24-1-2004 17:25:18


Interessant is ook dat je met de combinatie twist en helling in de boot je zeil veel vlakker kunt krijgen, althans zo ziet de wind dat. Kijk maar eens in onderstaand plaatje. De blauwe lijn is de bolling zoals de wind erlangs gaat

Bij het rechter bootje wordt de wind maar een klein beetje van richting veranderd. Zonder twist verandert de bolling zoals de wind die ziet nauwelijks. Zie de bootjes hieronder

Aangezien je met een vlakker zeil hoger kunt varen volgt hieruit dat je met wat helling in de boot ook hoger kunt varen. (Maar helaas wat minder snel). Bij relatief ruwe zeilen kan het ook gebeuren dat de stroming gewoon loslaat doordat er vteveel bolling is. De lucht moet dan halverwege je zeil te scherp de bocht om. Dit gebeurt typisch als je ruwe zeilen hebt, want dan krijg je meer grenslaag die als het ware in het gat aan lij blijft hangen. Dit is al beschreven in het Coanda verhaal



Heel leuk dit verhaal, maar hoe kun je zien hoe de stroming om je zeil verloopt? Bijvoorbeeld met telltales. Meer hierover in trim Ik zou je aanraden om voordat je daaraan begint eerst Koppels en Krachten Door te lezen. Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=proefjes.txt

Terug naar index Enkel huis tuin en keuken proefjes Het befaamde over een blaadje heen blazen. Het befaamde over een blaadje heenblazen om aan te tonen dat bij een hoge snelheid er lage druk is. Die redenering klopt in zoverre dat de druk daar lager is als de overdruk in je mond. Hoe zit het dan wel:

Volgens het Coanda effect wordt de lucht afgebogen. Hiervoor is een kracht nodig. De reactiekracht op het blaadje duwt het blaadje omhoog. nu echter het volgende experiment:

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Geef je het blaadje een ronding de andere kant op, dan wordt de lucht juist in de andere kant afgebogen en duwt de reactiekracht het blaadje naar beneden. Supervleugel??. Het volgende gedachte proefje gebruik ik om mensen aan het denken te zetten over de kansloze theorie: "de weg boven de vleugel is veel langer dan onder de vleugel en dus gaat de lucht boven de vleugel sneller en krijg je lift"

Welke heeft het grootste lengteverschil tussen boven en onderkant? Welke vleugel werkt het best? Waarom wordt het onderste type nooit gebruikt? http://zeilplan.net/leren/theorie/tekst.php?tekst=proefjes.txt (2 of 10)24-1-2004 17:26:44


Vleugel levert ook lift terwijl hij recht staat? Veel mensen zeggen: Ja, maar een vleugel profiel levert ook lift als hij recht staat!. Dit recht staan is een vorm van optisch bedrog. Hierinder een vleugel welke volgens die mensen "recht staat".

Dit vleugelprofiel levert inderdaad liftkracht terwijl hij recht lijkt te staan. Eigenlijk staat hij niet recht! Stel je voor ik neem dit vleugelprofiel:

En verdraai dat een beetje:

Om het nog echter te laten lijken verander ik een heel klein beetje aan de voorkant:

Dit is hetzelfde als het eerste plaatje!. De Kaars achter de fles uitblazen. Dit is een experimentje om te laten zien dat er bij een dikkere grenslaag sneller loslating is.



Neem een 1,5 L petfles en plaats er direct een brandend kaarsje achter (liefst volle petfles om te voorkomen dat hij in brand vliegt)). Blaas je nu recht tegenover het kaarsje tegen de fles dan zal de stroming om de fles heengaan en het kaarsje uiblazen:

Door het coanda effect bleef de stroming de fles volgen en blies je dus het kaarsje uit. Als je nu de fles heel ruw maakt rem je de stroming dicht bij de fles af, krijg je dus een veel dikkere grenslaag en zal de stroming loslaten. Dit ruwen kun je bijvoorbeeld doen door er een verfrommeld keukenpapiertje omheen te plakken. Dan is het moeilijker om het kaarsje uit te blazen. (let wel op dat keukenpapier niet in de fik vliegt).



Dit effect is nog duidelijker als je een nog dikkere cilinder weet te vinden, (zoals een bloempot) en minder duidelijk als je een kleinere cilinder neemt (een bierflesje). Dit is effect is de hoofdreden dat men bij vliegtuigen zo bang is voor ijsafzetting op de vleugels. Het ruwe ijs zorgt ervoor dat de stroming loslaat, de vleugel dus eigenlijk overtrokken raakt, en het vliegtuig naar beneden valt. (Andere reden is dat het ijs ook wel wat weegt). Waarom blijft een ballon boven een stofzuiger hangen. (als de stofzuiger omhoog blaast) (of waarom je een pingpongbal omhoog kunt houden met een haardroger)

Pingpongbal in lucht (foto gekopieerd van http://www.nal.go.jp/eng/newsletter/98autumn/ m106.htm) Vaak is de redenering dat de lucht daar sneller gaat en er daar dus een lagere druk is volgens bernouilli. (Die redenering klopt in zoverre dat de druk daar lager is als de overdruk in je mond.) Hoe zit dat dan wel: Als de ballon half in de hoofdstroom zit wordt de hoofdstroom afgebogen naar de zijkant, met als gevolg dat de ballon terug de hoofdstroom ingaat.



Doe je ditzelfde met een prop papier dan werkt het niet. Dit komt omdat het oppervlak te grof is en de stroming loslaat. De pingpongbal welke je niet uit de trechter kunt blazen Neem een pingpongbal en een trechter. Als je nu hard door de trechter blaast komt de pinpongbal niet uit de trechter (je kunt ook een trechter uit papier vouwen).



Hoe komt dit nou weer? De lucht wil de trechter volgen, de lucht wordt dus afgebogen naar de zijkant.

Het balletje buigt deze lucht weer terug, waardoor de reactiekracht de bal in de trechter trekt (en het balletje iets uit elkaar trekt)



Een druppel is niet druppelvormig Een druppel is niet druppelvormig als hij valt. Een druppel plat af:



Een druppel wil als er geen zwaartekracht enz is door de oppervlaktespanning rond worden. Laten we deze ronde druppel vallen dan zien we het bovenstaande krachtenspel doordat de lucht om de druppel heengaat. De boven en onderkant worden dus naar binnen geduwd, en de zijkanten naar buiten gedrukt. Iets soortgelijks zie je bij luchtbellen. Deze platten ook af. Omdat deze langzamer gaan kun je dat wel duidelijker zien. en ruw katoenen zeil wat luchtdoorlatend is minder goed werkt als een mooi vlak luchtdicht plastic zeil Dit komt namelijk omdat een katoenen zeil veel meer grenslaag maakt omdat de lucht vlakbij je zeil meer wordt afgeremd en er lucht door je zeil gaat. De stroming zal dan dus eerder loslaten volgens het coanda effect. O ja, veel mensen denken dat een grenslaag heel erg dun is. Dit valt wel mee. bij een binnenvaart schip is hij achter bij het schip al gauw 200mm dik. (afhankelijk van snelheid) Bij je polyvalk is hij nog enkele centimeters. In de friese wateren kun je dit best http://zeilplan.net/leren/theorie/tekst.php?tekst=proefjes.txt (9 of 10)24-1-2004 17:26:44


waarnemen. Ga is over de rand hangen en je zult zien dat bij het achterschip het water inderdaad enigzins wordt meegesleurd. Kun je meteen zien hoe een grenslaag er uitziet en hoe deze groeit, hoef ik dat niet te tekenen Lucht is als ik het goed heb begrepen wat lichter en wat visceuzer, maar het principe is hetzelfde, alleen is de grenslaag bij lucht niet zo snel groeiend als in water. Toch is de grenslaag achter op je zeil al gauw enkele mm dik, je kunt dit zien met behulp van wat sigarattenrook (let op dat je het zeil niet in de fik steekt. Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=MenF.txt

Terug naar index Dit hoofdstuk heb ik toegevoegd omdat blijkt dat hier mensen toch teveel op vastlopen. Mijns inzien kom dit doordat veel mensen allerlei krachten gaan lopen optellen aftrekken ontbinden erbij halen, en ook nog eens allerlei snelheden en koppels en momenten met pijlen aangeven, en nog met een aantal draaipunten werken. Daarom wil ik laten zien dat als je het simpel houdt het wel begrijpelijk is en je er wel wat aan hebt. Daarnaast heb ik al heel wat foute dingen hierover in zeilboeken gezien, vandaar dat ik toch nog iets hier op in ga, omdat dit bij de basis van theorie hoort. Krachten Een kracht is de neiging tot verplaatsing. (dat is heel wat anders dan de snelheid of de richting van bewegen.) Je kunt kracht leveren door te trekken of te duwen. Verspeiden we een kracht over een oppervlak dan noemen we dit druk (of onderdruk) Een kracht heeft een richting. Die geef ik aan met een pijl. Volgens Newton heeft elke kracht een even grote reactiekracht Vrij vertaald zei Newton actiekracht=reactiekracht. Voorbeeld: als ik een tafel verschuif dan heb ik daar een kracht voor nodig die even groot is als de wrijving van de tafel over de vloer. Ander voorbeeld: als ik een bal wegtrap is mijn trapkracht even groot als de kracht nodig om de bal te versnellen. (de versnellingskracht. De kunst van deze manier van krachten bekijken is het simpel houden: Er zijn namelijk nog meer krachten in het spel, zoals de vertragingskracht die de bal levert door de wrijving, de zwaarte kracht, de kracht op de bal door de hogere druk die er inzit, De kracht op het stiksel door de druk in de bal, de kracht op de buitenkant van de bal door de gewone luchtdruk, de kracht op de bal door de draaing van de aarde, de horizontaal ontbonden kracht van de torsie in de vezels van het stiksel etc. Ga je die allemaal er ook bij halen, vervolgens rekenen en dan de bal trappen dan is de kans groot dat je ergens een foutje hebt gemaakt hebt en hem in het verkeerde doel trapt. Daarom worden meestal niet alle krachten getekend, dat doe ik zelf ook niet. Bij het voorbeeld verderop van de tafels teken ik ook niet de reactiekracht, maar je moet wel beseffen dat die er is. Je kunt krachten bij elkaar optellen, maar dat lang niet altijd zo simpel voor degene zonder VWO 6 natuurkunde. Gebruik je boerenverstand: ● ● ●

● ●

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Trek je met zijn tweeen zachtjes aan een touw aan een tafel, dat is hetzelfde als in je eentje hard trekken. Als je als je tegenover elkaar zit allebij even hard tegen een tafel duwt is dat hetzelfde als dat er niemand duwt. Duw je allebij een tafel schuin naar voren, elk aan een kant, dan gaat de tafel toch rechtdoor. trek je allebij aan een hoekpunt van de tafel de tafel rechtdoor dan is dat hetzelfde als een persoon die hard in het midden trekt. Anders gezegd: Krachten in dezelfde richting kun je simpel bij elkaar optellen Krachten in precies de tegenovergestellde richting kun je van elkaar aftrekken. De uitkomst van twee krachten welke onder een hoek ten opzichte van elkaar staan ligt hier tussenin. Twee krachten in dezezelfde richting maar een stuk uitelkaar kun je vervangen door een grote kracht daar tussenin

Maar let op, een kracht zegt lang niet altijd wat over de richting en snelheid van bewegen:

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● ● ●

duw je tegen een stevige muur, dan is er een kracht welke niet tot beweging leidt. duw je schuin van achter tegen een kleein wagentje, dan gaat het toch rechtuit. Duw je heel erg hard tegen een grote vvrachtwagen, dan gaat hij misschien langzaam vooruit -wil je met een slee hard blijven gaan dan moet je hard blijven trekken.

Koppels Een koppel is de neiging om te draaien. Dit kan natuurlijk linksom en rechtsom, maar je kunt het natuurlijk ook oploeven en afvallen noemen. Liggen de actie en reactie kracht in elkaars verlengde dan is het koppel 0. Liggen ze echter iets naast elkaar dan is er een koppel. hoe groter de kracht en hoe verder ze uit elkaar liggen hoe groter het koppel. Anders gezegd: koppel = kracht X afstand. De afstand is de afstand tussen de werklijnen van de krachten. Laat je dus niet verlijden om de uiteinden van de krachten met elkaar te verbinden.

Nu gaan we dit toepassen op een zeilboot. de truuk als je sturen met de zeilen wil laten zien is: ● ● ●

teken altijd de kracht loodrecht op heet zeil ca. 1/3 van de voorkant van de mast. als je twee zeilen hebt neem deze bij elkaar. teken altijd de kracht op het onderwaterschip vanuit je kiel.



Op bovenstaand plaatje van een zeilschip wat halve wind vaart zie je een klein koppel linksom, in dit geval oploevend. Het bootje vaart bijna helemaal rechtdoor ondanks dat de krachten schuin lopen. Dit komt natuurlijk omdat door het zwaard welke bij een lage zijwaartse snelheid al een grote kracht levert, en in voorwaarte richting lang niet zoveel weerstand heeft. Sturen met de zeilen Teken ik een bootje met alleen een grootzeil nu: ●

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met een te los zeil (alleen achterlijkk van het zeil vangt nog wind, zeilkracht wordt minder en schuift naar achterlijk) -met het zeil goed. te strak zeil (achterlijk buigt de winnd niet meer goed om door loslaatwervels, zeilkracht schuift naar voren dan zie je het volgende.

En dit klopt ook met de praktijk bij dit soort boten (wel flink overdrijven en boot rechthouden, en mast niet buigen) http://zeilplan.net/leren/theorie/tekst.php?tekst=MenF.txt (3 of 7)24-1-2004 17:28:12


Nu getekend bij verschillende koersen met de juiste zeilstand:

Ze ziet dat het bootje aan de wind neutraal is en voor de wind graag wil oploeven. Ook dat klopt weer met de praktijk Vuistregel bij deze boot is dus: hoe verder het zeil naar binnen hoe minder loefgierig. (bij constante helling en masbuiging

Nu gaan we kijken wat een fok doet. zie de tekening hieronder van een boot met alleen een fok.



Eigenlijk zie je dat de fok een afvallende werking heeft, alleen voor de wind als je de fok heel los hebt is er een klein oploevend koppel Opvallend is dat je voor de wind vaak toch een afvallende werking hebt omdat men zelden de fok echt ver uitvierd.



De vuistregel "hoe strakker de fok hoe lijgieriger de boot wordt" klopt dus. Het is zelfs zo dat deze de fok vuistregel "hoe strakker het grootzeil hoe minder loefgierig de boot" tot onzin verklaard. Als we namelijk het grootzeil loslaten wordt de zeilkracht van het grootzeil minder, en het grootzeil levert maar een klein oploevend koppel, terwijl de fok een flink afvallend koppel heeft. Anders gezegd, als we het grootzeil loslaten blijft de werking van de fok over. Sommige mensen denken om deze reden dat een boot met een genua of een heel grote fok, lijgieriger wordt. meestal is het omgekeerde waar omdat het achterse gedeelte van de fok eigenlijk een oploevende werking heeft. Maak je dat groter dan wordt de boot dus loefgieriger. Met een flinke genua kun je over het algemeen best halve wind varen zonder dat het roer trekt. Sturen met de helling van het schip Hieronder zie je een bovenaanzicht van een schip wat halve wind over stuurboord vaart. Links hang zij naar loef, rechts hangt zij steeds meer naar lij. De zeilstand blijft gelijk.

Duidelijk is te zien dat de boot die naar loef hangt een afvallend koppel heeft, en de rechter boot een sterk oploevend koppel heeft Dit komt doordat het zeilpunt als het ware steeds verder naar buiten wordt gebracht. Veel meer is hier niet aan uit te leggen. Nog even een misvatting uit de weg ruimen die in veel zeiboeken staat: -Als het zeilpunt in de lengterichting gelijk staat met het lateraalpunt dan is de boot niet loef of lijgierigDIT IS KANSLOOS FOUT. Zie het plaatje hieronder:



Het zeilpunt en het lateraal punt vallen duidelijk niet samen in het zijaanzicht. Toch zie je duidelijk in het bovenaanzicht dat de boot neutraal vaart. Gebruik dus geen zijaanzichten om de sturende werking uit te leggen. De conclusie die zeilboeken aan hun foute uitspraak hangen: -Als je het zeilpunt naar achter verschuifd door bijvoorbeel de mast naar achter te zetten word de boot loefgierigeris wel juist (met uitzondering van pal voor de wind met je zeil haaks op de boot) Dat teken ik niet want dat kun je ook wel zelf uitvogelen. Remmende werking van sturen met het roer In andere hoofdstukken vergelijk ik enkele malen met "de remmende werking van het roer" Hoe zit dat dan: Stuur je niet dan heb je ook geen stuurkracht. Stuur je dan buigt het roer het water af en krijg je dus een roerkracht. Heb je een kleine roeruitslag dan is deze roerkracht bijna helemaal dwars gericht, zodat de kont van het schip die kant opgaat en je waarschijnlijk gaat draaien. Heb je een grote roeruitslag dan is deze roerkracht voor een gedeelte tegen de vaarrichting in. Heb je het roer helemaal dwars gezet dan is deze roerkracht volledig tegen de vaarrichting in.

De natuurkundingen onder u mogen natuurlijk best de krachten gaan ontbinden, maar het principe blijft hetzelfde. Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=onderwaterschip.txt

Terug naar index Het Onderwaterschip Hoe werkt je kiel Een kiel werkt volgens hetzelfde principe als een zeil en vleugel, alleen nu in het water. Je anti-verlijerende kracht is de liftkracht van je kiel. Als je normaal aan de wind vaart is je drifthoek al gauw enkele graden, je kiel wordt dus scheef door het water getrokken met die drifthoek. De kiel levert dan evenveel kracht als de zijwaartse component van je zeilkracht. Vaar je langzamer dan moet je nog steeds dezelfde zijwaartse component van je zeilkracht opheffen met je kiel. Je kiel gaat dan minder water veel ombuigen om toch die zijwaartse component van je zeilkracht op te heffen. Dat kan hij alleen voor elkaar krijgen door schuiner door het water te gaan. Vaar je echt heel langzaam dan raakt je kiel overtrokken,(=de stroming laat los)en kan hij kan die kracht helemaal niet leveren, en je begint nog veel meer te verlijeren. (Bij vliegtuigen is dit "overtrokken" raken heel dramatisch, een overtrokken vliegtuigvleugel valt als het ware uit de lucht. Bij een "overtrokken" kiel begint gewoon de boot veel meer te verlijeren. Vandaar dat je bij aanspringen als je weinig wilt verlijeren je je druk rustig dient op te bouwen met je snelheid of even iets lager moet varen om de zeilkracht even wat meer in de vaarrichting te kunnen richten. Natuurlijk is ook de vorm van je kiel en de ruwheid van je kiel (grenslaagbeinvloeding)erg van belang om loslating van de stroming te vookomen. Wat gebeurt er als een kiel overtrokken is? Als de kiel inderdaad overtrokken is en alleen als een weerstandsprofiel zijwaarts door het water gesleurd wordt is de kiel niet bijzonder efficient meer. De romp van een polyvalk en vele andere boten heeft achter bijna rechte zijkanten, terwijl ze voor nog onder een redelijke hoek staan. De achterkant van het schip is daardoor moeilijker dwars door het water te sleuren als de in zijwaartse richting meer gestroomlijnde voorkant. De achterkant houd zich nog enigszins "vast" in het water. Dat betekent dus dat de boot dan van de wind afdraait. Ga je dan tegensturen en laat je je zeil strak in het midden staan, dan wordt het verlijeren alleen maar erger. Voeg je daar een klapperende fok, een smalle drukke brug met een leuk terras aan toe, dan snap je meteen waarom dat terras zo goed loopt. Laat je roer dus enigzins gaan en zet je zeil in de juiste stand (losser dus) zodat de zeilkracht meer naar voren wordt gericht, je weer snelheid vooruit krijgt, het ernstig verlijeren ophoud, en stuur dan pas weer rustig op en trek het zeil aan.

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http://zeilplan.net/leren/theorie/tekst.php?tekst=onderwaterschip.txt

Roer Als je roerblad overtrokken raakt (zoals bij grote roeruitslagen) geeft het minder roerkracht. Dit effect kun je nog bijtellen bij het effect dat je roerkracht bij grote uitslagen voor een groot gedeelte remmende kracht geeft, ipv sturende kracht. Geef dus niet te veel roer. Overigens moet je niet vergeten dat als je hard aan het draaien bent het water niet recht meer onder je kont naar achter gaat, waardoor je weer meer roer mag geven. Ook opvallend is dat een profielroer beter werkt als een rechte plaat. Dit komt natuurlijk doordat de stroming aan de onderdruk kant van het roer minder snel loslaat, er kan dus meer water worden omgebogen. Wel heeft een profielroer als nadeel dat hij in een keer overtrokken raakt. Het uit het roer lopen is daardoor een vrij abrupt proces. Een vlakke plaat levert weliswaar minder roerkracht, maar raakt gelijdelijk overtrokken, omdat eigenlijk vanaf kleine hoeken de stroming aan lij loslaat. Je voelt uit het roer lopen dus beter aankomen bij een vlakke plaat. Je kunt dit verschil duidelijk merken als je met een nieuwe Hoora boot met profielroer en een oude polyvalk bij harde wind naast elkaar vaart en ze in een windvlaag allebij uit het roer lopen. Terug naar index

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http://zeilplan.net/leren/theorie/tekst.php?tekst=weerstand.txt

Terug naar index Weerstand Weerstand van een schip is globaal te verdelen in 5 zaken: wrijvingsweerstand drukweerstand golfmakende weerstand weerstand door drift Wrijvingsweerstand. Dit is de kracht door wrijving van het water langs de romp. Deze weerstand is afhankelijk van de grote van het oppervlak, de ruwheid van het oppervlak, en de vorm van de grenslaag en natuurlijk de snelheid. Door deze weerstand ontstaat de grenslaag. Het is dus eigenlijk de kracht welke nodig is om water vlak langs een oppervlakte mee te sleuren. Het is vrij makkelijk voor te stellen hoe de wandruwheid hier invloed op heeft. Hoe ruwer de wand hoe meer water wordt meegesleurd. Ook de invloed van de grote van oppervlak is makkelijk te voorspellen: Hoe groter het oppervlak hoe groter de wrijvingsweerstand. De vorm van de grenslaag is wat complexer. Globaal zijn er twee type grenslagen, de laminaire en de turbulente grenslaag. Een laminaire grenslaag is een mooie, vloeiend verlopende grenslaag: plaatje. Deze grenslaag geeft maar weinig wrijving. Hij is echter vrij makkelijk te verstoren zodat hij overgaat naar de turbulente stroming. Een turbulente grenslaag is veel "woester". De snelheidsverdeling is niet zo geleidelijk en niet constant. De snelheidsverdeling is globaal veel anders. Vlak bij het oppervlakte is de snelheid heel laag, waarna er een vrij dik gebied is waarin de grenslaag bijna de omringende snelheid heeft. De turbulente grenslaag heeft een grote weerstand in verhouding tot een laminaire grenslaag. Een turbulente grenslaag heeft vergeleken met een laminaire grenslaag maar voor een heel klein gedeelte een lage snelheid, en voor een groot gedeelte een snelheid welke net iets langzamer gaat als het ongestoorde water.

Wat je typisch ziet is dat aan de voorkant van het schip de grenslaag laminair is, en verder naar achter omslaat in een turbulente grenslaag. Hoe ruwer je romp en hoe harder je vaart hoe eerder de grenslaag omslaat.

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Dikte van de grenslaag is wel sterk overdreven voor deze lengte. Als je een mooie schone romp hebt en het waait eigenlijk nauwelijks en het water is als een spiegel dan kan de grenslaag helemaal laminair zijn. Ben je vet in planee met een aangegroeide romp en zijn er veel golven dan is de grenslaag bijna meteen turbulent. Natuurlijk wordt de grenslaag naar achter ook steeds dikker, Je remt tenslotte steeds meer water af. Bij een zeilboot kun je deze weerstand dus verminderen door een hele mooie vlakke en schone romp te kiezen, zo min mogelijk oppervlak in het water te hebben, en niet te stampen omdat dan de grenslaag omslaat van laminair naar turbulent (zeker niet bij weinig wind). Er zijn ook nog wat andere trucjes om de wrijvingsweerstand/grenslaag te beïnvloeden, maar deze gaan wat ver voor bij een zeilboot in mijn ogen. Dit zijn: -Afzuigen van de grenslaag. Je boort een boel kleine gaatjes in het oppervlak, en daar zuig je aan. Dan zuig je de grenslaag weg, Als gevolg heb je dus bijna geen grenslaag en dus bijna geen wrijvingweerstand. -Aanblazen van de grenslaag. Je boort een boel gaatjes in de richting schuin naar achter en laat daar het met hoge snelheid uitkomen. Je versnelt daarmee je grenslaag weer, met als gevolg bijna geen grenslaag, en dus bijna geen wrijvingsweerstand. Deze worden redelijk vaak gecombineerd, als je iets afzuigt moet je het ook ergens laten en omgekeerd. Grote nadeel van deze twee technieken is dat kleine gaatjes/spleetjes kunnen verstoppen, en je dus uit praktische overwegingen met grotere spleten/gaten moet werken -Bewegen van de wand. Laat je de wand meebewegen met de stroming dan is er geen snelheidsverschil en daarmee geen wrijving. -Versnellen van de moleculen vlak bij de wand door statische elctriciteit gecombineerd met ionisatie/plasma. Zie voor meer informatie bijvoorbeeld: JLN lab -Ter plaatse van je grenslaag iets laten stromen wat minder visceus is. Dit kan betekenen dat je een minimaal visceuze vloeistof aan de voorkant van je schip laat sijpelen, of bijvoorbeeld lucht perst onder je schip, of bijvoorbeeld de romp opwarmt. (warm water is minder visceus) -De wervels in de grenslaag weer netjes in de lengterichting legt, met kleine langsgroefjes. De "haaie huid" bij de nieuwste zwempakken is hier een voorbeeld van. 3M heeft ook wel eens haaiefolie gemaakt voor wedstrijdroeiboten, wat direct daarna werd verboden door de roeibond. Drukweerstand Loopt de stroom mooi om het voorwerp heen dan is de weerstand laag. Wordt de stroom beinvloed dan is de vormweerstand hoog. Deze weerstand wordt ook wel vormweerstand genoemd. Deze is afhankelijk van de vorm van de stroomlijnen. Als je weet hoe de stroomlijn loopt weet je ook de drukweerstand. Bekijk eens onderstaande tekening met de theorieën van hoe werkt een zeil in gedachte: Je kunt op twee manieren zien dat de driehoek meer drukweerstand heeft als de cirkel: een manier is dat bij de driehoek de stroomlijnen na de driehoek anders liggen. er is dus iets verplaatst, en de kracht voor die verplaatsing is de druk weerstand. Andere redenering is dat bij de driehoek kracht C, welke naar gedeeltelijk naar voren is gericht ontbreekt



De route van de stroomlijn is echter volgens de coanda theorie mede afhankelijk van de grenslaag. Zou de cirkel veel grenslaag hebben dan zou de stroomlijn er ook zo uit kunnen zien.

In de praktijk is gebleken dat een turbulente laag door zijn snelheidsverdeling waarbij maar weinig echt heel langzaan gaat, de stroming beter blijft aanliggen. Voor de drukweerstand is een turbulente grenslaag daardoor beter als een laminaire grenslaag. Vandaar dat bij top schaatsers strippen op de pakken worden geplakt om ervoor te zorgen dat de stroming turbulent wordt, en daardoor de schaatser een een lagere drukweerstand krijgt. Hetzelfde trucje verklaart de putjes van de golfbal.

Dit zijn leuke truuken, maar helaas niet toepasbaar op een scheepsromp, daar is de stroming na ca. een meter toch al turbulent. Strippen opplakken maakt de grenslaag dan alleen maar dikker, en dus een snellere loslating en dus de drukweerstand hoger. De romp moet dus gewoon zo gestroomlijnd mogelijk zijn. Opvallend is dat bij veel schepen de drukweerstand lager is als ze achteruitvaren, of onder een grote helling lager is. Dit zie je ook vaak bij autos terug. Meest gehoorde verklaring hiervoor is dat er voor een groot gedeelte op gevoel wordt ontworpen, en het gevoel zegt toch dat een driehoek met de punt naar voren minder druk opmaakt als andersom. Terwijl de werkelijkheid andersom zegt. Ook bij druk weerstand geld net zoals bij een zeil dat de achterkant het belangrijkst is omdat daar onderdruk heerst en daar dus de http://zeilplan.net/leren/theorie/tekst.php?tekst=weerstand.txt (3 of 7)24-1-2004 17:30:16


hoogste snelheden zijn. Golf makende weerstand. Een schip maakt golven als zij vaart. In die golven zit energie. Die energie is de golfweerstand. golf ontstaat door de scheepsronding, want die geeft een drukverschil, die zich nu behalve in een snelheids verandering ook uit in een niveau verschil. Hoe sneller en hoe meer het water de bocht om moet, hoe hoger het nivoverschil. Een hoge druk uit zich natuurlijk in een niveau verschil omhoog, zoals bij de boeg, waar het water door de boeg van de hartlijn van het schip wordt afgebogen, dus wordt weggedrukt. Dit noem ik nu even boeg berg. Een lage druk uit zich natuurlijk in een nivo verschil omlaag, zoals op de grootste breedte van het schip, waar het water juist weer naar binnen wordt afgebogen, dus weer richting romp wordt getrokken. Dit noem ik nu even schouderdal bij de kont wordt het richting hartlijn stromende water weer naar buiten afgebogen, het wordt door het water wat van de andere kant komt weggedrukt, dus nivoverhoging bij de kont. Dit noem ik nu even achterberg

Nu heeft water de eigenschap dat een nivoverhoging graag geegaliseerd wil worden, vandaar dat je sluizen enzo nodig hebt en het water niet gewoon met een graafmachine kunt ophogen. Het bergje valt dus als het ware naar beneden door de zwaartekracht, en schiet zelfs door als het het niveau van het omringende water heeft bereikt. De golftop wordt dus na een tijdje een golfdal, en even later weer een golftop In dat tijdje heb je wel een stukje gevaren. Heb je in het tijdje dat je boegberg een boegdal werd een halve scheepslente gevaren, dan valt je boegdal dus samen met je schouderdal. Hierdoor heb je dus een dieper dal halverwege je schip. In diezelde tijd is je schouderdal een schouderberg geworden, en valt samen met je achterberg, waardoor je achterberg dus extra hoog wordt. In deze situatie maak je dus heel veel golven, en heb je dus een heel hoge golfmakende weerstand. Deze stuatie is bekend onder de naam rompsnelheid. Je golfpatroon ziet er dan ongeveer zo uit:

((De rompsnelheid van een schip is in km/u als je lengte in meters invult ongeveer: 4,5*wortel(lengte). Een valk (6m) heeft dus een rompsnelheid van 11 km/u)). Vaar je in de tijd dat je boegberg een boegdal en weer een boegberg werd een halve scheepslengte dan heft je boegberg juist je schouderdal op en maak je dus heel weinig golven, en heb je dus een lage golmakende weerstand. Vaar je sneller dan de rompsnelheid, dan noemt men dat planeren. Typisch zie je dan juist een kuil direct achter het schip ontstaan en maak je opeens veel minder golven. Je vaart als het ware op je boeggolf. hierdoor kom je een stukje omhoog (in vergelijking met je rompsnelheid, waarbij je eigenlijk in je boegdal vaart). Dit omhoogkomen is wat anders dan de kracht waarmee waterskiers niet zinken! Het niet zinken van een waterskier komt omdat de achterkant van de skie lager zit als de voorkant, en daardoor het water naar beneden wordt afgebogen, en dus de reactiekracht op de skie omhoog is. Dit "waterski-effect" moet je wel enigzins hebben bij een planerende boot, anders "zuigt" de boot zich verder naar beneden, zoals bijvoorbeeld bij sleepboten gebeurt, die wel vaak wel grote motoren hebben, maar een oplopende kont welke het water juist omhoog richt, en de boot dus naar beneden trekt.



Vandaar dat sommige sloepen "planeer vlakken" hebben (onder water bij de spiegel). Dit is om niet te veel naar beneden te worden gezogen, zodat ze dus wel kunnen planeren.

Een catamaran, kano, of wedstrijdroeiboot romp heeft heel weinig ronding in de lengterichting. Dit betekent dat de boegbergen, schouderdalen en hekbergen in vergelijking met een gewone romp erg klein zijn. De golfmakende weerstand is daardoor maar een klein gedeelte van de totale weerstand. Hierdoor is de overgang tussen rompsnelheid en planeren lang niet zo duidelijk als bij een zwaardboot, en zul je deze mensen wat minder over planeren horen praten. Bij sommige boten worden de rondingen in het water minder door haar scheef te hangen, wat kan resulteren in een lagere golfmakende weerstand. (en vorm en wrijvings weerstand). Tegelijkertijd wordt dan het waterskie-effect minder door het mindere oppervlakte waardoor planeren wel kansloos is. Vandaar dat een zwaardboot bij weinig wind vaak toch beter vaart onder een kleine hoek. Bij een valk is dit volgens sommige een van de redenen dat hij onder helling hoger aan de wind kan varen. Andere reden is dat de valk onder grote helling loefgieriger is, en dus om rechtdoor te blijven varen zo roer moet worden gegeven dat je met het roer het water afbuigt naar lij, en dus als het ware de boot naar loef duwt. Dit roergeven remt de boot weer af waardoor de schijnbare wind ook wat ruimer inkomt. Ook wordt de romp assymetrisch waardoor deze misschien het water naar lij afbuigt, en de boot dus naar loef duwt, maar dat lijkt me sterk Laatste mogelijke verklaring (die ik het best vindt) is dat de bolling van het zeil anders wordt gevolgd, doordat de wind als het ware vanuit de halshoek naar boven gaat, en dus minder bolling tegenkomt, waardoor het zeil vlakker lijkt voor de wind, en je dus hoger kan. Dit vlakkere zeil wordt nog eens extra vlak doordat bij grote hellingshoeken de twist van het grootzeil beduidend meer wordt doordat de gaffel onder die hoeken meer naar beneden valt. Vandaar dat juist bij gaffelgetuigde schepen zonder neerhouder dit effect dat ze hoger kunnen varen onder grotere hellingshoeken optreedt. Weerstand door drift. Vooral bij aan de wind varen is er een grote zijdelingse kracht en slechts een kleine voorwaartse kracht. Daardoor wordt een zeilboot eigenlijk scheef door het water getrokken in plaats van rechtdoor. Hierdoor ontstaat drift.



Heb je veel drift dan heb je ook een hoge weerstand door drift. Dit is vergelijkbaar met de remmende werking van roergeven. Als we inzoomen op de kiel: De kracht op de kiel werkt (net zo als bij een zeil) loodrecht op de kiel. Bij drift is dit niet meer loodrecht op de vaarrichting. Dat betekent dat de dwarskracht van de kiel een beetje tegen de vaarrichting komt in te staan. Anders gezegd, om de boot dwars door het water te trekken heb je vrij veel kracht nodig



Wat je eigenlijk wil is dus zo min mogelijk drift. Dat kan je bereiken door een grote, goed gevormde kiel te kiezen welke al bij een kleine drifthoek voldoende kracht geeft om de dwarskracht van je zeil op te vangen. Hierbij geld net zoals bij een zeil dat een diepstekende in de lengterichting korte kiel dat het beste doet. Kun je niet zo diep, dan moet je het in de lengte zoeken. Een diepstekende kiel brengt echter het aangrijpingspunt van de dwarskracht lager, wat resulteerd in een groter hellend koppel. dit wordt wel weer enigszins gecompenseerd door hel lagere zwaartepunt. Meestal kun je echter niet makkelijk iets aan de kiel veranderen, Het enige wat je dant kunt doen is ervoor zorgen dat de kiel mooi glad is, zodat de stroming goed blijft aanliggen. Dit kan soms best veel schelen. Vergeet niet dat de krachten op je zeil ongeveer even groot zijn als op je kiel. Overigens helpt je romp ook tegen verlijeren, principe van driftweerstand blijft echter hetzelfde. De dwarskracht van je romp door het water is soms wel te beinvloedden. Denk bijvoorbeeld eens aan een knikspant welke meer "knik" aan lij in het water duwt bij grote hoeken, of aan een catamaran met a-symmetrische rompen, etc Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=stabiliteit.txt

Terug naar index Terug naar index Stabiliteit Ook stabiliteit wordt door veel mensen gezien als iets heel ingewikkelds. Omdat naar mijn mening de de zeilboeken vooral dwaalsporen geven vertel ik het hier nog een sop een andere manier. Stabiliteit is de mate waarin het schip weer overeind wil komen. Met aanvangstabiliteit wordt bedoelt de mate waarin een schip overeind wil komen bij kleine hoeken. Met stabiliteits omvang wordt bedoelt tot welke hoek het schip nog overeind wil komen. Globaal is stabiliteit opgebouwd uit: vormstabiliteit gewichts stabiliteit Snelheids stabiliteit Elke boot heeft met al deze stabiliteitsvormen te maken. Soms is een veruit het belangrijkst en wordt gezegd dat die boot "zus en zo" stabilititeit heeft. Vormstabiliteit Als je een bal onderwater duwt heb je hiervoor een kracht nodig. Bij een skippy bal heb je meer kracht nodig dan een pinpongbal. De skippybal kun je maar een klein stukje onder water trekken. Eigenlijk is het zo dat hoe meer volume je onderdompelt hoe meer kracht je nodig hebt. (Om een melkpak van 1 liter onder te dompelen door zijn eigen gewicht moet je het vullen met 1 kg water.) (Dit is nou de wet van Archimedes). Stel je nou de volgende situatie voor: Ik heb een vlotje gebouwd met twee dicht bij elkaar elkaar geknoopte ballen. De ballen worden enigzins in het water geduwd door het gewicht van het vlot. Als ik het scheef trek gaat de ene bal dieper en de andere bal juist ondieper. Dat betekent dat de bal die dieper gaat graag weer omhoog wil, en de bal die uit het water is niet meer omhoog wil. De lage kant wil dus omhoog.

Dat is nu de basis van vormstabiliteit. Dan nu het plaatje met met een vierkante bakken, allebij even scheef getrokken, maar de een veel breder als de ander

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Je ziet duidelijk dat aan de lage kant de bak verder het water wordt ingedrukt, en aan de hoge kant minder het water wordt ingedrukt. de verandering tov rechtop is aangegeven met de blauwe vlakjes. Ook zie je dat bij de brede bak de blauwe vakjes veel groter zijn, en ze liggen ook verder van het midden als bij de smalle bak, de opdrijfkracht komt daardoor ver uit het midden te liggen. De drijfkracht verschuift daardoor veel meer bij de brede bak als bij de smalle bak. Dit effect zie je net zo goed bij een ronde vorm.

Vormstabiliteit is dus afhankelijk van je breedte! Hoe zit dat nu met dat verhaal uit de zeilboeken dat vormstabiliteit afneemt als je erg scheef ligt? kijk maar naar onderstaand plaatje en let op de grote en vooral afstand tot het midden van de blauwe vlakken:

Tot nu toe is het eigenlijk hetzelfde verhaal als in het zeilboek alleen op een andere manier verteld. In de zeilboeken heeft men het over het verschuiven van het drukkingspunt, waarbij het drukkingspunt het aangrijpingspunt van de opdrijvende kracht is. Een hele mond vol, maar hoe weet je nu waar het drukkingspunt zit? welnu, het drukkingspunt is het midden van het onderwaterschip. Dus zoals op onderstaand plaatje voor een rechte bak (let op de gestippelde hulplijnen)



In deze tekening kun je zien dat het drukkingspunt inderdaad iets verschuift als er helling onstaat. Zonder de hulplijnen teken je jezelf echter als snel klem Je zult zien als je een flink bredere bak tekent dat inderdaad het drukkingspunt bij een brede bak meer van het midden komt te liggen. Teken dit zelf maar eens. Gewichtsstabiliteit in het verhaaltje over vormstabiliteit kijk ik alleen maar naar hoe opdrijvende krachten van het midden schuiven. Het omgekeerde van de opdrijvingskracht is de zwaartekracht. (Als het goed is, anders zink je of ga je juist vliegen) Zit er een afstand tussen de werklijnen van de opdrijvende kracht en de zwaartekracht dan heb je een koppel, dat is nu het oprichtend koppel (of kenterend koppel.) Ga je hier nu fijn aan zitten tekenen, dan zul je zien dat bij grotere hoeken de hoogte van je zwaartepunt er erg toe doet. Hoe lager je zwaartepunt hoe meer opichtend koppel. Zie het plaatje hieronder met links een mega zware kiel en rechts iemand boven in de mast. Werklijnen zijn de rode stippel lijnen.



Snelheids stabiliteit Dit heeft te maken met het waterski effect, buigt de boot het water naar beneden af en komt ze schuin te liggen dan krijg je aan de lage kant meer lift met als gevolg een richtend moment. Buigt de boot het water juist omhoog af zoals de sleepboot, dan heeft dit juist een negatieve uitwerking op de stabiliteit. Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=trim.txt

Terug naar index Trimmen Ook dit theorie onderdeel wordt als erg ingewikkeld gezien. De beste zeiltrim is erg moeilijk omdat er zoveel factoren zijn die ook nog constant veranderen. Veel zeilers komen niet veel verder dan: "Bij harde wind moet je alles strak zetten, en bij weinig wind alles zo zetten dat net je vouwen uit je zeil zijn." Veel zitten deze mensen er niet naast, maar ze missen wel de uitdaging om "de boot zo lekker mogelijk te laten lopen", wat een hele nieuwe dimensie aan zeilen toevoegt. Een belangrijk hulpmiddel hierbij zijn de telltales, die de stroming om het zeil vertellen. Dat je behalve met je zeilen ook met je gewicht kunt trimmen wordt vaak niet eens beseft. Volgens mij is het idee achter trim door bijna alle zeilers te volgen die al zo goed zijn dat ze zelfstandig kunnen varen: Over hoe je je zeil in de juiste vorm krijgt ga ik niet op in aangezien dit teveel mogelijkheden zijn en omdat de precieze uitwerking nogal eens per boot verschilt. Als voorbeeld: zet je de neerhouder wat losser om meer twist te krijgen dan krijg je bij een flexibele mast ook minder mastbuiging en daarmee meer bolling. Zeiltrim Bij optimale zeiltrim wordt zoveel mogelijk wind zoveel mogelijk naar achter afgebogen. Hoe buig je nou zoveel mogelijk wind om naar achter: Neem zoveel mogelijk bolling en zorg dat je achterlijk zoveel mogelijk naar achter wijst. maar let op het volgende: ●

●

●

●

De stroming om het zeil niet mag niet loslaten, wat je kunt zien aan je achterste telltales die niet meer naar achter gaan. Als dit gebeurt kan dit komen omdat: ❍ je je zeil te ver hebt aangetrokken, de intredehoek is te groot ten opzichte van de hoek met de wind ❍ Je bollingscurve ergens te veel is voor de stroming om nog te volgen. Het voorlijk mag niet killen. Als dit gebeurt kan dit komen omdat: ❍ je zeil niet genoeg is aangetrokken. De wind komt dan in het voorste stukje simpelweg de verkeerde kant het zeil binnen. ❍ De intredehoek van het zeil te groot is (Dit geld natuurlijk als je je zeil al zover hebt aangetrokken dat het achterlijk al naar achter wijst). De telltales horen over de volle hoogte van het zeil naar achter te wijzen. Als dit niet zo is kan dit komen omdat: ❍ als alleen je bovenste telltales naar lij gaan: te weinig twist. ❍ als allen je onderste telltales naar lij gaan: te veel twist. Dat je de boot nog kunt houden, en je niet teveel helling krijgt. Je bent dan overpowered, je buigt dus teveel wind teveel om, je kunt dan: ❍ Je zeil wat losser te laten, dan buig je de lucht wat minder om.(en dan begint je voorlijk te klapperen, en hoor je intredehoek dus verkleinen) ❍ Meer twist te nemen waardoor de bovenkant van het zeil (wat het grootste hellend koppel levert) wat minder wind ombuigt. ❍ Te reven, waardoor je gewoonweg minder wind ombuigt, en juist het meest hellend

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koppel leverend stuk zeil bovenin omlaag schuift. Als je nu afgehaakt bent komt dit waarschijnlijk door de enorme berg begrippen in bovenstaand verhaal. Zie hieronder wat ik met deze begrippen bedoel. begrippen:



Over de begrippen is meestal nogal wat verwarring omdat een eigenschap meestal samenhang met een andere eigenschap. Zo heeft bijvoorbeeld een zeil met de bolling ver naar achter een wat meer scheppend achterlijk. De term achterlijk open zetten kun je op twee manieren zien: ●

●

Twist zorgt ervoor dat je achterlijk als het ware wegwaait, wat een lagere uittredehoek geeft. Je uittredehoek verkleinen door je bolling minder te maken en verder naar voren te zetten geeft ook een lagere uittredehoek.

Twist

Hoe hoger je komt hoe harder het waait. De wind wordt namelijk afgeremd door de wrijving met http://zeilplan.net/leren/theorie/tekst.php?tekst=trim.txt (3 of 11)24-1-2004 17:31:55


het water. Hoe verder van het water hoe minder afgeremd. (dit is dus eigenlijk ook een grenslaag). Als je halve wind vaart komt de schijnbare wind bovenin je zeil dus ruimer in als onderin je zeil. Bovenin moet je zeil dus meer uitstaan. Halve wind is een verdraaiing van je bovenste zeillat van ca 15 graden normaal, aan de wind is 5 graden normaal. Natuurlijk is dit maar een richtwaarde, die van meerdere dingen afhankelijk is. Een beetje twist is dus goed. Opvallend is dat twist onder helling een grote invloed heeft. Als het schip onder helling komt lijkt de lucht meer vanaf onderen te komen, dat wil zeggen uit de richting van de giek. Het profiel van het zeil wordt dan dus wat anders gevolgd. Bij veel twist en grote hellingshoeken heeft dit een zeer grote invloed: In de plaatjes hieronder in blauw de route van de lucht langs het zeil. Beide van boven af gezien, rechtse schip ligt onder helling.

Nu hetzelfde maar dan met twist:



Hopelijk heb ik hiermee duidelijk gemaakt dat twist een heel grote invloed heeft onder helling. Wanneer is de twist juist gekozen: Als alle telltales in je achterlijk naar achter gaan. Heb je geen telltales in het achterlijk (zoals bij je fok)dan is het een goede vuistregel dat als je wat oploeft het zeil gelijkmatig van boven tot onder begint te killen, en niet eerst boven of onder.

ourworld.compuserve.com/homepages/lestergilbert/

plaatje van http://

Telltales

Telltales zijn verklikkers die de stromingsrichting weergeven. Ze bestaan in vele afmetingen en uitvoeringen. Een telltale is niets anders dan een dun,licht touwtje of dun strookje dat aan je zeil is



vastgemaakt. Aangezien je lucht niet kunt zien heb je telltales nodig om te zien hoe de stroming om je zeil heen verloopt. De basis voor een goede zeiltrim. Ze zijn verkrijgbaar in de betere watersportwinkel. Uiteraard kun je ze ook zelf maken, wat veel goedkoper is aangezien een dun touwtje/reepje nogal gevoelig is voor beschadiging. Simpelste uitvoering is een wollen draadje (liefst synthetisch in verband met rot) dat met een naald door je zeil wordt gehaald, en aan bijde zijden een knoopje zodat het niet los kan schieten. Ik ben geen voorstander van gaatjes in je zeil prikken dus mijn versie is een synthetisch wollen draadje vastgeplakt met een rondje geknipt uit zeilreparatie tape. Op huurboten gebruik ik meestal een stukje cassetteband (heb ik in overvloed aangezien mijn autoradio graag bandjes lust)met een gewoon stukje plakband. Gebruik liever geen grijze tape (ducktape) of bruine tape (dozentape) omdat deze soms enorm smerige lijmsporen achterlaten bij verwijderen. Een strookje knippen van licht spinnakerdoek (5 oz)is nog mooier aangezien deze bij regen deze iets minder snel an je zeil kleven. Ik probeer de telltales niet in de buurt te plakken van stiksels aangezien ze hier nogal eens aan willen blijven kleven. Als laatste zou ik willen opmerken om ze donker van kleur te maken zodat je ze goed kunt zien. Plaats van de telltales: Eigenlijk ben je alleen geïnteresseerd in of de stroming bij de achterkant van het zeil nog mooi verloopt. Logisch is dus om ze daar te zetten, 3 is meestal voldoende, mooi verdeelt over je achterlijk. Bij je fok echter is dit meestal vrij zinloos omdat je ze dan niet kunt zien als je je aan loef bevind doordat ze dan achter het grootzeil zitten. Daarom zet je ze bij de fok zover naar voren dat je ze als stuurman kunt zien. Ga je voor de eerste keer fanatiek aan de gang met de zeiltrim dan is het aan te bevelen om er meerdere horizontaal te plaatsen zodat je de stroming om het zeil goed kunt zien. dat ziet er helaas al snel uit alsof er iemand jarig is aan boord, dus is het aan te raden dese tijdelijk te gebruiken. Ga je zo eens rondvaren dan zul je waarschijnlijk merken dat het heel moeilijk is om alle telltales naar achter te krijgen. Bedenk dan dat de stroming aan lij het belangrijkst is aangezien deze de hoogste snelheid heeft. Aan loef in je voorlijk lukt het vaak niet om de telltales naar achter te krijgen. Dit komt omdat als het goed is de stroming aan loef langzamer gaat als aan lij, al gauw is dit zoveel langzamer dat de stroming daar zelfs stil komt te staan, en je telltales gewoon wat ronddwarrelen. Let dus alleen op je telltales aan lij en aan je telltales aan je achterlijk. Krijg je ook je telltales aan lij niet goed bedenk dan dat het eigenlijk alleen gaat om je telltales aan je achterlijk. Het gaat erom wat je zeil in totaal doet en niet wat het voorste stukje doet. Uiteraard is het wel het streven om de telltales aan lij en in het achterlijk goed te krijgen. Zeiltrim hoog aan de wind Snelheid versus hoogte. Hoog aan de wind is het meestal de bedoeling om zo hard en zo hoog mogelijk te gaan. Vaak zie je de neiging om het zeil dan ook maar helemaal binnen te trekken. Dit heeft geen zin als daarbij de lucht naar loef wordt afgebogen, dan ben namelijk lucht naar



loef aan het ombuigen, en maak je dus meer zijwaartse kracht, die leid tot een grotere driftweerstand. Met een vlakker zeil kun je het zeil wel verder aantrekken. Helaas is het zo dat een vlakker zeil de lucht minder ombuigt en je dus minder kracht krijgt.

Vaak is het niet mogelijk om even snel het zeil vlakker te maken, denk dan ook eens aan de truc om iets meer helling te nemen, dan gaat de lucht "vlakker langs je zeil zoals hierboven in het verhaal over twist beschreven. Dus: hoe vlakker het zeil, hoe verder je het mag aantrekken, hoe minder kracht je krijgt. Let trouwens ook eens op bovenstaand plaatje op de lucht voordat die bij het zeil is. De lucht voelt namelijk al voor het zeil de onderdruk aan lij en wil daar naar toe stromen. Dit effect noemt men ook wel "upwash". de ombuiging achter het zeil noemt men "downwash". Deze termen komen uit de wereld van vliegtuigvleugels, waar de up en de down wat logischer is.

Als laatste is op te merken dat aan de wind de bolling iets verder naar voren wordt gezet. Dit zorgt (naast afvlakking van het achterlijk, dus minder afbuiging naar loef) ervoor dat je zeilkracht iets meer naar voren wordt gericht. http://zeilplan.net/leren/theorie/tekst.php?tekst=trim.txt (7 of 11)24-1-2004 17:31:55


Zeiltrim ruimere koersen Op ruimere koersen speelt hoogte halen natuurlijk weinig rol. Ook is de helling meestal niet zo gauw een probleem omdat de zeilkracht nauwelijks naar de zijkant gaat. Op ruimere koersen gaat het dus om zoveel mogelijk kracht. . Dit betekent dus een zo groot mogelijke intredehoek en een zo groot mogelijke uitredehoek en geen loslating van de stroming. Dus een zo groot mogelijke diepte en een een curve die maar langzaam afneemt. Extra bolle zeilen zoals bolle jannen, gennakers en spinnakers kunnen nu meestal hun werking doen. Bij deze extra bolle zeilen dient natuurlijk ook gelet te worden dat de lucht niet naar loef wordt afgebogen, maaar een beetje is niet zo erg zolang er maar wat lucht naar achter wordt omgebogen. Het alternatief is al gauw dat zeil te strijken, waarmee het natuurlijk helemaal niks meer doet. Zeiltrim met golven Golven zorgen ervoor dat het schip en daarmee de zeilen heen en weer bewegen. Zowel ten opzichte van de wind als ten opzichte van het water. Als je zeil heen en weer beweegt betekent dit dat de wind door het schommelen ook steeds iets anders binnenkomt. Voor je zeiltrim betekent dit dus dat deze niet superkritisch kan zijn. Als de wind iets voorlijker inkomt of iets dwarser inkomt moet ze zeiltrim ook nog redelijk zijn. Aan de wind dus iets meer op snelheid varen in plaats van op hoogte. Voor je kiel geld dat deze ook heen en weer beweegt. Je kiel ziet dit als een constant veranderende drifthoek, soms een negatieve drifthoek (geeft ook drift weerstand) en vaak een heel grote drifthoek. Een grote drifthoek geeft veel driftweerstand. Je hebt dus meer driftweerstand, met als gevolg dat je langzamer gaat varen en nog meer driftweerstand krijgt. Dit is het simpelst te verbeteren door iets meer op snelheid te varen waardoor de drifthoek en daarmee de driftweerstand minder wordt. Je bent het grootse gedeelte van de tijd bezig om golven te beklimmen, aangezien golfaf sneller gaat, en golfaf dus minder tijd kost. anneer er tegen een golf op wordt gevaren zal met name de snelheid van de top van de mast erg laag zijn. Hierdoor zal de schijnbare wind in de top veel ruimer inkomen. Met wat extra twist staat je zeil op dat moment optimaal. Verder is het gebruikelijk om de bolling iets verder naar voren te zetten. Ik weet niet precies waarom maar een redenering hierachter is dat hoe verder de bolling naar voren staat hoe beter de bolling op die plek blijft. Als je bolling steeds van voor naar achter beweegt heeft de stroming wat meer moeite om aan te blijven liggen. Weer een andere redenering om de bolling wat verder naar voren te leggen is dat de stroming wat onregelmatiger is, en hoe verder de bolling naar voren in het zeil ligt hoe minder snel de stroming daar loslaat, omdat daar minder grenslaag is en omdat de fok daar helpt om de



stroming aan te laten liggen. Een andere redenering die ik wel eens van een grootzeil trimmer heb gehoord is dat de zeilkracht daarmee wat meer naar voren wordt gericht. Die laatste heb ik mijn twijfels bij omdat je je dan kunt afvragen of dit niet altijd verstandig is. Balanceren zeilkracht en onderwaterkracht. Heb je nu eindelijk een heleboel power in je zeil getrimd, dan kan het gebeuren dat de boot vreselijk loefgierig is zodat je een flinke roeruitslag moet geven. Veel roeruitslag geven remt natuurlijk, dus dat is geen optimale trim. Een beetje druk op je roer (loefgierig natuurlijk) is gunstig. Dit is makkelijker voor te stellen als je de werking van het grootzeil en de fok vergelijkt met de werking van kiel en roer. Je vaart toch ook niet met de fok los! Helmaal uit den boze is een lijgierig schip, dat kun je vergelijken met rondvaren met een onderwaterschip met de fok bak. Je kunt het ook anders zien: Buig je met het roer water naar lij om dan duw je de boot naar loef, je verlijerd dan dus minder en je hebt dan ook minder driftweerstand. De discussie is eigenlijk hoeveel roeruitslag moet je hebben bij het rechtdoor varen. Om dit voor te kunnen stellen ga ik weer naar de vergelijking met de zeilen. Je fok heeft de wind al wat omgebogen voor je grootzeil. Daarom is het grootzeil altijd al wat extra naar binnen getrokken ten opzichte van de fok. (overigens zou je ook kunnen zeggen dat de fok wat losser staat als het grootzeil omdat de fok in de upwash van het grootzeil zit.) Je kunt net zo goed zeggen dat de kiel het water voor het roer al wat heeft omgebogen, en je roer dus wat "strakker" moet. Gevangen in een sterk overdreven plaatje de juiste stand waarbij het de blauwe lijn de route van het water voorsteld:



De roerstand hoort dus een stukje naar lij (=helmstok naar loef)te zijn, maar minder dan de drifthoek. Als de drifthoek dus 5º is zal de helmstok ca 1-4 graden naar loef horen te zijn. De kleine loefgierigheid die hierbij hoort is te verwezenlijken door met de plaats van de zeilkracht te spelen, Dit kan met de helling gebeuren (=dwarscheepse verplaatsing van het zeilpunt) en/of met de zeilen en/of bolling naar voor of achter schuiven (=verschuiven van zeilpunt in lengterichting). Trimmen met de helling Los van de grote invloed van de helling op de zeilen, kan dit een grote invloed op de scheepsweerstand hebben. Meestal is het zo dat een schip het beste vaart met een kleine helling omdat het nat oppervlak iets kleiner wordt. Is men aan het planeren dan moet men juist zo recht mogelijk varen, omdat men de liftkracht naar beneden wil richten. Ook gaat het water scheef over je kiel wat ook invloed heeft. http://zeilplan.net/leren/theorie/tekst.php?tekst=trim.txt (10 of 11)24-1-2004 17:31:55


Meer durf ik hier nu niet over te zeggen omdat dit erg scheepstype afhankelijk is. Trimmen met de helling in lengterichting Als de boot wat achterover hangt dreigt als snel de spiegel in het water te komen. Als de spiegel gedeeltelijk onder water komt betekent dit meestal dat het water daar een raar hoekje om moet, en daarmee extra weerstand maakt. Als de spiegel te ver boven water steekt zit er een heel stuk romp niet in het water. Dit heeft tot gevolg heeft dat de lengte van de waterlijn korter is en daardoor de golfmakende weerstand dreigt toe te nemen. Meer durf ik hier nu niet over te zeggen omdat dit erg scheepstype afhankelijk is. Waarmee te trimmen Zoals ik bovenaan al zei is dit erg bootafhankelijk. Beste is dus naar mijn mening hier gewoon eens mee te spelen op een dag met weinig wind zodat je goed kunt zien wat er gebeurt als je aan een lijntje trekt. Liefst zelfs zonder "advies" van pottenkijkers die het "beter" weten hoe het moet, zodat je nooit eens dingen kunt overdrijven als experiment. Enige tips die ik geef is dat als je je neerhouder strak doorzet als je ook je grootschoot helemaal hebt aangetrokken, en dan de grootschoot weer loslaat er een kans bestaat dat je mast knikt bij de giek. Andere tip is dat dat je een stijve vast niet moet proberen te buigen. Voor de rest niet te bang zijn om een lijntje goed strak te zetten, maar blijf je gezond verstand gebruiken. Volgende stap is dat je een zelfde boot zoek en daar dicht bij gaat varen zodat je kunt zien bij welke trim je sneller gaat. Veelal is de gelegenheid hiertoe op een dagtocht bij een zeilschool, of indien je een eigen boot hebt op gezellige wedstrijden die meestal wel door een lokale club worden georganiseerd. Meestal zijn die lokale competities ook leuk voor beginners omdat het nivo meestal niet belachelijk hoog is en er veel verschil zit tussen de boten, er zit altijd wel iemand tussen waaran je gewaagd bent, en anders kun je altijd de schuld geven aan het materiaal. Ook heeft dit als voordeel dat je aan de bar de wedstrijd in een ontspannen sfeer kunt evalueren, (wat voor veel mensen misschien nog wel belangrijker is als het zeilen zelf). Is je materiaal echt bar slecht dan kun je vast wel iemand vinden die volgende week een fokkemaat nodig heeft. Terug naar index


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Terug naar index Veelgestelde vragen Uiteraard kun je nieuwe vragen aan mij mailen. ● ● ● ●

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1 Waarom verlijer ik zoveel na de overstag? 2 Hoe werkt een vleugelkiel? 3 Wanneer moet ik kiezen voor een High Aspect fok en wanneer voor de genua? 4 Molens draaien altijd linksom en mijn boot loopt ook lekkerder over bakboord, Is dit om dezelfde reden? 5 Ga ik harder in de vaargeul of op het meer? 6 Als ik ruime wind in de trapeze blijf hangen, en de stuurman naar de andere kant gaat zodat we toch rechtop blijven, ga ik harder. Hoe kan dit? 7 Kan een boot sneller dan de wind varen? 8 Kan ik bij de hogerwal aanleg met sliplanding niet beter de fok wegrollen? 9 Iemand heeft heeft het drukverschil tussen loef en lij gemeten en kwam op 0 uit. Hoe kan dat? 10 In je koppels en krachten verhaal zeg je dat de zeilkracht loodrecht op de giek is. In je verhaal over zeiltrim zeg je dat de bolling naar voren plaatsen de zeilkracht meer naar voren richt. Je verhaal klopt dus niet en je bent een prutser! 11 Met dat theorie verhaal van jou kom ik nergens tegen dat de fok meer doet dan het grootzeil. Toch heb ik dat al meerdere keren gehoord. hoe zit dat ? 12 In het blad "Zeilen" stond dat een cunningham hole weinig zin heeft bij moderne boten. hoe zit dat? 13 Hoe werkt een zelflozer? 14 Een opgeklapt roer (wat nog steeds onder water zit) stuurt slechter als een roer dat netjes naar beneden zit. Hoe kan dat? 15 Die foto van dat vliegtuig die een sleuf achter zich maakt in de wolken, is die echt? 16 mag ik dingen kopieren uit je site? 17 Je zegt dat de stroming achter op je zeil turbulent is, en dat de stroming moet blijven aanliggen. Dat kan toch niet?

1 Waarom verlijer ik zoveel na de overstag? Lees het stuk zeilpuntverplaatsing en onderwaterschip. Het heeft dus te maken met het overtrokken zijn van je kiel en/of overtrokken zijn van je zeil. Als je je zeilstand netjes aanpast (ook je fok)aan hoe de wind tijdens je overstag staat, en pas weer hoog stuurt als je snelheid hebt zou dit moeten ophouden. 2 Hoe werkt een vleugelkiel? Een vleugelkiel trekt de boot dieper het water in, de vleugel trekt de boot dus naar loef bij grotere helling waardoor de rest van de kiel minder kracht naar loef hoeft te geven. http://zeilplan.net/leren/theorie/tekst.php?tekst=faq.txt (1 of 5)24-1-2004 17:32:51


Meestal is de prestatie van een schip met een vleugelkiel slechter als van een schip met een diepstekende kiel. Een vleugelkiel steekt meestal minder diep, waardoor het de oplossing kan zijn voor boten die wat ondieper vaarwater willen kunnen aandoen. 3 Wanneer moet ik kiezen voor een High Aspect fok en wanneer voor de genua? Als je hoog wil kunnen gaan is de HA beter, zeker als het hard waait op vlak water. Wil je wat meer op snelheid varen, zoals bij golven dan is de Genua beter. Dit komt omdat je Genua meer "tipwervels" heeft, en vaak wat boller is. Minder efficient dus als je hoog wil. Daarentegen is een Genua wel gewoon groter, en daarom gunstiger op de ruime koersen. 4 Molens draaien altijd linksom en mijn boot loopt ook lekkerder over bakboord, Is dit om dezelfde reden? Nee, Molens draaien linksom omdat dit historisch zo gegroeid is omdat maalstenen vroeger maar op een manier werden gemaakt. Eigenlijk draaien alleen de molens in Nederland en Belgie linksom. (bron Informatie-XVI het gilde van vrijwillige molenaars, Evert Smit, die dit heel duidelijk uitlegt in ca 50 kantjes die ik jullie wil besparen) Waarschijnlijk staat je mast scheef, is krom, of je boot is scheef, of je gewicht is scheef verdeeld. Overigens is het wel zo dat windvlagen over de ene boeg ruim inkomen, en over de andere boeg juist hoger, afhankelijk aan welke kant van het lagedrukgebied je zit. Zie zeilplan.net onder weer, en onder wedstijdzeilen en vervolgens tactiek. Dit effect zorgt ervoor dat de boot over de ene boeg lekkerder loopt. Ook zou het zo kunnen zijn dat bij de bovenkant van je zeil de wind onder een iets andere hoek binnenkomt, waardoor je over de ene boeg met teveel twist vaart, en over de andere boeg met te weinig twist. 5 Ga ik harder in de vaargeul of op het meer? In de vaargeul ga je harder (als deze dieper is als het meer) Dat komt omdat in ondiep water golven langzamer gaan. Je kunt dit ook zien bij de kust waar het ondieper wordt, daar komen de golven dichter achter elkaar te zitten terwijl het er niet meer worden. De golven gaan dus langzamer. Dit betekent ook dat je rompsnelheid lager wordt. Is het meer erg ondiep dan krijg je ook nog eens het effect wat je ook in een kleine sloot hebt: Zuiging Als je door een kleine sloot vaart moet het water onder de boot door naar achteren. Het water wordt dan als het ware door de spleet tussen bodem en boot geperst. Deze "pers druk" is extra weerstand. 6 Als ik ruime wind in de trapeze blijf hangen, en de stuurman naar de andere kant gaat zodat we toch rechtop blijven, ga ik harder. Hoe kan dit? Waarschijnlijk omdat dan de boot minder beweegt en je dus minder last hebt van beweging van je zeil die de stroming verstoord. Verder zou ik me kunnen voorstellen dat je zelf meer wind vangt als je in de trapeze hangt. Als laatste zou ik me kunnen voorstellen dat je minder mastbuiging hebt doordat je trapezedraad de functie van zijstag enigszins overneemt, en dat beter doet dan het zijstag omdat je trapezedraad horizontaler trekt dan het zijstag.

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7 Kan een boot sneller dan de wind varen? Voor de wind: natuurlijk niet (nou ja zonder motor en andere flauwe grappen dan). Rond halve wind: als je een boot hebt met niet te veel weerstand moet dit makkelijk kunnen. Surfers en catamarans varen regelmatig twee keer harder dan de wind. Met een gewone zeilboot haal je dit meestal niet. Misschien kun je begrijpen dat als je halve wind vaart, met dezelfde snelheid als de wind, de schijnbare wind schuin van voren komt, en je net zoals bij aan de wind varen daar gewoon mee kunt doorgaan. 8 Kan ik bij de hogerwal aanleg met sliplanding niet beter de fok wegrollen? Als je de fok laat klapperen kun je hem beter wegrollen. Als je hem netjes bedient zal je een stuk minder verlijeren. Laten staan dus. 9 Iemand heeft heeft het drukverschil tussen loef en lij gemeten en kwam op 0 uit. Hoe kan dat? Omdat de drukken niet zo hoog zijn en daarmee moeilijk te meten. Makkelijker is het gemiddelde uitrekenen door de kracht op je zeilen te meten en te delen door het zeiloppervlak. 10 In je koppels en krachten verhaal zeg je dat de zeilkracht loodrecht op de giek is. In je verhaal over zeiltrim zeg je dat de bolling naar voren plaatsen de zeilkracht meer naar voren richt. Je verhaal klopt dus niet en je bent een prutser! Ik ben inderdaad een prutser, maar van prutsen kun je heel wat leren. Het koppels en krachten verhaal wou ik begrijpbaar houden, en dus niet ingewikkelder maken door te zeggen dat de zeilkracht meestal iets meer naar voren gericht is ten opzichte van je giek. De zeilkracht is inderdaad iets meer naar voren gericht als loodrecht op de giek. Dit komt door: ●

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Twist, De bovenkant van je zeil is wat meer naar voren gericht, en je zeilkracht dus ook.

Bolling voor het midden. De zeilkracht is afhankelijk van hoeveel je ombuigt en dus van je curve. Voorin heb je de meeste curve en dus de meeste kracht, en het zeil is voorin meer vaar voren gericht. Overdreven:



Wat je hieruit leert is vooral dat je eigenlijk niet naar je giek moet kijken maar naar je zeil. Gebruik je het voor koppels en krachten dan wordt je al gauw gek, dan niet doen dus. 11 Met dat theorie verhaal van jou kom ik nergens tegen dat de fok meer doet dan het grootzeil. Toch heb ik dat al meerdere keren gehoord. hoe zit dat ? De fok levert inderdaad een relatief grotere kracht dan je grootzeil. Dit komt omdat de fok in de snellere lucht van het grootzeil zit, en het grootzeil in de langzamere lucht van de fok. leest het bernoulli verhaal maar door. Je fok doet dus meer door het grootzeil, en je grootzeil minder door de fok. Kortom, de prestatie van je fok is dus sterk afhankelijk van wat je precies met je grootzeil doet. Als ik een uitspraak zou doen dat de fok meer doet dan het grootzeil, dan betekent dat dus niet dat je je helemaal moet focussen op de fok. 12 In het blad "Zeilen" stond dat een cunningham hole weinig zin heeft bij moderne boten. hoe zit dat? Sorry, dat weet ik niet want ik heb dat nooit gelezen. Lijkt mij dat een cunningham handig blijft, zelfs bij zeilen die niet rekken en die al een boel andere trimmogelijkheden hebben. Een cunningham is erg makkelijk om spanning op je voorlijk te trekken. Met je val is dit vaak lastiger omdat deze niet (of weinig) vertraagd is. 13 Hoe werkt een zelflozer? Er zijn twee soorten zelflozers. De een steekt niet door de romp heen. De ander steekt wel door de romp heen of heeft een kapje op de romp. De versie welke niet door de romp steekt zuigt eigenlijk niet, terwijl de versie welke wel doorsteekt zuigt als een tiet. Hoe kan dat nu? De niet doorstekende zelflozer buigt geen water om. Volgens De wet van Bernoulli is er dus ook geen drukverandering. De wel doorstekende zelflozer buigt het water wel om. Dit geeft natuurlijk krachten. Het looswater komt er op een plek uit waar het water in de richting van de romp wordt



omgebogen, daar waar dus een reactiekracht is van de romp af is.

Dit is nou ook de reden dat een hoogtemeter van een vliegtuig(welke eigenlijk de luchtdruk meet) op een vlak gedeelte van een vliegtuig zit. Dan wordt hij namelijk niet beinvloed door de snelheid. 14 Een opgeklapt roer (wat nog steeds onder water zit) stuurt slechter als een roer dat netjes naar beneden zit. Hoe kan dat? Dit komt namelijk doordat de druk welke je opbouwt "weglekt" om de bovenkant en onderkant van je roerblad heen: De "tipwervels". Bij een opgeklapt roer heeft het water veel meerde tijd en ruimte om tipwervels te maken omdat je naar verhouding veel meer tip hebt. 15 Die foto van dat vliegtuig die een sleuf achter zich maakt in de wolken, is die echt? Zover ik weet wel, Het enige wat eraan getruukt is is dat er een vliegtuig voor vliegt. De fotograaf zat namelijk in dat vliegtuig ervoor. Zover ik weet vloog dit vliegtuig ook gewoon rechtdoor en steeg niet op ofzo. Dit heb ik niet gecheckt. Foto is van het bedrijf die dat vliegtuig bouwt (Cessna. 16 mag ik dingen kopieren uit je site? Ja hoor, daar is hij voor. Wel zou ik het fijn vinden als je het internetadres erbij vermeld, in plaats van mijn naam. Dit omdat als je een stukje uit zijn verband trekt ik liever heb dat mensen het hele verhaal kunnen lezen, dan dat ze denken dat ik dom ben. 17 Je zegt dat de stroming achter op je zeil turbulent is, en dat de stroming moet blijven aanliggen. Dat kan toch niet? Ik zeg niet dat de hele stroming achter op je zeil turbulent is, ik zeg dat de grenslaag bij je achterlijk turbulent is geworden. Let op dat als het dunne (enkele mm)luchtlaagje wat op je zeil "kleeft" turbulent is dit absoluut niet betekent dat de stroming daar als geheel turbulent is. Terug naar index


http://zeilplan.net/leren/theorie/tekst.php?tekst=links.txt

Terug naar index Enkele links misinterpretations of bernoulli lawDit is eigenlijk hetzelfde als mijn verhaal over hoe een zeil werkt, alleen heb ik de wiskunde geskipt. geen aanrader als je niet van wiskunde houd (Weet je ook meteen waar mijn coanda plaatje vandaan komt). a physical description of lift Dit is een verhaal over hoe een vleugel werkt wat wiskundig niet zo diep gaat, en daardoor wat makkelijker leesbaar is. Wel is het coanda verhaal hier niet helemaal duidelijk. see how it flies Deze vlieger kan het mooi vertellen. Let op hoe hij de magere uitleg van vorige link over coanda weet te gebruiken om te vertellen dat dit nergens op slaat. Lees je mijn verhaal, of de eerste link, dan zie je dat zijn huis-tuin en keuken proefje dit juist bewijst. Rest van de site is best intressant als je ook over vliegtuigen wilt meepraten, verder is dit iemand die de circulatie theorie ziet als werkelijkheid. jef raskin Geloof je toch nog een beetje dat een vleugel werkt doordat de weg boven de vleugel langer is, dan wordt dat helemaal de grond ingestampt. Erg leuk geschreven vind ik. Door deze site ben ik aan het denken gezet en heb heel dit verhaal geschreven. veenhoop Deze zeilschool vertelt op zijn website iets over theorie. Dit is wel een typisch voorbeeld van toveren met de circulatie theorie van prandtl, waar je dus eigenlijk niks aan hebt. Rest van het verhaal vind ik heel goed. Weet je ook meteen waar ik die foto van dat jacht met reefknuttels vandaan heb. vleugel in rook Hier wat plaatjes van vleugels met wat rook. aan lijzijde is bovenkant gaat de lucht veel sneller draaiende ballonnen theorie Een hele intressante theorie die de werking van een vleugel verklaard op een andere manier. niet alleen de vliegtuigvleugel wordt fout uitgelegd Voor de technische nerd (zoals ik) leuk om eens door te lezen, ook de rest van deze site heeft leuke dingen, zoals practical jokes http://www.amasci.com/ virtuele windtunnel Deze mensen hebben een virtuele windtunnel op hun site staan. Je kunt hem zelf downloaden op nasa simulator site

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http://zeilplan.net/leren/theorie/tekst.php?tekst=links.txt

Had je zelf nog een mooie toevoeging/opmerking/vraag mail me dan op [email protected] (domme vragen bestaan niet) Ook kun je een bericht achterlaten op het forum. Ik ben erg benieuwd vanaf waar je niks meer snapt van bovenstaand verhaal, dan moet ik dat nog eens wat beter neerzetten. Ik wil heel graag feedback!! Terug naar index

http://zeilplan.net/leren/theorie/tekst.php?tekst=links.txt (2 of 2)24-1-2004 17:33:43

Misinterpretations of Bernoulli's Equation

Misinterpretations of Bernoulli's Law Weltner, Klaus and Ingelman-Sundberg, Martin Department of Physics, University Frankfurt, Postfach 11 19 32, 60054 Frankfurt, Germany; Stockholm

Abstract. Bernoulli's law and experiments attributed to it are fascinating. Unfortunately some of these experiments are explained erraneously, e.g.: the function of a vaporizer and the soaring of a ping-pong ball in a jet stream of a hair dryer can not be used as applications of Bernoulli's law. The static pressure in a free jet stream is equal to the static pressure in the environmental atmosphere regardless of the streaming velocity of the jet. This can be shown by classroom experiments. Acceleration of air is caused by pressure gradients. Air is accelerated in direction of the velocity if the pressure goes down. Thus the decrease of pressure is the cause of a higher velocity. It is wrong to say that a lower pressure is caused by a higher velocity. Pressure gradients perpendicular to the streamlines are caused by the deflection of streaming air. The deflection of air generates regions of lower and higher pressure according to the curvature of the streamlines. Vaporizer, the soaring ping-pong ball as well as the physics of flight are only to be explained regarding the acceleration perpendicular to the streamlines.

1. Common derivation and applications of Bernoulli's law In a recent paper Baumann and Schwaneberg [1] state: Bernoulli's Equation is one of the more popular topics in elementary physics. It provides striking lecture demonstrations, challenging practice problems, and plentiful examples of practical applications from curving baseballs to aerodynamic lift. Nevertheless, Students and Instructors are often left with an uncomfortable feeling that the equation is clear and its predictions are verified, but the real underlying cause of the predicted pressure changes is obscure.

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Figure 1

This statement is correct and it should be added that the common treatment of Bernoulli's equation is also misleading. Generally a flow of an incompressible fluid through a tube with different cross-sections is observed and the theory of conservation of energy is applied to the flow.

The energy of a volume V at any point is the sum of its kinetic energy and its potential energy (pV). Effects of gravitation and viscosity are neglected. The energy of a given volume of the fluid which moves from point 1 to point 2 is the same at both points. The related energy equation is

(1) Using

and rearranging we arrive at Bernoulli's Law:

(2) The equation states a reversed relation between static pressure and streaming velocity which is often demonstratet by experiments like ●

Soaring ball: A light ball (e.g. ping pong ball) can be kept soaring in an upwards directed air stream of a hair dryer. The ball remains within the stream even if the stream is inclined and not vertical. The explanation given is that the static pressure within the stream is less due to the higher velocity.



●

Evaporator: If a fast stream of air passes over the opening of a pipe, the pressure inside is lowered and it is possible to suck in liquids. This effect is used as an application of Bernoulli's law [2] referring to the high streaming velocity within the air stream and claiming pstream < patmosphere .

Figure 2 ●

Aerodynamic lift: The higher streaming velocity of the air at the upper surface of the wing is stated to be the cause of the lower pressure. Different reasons are given for the generation of the higher streaming velocity. The most popular one is a comparison of path lengths of the flow above and below the aerofoil and the statement that due to a longer path length at the upper side the flow has to be faster [3], [1].

2. Misinterpretations and misapplications of Bernoulli's law 2.1 Static pressure in a free air stream Static pressure is the pressure inside the stream measured by a manometer moving with the flow. At the same time, the static pressure is the pressure which is excerted on a plane parallel to the flow. Thus the static pressure within an air stream has to be measured carefully using a special probe. A thin disk must cover the probe except for the opening. The disk must be positioned parallel to the streaming flow, so that the flow is not interfered with. If the static pressure is measured in the way outlined above within a free air stream generated by a fan or a hair dryer it can be shown that the static pressure is the same as in the surrounding atmosphere. Bernoulli's law cannot be applied to a free air stream because friction plays an important role. It may be noted that the situation is similar to the laminar flow of a liquid with viscosity inside a tube. The different velocity of the stream layers is caused by viscosity. The static pressure is the same throughout http://www.informatik.uni-frankfurt.de/~plass/MIS/mis6.html (3 of 13)24-1-2004 17:41:01


the whole cross-section. A free air stream in the atmosphere is exlusively decelerated by friction. If static pressure in a free air stream is equal to atmospheric pressure, some of the striking lecture demonstrations are interpreted incorrectly since the effects observed are not caused by Bernoulli's law. 2.2. Aerodynamic lift The explications referring to differences of path lengths are wrong. Air volumes which are adjacent before separation at the leading edge of the aerofoil do not meet again at the trailing edge [4]. This explanation is erranous. The higher streaming velocity at the upper surface of the aerofoil is not the cause of lower pressure. It is the other way round as will be shown below. As a matter of fact the higher streaming velocity is the consequence of the lower pressure at the upper surface of the wing [4]. These contradictions and misunderstandings can only be clarified by means of the basic physics of fluid mechanics. 3. Fluid dynamics, Newtons laws and the Euler equations Fluid dynamic is an extension of Newton’s mechanic. It was Euler who applied the fundamental laws of Newton to fluid motion. He succeeded in establishing equations for the three dimensional fluid motion the Euler equations. For simplicity reasons we restrict our considerations to stationary flow and we neglect effects of gravitation and viscosity [5]. We refer to an elementary cubical volume within curved streamlines. Figure 3. The reference system is chosen deliberately to separate the direction of velocity and its perpendicular. We analyse the acceleration of a mass element components of the acceleration:

. We separate the

Tangential acceleration = acceleration in direction of the velocity (Figure 3) Normal acceleration = acceleration perpendicular to the direction of velocity (Figure 4) Tangential acceleration in s-direction.

Figure 3



An acceleration is the result of a force acting on the mass element. A force in direction of the velocity can only be generated by a pressure difference. The static pressure acting on the aera A at the back must exceed the pressure on the aera A at the front. Acceleration in direction of the motion is the effect of a decrease of pressure. The force F is given by: (3) Thus Newton’s equation reads

(4)

Using

and

we obtain

(5) This equation can be transformed to

(6) The definite integral for two positions 1 and 2 is

(7) The solution is

(8) This is Bernoulli's law.



This derivation of Bernoulli's law is more instructive compared to the derivation generally used in textbooks because it shows the physics behind the law. The streaming fluid accelerates as a result of decreasing pressure (i.e. or a negative pressure gradient). This derivation clearly shows that an acceleration can never be the cause of decreasing pressure. Normal acceleration exists if streamlines are curved. A normal acceleration is the effect of a force in direction of the radius of curvature. In the case of the elementary volume the pressure acting on the outside area must exceed the pressure on the inside area.

Figure 4

The force referring to the z-axis is: The negative sign is due to the fact that the force has the opposite direction of a positive pressure gradient. Newton’s equation reads

(9) The normal acceleration is well known for circular motions with a radius R and a velocity v:



(10)

Inserting

and

in (9) we obtain the pressure gradient in z-direction.

(11) Unfortunately this equation can only be integrated if the total field of the flow is known. However the relation can be demonstrated in a simple and impressing way. If we make water rotate in a disk or a pot, the surface of the water rises at the outer parts. The level of the surface is a manometer indicating the pressure beneath. Assuming homogenous angular velocity of the circular flow of water the velocity is . Thus equation (9) may be solved for a horizontal level beneath the surface neglecting gravitational pressure:

(12)

(13) The pressure is proportional to the square of the radius generating a parabolic surface.

(14) This result is also well known for centrifuges. As a rule, physics textbooks neglect the treatment of normal acceleration of fluids. They do not discuss the pressure gradients normal to the velocity if streamlines are curved. By the way, this is different from textbooks on technical fluid dynamics which treat the flow of fluids in curved tubes. The neglect of pressure gradients related to curved streamlines is disastrous because the mechanism producing low pressure is thus made impossible to understand. Obstacles cause curved streamlines and generate pressure gradients of air and as a consequence regions of higher or lower pressure. The deflection of the streaming is the cause for the generation of pressure gradients perpendicular to the streamlines and thus the cause for the generation of pressure differences.



3.2 Coanda-effect. The flow near limiting surfaces follows the geometrical shape of these surfaces. This behaviour is called Coanda-effect. It is neither trivial nor general. The flow must not be forced to change its direction abruptly as to avoid the generation of turbulence and separation. The classic example for the Coandaeffect is a flow blown across a flat plane with an adjacent half cylinder. At first the flow follows the surface of the cylinder and separates later.

Figure 5

Figure 6

This is important because this behaviour holds for all flows limited by smoothly curved surfaces like aerofoils, streamlined obstacles, sails and - with a certain reservation - roofs. The Coanda-effect can be understood taking viscosity into consideration. In figure 6 we assume the stream to start. It will flow horizontally. But due to viscosity some layers of the adjacent air will be taken away by the stream. In this adjacent region - dotted in figure 6 - the air is sucked away and hence, gives rise to a reduction of pressure, consequently producing a normal acceleration of the stream. By the end of the process the stream fits the shape of the curved surface. This Gedankenversuch illustrates the importance of viscosity in generating of stationary flow. Also the stationary flow around an aerofoil which produces lift is only possible due to the Coanda-effect and the air's viscosity. 4. Generation of high and low pressure within a flow 4.1 Measurement of static pressure within a free stream of air A sufficiently sensitive manometer can be produced easily if not available in the lab. A fine pipe of glas is bent at one side to dip in a cup and to be fixed according to figure 7. The meniscus must be positioned in the middle of the pipe. The suitable inclination should be 1:15 - 1:30. A rubber tube connects the glas pipe with a probe. As has been pointed out before a flat disk must be glued on top of the probe leaving the opening free. The disk has to be held parallel to the streaming. If the static pressure is measured in http://www.informatik.uni-frankfurt.de/~plass/MIS/mis6.html (8 of 13)24-1-2004 17:41:01


such a way it can be shown that it is equal to the pressure in the environmental atmosphere.

Figure 7

4.2 Generation of high and low pressure by deflection of an air stream According to figure 8 we place a curved plane into the air stream of a fan. A curved plane can be produced by glueing two postcards on top of each other. By fixing them around a bottle with a rubber, an appropriate curvature can be achieved.

Figure 8

Due to the Coanda-effect, the air stream follows the shape of the curved plane at the lower side. The stream follows the upper side because there is no other way left to move.



Figure 9

The curved plane forces a curved flow resulting in a radial pressure gradient. Outside of the flow there is atmospheric pressure. Due to the pressure gradient inside the curved air stream an increase of pressure is to be expected at the inner or convex side of the plane in relation to the center center of curvature. Figure 8. At the outer or concave side a decrease of pressure is to be expected. Figure 9. This increase and decrease exists indeed and can be demonstrated using the manometer described above. See figure 8 and figure 9. The experiment shows that by deflecting of an air stream regions of increased or decreased pressure may be generated. This experiment is fundamental for the understanding of the production of pressure differences if air passes obstacles. By analysing the curvature of the evasive flow we can predict pressure distribution. It should be added that in this case Bernoulli's law still holds since friction may be neglected. Since in figure 8 the pressure increases at the inner surface the local streaming velocity is reduced. In figure 9 the pressure decreases at the outer surface and the streaming velocity increases. The physical mechanism is quite obvious. The curved plane causes a curved streaming flow and a decrease of pressure. Hence incoming air is accelerated by the decrease of pressure. The experiment requires an air stream the cross section of which should exceed the width of the postcard. If a hair-dryer is used which produces a narrow air stream it is advisable to glue the curved plane between two even planes of glass or plastic to confine the air stream. The distance of the limiting planes should be equal to the diameter of the air stream produced by the hair-dryer. 4.3 Examples and applications Hill: If air passes a hill - figure 10- it follows the shape of it. A deformation of the original horizontal



flow occurs only in the surroundings of the hill. Further away we observe normal atmospheric pressure and horizontal flow.

Figure 10

We first analyse the curvature following the trajectory A.The trajectory starts from the bottom of the hill and is continued perpendicular to the streamlines. The streaming air is deflected upwards. The air is accelerated upwards too. Starting from the bottom and going outwards the pressure has to decrease in order to produce the acceleration upwards. Because of the atmospheric pressure further away there must be a higher pressure at the bottom of the trajectory A. In the case of trajectory B starting from the top of the hill the curvature of the streamlines is reversed. The streaming air is accelerated downwards throughout the whole trajectory. Following this trajectory the pressure increases starting from the top until it reaches its normal value further away. Thus at the top of the hill we expect a reduced pressure. In the case of the trajectory C we expect the same as for trajectory A. Modelling the hill with bent postcards these results can be demonstrated experimentally as well. Evaporator: These considerations give an explanation of the mechanism for the evaporator. A pipe dipping in a flow of air forces an evasive flow (see figure 11). This is a situation similar to that of the hill. The streaming is curved over the nozzle of the pipe and the acceleration directs to the aperture. Therefore lower pressure is generated at the nozzle.

Figure 11



Forces on a roof: If wind passes a house the stream is to flow around it. Due to the curvature of the evasive flow there is higher pressure at the front side and lower pressure at the peak of the roof.

Figure 12

The flow is by no means smooth and laminar. At the peak of the roof it definitely becomes turbulent and separates. (Thus at the rear side we cannot expect the same as for the hill.) Behind the peak of the roof the same reduced pressure can be found as at the peak. This is why the situation at the rear side of the house cannot be the same as for the hill. The effect of pressure differences on the roof is maximized if front doors or windows are opened. In this case there is high pressure inside the house. The pressure difference acting on the roof is increased. If windows or doors at the rear are opened there is a lower pressure inside the house that reduces the pressure difference acting on the roof. Aerodynamic lift: The aerodynamic lift, too, is a result of the evasive flow caused by the aerofoil. The streamlines near the wing are determined by the latter’s shape and position. As a whole the stream is deflected downwards. (See figure 8 and figure 9.) Propulsion by a sail: The same phenomenon can be observed in the case of a sail. A sail is a curved plane similar to figure 8 and 9. The sail deflects the air flow and produces an increase of pressure at the inner side in relation to the center of curvature and a decrease of pressure at the outer side. By this way it generates a force normal to the sail. Skilled sailors keep the streaming of the air smooth and laminar and avoid turbulent and separating flow. 6. Conclusion http://www.informatik.uni-frankfurt.de/~plass/MIS/mis6.html (12 of 13)24-1-2004 17:41:01


The deliberation of Bernoulli's law in schools and textbooks has serious drawbacks. Unfortunately many applications are erranous and misleading. One source of confusion is the derivation of Bernoulli's law based on the theorem of energy conservation. Bernoulli's law should be derived from the tangential acceleration as a consequence of declining pressure. Another source of difficulties is the fact that many physics textbooks do not mention normal acceleration of flow and the resulting pressure gradients perpendicular to the flow. Both, Bernoulli's law and the generation of pressure gradients perpendicular to the flow are consequences of Newton’s laws. None of them contradicts those. Bernoulli's law is insufficient to explain the generation of low pressure. A faster streaming velocity never produces or causes lower pressure. The physical cause of low or high pressure is the forced normal acceleration of streaming air caused by obstacles or curved planes in combination with the Coanda-effect. Pressure gradients generated by the deflection of streaming air can be clearly demonstrated by simple experiments which would substantially improve the discussion of fluid mechanics in schools and textbooks.

Literature [1] Baumann, R.; Schwaneberg, R.: "Interpretation of Bernoulli's Equation", The Physics Teacher, Vol. 32, Nov. 1994, pp. 478 - 488 [2] Paus, H.J.: "Physik in Experimenten und Beispielen" München/Wien, 1995. [3] Mansfield, M; O’Sullivan, C.: "Understanding Physics", Chichester, New York, 1998 [4] Weltner, K.; Ingelman-Sundberg, M.: "Physics of Flight - reviewed", submitted to Eurpean Journal of Physics


Stalls and Spins [Ch. 18 of See How It Flies]

_ [Previous] [Contents] [Next] [Comments or questions] Copyright © 1996-2001 jsd

*

18 Stalls and Spins Caution: Cape does not enable user to fly. — warning label on Superman costume sold at Walmart

Spins are tricky. After reading several aerodynamics texts and hundreds of pages of NASA spin-tunnel research reports, I find it striking how much remains unknown about what happens in a spin.

18.1 Stalls: Causes and Effects Here's a basic yet important fact: if you don't stall the airplane, it won't spin. Therefore, let's begin by reviewing stalls. As discussed in section 5.3, the stall occurs at the critical angle of attack, which is defined to be the point where a further increase in angle of attack does not produce a further increase in coefficient of lift. Nothing magical happens at the critical angle of attack. Lift does not go to zero; indeed the coefficient of lift is at its maximum there. Vertical damping goes smoothly through zero as the airplane goes through the critical angle of attack, and roll damping goes through zero shortly thereafter. An airplane flying 0.1 degree beyond the critical angle of attack will behave itself only very slightly worse than it would 0.1 degree below. If we go far beyond the critical angle of attack (the ``deeply stalled'' regime) the coefficient of lift is greatly reduced, and the coefficient of drag is greatly increased. The airplane will descend rapidly, perhaps at thousands of feet per minute. Remember, though: the wing is still supporting the weight of the airplane. If it were not, then there would be an unbalanced vertical force, and by Newton's law the airplane would be not only descending but accelerating downward. If the wings were really producing zero force (for instance, if you snapped the wings off the airplane) the fuselage would accelerate downward until it reached a vertical velocity (several hundred knots) such that weight was balanced by fuselage drag.

18.2 Stalling Part vs. All of the Wing http://www.av8n.com/how/htm/spins.html (1 of 20)24-1-2004 17:54:49


We can arbitrarily divide the wing into sections; each section contributes something to the lift of the whole wing. It is highly desirable (as discussed in section 5.4.3) to have the coefficient of lift for sections near the wing-root reach its maximum early, and start decreasing, while the coefficient of lift 1

for sections near the tips continues increasing (as a function of angle of attack). Therefore it makes perfect sense to say that the sections near the roots are stalled while the sections near the tips are not stalled. If only a small region near the root is stalled, the wing as a whole will still have an increasing coefficient of lift — and will therefore not be stalled. We see that the wing will continue to produce lots of lift well beyond the point where part of it is stalling. This is the extreme slow-flight regime — you can fly around all day with half of each wing stalled (although it takes a bit of skill and might overheat the engine).

18.3 Boundary Layers There is a very simple rule in aerodynamics that says the velocity of the fluid right next to the wing (or any other surface) is zero. This is called the no-slip boundary condition. Next to the surface there is a thin layer, called the boundary layer, in which the velocity increases from zero to its full value. 18.3.1 Separated versus Attached Flow The wing works best when the airflow is attached to the wing surface by a simple boundary layer. The opposite of attached flow is separated flow. For attached flow, as we move through the boundary layer from the wing surface out to the full-speed flow, there is practically no pressure change. Sometimes it helps to think about attached flow in the following way: Imagine removing the boundary layer and replacing it with a layer of putty that redefines the shape of the wing. Then imagine ``lubricating'' the new wing so that the air slides freely past it; the no-slip condition no longer applies. Bernoulli's principle can be used to calculate the pressure on the surface of the putty; obviously it could never be applied inside the boundary layer. The putty-covered wing may not be the most desirable shape, but it won't necessarily be terrible. For separated flow, the putty model does not work. Suppose I want to pick up a piece of lint from the floor using a high-powered vacuum cleaner. If I keep the hose 3 feet away from the floor, it will never work; I could have absolute zero pressure at the mouth of the hose, but the low pressure region would be ``separated'' from the floor and the lint. If I move the hose closer to the floor, eventually it will develop low pressure near the floor. This is part of the problem with separated flow: there is low pressure somewhere, but not where you need it. Separation can have multiple evil effects: ●

Separation means the air doesn't follow the contour of the wing. This is somewhat like having a really thick boundary layer. The wing can't force the air into the optimal flow pattern, so not as

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●

●

much low pressure is produced. Whatever low pressure is produced isn't all attached to the wing surface. This is a new problem that an attached flow would not have, no matter how thick the boundary layer. On a non-streamlined object such as a golf ball, there is a lot of drag (specifically: form drag, as discussed in section 4.4) because separation disrupts a desirable high-pressure area behind the ball.

18.3.2 Laminar versus Turbulent Flow In the simplest case, there is laminar flow, in which every small parcel of air has a definite velocity, and the velocity varies smoothly from place to place. The other possibility is called turbulent flow, in which: ● ●

at any given point the velocity fluctuates as a function of time, and at any given time the velocity changes rapidly as we move from point to point, even for nearby points.

The closer we look, the more fluctuations we see. Attached turbulent flow produces a lot of mixing. Some bits of air move up, down, left, right, faster, and slower relative to the average rearward flow.

For separated laminar flow, there will be some reverse flow (noseward, opposing the overall rearward flow) but the pattern in space will be much smoother than it would be for turbulent flow, and it will not fluctuate in time.

You can tell whether a situation is likely to be turbulent if you know the Reynolds number. You don't need to know the details, but roughly speaking small objects moving slowly through viscous fluids (like honey) have low Reynolds numbers, while large objects moving quickly through thin fluids (like air) have high Reynolds numbers. Any system with a Reynolds number less than about 10 is expected to have laminar flow everywhere. If you drop your FAA ``Pilot Proficiency Award'' wings into a jar of honey, they will settle to the bottom very slowly. The flow will be laminar everywhere, since the Reynolds is slightly less than 1. There will be no separation, no turbulence, and no form drag — just lots of skin-friction drag. Systems with Reynolds numbers greater than 10 or so are expected to create at least some turbulence. Airplanes operate at Reynolds numbers in the millions. The wing will have a laminar boundary layer near the leading edge, but as the air moves back over the wing, at some point the boundary layer will become turbulent. This is called the transition to turbulence or simply the boundary layer transition. Also at some point (before or after the transition to turbulence) the airflow will become separated. The designers try to keep the region of separation rather small and near the trailing edge. In order to make a wing develop a lot of lift without stalling, it helps to minimize the amount of separation. 18.3.3 Boundary Layer Control http://www.av8n.com/how/htm/spins.html (3 of 20)24-1-2004 17:54:49


2

One scheme for controlling separation involves the use of vortex generators (VGs). The VGs are the little blades you see on the top of some wings, sticking up into the airstream at funny angles. Each blade works like a turnplow, reaching out into the high-velocity airstream and turning the layers over — plowing energy into the inner layers. Re-energizing the boundary layer allows the wing to fly at higher angles of attack (and therefore higher coefficients of lift) without stalling. This improves your ability to operate out of short and/or obstructed fields. The vorticity created by these little VGs should not be confused with the bound vortex, the big vortex that generates the circulation that supports the weight of the airplane. As discussed in section 3.12, to create lift you have to make the air circulate around the wing; that is, there must be vortex line running 3

along the span. VGs don't do that; their vortex lines run chordwise, not spanwise.

Boundary-layer turbulence (whether created by VGs or otherwise) also helps prevent separation, once again by stirring additional energy into the inner sublayers of the boundary layer. On a golf ball, 99% of the drag is form drag, and only 1% is skin-friction drag. The dimples in the golf ball provoke turbulence, adding energy to the boundary layer. This allows the flow to stay attached longer, maintaining the high-pressure region behind the ball, thereby decreasing the amount of form drag. The turbulence of course increases the amount of skin-friction drag, but it is worth it.

4

Bernoulli's principle does not apply inside the boundary layer, separated or otherwise. As discussed in section 3.4, Bernoulli's principle applies in situations where pressure (potential energy) and airspeed squared (kinetic energy) add up to a constant. This is not the case in the boundary layer, because friction there converts a significant amount of the energy into heat. Do VGs play the same role as dimples on a golf ball? Not exactly. Unlike a golf ball, a wing is supposed to produce lift. Also unlike a golf ball, a wing is highly streamlined; consequently, its form drag is not predominant over skin-friction drag. VGs are typically used to improve lift at high angles of attack (by fending off loss of lift due to separation). They may or may not improve performance at low angle of attack (by decreasing form drag at the expense of skin-friction drag). If you want ultra-low drag, and don't care about short-field performance, you want a wing with as much laminar flow as possible. Designing a ``laminar flow wing'' is exquisitely difficult, especially in the real world where the laminar flow could be disturbed by rain, ice, mud, and splattered bugs on the leading edge. There is always some separation on every airfoil section. The separation grows as the angle of attack increases. If there is too much separation, it cuts into the wing's ability to produce lift. If there were no separation, the wing could continue producing lift up to very high angles of attack (thereby achieving very high coefficients of lift). http://www.av8n.com/how/htm/spins.html (4 of 20)24-1-2004 17:54:49


5

Having lots of separation is the dominant cause (but not the definition) of stalling. Remember: the stall occurs at the critical angle of attack, i.e. the point where max coefficient of lift is attained. 18.3.4 Summary A full discussion of turbulence and/or separated flow is beyond the scope of this book; indeed, trying to really understand and control these phenomena is a topic of current research. There is nothing simple about it. But there are a few things we can say. ● ● ● ● ●

The opposite of separated flow is attached flow. The opposite of turbulent flow is laminar flow. Separated flow need not be very turbulent, nor vice versa. Laminar flow need not be attached, nor vice versa. Turbulence doesn't cause separation (and indeed oftenhelps prevent it).

For more information, see e.g. reference 17.

18.4 Coanda Effect, etc. The name Coanda effect is generally applied to any situation where a thin, high-speed jet of fluid meets a solid surface and follows the surface around a curve. Depending on the situation, one or more of several different physical processes might be involved in making the jet follow the surface. As a pilot, you absolutely do not need to know about the Coanda effect or what causes it. Indeed, many professional aerodynamicists get along just fine without really understanding such things. The main purpose of this section is to dispel the notion that a normal wing produces lift ``because'' of some type of Coanda effect. Using the Coanda effect to explain the operation of a normal wing makes about as much sense as using bowling to explain walking. To be sure, bowling and walking use some of the same muscle groups, and both at some level depend on Newton's laws, but if you don't already know how to walk you won't learn much by considering the additional complexity of the bowling situation. 18.4.1 Tissue-paper Demonstration You can demonstrate one type of Coanda effect for yourself using a piece of paper. Limp paper, such as tissue paper, works better than stiff paper. Drape the paper over your fingers, and then blow horizontally, as shown in the following figures.

6



Figure 18.1: Tissue Paper; No Coanda Effect If the jet passes just above the paper, as shown in figure 18.1, nothing very interesting happens. The jet just keeps on going. The paper is undisturbed.

Figure 18.2: Tissue Paper; Coanda Effect On the other hand, if the jet actually hits the paper as shown at point C in figure 18.2, the downstream part of the paper will rise up. This is because the air follows the curved surface; as it does so, it creates enough low pressure to lift the weight of the paper. The air in your lungs, at point A, is at a pressure somewhat above atmospheric. At point B, after emerging from the nozzle, the air in the jet is at atmospheric pressure. As discussed in section 3.3, the fact that the fluid follows a curved path proves that there is a force on it. This force must be due to a pressure difference. In this case, the pressure on the lower edge of the jet (where it follows the curve of the tissue paper near point D) is less than atmospheric, while the pressure on the upper edge of the jet (near point E) remains more-or-less atmospheric. This pressure difference pulls down on the jet, making it curve. By the same token it also pulls up on the paper, creating lift. People who only half-understand Bernoulli's principle will be surprised to hear that the jet leaves the nozzle at high speed at atmospheric pressure. It's true, though. In particular, the crude statement that ``high velocity means low pressure'' is an oversimplification that cannot be used in this situation. The correct basis of Bernoulli's principle is that for a particular parcel of air the mechanical energy (pressure plus kinetic energy per unit volume) remains more-or-less constant. If you want to compare two different parcels of air, you'd better make sure that they started out with the same mechanical energy. In this case, the air in the jet leaves the nozzle with a higher mechanical energy than the ambient air. Your lung-muscles are the source of the extra energy. When this high-velocity, atmospheric-pressure air smacks into the paper at point C, it actually creates above-atmospheric pressure there. Indeed, we can use the streamline-curvature argument again: if the air http://www.av8n.com/how/htm/spins.html (6 of 20)24-1-2004 17:54:49


turns a sharp corner, there must be a very large pressure difference. In order to make this sharp turn, the air needs something to push against. A good bit of the required momentum comes from the air that splatters backward, as suggested by the squiggles just below and upstream of the point of contact. This process is extremely messy. It is much more complicated than anything that happens near a wing in normal flight. To visualize this splatter, blow a jet of air onto a 7

dusty surface. Even if you blow at a very low angle, some of the dust particles blow away in the direction opposite to the main flow. 18.4.2 Blowing the Boundary Layer Since we saw in section 18.3 that de-energizing the boundary layer is bad, you might think adding energy to the boundary layer should be good... and indeed it is. One way of doing so uses vortex generators, as discussed in section 18.3. Figure 18.3 shows an even more direct approach. ● ●

We use a pump to create a supply of air at very high pressure. The air comes out a nozzle. The result is a jet of high-velocity air at the same pressure as the 8

●

local air. The jet shoots out of a slot in the top of the wing, adding energy to the boundary layer at a place where this could be very helpful.

Figure 18.3: Blowing the Boundary Layer Once again, the Coanda effect cannot explain how the wing works; you have to understand how the wing works before you consider the added complexity of the blower. In this case we expect one spectacular added complexity, namely curvature-enhanced turbulent mixing. This phenomenon will not be discussed in this book, except to say that it does not occur near a normal wing, while it is likely to be quite significant in the situation shown in figure 18.3. Curving flows with lots of shear can be put to a number of other fascinating uses, but a discussion is beyond the scope of this book. See reference 9. 18.4.3 Teaspoon Demonstration



Another example of a jet following a curved surface uses a jet of water. You can easily perform the following experiment: let a thin stream of water come out of the kitchen faucet. Then touch the left side of the stream with the convex back side of a spoon. The stream will not be pushed to the right, but instead it will follow the curve of the spoon and be pulled to the left. The stream can be deflected by quite a large amount. In accordance with Newton's third law of motion, the spoon will be pulled to the right. I don't understand everything I know about this situation, but it is safe to say the following: 1. This water-in-air jet differs in fundamental ways from the air-in-air jet situation described above. 2. This effect has practically nothing to do with the way a normal wing produces lift. To convince yourself of these facts, it helps to have a higher velocity and/or a larger diameter than you can conveniently get from a kitchen faucet. A garden hose will give you a bigger diameter, and if you add a nozzle you can get a higher velocity. You can easily observe: ●

●

●

●

●

9

The amount of lift you can produce is pathetically small, compared to the dynamic pressure and area of the water jet. The lift-to-drag ratio is terrible. Indeed this makes it very hard to measure the lift; if you get the angle slightly wrong you will inadvertently measure a drag component instead. The water spreads out when it hits the surface, making a thin coating over a wide area of the surface. This is in marked contrast to what happens in the air-in-air jet, as you can demonstrate by placing thin strips of tissue paper side by side. You can easily blow on one strip and lift it without disturbing its neighbors. Some of the spreading layer flows backwards, ahead of the point of contact of the jet, corresponding to a negative amount of upwash. This is grossly different from what happens near a real wing. The effect does not depend on curvature-enhanced turbulent mixing with the ambient air. This is quite unlike what happens in a real airplane with boundary-layer blowing.

It appears that surface tension plays two very important roles: 1. At the water/air interface it prevents mixing of the air and water. 2. At the water/wing interface it plays a dominant role in making the water stick to the surface. In both respects this is quite unlike the air-in-air jet, where the air/wing surface tension has no effect and there is no such thing as air/air surface tension. To convince yourself of this: Take a thin sheet of plastic. Get it wet on both sides, and drape it over a cylinder. You will not be able to lift it off the cylinder using a tangential water jet. The surface tension holding the wet plastic to the cylinder is just as strong as the tension between the plastic and the jet. In contrast, when the same piece of plastic has air on both sides, you can easily lift it off the cylinder using http://www.av8n.com/how/htm/spins.html (8 of 20)24-1-2004 17:54:49


an air jet. 18.4.4 Fallacious Model of Lift Production You may have heard stories saying that the Coanda effect explains how a wing works. Alas, these are just fairy tales. They are worse than useless. 1. For starters, these fairy tales often claim that blowing on tissue paper (as described just above) proves that ``high velocity means low pressure'' which is absolutely not what is being demonstrated. The high-velocity air coming out of your mouth is at atmospheric pressure. If you blow across the top of a flat piece of paper, it will not rise, no matter what you do. There is no low pressure in the jet (unless and until it gets pulled around a corner). Therefore the Coanda stories give a wrong explanation of normal wings and basic aerodynamics. And by the way, such stories cannot even begin to explain the operation of flat wings — yet we have seen in section 3.10.1 that a barn door doesn't behave very differently from other airfoils. 2. The Coanda-like notion of airflow following a curved surface cannot possibly explain why there is upwash in front of the wing. In figure 18.2 there must be a stagnation point on the upper surface of the paper near point C. This is completely different from the situation near a normal wing, where the stagnation line must be somewhere below the leading edge of the wing. Upwash is important, since it contributes to lift while creating a negative amount of induced drag. A further consequence, by the way, is that these Coanda-like stories cannot possibly explain the operation of stall-warning devices, as discussed in section 3.5. 3. As mentioned above, the distribution of velocities necessary to create curvature-enhanced turbulent mixing is produced by a high-speed jet but is not produced by a normal wing. 4. Sometimes the fairy tales say that the jet ``sticks'' to the surface because of viscosity. This implies that if the viscosity of the fluid changes, the amount of lift an airfoil produces should change in proportion. In fact, though, the amount of lift produced by a real wing is independent of viscosity over a wide range. Also, many of the processes responsible for the real Coanda effect require the 10

production of turbulence, so they only work if the viscosity is sufficiently low. 5. In the real Coanda effect, we know where the high-velocity air comes from. It comes from a nozzle. Upstream of the nozzle is a pump (or a rocket engine, or some other device) to supply the necessary energy. The jet makes high-velocity air above the wing, not below, because that's where we aim the nozzle. An ordinary wing is completely different. It is wonderfully effective at creating high-velocity air above itself, without nozzles, without pumps, and without transferring 11

energy to the air. 6. The fairy tales generally neglect the fact that the wing speeds up the air in its vicinity, and just assume that the relative wind meets the wing at the free-stream velocity and follows the curve in a Coanda-like way. As a consequence, they miscalculate the pressure gradients by a factor of ten or so. 7. Finally, in the real Coanda effect we know how big the jet is. Its initial size is determined by the nozzle. The amount of mixing depends on the speed of the jet, the speed of the ambient air, the



curvature of the surface, and other known quantities. Awareness of the Coanda effect is a small part of — not a replacement for — a full analysis of the wing in figure 18.3. In contrast, (a) the typical fairy tales imply that the entire flow pattern of a normal wing can be explained by mentioning the magic words ``Coanda effect'', yet (b) they cannot explain how thick a chunk of air is deflected by the wing. One inch? Six inches? A chord-length? A span-length? Some amount proportional to the viscosity of the air? It would be very hard to calculate how much.

12

18.4.5 Summary Don't let anybody tell you that squirting a spoon or blowing on tissue paper is a good model of how a wing works. If you want to ``get the feel'' of lift production, the obvious methods are the best. These include holding a model airfoil

13

downstream of a household fan, or sticking it out of a car window.

18.5 Spin Entry Case 1: In normal flight, rolling motions are very heavily damped, as discussed in section 5.4. Even though the static stability of the bank angle is small or even negative, you cannot get a large roll rate without a large roll-inducing force; when you take away the force the roll rate goes away. Case 2: Near the critical angle of attack, the roll damping goes away. Suppose you start the aircraft rolling to the right. The roll rate will just continue all by itself. The right wing will be stalled (beyond max lift angle of attack) and the left wing will be unstalled (below max lift angle of attack). Case 3: At a sufficiently high initial angle of attack (somewhat greater than the critical angle of attack), 14

the roll will not just continue but accelerate, all by itself. This is an example of the ``departure'' that constitutes the beginning of a snap roll or spin. The resulting undamped rolling motion is called autorotation. At a high enough angle of attack, the ailerons lose effectiveness, and at some point they start working in 15

reverse. Figure 18.4 shows how this reversal occurs. Suppose you deflect the ailerons to the left. This raises the angle of attack at the right wingtip and lowers it at the left wingtip. Normally, this would increase the lift on the right wing (and lower it on the left), creating a rolling moment toward the left. Near the critical angle of attack, though (as seen in the left panel of the figure), raising or lowering the angle of attack has about the same effect on the coefficient of lift, so no rolling moment is produced (for now, at least).



Figure 18.4: Lift and Drag at Departure We see that at this angle of attack, anything that creates a rolling mo-ment will cause the aircraft to roll like crazy, and indeed to keep accelerating in the roll-wise direction. There will be no natural roll damping, and you will be unable to oppose the roll with the ailerons. There are two main ways of provoking a spin at this point: 1. Suppose the airplane is in a steady slip to the left. That is, you are steadily pushing on the right rudder pedal. Then the slip/roll cou-pling (as discussed in section 9.1 and section 9.2) will cause it to spin to the right. 2. Suppose the airplane is not in much of a slip, but you suddenly cause it to yaw to the right. The left wingtip will temporarily be moving faster, and the right wingtip will temporarily be moving slower. This difference in airspeeds will create a difference in lift, causing a spin to the right. The initial yawing motion could come from a sudden application of rudder, or from adverse yaw, or what-ever. Note that in the right panel of figure 18.4, the ai-leron deflection has a tremendous effect on the drag. This means that ailerons deflected to the left cause a yaw to the right which in turn provokes a roll to the just the opposite of what ailerons normally do.

18.6 Types of Spin 18.6.1 Spin Modes The word ``spin'' can be used in several different ways, which we will discuss below. The spin family tree includes: ● ● ●

``departure'', i.e. onset of undamped rolling; incipient spin — i.e. one that has just gotten started; or well-developed spin, which could be ❍ a steep spin, or ❍ a flat spin.

Figure 18.5 shows an airplane in a steady spin. You can see that the direction of flight has two components: a vertical component (down, parallel to the spin axis) and a horizontal component (forward and around).



Figure 18.5: Airplane in a Steady Spin Figure 18.6 is a close-up of a wing in a steep spin. We have welded a pointer to each wingtip, indicating the direction from which the relative wind would come if the wing were producing zero lift; we call this the Zero-Lift Direction (ZLD). (For a symmetric airfoil, the ZLD would be aligned with the chord line of the wing.) Remember that the angle between the direction of flight and the ZLD pointer is the angle of attack.

Figure 18.6: Steep Spin — Geometry



Figure 18.7: Steep Spin — Coefficient of Lift In this situation, both wingtips have the same vertical speed, but they have significantly different horizontal speeds — because of the rotation. Consequently they have different directions of flight, as shown in the figure. This in turn means that the two wingtips have significantly different angles of attack, as shown in figure 18.7. The two wings are producing equal amounts of lift, even though one is in the stalled regime and one in the unstalled regime. Figure 18.8 shows another spin mode. This time the rotation rate is higher than previously. The spin axis is very close to the right wingtip. The outside wing is still unstalled, while the inside wing is very, very deeply stalled, as shown in figure 18.9.

Figure 18.8: Flat Spin — Geometry

Figure 18.9: Flat Spin — Coefficient of Lift



Figure 18.10: Doubly Stalled Flat Spin — Coefficient of Lift Figure 18.10 shows yet another possible spin mode. In this case, the outside wing is stalled, while the inside wing is, of course, much more deeply stalled. Whether this spin mode, or the one shown in figure 18.9 (or both or neither) is stable depends on dozens of details (aircraft shape, weight distribution, et cetera). There is a common misconception that in a spin, one wing is stalled and the other wing is always unstalled. This is true of ``most'' spins but it is not a defining property. It would be safer to use the following definition: In a spin, at least one wing is stalled, and the two wings are operating at very different angles of attack.

18.6.2 Samaras, Flat Spins, and Centrifugal Force A samara is a winged seed. Maples are a particularly well known and interesting example. Maple samaras have only one wing, with the seed all the way at one end. Its mode of flight is analogous to an airplane in a flat spin. In an airplane, the inside wing is deeply stalled, while in the samara the inside wing is missing entirely. In a non-spinning airplane, if one wing were producing more lift than the other, that wing would rise. So the question is, why is a flat spin stable? Why doesn't the outside wing continue to roll to ever-higher 16

bank angles? The secret is centrifugal force. Suppose you hold a broomstick by one end while you spin around and around; the broomstick will be centrifuged outward and toward the horizontal. In an airplane spinning about a vertical axis, the high (outside) wing will be centrifuged outward and downward (toward the horizontal), while the low (inside) wing will be centrifuged outward and upward (again toward the horizontal). In a steady flat spin, these centrifugal forces cancel the rolling moment that results from one wing producing a lot more lift than the other. This is the only example I can imagine where an airplane is in a steady regime of flight but one wing is producing more lift than the other. As discussed in reference 6, an aircraft with a lot of mass in the wings will have a stronger centrifugal force than one with all the mass near the centerline of the fuselage. In particular, an aircraft with one http://www.av8n.com/how/htm/spins.html (14 of 20)24-1-2004 17:54:49


pilot and lots of fuel in the wing tanks could have completely different spin characteristics than the same aircraft with two pilots and less fuel aboard. 18.6.3 NASA Spin Studies In the 1970s, NASA conduced a series of experiments on the spin behavior of general-aviation aircraft; see reference 8 and reference 7 and other papers cited therein. They noted that there was ``considerable confusion'' surrounding the definition of steep versus flat spin modes, and offered the classification scheme shown in table 18.1. spin mode

Steep

Mod'ly Steep Mod'ly Flat

angle of attack

20 to 30

30 to 45

45 to 65

Flat 65 to 90

nose attitude

extreme nose-down

less nose-down

rate of descent

very rapid

less rapid

rate of roll

extreme

moderate

rate of yaw

moderate

extreme

wingtip-to-wingtip difference in angle of attack

modest

large

nose-to-tail difference in slip

large

large

Table 18.1: Spin Mode Classification The angle of attack that appears in this table is measured in the aircraft's plane of symmetry; the actual angle of attack at other positions along the span will depend on position. The NASA tests demonstrated that general aviation aircraft not approved for intentional spins commonly had unrecoverable flat spin modes. 18.6.4 Effects of Changes in Orientation of Spin In all cases NASA studied, the flat spin had a faster rate of rotation (and a slower rate of descent) than the steep spin. Meanwhile, reference 15 reports experiments in which the flatter pitch attitudes were associated with the slower rates of rotation. This is not a contradiction, because the latter dealt with an unsteady spin (with frequent changes in pitch attitude), rather than a fully stabilized flat spin. A sudden change to a flatter pitch attitude will cause a temporary reduction in spin rate, for the following reason. In any system where angular momentum is not changing, the system will spin faster when the mass is more concentrated near the axis of rotation. The general concept is discussed in section 19.8. By the same token, if the mass of a spinning object is redistributed farther from the axis, the rotation will slow down. http://www.av8n.com/how/htm/spins.html (15 of 20)24-1-2004 17:54:49


When the spinning airplane pitches up into a flatter attitude, whatever mass is in the nose and tail will move farther from the axis of rotation. Angular momentum doesn't change in the short run, so the rotation will slow down in the short run. In the longer run — in a steady flat spin — the aerodynamics of the spin will pump more angular momentum into the system, and the rotation rate will increase quite a lot. The rotation rate of the established flat spin is typically twice that of the steep spin. Recovering from an established flat spin requires forcing the nose down. This brings the mass in the nose and tail closer to the axis of rotation. Once again using the principle of conservation of angular momentum, you can see that the rotation rate will increase (at least in the short run) as you do so — which can be disconcerting.

18.7 Recovering from a Spin If you find yourself in an unusual turning, descending situation, the first thing to do is decide whether you are in a spiral dive or in a spin. In a spiral dive, the airspeed will be high and increasing; in a spin the airspeed will be low. You should be able to hear the difference. Also, the rate of rotation in a spiral is much less; the high speed means the airplane has lots of momentum and can't turn on a dime. In a spin, the aircraft will be turning a couple hundred degrees per second. 17

To get out of a spin, follow the spin-recovery procedures given in the Pilot's Operating Handbook for your airplane. The literature is full of home-brew spin recovery procedures that probably work most of the time in most airplanes, but if you want a procedure that works for sure, follow the handbook for your airplane. For typical airplanes, the spin recovery procedure contains the following items: ● ● ● ● ●

Retard the throttle to idle Retract the flaps Neutralize the ailerons Apply full rudder in the direction opposing the spin Briskly move the yoke to select zero angle of attack.

Now let's discuss each of these items in a little more detail. Retarding the throttle is a moderately good idea for a couple of reasons. For one thing (especially if you have a fixed-pitch prop) it keeps the engine from overspeeding during the later stages of the spin recovery. More importantly, gyroscopic precession of the rotating engine and propeller can hold the nose up, flattening the spin and interfering with the recovery (depending on the direction of spin). Propwash might increase the effectiveness of the horizontal tail and therefore assist in the spin recovery, http://www.av8n.com/how/htm/spins.html (16 of 20)24-1-2004 17:54:49


but (especially in a flat spin) the propwash could be blown somewhere else by the abnormal airflow — so you may not be able to count on this. Retracting the flaps is a moderately good idea because you might exceed the ``max flaps-extended speed'' if you mishandle the later stages of the spin recovery and you don't want to damage the flaps. Retracting the flaps may help with the spin recovery itself. Recall from section 5.4.3 that the flaps effectively increase the washout of the wings. Washout ensures that the airplane will stall before it runs out of roll damping. (This produces a nice straight-ahead stall.) In the spin, though, when you have lost all vertical damping and roll damping, the washout doesn't help. The early stages of spin recovery are not like the early stages of stall entry. Neutralizing the ailerons is usually a good idea for the simple reason that it is hard to think of anything better to do with them. Deflecting the ailerons effectively increases the angle of attack of one wingtip and decreases the angle of attack of the other wingtip. In a spin, the part of the wing where the ailerons are may (or may not) be in the stalled regime — so deflecting the ailerons to the left may (or may not) produce a paradoxical rolling moment to the right. Depressing the rudder to oppose the spin is obviously a good thing to do. Finally, you want to move the yoke to select zero angle of attack. In typical trainers, this means shoving the yoke all the way forward, but in other aircraft, especially aerobatic aircraft, all the way forward might select a large negative angle of attack. Shoving the yoke all the way forward in such a plane would likely convert the spin to an inverted spin — hardly an improvement. This is just one example of why you want to know and follow the spin recovery procedure for your specific airplane. The relative significance of the rudder compared to the flippers in breaking the spin depends radically on the design of the airplane, the loading of the airplane, and on the spin mode, as discussed in reference 6. In normal non-spinning flight, you should apply smooth pressures to the controls. Spin recovery is the exception: it calls for brisk, mechanical motions of the controls, almost without regard to the pressures involved. If you get into a spin in instrument conditions, you should rely primarily on the airspeed indicator and the rate-of-turn gyro. The inclinometer ball cannot be trusted; it is likely to be centrifuged away from the center of the airplane — giving an indication that depends on where the instrument is installed, telling you nothing about the direction of spin. The artificial horizon (attitude indicator) cannot be trusted since it may have tumbled. The rate-of-turn gyro is more trustworthy, since it is a rate gyro, not a free gyro; that is, it has no gimbals and cannot possibly suffer from gimbal lock. Recovery from a so-called incipient spin (one that has just gotten started) is easier than from a well18

developed spin. Normal-category single-engine certification requirements say that an airplane must be able to recover from a one-turn spin (or a 3-second spin, whichever takes longer) in not more than one additional turn. If you let the spin go on for several turns, you might progress from a steep spin to a flat http://www.av8n.com/how/htm/spins.html (17 of 20)24-1-2004 17:54:49


spin. Recovery could take a lot longer — if it is possible at all. If you load the airplane beyond the aft limit of the weight and balance envelope, even the incipient spin may be unrecoverable; see section 6.1.1. Imperfect repairs to the wing, or slack in the control cables, could also impede spin recovery. Finally, the spin is yet another reason why it is NOT SAFE to think of the yoke as simply the up/down 19

control. In a spin you have a low airspeed and a high rate of descent. If you think of the yoke as the up/ down control, you will be tempted to pull back on the yoke, which is exactly the wrong thing to do. On the other hand, if you think of the yoke as (primarily) the fast/slow control, you will realize that you need to push forward on the yoke, to solve the airspeed problem.

18.8 Don't Mess With Spins It is quite impressive how well a samara works. A maple seed descends very slowly, riding the wind much better than a parachute of similar size and weight ever could. Flat spins can be extremely stable; a wing by itself loves to spin. That's why spins (and flat spins in particular) are so dangerous: it takes a lot of rudder force to persuade a wing to stop spinning. Spins are extremely complex. Even designers and top-notch test pilots are routinely surprised by the behavior of spinning airplanes. Spin-test airplanes are equipped with cannon-powered spin-recovery parachutes on the airframe, and quick-release doors in view of the distinct possibility that the pilot will have to bail out. Tests are conducted at high altitude over absolutely unpopulated areas. Therefore please don't experiment with spinning a plane except exactly as approved by the manufacturer — one unrecoverable spin mode can ruin your whole day.

1

This happens naturally on a rectangular wing; it can be enhanced by washout and other designers' tricks. 2

An even more direct method of adding energy to the boundary layer uses a jet of high-velocity air, as discussed in section 18.4.2. 3

Of course, the VGs contribute indirectly to maintaining the health of the big bound vortex, since they help maintain attachment and therefore help create lots of circulation. 4

See reference 17 for a nice discussion of golf balls, cricket balls, and boundary layers in general. 5

... just as having lots of water is the cause, but not the definition, of drowning — you can get very wet without drowning. http://www.av8n.com/how/htm/spins.html (18 of 20)24-1-2004 17:54:49


6

You can blow directly from your lips, but it's better to use a flexible straw or a thin piece of tubing, so that you can get a better view of what's happening. If you put a nozzle at the end of the tube, the jet will keep its shape better. 7

Ground pepper is a convenient source of suitable dust. 8

This won't be exactly atmospheric, since the local pressure has been affected by the wing. 9

Remember, lift is the force perpendicular to the flow and perpendicular to the surface. 10

Indeed, as long as the viscosity is not exactly zero, the smaller the viscosity, the greater the turbulence. 11

Of course some energy is transferred, in the form of friction and induced drag, but this is very small, out of all proportion to the energy that the air parcel transfers from its own speed to pressure and back again. 12

Nonsensical things are often rather hard to calculate. 13

If you don't have a good model airfoil, start with a flat piece of cardboard. 14

This refers to ``departure from normal flight''. It has nothing to do with takeoff or with a ``departure stall'' which merely refers to a stall in the takeoff configuration. 15

Under present-day certification rules, the ailerons are required to work normally up to at least stalling angle of attack. However, some older airplanes were built under older rules. These planes, including many aerobatic aircraft, have much less washout, and therefore lose aileron effectiveness earlier. All planes lose effectiveness eventually. For simplicity, this section ignores washout. 16

See section 19.4 for a discussion of the nature of centrifugal fields. 17

Recovery from a spiral dive is discussed in section 6.2.4. 18

Multi-engine aircraft are not required to be recoverable from any sort of spin, incipient or otherwise. 19

This point is discussed in chapter 7. [Previous] [Contents] [Next] [Comments or questions] _ http://www.av8n.com/how/htm/spins.html (19 of 20)24-1-2004 17:54:49


Copyright © 1996-2001 jsd


A Physical Description of Lift

This material can be found in more detail in "Understanding Flight", by David Anderson and Scott Eberhardt, McGraw-Hill, 2001, ISBN: 0-07-136377-7

Reviews

Review from Discovery

Review from Pilot Training

A Physical Description of Flight © David Anderson Fermi National Accelerator Laboratory Ret. [email protected]

& Scott Eberhardt Dept. of Aeronautics and Astronautics University of Washington Seattle WA 91895-2400 [email protected]

http://www.aa.washington.edu/faculty/eberhardt/lift.htm (1 of 18)24-1-2004 18:04:54


Almost everyone today has flown in an airplane. Many ask the simple question "what makes an airplane fly?" The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton’s laws rather than the Bernoulli principle. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down. Most of this diverted air is pulled down from above the wing. Let us start by defining three descriptions of lift commonly used in textbooks and training manuals. The first we will call the Mathematical Aerodynamics Description of lift, which is used by aeronautical engineers. This description uses complex mathematics and/or computer simulations to calculate the lift of a wing. It often uses a mathematical concept called "circulation" to calculate the acceleration of the air over the wing. Circulation is a measure of the apparent rotation of the air around the wing. While useful for calculations of lift, this description does not lend themselves to an intuitive understanding of flight. The second description we will call the Popular Description, which is based on the Bernoulli principle. The primary advantage of this description is that it is easy to understand and has been taught for many years. Because of its simplicity, it is used to describe lift in most flight training manuals. The major disadvantage is that it relies on the "principle of equal transit times", or at least on the assumption that because the air must travel farther over the top of the wing it must go faster. This description focuses on the shape of the wing and prevents one from understanding such important phenomena as inverted flight, power, ground effect, and the dependence of lift on the angle of attack of the wing. The third description, which we are advocating here, we will call the Physical Description of lift. This description of lift is based primarily on Newton's three laws and a phenomenon called the Coanda effect. This description is uniquely useful for understanding the phenomena associated with flight. It is useful for an accurate understanding the relationships in flight, such as how power increases with load or how the stall speed increases with altitude. It is also a useful tool for making rough estimates ("back-of-theenvelope calculations") of lift. The Physical Description of lift is also of great use to a pilot who needs an intuitive understanding of how to fly the airplane.

The popular description of lift Students of physics and aerodynamics are taught that an airplane flies as a result of the Bernoulli principle, which says that if air speeds up the pressure is lowered. (In fact this is not always true. The air flows fast over the airplane’s static port but the altimeter still reads the correct altitude.) The argument goes that a wing has lift because the air goes faster over the top creating a region of low pressure. This explanation usually satisfies the curious and few challenge the conclusions. Some may wonder why the air goes faster over the top of the wing and this is where the popular explanation of lift falls apart. In order to explain why the air goes faster over the top of the wing, many have resorted to the geometric http://www.aa.washington.edu/faculty/eberhardt/lift.htm (2 of 18)24-1-2004 18:04:54


argument that the distance the air must travel is directly related to its speed. The usual claim is that when the air separates at the leading edge, the part that goes over the top must converge at the trailing edge with the part that goes under the bottom. This is the so-called "principle of equal transit times". One might ask if the numbers calculated by the Popular Description really work. Let us look at an example. Take the case of a Cessna 172, which is popular, high-winged, four-seat airplane. The wings must lift 2300 lb (1045 kg) at its maximum flying weight. The path length for the air over the top of the wing is only about 1.5% greater than under the wing. Using the Popular Description of lift, the wing would develop only about 2% of the needed lift at 65 mph (104 km/h), which is "slow flight" for this airplane. In fact, the calculations say that the minimum speed for this wing to develop sufficient lift is over 400 mph (640 km/h). If one works the problem the other way and asks what the difference in path length would have to be for the Popular Description to account for lift in slow flight, the answer would be 50%. The thickness of the wing would be almost the same as the chord length. But, who says the separated air must meet at the trailing edge at the same time? Figure 1 shows the airflow over a wing in a simulated wind tunnel. In the simulation, smoke is introduced periodically. One can see that the air that goes over the top of the wing gets to the trailing edge considerably before the air that goes under the wing. In fact, the air is accelerated much faster than would be predicted by equal transit times. Also, on close inspection one sees that the air going under the wing is slowed down from the "free-stream" velocity of the air. The principle of equal transit times holds only for a wing with zero lift.

Fig 1 Simulation of the airflow over a wing in a wind tunnel, with "smoke". The popular explanation also implies that inverted flight is impossible. It certainly does not address acrobatic airplanes, with symmetric wings (the top and bottom surfaces are the same shape), or how a wing adjusts for the great changes in load such as when pulling out of a dive or in a steep turn? So, why has the popular explanation prevailed for so long? One answer is that the Bernoulli principle is easy to understand. There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing. But, as the above discussion suggests, our understanding is not http://www.aa.washington.edu/faculty/eberhardt/lift.htm (3 of 18)24-1-2004 18:04:54


complete with this explanation. The problem is that we are missing a vital piece when we apply Bernoulli’s principle. We can calculate the pressures around the wing if we know the speed of the air over and under the wing, but how do we determine the speed? As we will soon see, the air accelerates over the wing because the pressure is lower, not the other way around. Another fundamental shortcoming of the popular explanation is that it ignores the work that is done. Lift requires power (which is work per time). As will be seen later, an understanding of power is key to the understanding of many of the interesting phenomena of lift.

Newton’s laws and lift So, how does a wing generate lift? To begin to understand lift we must review Newton’s first and third laws. (We will introduce Newton’s second law a little later.) Newton’s first law states a body at rest will remain at rest, or a body in motion will continue in straight-line motion unless subjected to an external applied force. That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, a force is acting on it. Newton’s third law states that for every action there is an equal and opposite reaction. As an example, an object sitting on a table exerts a force on the table (its weight) and the table puts an equal and opposite force on the object to hold it up. In order to generate lift a wing must do something to the air. What the wing does to the air is the action while lift is the reaction. Let’s compare two figures used to show streamlines over a wing. In figure 2 the air comes straight at the wing, bends around it, and then leaves straight behind the wing. We have all seen similar pictures, even in flight manuals. But, the air leaves the wing exactly as it appeared ahead of the wing. There is no net action on the air so there can be no lift! Figure 3 shows the streamlines, as they should be drawn. The air passes over the wing and is bent down. Newton’s first law says that them must be a force on the air to bend it down (the action). Newton’s third law says that there must be an equal and opposite force (up) on the wing (the reaction). To generate lift a wing must divert lots of air down.

Fig 2 Common depiction of airflow over a wing. This wing has no lift.



Fig 3 True airflow over a wing with lift, showing upwash and downwash. The lift of a wing is equal to the change in momentum of the air it is diverting down. Momentum is the product of mass and velocity (mv). The most common form of Newton’s second law is F= ma, or force equal mass times acceleration. The law in this form gives the force necessary to accelerate an object of a certain mass. An alternate form of Newton’s second law can be written: The lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air. It is that simple. For more lift the wing can either divert more air (mass) or increase its vertical velocity. This vertical velocity behind the wing is the vertical component of the "downwash". Figure 4 shows how the downwash appears to the pilot (or in a wind tunnel). The figure also shows how the downwash appears to an observer on the ground watching the wing go by. To the pilot the air is coming off the wing at roughly the angle of attack and at about the speed of the airplane. To the observer on the ground, if he or she could see the air, it would be coming off the wing almost vertically at a relatively slow speed. The greater the angle of attack of the wing the greater the vertical velocity of the air. Likewise, for a given angle of attack, the greater the speed of the wing the greater the vertical velocity of the air. Both the increase in the speed and the increase of the angle of attack increase the length of the vertical velocity arrow. It is this vertical velocity that gives the wing lift.

Fig 4 How downwash appears to a pilot and to an observer on the ground. As stated, an observer on the ground would see the air going almost straight down behind the plane. This can be demonstrated by observing the tight column of air behind a propeller, a household fan, or under the rotors of a helicopter; all of which are rotating wings. If the air were coming off the blades at an angle the air would produce a cone rather than a tight column. The wing develops lift by transferring momentum to the air. For straight and level flight this momentum eventually strikes the earth in. If an airplane were to fly over a very large scale, the scale would weigh the airplane. http://www.aa.washington.edu/faculty/eberhardt/lift.htm (5 of 18)24-1-2004 18:04:54


Let us do a back-of-the-envelope calculation to see how much air a wing might divert. Take for example a Cessna 172 that weighs about 2300 lb (1045 kg). Traveling at a speed of 140 mph (220 km/h), and assuming an effective angle of attack of 5 degrees, we get a vertical velocity for the air of about 11.5 mph (18 km/h) right at the wing. If we assume that the average vertical velocity of the air diverted is half that value we calculate from Newton's second law that the amount of air diverted is on the order of 5 ton/s. Thus, a Cessna 172 at cruise is diverting about five times its own weight in air per second to produce lift. Think how much air is diverted by a 250-ton Boeing 777 on takeoff. Diverting so much air down is a strong argument against lift being just a surface effect (that is only a small amount of air around the wing accounts for the lift), as implied by the popular explanation. In fact, in order to divert 5 ton/sec the wing of the Cessna 172 must accelerate all of the air within 18 feet (7.3 m) above the wing. One should remember that the density of air at sea level is about 2 lb per cubic yard (about 1kg per cubic meter). Figure 5 illustrates the effect of the air being diverted down from a wing. A huge hole is punched through the fog by the downwash from the airplane that has just flown over it.

Fig 5 Downwash and wing vortices in the fog. (Photographer Paul Bowen, courtesy of Cessna Aircraft, Co.)

So how does a thin wing divert so much air? When the air is bent around the top of the wing, it pulls on the air above it accelerating that air downward. Otherwise there would be voids in the air above the wing. Air is pulled from above. This pulling causes the pressure to become lower above the wing. It is the acceleration of the air above the wing in the downward direction that gives lift. (Why the wing bends the air with enough force to generate lift will be discussed in the next section.) As seen in figure 3, a complication in the picture of a wing is the effect of "upwash" at the leading edge http://www.aa.washington.edu/faculty/eberhardt/lift.htm (6 of 18)24-1-2004 18:04:54


of the wing. As the wing moves along, air is not only diverted down at the rear of the wing, but air is pulled up at the leading edge. This upwash actually contributes to negative lift and more air must be diverted down to compensate for it. This will be discussed later when we consider ground effect. Normally, one looks at the air flowing over the wing in the frame of reference of the wing. In other words, to the pilot the air is moving and the wing is standing still. We have already stated that an observer on the ground would see the air coming off the wing almost vertically. But what is the air doing below the wing? Figure 6 shows an instantaneous snapshot of how air molecules are moving as a wing passes by. Remember in this figure the air is initially at rest and it is the wing moving. Arrow "1" will become arrow "2" and so on. Ahead of the leading edge, air is moving up (upwash). At the trailing edge, air is diverted down (downwash). Over the top the air is accelerated towards the trailing edge. Underneath, the air is accelerated forward slightly.

Fig 6 Direction of air movement around a wing as seen by an observer on the ground.

So, why does the air follow this pattern? First, we have to bear in mind that air is considered an incompressible fluid for low-speed flight. That means that it cannot change its volume and that there is a resistance to the formation of voids. Now the air has been accelerated over the top of the wing by of the reduction in pressure. This draws air from in front of the wing and expels if back and down behind the wing. This air must be compensated for, so the air shifts around the wing to fill in. This is similar to the circulation of the water around a canoe paddle. This circulation around the wing is no more the driving force for the lift on the wing than is the circulation in the water drives the paddle. Though, it is true that if one is able to determine the circulation around a wing the lift of the wing can be calculated. Lift and circulation are proportional to each other. One observation that can be made from figure 6 is that the top surface of the wing does much more to move the air than the bottom. So the top is the more critical surface. Thus, airplanes can carry external stores, such as drop tanks, under the wings but not on top where they would interfere with lift. That is also why wing struts under the wing are common but struts on the top of the wing have been historically rare. A strut, or any obstruction, on the top of the wing would interfere with the lift.



Coanda Effect A natural question is "how does the wing divert the air down?" When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface. To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass as is shown in figure 7. This tendency of fluids to follow a curved surface is known as the Coanda effect. From Newton’s first law we know that for the fluid to bend there must be a force acting on it. From Newton’s third law we know that the fluid must put an equal and opposite force on the glass.

Fig 7 Coanda effect.

So why should a fluid follow a curved surface? The answer is viscosity; the resistance to flow which also gives the air a kind of "stickiness". Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. At the surface the relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car.) Just above the surface the fluid has some small velocity. The farther one goes from the surface the faster the fluid is moving until the external velocity is reached. Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface by shear forces. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially stuck to the wing is called the "boundary layer" and is less than one inch (2.5 cm) thick, even for a large wing. Look again at Figure 3. The magnitude of the forces on the air (and on the wing) are proportional to the "tightness" of the bend. The tighter the air bends the greater the force on it. One thing to notice in the figure is that most of the lift is on the forward part of the wing. In fact, half of the total lift on a wing is http://www.aa.washington.edu/faculty/eberhardt/lift.htm (8 of 18)24-1-2004 18:04:54


typically produced in the first 1/4 of the chord length.

Lift as a function of angle of attack There are many types of wing: conventional, symmetric, conventional in inverted flight, the early biplane wings that looked like warped boards, and even the proverbial "barn door". In all cases, the wing is forcing the air down, or more accurately pulling air down from above. (Though the early wings did have a significant contribution from the bottom.) What each of these wings has in common is an angle of attack with respect to the oncoming air. It is the angle of attack that is the primary parameter in determining lift. To better understand the role of the angle of attack it is useful to introduce an "effective" angle of attack, defined such that the angle of the wing to the oncoming air that gives zero lift is defined to be zero degrees. If one then changes the angle of attack both up and down one finds that the lift is proportional to the angle. Figure 8 shows the lift of a typical wing as a function of the effective angle of attack. A similar lift versus angle of attack relationship is found for all wings, independent of their design. This is true for the wing of a 747, an inverted wing, or your hand out the car window. The inverted wing can be explained by its angle of attack, despite the apparent contradiction with the popular explanation of lift. A pilot adjusts the angle of attack to adjust the lift for the speed and load. The role of the angle of attack is more important than the details of the wings shape in understanding lift. The shape comes into play in the understanding of stall characteristics and drag at high speed.

Fig 8 Lift versus the effective angle of attack.

Typically, the lift begins to decrease at a "critical angle" of attack of about 15 degrees. The forces http://www.aa.washington.edu/faculty/eberhardt/lift.htm (9 of 18)24-1-2004 18:04:54


necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing. This separation of the airflow from the top of the wing is a stall.

The wing as air "scoop" We now would like to introduce a new mental image of a wing. One is used to thinking of a wing as a thin blade that slices though the air and develops lift somewhat by magic. The new image that we would like you to adopt is that of the wing as a scoop diverting a certain amount of air from the horizontal to roughly the angle of attack, as depicted in Figure 9. For wings of typical airplanes it is a good approximation to say that the area of the scoop is proportional to the area of the wing. The shape of the scoop is approximately elliptical for all wings, as shown in the figure. Since the lift of the wing is proportional to the amount of air diverted, the lift of is also proportional to the wing’s area.

Fig 9 The wing as a scoop.

As stated before, the lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air. As a plane increases speed, the scoop diverts more air. Since the load on the wing does not increase, the vertical velocity of the diverted air must be decreased proportionately. Thus, the angle of attack is reduced to maintain a constant lift. When the plane goes higher, the air becomes less dense so the scoop diverts less air at a given speed. Thus, to compensate the angle of attack must be increased. The concepts of this section will be used to understand lift in a way not possible with the popular explanation.

Lift requires power When a plane passes overhead the formally still air gains a downward velocity. Thus, the air is left in motion after the plane leaves. The air has been given energy. Power is energy, or work, per time. So, lift requires power. This power is supplied by the airplane’s engine (or by gravity and thermals for a sailplane).



How much power will we need to fly? If one fires a bullet with a mass, m, and a velocity, v, the energy given to the bullet is simply ½mv2. Likewise, the energy given to the air by the wing is proportional to the amount of air diverted down times the vertical velocity squared of that diverted air. We have already stated that the lift of a wing is proportional to the amount of air diverted times the vertical velocity of that air. Thus, the power needed to lift the airplane is proportional to the load (or weight) times the vertical velocity of the air. If the speed of the plane is doubled the amount of air diverted down doubles. Thus to maintain a constant lift, the angle of attack must be reduced to give a vertical velocity that is half the original. The power required for lift has been cut in half. This shows that the power required for lift becomes less as the airplane's speed increases. In fact, we have shown that this power to create lift is proportional to 1/speed of the plane. But, we all know that to go faster (in cruise) we must apply more power. So there must be more to power than the power required for lift. The power associated with lift is often called the "induced" power. Power is also needed to overcome what is called "parasitic" drag, which is the drag associated with moving the wheels, struts, antenna, etc. through the air. The energy the airplane imparts to an air molecule on impact is proportional to the speed2 (form ½mv2) . The number of molecules struck per time is proportional to the speed. The faster one goes the higher the rate of impacts. Thus the parasitic power required to overcome parasitic drag increases as the speed3. Figure 10 shows the "power curves" for induced power, parasitic power, and total power (the sum of induced power and parasitic power). Again, the induced power goes as 1/speed and the parasitic power goes as the speed3. At low speed the power requirements of flight are dominated by the induced power. The slower one flies the less air is diverted and thus the angle of attack must be increased to increase the vertical velocity of that air. Pilots practice flying on the "backside of the power curve" so that they recognize that the angle of attack and the power required to stay in the air at very low speeds are considerable.

Fig 10 Power requirements versus speed. http://www.aa.washington.edu/faculty/eberhardt/lift.htm (11 of 18)24-1-2004 18:04:54


At cruise, the power requirement is dominated by parasitic power. Since this goes as the speed3 an increase in engine size gives one a faster rate of climb but does little to improve the cruise speed of the plane. Doubling the size of the engine will only increase the cruise speed by about 25%. Since we now know how the power requirements vary with speed, we can understand drag, which is a force. Drag is simply power divided by speed. Figure 11 shows the induced, parasitic, and total drag as a function of speed. Here the induced drag varies as 1/speed2 and parasitic drag varies as the speed2. Taking a look at these figures one can deduce a few things about how airplanes are designed. Slower airplanes, such as gliders, are designed to minimize induced power, which dominates at lower speeds. Faster propeller-driven airplanes are more concerned with parasite power, and jets are dominated by parasitic drag. (This distinction is outside of the scope of this article.)

Fig 11 Drag versus speed.

Wing efficiency At cruise, a non-negligible amount of the drag of a modern wing is induced drag. Parasitic drag of a Boeing 747 wing is only equivalent to that of a 1/2-inch cable of the same length. One might ask what affects the efficiency of a wing. We saw that the induced power of a wing is proportional to the vertical velocity of the air. If the area of a wing were to be increased, the size of our scoop would also increase, http://www.aa.washington.edu/faculty/eberhardt/lift.htm (12 of 18)24-1-2004 18:04:54


diverting more air. So, for the same lift the vertical velocity (and thus the angle of attack) would have to be reduced. Since the induced power is proportional to the vertical velocity of the air, it is also reduced. Thus, the lifting efficiency of a wing increases with increasing wing area. The larger the wing the less induced power required to produce the same lift, though this is achieved with and increase in parasitic drag. As will be briefly discussed in the section on ground effect, the additional loading on the wing in straight and level flight due to upwash is equal to the weight of the airplane time 2/AR. Where AR is the wing’s aspect ratio (span divided by the mean chord). Thus, when considering two wings with the same area but different aspect ratios, the wing with the greater aspect ratio will be the most efficient. There is a misconception by some that lift does not require power. This comes from aeronautics in the study of the idealized theory of wing sections (airfoils). When dealing with an airfoil, the picture is actually that of a wing with infinite span. Since we have seen that the power necessary for lift decrease with increasing area of the wing, a wing of infinite span does not require power for lift. If lift did not require power airplanes would have the same range full as they do empty, and helicopters could hover at any altitude and load. Best of all, propellers (which are rotating wings) would not require power to produce thrust. Unfortunately, we live in the real world where both lift and propulsion require power.

Power and wing loading Now let us consider the relationship between wing loading and power. At a constant speed, if the wing loading is increased the vertical velocity must be increased to compensate. This is accomplished by increasing the angle of attack of the wing. If the total weight of the airplane were doubled (say, in a 2g turn), and the speed remains constant, the vertical velocity of the air is doubled to compensate for the increased wing loading. The induced power is proportional to the load times the vertical velocity of the diverted air, which have both doubled. Thus the induced power requirement has increased by a factor of four! So induced power is proportional to the load2. One way to measure the total power is to look at the rate of fuel consumption. Figure 12 shows the fuel consumption versus gross weight for a large transport airplane traveling at a constant speed (obtained from actual data). Since the speed is constant the change in fuel consumption is due to the change in induced power. The data are fitted by a constant (parasitic power) and a term that goes as the load2. This second term is just what was predicted in our Newtonian discussion of the effect of load on induced power.



Fig 12 Fuel consumption versus load for a large transport airplane traveling at a constant speed.

The increase in the angle of attack with increased load has a downside other than just the need for more power. As shown in figure 8 a wing will eventually stall when the air can no longer follow the upper surface. That is, when the critical angle is reached. Figure 13 shows the angle of attack as a function of airspeed for a fixed load and for a 2-g turn. The angle of attack at which the plane stalls is constant and is not a function of wing loading. The angle of attack increases as the load and the stall speed increases as the square root of the load. Thus, increasing the load in a 2-g turn increases the speed at which the wing will stall by 40%. An increase in altitude will further increase the angle of attack in a 2-g turn. This is why pilots practice "accelerated stalls" which illustrates that an airplane can stall at any speed, since for any speed there is a load that will induce a stall.

Fig 13 Angle of attack versus speed for straight and level flight and for a 2-g turn.



Wing vortices One might ask what the downwash from a wing looks like. The downwash comes off the wing as a sheet and is related to the details on the load distribution on the wing. Figure 14 shows, through condensation, the distribution of lift on an airplane during a high-g maneuver. From the figure one can see that the distribution of load changes from the root of the wing to the tip. Thus, the amount of air in the downwash must also change along the wing. The wing near the root is "scooping" up much more air than the tip. Since the wing near the root is diverting so much air the net effect is that the downwash sheet will begin to curl outward around itself, just as the air bends around the top of the wing because of the change in the velocity of the air. This is the wing vortex. The tightness of the curling of the wing vortex is proportional to the rate of change in lift along the wing. At the wing tip the lift must rapidly become zero causing the tightest curl. This is the wing tip vortex and is just a small (though often most visible) part of the wing vortex. Returning to figure 5 one can clearly see the development of the wing vortices in the downwash as well as the wing tip vortices.

Fig 14 Condensation showing the distribution of lift along a wing. (from Patterns in the Sky, J.F. Campbell and J.R. Chambers, NASA SP-514.)

Winglets (those small vertical extensions on the tips of some wings) are used to improve the efficiency of the wing by increasing the effective length, and thus area, of the wing. The lift of a normal wing must go to zero at the tip because the bottom and the top communicate around the end. The winglet blocks this communication so the lift can extend farther out on the wing. Since the efficiency of a wing increases with area, this gives increased efficiency. One caveat is that winglet design is tricky and winglets can actually be detrimental if not properly designed.



Ground effect Another common phenomenon that is often misunderstood is that of ground effect. That is the increased efficiency of a wing when flying within a wing length of the ground. A low-wing airplane will experience a reduction in drag by as much as 50% just before it touches down. This reduction in drag just above a surface is used by large birds, which can often be seen flying just above the surface of the water. Pilots taking off from deep-grass or soft runways also use ground effect. Many pilots mistakenly believe that ground effect is the result of air being compressed between the wing and the ground. To understand ground effect it is necessary to look again at the upwash. Notice in Figure 15 that the air bends up from its horizontal flow to form the upwash. Newton's first law says that there must be a force acting on the air to bend it. Since the air is bent up the force must be up as shown by the arrow. Newton's third laws says that there is an equal and opposite force on the wing which is down. The result is that the upwash increases the load on the wing. To compensate for this increased load, the wing must fly at a greater angle of attack, and thus a greater induced power. As the wing approaches the ground the circulation below the wing is inhibited. As shown in Figure 16, there is a reduction in the upwash and in the additional loading on the wing caused by the upwash. To compensate, the angle of attack is reduced and so is the induced power. The wing becomes more efficient.

Fig 15 Wing out of ground effect



Fig 16 Wing in ground effect

The additional load due to upwash is equal to the weight of the airplane time 2/AR. Most small airplanes have aspect ratios of 7-8. An airplane with an aspect ratio of 8 can experience as much as a 25% reduction in wing loading due to ground effect. Since induced power is proportional to the load2, this corresponds to a 50% reduction in induced power. Earlier, we estimated that a Cessna 172 flying at 110 knots must divert about 5 ton/sec to provide lift. In our calculations we neglected the contribution of upwash. The amount of air diverted is probably closer to 6 ton/sec.

Conclusions Let us review what we have learned and get some idea of how the physical description has given us a greater ability to understand flight. First what have we learned: ●

●

● ●

The amount of air diverted by the wing is proportional to the speed of the wing and the air density. The vertical velocity of the diverted air is proportional to the speed of the wing and the angle of attack. The lift is proportional to the amount of air diverted times the vertical velocity of the air. The power needed for lift is proportional to the lift times the vertical velocity of the air.

Now let us look at some situations from the physical point of view and from the perspective of the popular explanation. ●

The plane’s speed is reduced. The physical view says that the amount of air diverted is reduced so the angle of attack is increased to compensate. The power needed for lift is also increased. The popular explanation cannot address this.


A Physical Description of Lift ●

●

The load of the plane is increased. The physical view says that the amount of air diverted is the same but the angle of attack must be increased to give additional lift. The power needed for lift has also increased. Again, the popular explanation cannot address this. A plane flies upside down. The physical view has no problem with this. The plane adjusts the angle of attack of the inverted wing to give the desired lift. The popular explanation implies that inverted flight is impossible.

As one can see, the popular explanation, which fixates on the shape of the wing, may satisfy many but it does not give one the tools to really understand flight. The physical description of lift is easy to understand and much more powerful. This material can be found in more detail in "Understanding Flight", by David Anderson and Scott Eberhardt, McGraw-Hill, 2001, ISBN: 0-07-136377-7


Jef Raskin

Jef Raskin's

Coanda Effect: Understanding Why Wings Work Site Directory

MODEL AIRPLANES, THE BERNOULLI EQUATION, AND THE COANDA EFFECT © 1994 by Jef Raskin "In aerodynamics, theory is what makes the invisible plain. Trying to fly an airplane without theory is like getting into a fistfight with a poltergeist." --David Thornburg [1992]. "That we have written an equation does not remove from the flow of fluids its charm or mystery or its surprise." --Richard Feynman [1964] INTRODUCTION http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (1 of 29)24-1-2004 18:11:58

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A sound theoretical understanding of lift had been achieved within two decades of the Wright brothers' first flight (Prandtl's work was most influential 1), but the most common explanation of lift seen in elementary texts and popular articles today is

The common explanation, from The Way Things Work [Macaulay 1988] The reasoning--though incomplete--i s based on the Bernoulli effect, which correctly correlates the increased speed with which air moves over a surface and the lowered air pressure measured at that surface. In fact, most airplane wings do have considerably more curvature on the top than the bottom, lending credence to this explanation. But, even as a child, I found that it presented me with a puzzle: how can a plane fly inverted (upside down). When I 1IMAGE Ludwig imgs/conda02. Prandtl (1875-1953), a German physicist, often called the

gif "father of aerodynamics." His famous book on the theory of wings, Tragflü geltheorie, was published in 1918.

http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (2 of 29)24-1-2004 18:11:58

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1

pressed my 6th grade science teacher on this question, he just got mad, denied that planes could fly inverted and tried to continue his lecture. I was very frustrated and argued until he said, "Shut up, Raskin!" I will relate what happened next later in this essay. A few years later I carried out a calculation according to a naive interpretation of the common explanation of how a wing works. Using data from a model airplane I found that the calculated lift was only 2% of that needed to fly the model. [See Appendix 1 for the calculation]. Given that Bernoulli's equation is correct (indeed, it is a form of the law of conservation of energy), I was left with my original question unanswered: where does the lift come from? In the next few sections we look at attempts to explain two related phenomena--what makes a spinning ball curve and how a wing's shape influences lift--and see how the common explanation of lift has led a surprising number of scientists (including some famous ones) astray. THE SPINNING BALL The path of a ball spinning around a vertical axis and moving forward through the air is deflected to the right or the left of a straight path. Experiment shows that this effect depends both on the fact it is spinning and that it is immersed in a fluid (air). Non-spinning balls or spinning balls in a vacuum go straight. You might, before going on, want to decide for yourself


Jef Raskin

which way a ball spinning counterclockwise (when seen from above) will turn. Let's see what five books say about this problem. Three are by physicists, one is a standard reference work, and the last, just for kicks, is from a book by my son's soccer coach. We'll start with physicist James Trefil, who writes [Trefil 1984],

Before leaving the Bernoulli effect, I'd like to point out one more area where its consequences should be explored, and that is the somewhat unexpected activity of a baseball. Consider, if you will, the curve ball. This particular pitch is thrown so that the ball spins around an axis as it moves forward, as shown in the top in figure 11-4. Because the surface of the ball is rough, the effect of viscous forces is to create a thin layer of air which rotates with the surface. Looking at the diagram, we see that the air at the point labeled A will be moving faster than the the air at the point labeled B, because in the first case the motion of the ball's surface is added to the ball's overall velocity, while in the second it is subtracted. The effect, then is a 'lift' force, which tends to move the ball in the direction shown. 2 imgs/conda02. 2IMAGE The surface roughness is not essential. The effect is observed no gif smooth the ball.

matter how


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2

Trefil's figure 11-4. It does not agree with some other sources. Baseball aficionados would say that the ball curves toward third base. Trefil then shows a diagram of a fast ball, shown as deflecting downward when spinning so that the bottom of the ball is rotating forward. It is the same phenomenon with the axis of rotation shifted 90 degrees. In The Physics of Baseball , Robert K. Adair [Adair 1990] imagines a ball thrown toward home plate, so that it rotates counterclockwise as seen from above--as in Trefil's diagram. To the left of the pitcher is first base, to his right is third base. Adair writes: We can then expect the air pressure on the third-base side of the ball, which is travelling faster through the air, to be greater than the pressure on the on the first-base side, which is travelling more slowly, and the ball will be deflected toward first base. This is exactly the opposite of Trefil's conclusion though they agree that the side spinning forward is moving faster http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (5 of 29)24-1-2004 18:11:58

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through the air. We have learned from these two sources that going faster through the air either increases or decreases the pressure on that side. I won't take sides in this argument as yet. The Encyclopedia Brittanica [1979] gives an explanation which introduces the concept of drag into the discussion. "The drag of the side of the ball turning into the air (into the direction the ball is travelling) retards the airflow, whereas on the other side the drag speeds up the airflow. Greater pressure on the side where the airflow is slowed down forces the ball in the direction of the low-pressure region on the opposite side, where a relative increase in airflow occurs." Now we have read that spinning the ball causes the air to move either faster or slower past the side spinning forward, and that faster moving air increases or decreases the pressure, depending on the authority you choose to follow. Speaking of authority, it 3

might be appropriate to turn to one of the giants of physics of this century, Richard Feynman. He takes the side of Trefil, and uses a cylinder rather than a sphere [Feynman et. al. 1964. Italics are theirs. The lift force referred to is shown pointing upwards.]: "The flow velocity is higher on the upper side of a cylinder [shown rotating so that its top is moving in the same direction as its forward travel] than on http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (6 of 29)24-1-2004 18:11:58

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the lower side. The pressures are therefore

lower on

the upper side than on the lower side. So when we have a combination of a circulation around a cylinder and a net horizontal flow, there is a net vertical force on the cylinder--it is called a lift force ." Now for my son's coach's book. The coach in this case is the world-class soccer player, George Lamptey. There is almost no theory given, but we can be reasonably sure that Lamptey has repeatedly tried the experiment and should therefore report the direction the ball turns correctly. He writes [Lamptey 1985]: "The banana kick is more or less an off-center instep drive kick which adds a spin to the soccer ball. Kick off center to the right, the soccer ball curves to the left. Kick off center to the left, the soccer ball curves to the right... The amount the soccer ball curves depends on the speed of the spin."


Jef Raskin

Lamptey, like Adair, has the high pressure on the side moving into the air. I will not relate more accounts, some having the ball swerve one way, some the other. Some explanations depend on the author's interpretation of the Bernoulli effect, some on viscosity, some on drag, some on turbulence. We will return to the subject of spinning balls, but we are not yet finished finding problems with the common explanation of lift. OTHER PARADOXES The common explanation of how a wing works leads us to conclude, for example, that a wing which is somewhat concave on the bottom, often called an "undercambered" wing, will always generate less lift (under otherwise fixed conditions) than a flat 4

bottomed one. This conclusion is wrong.

We then have to ask how a flat wing like that of a paper airplane, with no curves anywhere, can generate lift. Note that the flat wing has been drawn at a tilt, this tilt is called "angle of attack" and is necessary for the flat wing to generate lift. The topic of angle of attack will be returned to presently.


Jef Raskin

A flat wing can generate lift. This is a bit difficult to explain given the traditional mental model.

The cross-sectional shapes of wings, like those illustrated here, are called "airfoils." A very efficient airfoil for small, slow-flying models is an arched piece of thin sheet material, but it is not clear at all from the common explanation how it can generate lift at all since the top and bottom of the airfoil are the same length.

If the common explanation is all there were to it, then we should be making the tops of wings even curvier than they now are. Then the air would have to go even faster, and we'd get more lift. In this diagram the wiggliness is exaggerated. More realistic lumpy examples will be encountered in a few moments.

If we make the top of the wing like this, the air on top has a lot longer path to follow, so the air will go even faster than with a conventional wing. You might conclude that this kind of airfoil should have lots of lift. In fact, it is a disaster.

Enough examples. While Bernoulli's equations are correct, their proper application to aerodynamic lift proceeds quite differently than the common explanation. Applied properly or not, the equations result in no convenient visualization that links the 5


Jef Raskin

shape of an airfoil with its lift, and reveal nothing about drag. This lack of a readily-visualized mental model, combined with the prevalence of the plausible-sounding common explanation, is probably why even some excellent physicists have been misled. ALBERT EINSTEIN'S WING My friend Yesso, who works for the aircraft industry (though not as a designer), came up with a proposed improved airfoil. Reasoning along the lines of the common explanation he suggested that you should get more lift from an airfoil if you restarted the top's curve part of the way along:

An extra lump for extra lift?

This is just a "reasonable" version of the lumpy airfoil that I presented above. Yesso's idea was, of course, based on the concept that a longer upper surface should give more lift. I was about to tell Yesso why his foil idea wouldn't work when I happened to talk to Jö rgen Skogh 3. He told me of a humped airfoil Albert Einstein 4 designed during WWI that was based on much the same reasoning Yesso had used [Grosz 1988].


Jef Raskin

Albert Einstein's airfoil. It had no aerodynamic virtues.

This meant that instead of telling Yesso merely that his idea wouldn't work, I could tell him that he had created a modernized version of Einstein's error! Einstein later noted, with chagrin, that he had goofed 5. [Skogh 1993] EVIDENCE FROM EXPERIMENTS If it were the case that airfoils generate lift solely because the airflow across a surface lowers the pressure on that imgs/conda02. 3IMAGE Mr. Skogh worked on aircraft design for Saab in Sweden and for gif Lockheed in the United States. 4Albert Einstein [1879-1955], a German-American physicist, was one of

the greatest scientists of all time. His small error in wing

design does not detract from the massive revolution his thinking brought about in physics. 5Jö rgen Skogh writes, "During the First World War Albert Einstein was

for a time hired by the LVG (Luft-Verkehrs-Gesellshaft) as a

consultant. At LVG he designed an airfoil with a pronounced mid-chord hump, an innovation intended to enhance lift. The airfoil was tested in the Gö ttingen wind tunnel and also on an actual aircraft and found, in both cases, to be a flop." In 1954 Einstein wrote "Although it is probably true that the principle of flight can be most simply explained in this [Bernoullian] way it by no means is wise to construct a wing in such a manner!" See [Grosz, 1988] for the full text. 6


Jef Raskin

surface then, if the surface is curved, it does not matter whether it is straight, concave, or convex; the common explanation depends only on flow parallel to the surface. Here are some experiments that you can easily reproduce to test this idea. 1. Make a strip of writing paper about 5 cm X 25 cm. Hold it in front of your lips so that it hangs out and down making a convex upward surface. When you blow across the top of the paper, it rises. Many books attribute this to the lowering of the air pressure on top solely to the Bernoulli effect. blow air

Now use your fingers to form the paper into a curve that it is slightly concave upward along its whole length and again blow along the top of this strip. The paper now bends downward. 2. As per the diagrams below, build a box of thin plywood or cardboard with a balsa airfoil held in place with pins that allow it to flap freely up and down. Air is introduced with a soda straw. That's one of the nice things about science. You don't have to take anybody's word for a claim, you can try it yourself! 6 In this wind tunnel the air flows only across the top of the shape. A student friend of mine made another where a leaf blower blew on both top and bottom and he got the same results, but that design takes more effort to build and the airfoil models require leading http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (12 of 29)24-1-2004 18:11:58

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and trailing edge refinement. Incidentally, I tried to convince a company that makes science demonstrators to include this in their offerings. They weren't interested in it because "it didn't give the right results." "Then how does it work?" I asked. "I don't know," said the head designer. An experiment may be difficult to interpret but, unless it is fraudulent, it cannot give the wrong results. imgs/conda02. 6IMAGE In some fields, e.g. the study of sub-atomic particles, you gif megabucks and a staff of thousands to build

might need

an accelerator to do an independent check, but the principle is still there. 7

CROSS SECTION


Jef Raskin

SIDE VIEW

AIRFOIL DEMONSTRATOR. These drawings are full size, but the exact size and shape aren't important. I made a number of airfoils to test. Here are drawings of the ones I made: 8

NORMAL

CONCAVE

RECURVED

FLAT

FLAT WITH DOWNTURN FLAT WITH UPTURN

EXPERIMENTAL RESULTS When the straw is blown into, the normal airf oil promptly lifts off the bottom and floats up. When the blowing stops, it goes back down. This is exactly what everybody expects. Now consider the concave shape; the curve is exactly the same as the first airfoil , though turned upside down. If the common explanation were true, then, since the length along the curve is the same as with the "normal" example, you'd expect this one to rise, too. After all, the airflow along the surface must be lowering the pressure, allowing the normal ambient air http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (14 of 29)24-1-2004 18:11:58

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pressure below to push it up. Nonetheless, the concave airfoil stays firmly down; if you hold the apparatus vertically, it will be seen to move away from the airflow. In other words, an often-cited experiment which is usually taken as demonstrating the common explanation of lift does not do so; another effect is far stronger. The rest of the airfoils are for fun--try to anticipate the direction each will move before you put them in the apparatus. It has been noted that "progress in science comes when experiments contradict theory" [Gleick 1992] although in this case the science has been long known, and the experiment contradicts not aerodynamic theory, but the oftentaught common interpretation. Nonetheless, even if science does not progress in this case, an individual's understanding of it may. Another simple experiment will lead us toward an explanation that may help to give a better feel for these aerodynamic effects. THE COANDA EFFECT

9


Jef Raskin

If a stream of water is flowing along a solid surface which is curved slightly away from the stream, the water will tend to follow the surface. This is an example of the Coanda effect 7 and is easily demonstrated by holding the back of a spoon vertically under a thin stream of water from a faucet. If you hold the spoon so that it can swing, you will feel it being pulled toward the stream of water. The effect has limits: if you use a sphere instead of a spoon, you will find that the water will only follow a part of the way around. Further, if the surface is too sharply curved, the water will not follow but will just bend a bit and break away from the surface.

The Coanda effect works with any of our usual fluids, such as air at usual temperatures, pressures, and speeds. I make these qualifications because (to give a few examples) liquid helium, gasses at extremes of low or high pressure or temperature, and fluids at supersonic speeds often behave rather differently. Fortunately, we don't have to worry about all of those extremes http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (16 of 29)24-1-2004 18:11:58

Jef Raskin

with model planes. imgs/conda02. 7IMAGE In the 1930's the Romanian aerodynamicist Henri-Marie Coanda gif

(1885-

1972) observed that a stream of air (or other fluid) emerging

from a nozzle tends to follow a nearby curved or flat surface, if the curvature of the surface or angle the surface makes with the stream is not too sharp. 10

A stream of air, such as what you'd get if you blow through a straw, goes in a straight line

A stream of air alongside a straight surface still goes in a straight line A stream of air alongside a curved surface tends to follow the curvature of the surface. Seems natural enough.

Strangely, a stream of air alongside a curved surface that bends away from it still tends to follow the curvature of the surface. This is the Coanda effect.

Another thing we don't have to wonder about is why the Coanda effect works, we can take it as an experimentally given fact. But I hope your curiosity is unsatisfied on this point and that you will seek further. A word often used to describe the Coanda effect is to say that the airstream is "entrained" by the surface. One advantage of http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (17 of 29)24-1-2004 18:11:58

Jef Raskin

discussing lift and drag in terms of the Coanda effect is that we can visualize the forces involved in a rather straightforward way. The common explanation (and the methods used in serious texts on aerodynamics) are anything but clear in showing how the motion of the air is physically coupled to the wing. This is partly because much of the approach taken in the 1920s was shaped by the need for the resulting differential equations (mostly based on the Kutta- Joukowski theorem 8) to have closed-form solutions or to yield useful numerical results with paper-and-pencil methods. Modern approaches use computers and are based on only slightly more intuitive constructs. We will now develop an alternative way of visualizing lift that makes predicting the basic phenomena associated with it easier. imgs/conda02. 8IMAGE Discovered independently by the German mathematician M. Wilheim gif 1944) and the Russian physicist Nikolai Kutta (1867-

Joukowski (1847-1921). 11

A MENTAL MODEL OF HOW A WING GENERATES LIFT AND DRAG As is typical of physicists, I have often spoken of the air moving past the wing. In aircraft wings usually move through the air. It makes no real difference, as flying a slow plane into the wind so that the plane's ground speed is zero demonstrates. So I will speak of the airplane moving or the wind moving whichever makes the point more clearly at the time. In the next illustration , it becomes convenient to look at


Jef Raskin

the air molecules, attracted to the surface, are pulled down.

Think of the wing moving to the left, with the air standing still. The air moves toward the wing much as if it was attached to the wing with invisible rubber bands. It is often helpful to think of lift as the action of the rubber bands that are pulling the wing up. Another detail is important: the air gets pulled along in the direction of the wing's motion as well. So the action is really more like the following picture.

The air is pulled forward as well as down by the motion of the wing.

If you were in a canoe and tried pulling someone in the water toward you with a rope, your canoe would move toward the person. It is classic action and reaction. You move a mass of air down and the wing moves up. This is a useful visualization of the lift generated by the top of the wing. As the diagram suggests, the wing has also spent some of its energy, necessarily, in moving the air forward. The imaginary rubber bands pull it back some. That's a way to think about the drag that is caused by the lift the wing generates. Lift cannot be had without drag. The acceleration of the air around the sharper curvature near the front of the top of the wing also imparts a downward and http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (19 of 29)24-1-2004 18:11:58

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forward component to the motion of the molecules of air (actually a slowing of their upward and backward motion, which is equivalent) and thus contributes to lift. The bottom of the wing is easier to understand, and an explanation is left to the reader. The experiments with the miniature wind tunnel described earlier are readily understood in terms of the Coanda effect: the downward-curved wing entrained the airflow to move downward, and a force upward is developed in reaction. The upward-curved (concave) airfoil entrained the airflow to move upwards, and a force downward was the result. The lumpy wing generates a lot of drag by moving air molecules up and down repeatedly. This eats up energy (by generating frictional heat) but doesn't create a net downward motion of the air and therefore doesn't create a net upward

12

movement of the wing. It is easy, based on the Coanda effect, to visualize why angle of attack (the fore-and-aft tilt of the wing, as illustrated earlier) is crucially important to a symmetrical airfoil, why planes can fly inverted, why flat and thin wings work, and why Experiment 1 with its convex and concave strips of paper works as it does. What has been presented so far is by no means a physical account of lift and drag, but it does tend to give a good picture of the phenomena. We will now use this grasp to get a reasonable hold on the spinning ball problem. http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (20 of 29)24-1-2004 18:11:58

Jef Raskin

WHY THE SPINNING BALL'S PATH CURVES, IN TERMS OF THE COANDA EFFECT The Coanda effect tells us the air tends to follow the surface of the ball. Consider Trefil's side A which is rotating in the direction of flight. It is trying to entrain air with it as it spins, this action is opposed by the oncoming air. Thus, to entrain the air around the ball on this side, it must first decelerate it and then reaccelerate it in the opposite direction. On the B side, which is rotating opposite the direction of flight, the air is already moving (relative to the ball) in the same direction, and is thus more easily entrained. The air more readily follows the curvature of the B side around and acquires a velocity toward the A side. The ball therefore moves toward the B side by reaction. It is again time for a simple experiment. It is difficult to experiment with baseballs because their weight is large compared to the aerodynamic forces on them and it is very hard to control the magnitude and direction of the spin, so let us look at a case where the ball is lighter and aerodynamic effects easier to see. I use a cheap beach ball (expensive ones are made of heavier materials and show aerodynamic effects less). Thrown with enough bottom spin (bottom moving forward) such a ball will actually rise in a curve as it travels forward.The lift due to spin can be so strong that it is greater than the downward force of gravity! Soon, air resistance stops both the spin and the forward


Jef Raskin

motion of the ball and it falls, but not before it has shown that Trefil's explanation of how spin affects the flight of a ball is wrong. The lift due to spinning while moving through the air is usually called the "Magnus 9 effect." Some books on aerodynamics also describe the "Flettner Rotor," which is a long-since abandoned attempt to use the Magnus effect to make an efficient boat sail. Many sources besides Trefil get the effect backwards including the usually reliable Hoerner [Hoerner 1965]. Collegelevel texts tend to get it right [Kuethe and Chow 1976; Houghton and Carruthers 1982] but, as noted above, Feynman's Lectures on Physics has the rotation backwards. I was relieved to see that the classic Aerodynamics [von Ká rmá n 1954] gets the lift force on a imgs/conda02. 9IMAGE H. G. Magnus (1802-1870), a German physicist and chemist, demonstrated gif this effect in 1853. 13

spinning ball in the correct direction though the reasoning seems a bit strained. I wish I could send this essay to the 6th grade science teacher who could not take the time to listen to my reasoning. Here's what happened: he sent me to the principal's office when I came in the next day with a balsa model plane with dead flat wings. It would fly with either side up depending on how an aluminum foil elevator adjustment was set. I used it to demonstrate that the explanation the class had been


Jef Raskin

given must have been wrong, somehow. The principal, however, was informed that my offense was "flying paper airplanes in class" as though done with disruptive intent. After being warned that I was to improve my behavior, I went to my beloved math teacher who suggested that I go to the library to find out how airplanes fly--only to discover that all the books agreed with my science teacher! It was a shock to realize that my teacher and even the library books could be wrong. And it was a revelation that I could trust my own thinking in the face of such concerted opposition. My playing with model airplanes had led me to take a major step toward intellectual independence--and a spirit of innovation that later led me to create the Macintosh computer project (and other, less-well-known inventions) as an adult. APPENDIX 1 A QUANTITATIVE APPLICATION OF THE COMMON (INCORRECT) EXPLANATION If the pressure, in Newtons per square meter (Nm -2 kgm-1s- 2), on the top of a wing is notated p top , the

=

pbottom , the velocity (ms -1) on the top of the wing v , and the velocity on the bottom v top bottom, and where __ is the pressure on the bottom

density of air (approximately 1.2 kgm -3), then the pressure difference across the wing is given by the first term of Bernoulli's equation: ptop - pbottom = 1/2 _ (vtop2 - vbottom2) A rectangular planform (top view) wing of one meter span was measured as having a length chordwise along the bottom of 0.1624 m


Jef Raskin

while the length across the top was 0.1636 m. The ratio of the lengths is 1.0074. This ratio is typical for many model and fullsize aircraft wings. According to the common explanation which has two adjacent molecules separated at the leading edge mysteriously meeting at the trailing edge, the average air velocities on the top and bottom are also in the ratio of 1.0074. A typical speed for a model plane of 1m span and 0.16m chord with a mass of 0.7 kg (a weight of 6.9 N) is 10 ms -1 , so v is 10 ms-1 which makes v top 10.074 ms -1. Given these numbers, webottom find a pressure difference from the equation of about 0.9 kgm -1 - 2. The area of the wing is 0.16 m 2 s

giving a total force of 0.14 N. This is not nearly enough--it misses lifting the weight of 6.9 N by a factor of about 50. We would need an air velocity difference of

14

about 3 ms -1 to lift the plane. The calculation is, of course, an approximation since Bernoulli's equation assumes nonviscous, incompressible flow and air is both viscous and compressible. But the viscosity is small and at the speeds we are speaking of air does not compress significantly. Accounting for these details changes the outcome at most a percent or so. This treatment also ignores the second term (not shown) of the Bernoulli equation--the static http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (24 of 29)24-1-2004 18:11:58

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pressure difference between the top and bottom of the wing due to their trivially different altitudes. Its contribution to lift is even smaller than the effects already ignored. The use of an average velocity assumes a circular arc for the top of the wing. This is not optimal but it will fly. None of these details affect the conclusion that the common explanation of how a wing generates lift--with its naï ve application of the Bernoulli equation--fails quantitatively. FURTHER READING: There are many fine books and articles on the subject of model airplane aerodynamics (and many more on aerodynamics in general). Commendably accurate and readable are books and articles for modelers by Professor Martin Simons [e.g. Simons 1987]. Much can be learned from Frank Zaic's delightful, if not terribly technical, series [Zaic 1936 to Zaic 1964] (Available from the Academy of Model Aeronautics in the United States), and no treatments are more professional or useful than those of Professor Michael Selig and his colleagues [e.g. Selig et. al. 1989]. All of these authors are also well-known modelers. The other references on aerodynamics, e.g. Kuethe and Chow [1976] and Houghton and Carruthers [1982] are graduate or upper-level undergraduate texts, they require a knowledge of physics and calculus including partial differential equations. Jones [1988] is an informal treatment by a master and Hoerner [1965] is a http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (25 of 29)24-1-2004 18:11:58

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magnificent compendium of experimental results, but has 5 little theory--practical designers find his work invaluable.

15

REFERENCES * Adair, Robert K. The Physics of Baseball , Harper and Row, NY, 1990. pg. 13 * Feynman, R. et. al. Lectures on Physics, Vol II , Addison-Wesley 1964 pg. 40-9, 40-10, 41-11 * Gleick, J. Genius. Pantheon Books, NY 1992 pg. 234 * Grosz, Peter M. "Herr Dr Prof Albert Who? Einstein the Aerodynamicist, That's Who!" WWI Aero No. 118, Feb. 1988 pg. 42 ff * Hoerner, S.F. Fluid-Dynamic Drag , Hoerner Fluid Dynamics, 1965 pg. 7-11 * Houghton and Carruthers. Aerodynamics for Engineering Students , Edward Arnold Publishers, Ltd. London, 1982 * Jones, R.T. Modern Subsonic Aerodynamics . Aircraft Designs Inc., 1988. pg.36 * Lamptey, George. The Ten Bridges to Professional Soccer, Book 1: Bridge of Kicking . Academy Press, Santa Clara CA, 1985. * Levy, Steven. "Insanely Great." Popular Science, February, 1994. pg. 56 ff. * Linzmayer, Owen. The Mac Bathroom Reader , Sybex 1994 * Kuethe and Chow. Foundations of Aerodynamics , Wiley, 1976 * Macaulay, David. The Way Things Work . Houghton Mifflin Co. Boston, 1988. pg. 115 http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (26 of 29)24-1-2004 18:11:58

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* Selig, M. et. al. Airfoils at Low Speeds . Soartech 8. Herk Stokely, 1504 Horseshoe Circle, Virginia Beach VA 23451, 1989 * Simons, M. Model Aircraft Aerodynamics , 2nd ed.. Argus Books Ltd., London, 1987. * Skogh, Jö rgen. Einstein's Folly and The Area of a Rectangle , in publication * Thornburg, Dave. Do You Speak Model Airplane? Pony X Press, 5 Monticello Drive, Albuquerque NM 87123, 1992 * Trefil, James S. A Scientist At The Seashore. Collier Books, Macmillan Publishing Co., 1984, pp 148-149 * von Ká rmá n, T. Aerodynamics . Oxford Univ. Press 1954 pg. 33 * Zaic, Frank. Model Aeronautic Yearbooks. Published from the 30's to the 60's * Zaic, Frank. Circular Airflow . Model Aeronautic Publications, 1964. ACKNOWLEDGMENTS I am very appreciative of the suggestions I have received from a number of careful readers, including Dr. Bill Aldridge, Professors 16


Jef Raskin

Michael Selig, Steve Berry, and Vincent Panico, and Linda Blum. They have materially improved both the content and the exposition, but where I have foolishly not taken their advice my own errors may yet shine through. AUTHOR'S BIOGRAPHY Jef Raskin was a professor at the University of California at San Diego and originated the Macintosh computer at Apple Computer Inc [Levy 1994; Linzmayer 1994]. He is a widelypublished writer, an avid model airplane builder and competitor, and an active musician and composer. 17

Site Directory: The Humane Interface - The Humane Interface Cover - Summary of The Humane Interface Home - Updates and Additions - Curriculum Vitae Music - Organ Picture - Pipe Organ Pictures - Airplane Pictures - Double Rainbow - Picture of Jef Raskin - Blue Butterfly - Paper Beams - The Electric Van Pictures - Western Wind - Apple's Publications Department Some Published Works - Bible Hoax Program http://www.jefraskin.com/forjef2/jefweb-compiled/published/coanda_effect.html (28 of 29)24-1-2004 18:11:58

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- Shanghaied to the Windward Islands - Coanda Effect: Understanding Why Wings Work - Holes In The Histories - There is No Such Thing as Information Design - Pacifica Moods - Humbug: Nursing Theory - The Bible Hoax - How to Balance a Model Airplane Talks and Workshops - Turning the Art of Interface Design into Engineering Fun and Games - Alien Arithmetic: An Experiment - Aza's Sparkler Humor - The Body Weight of a Bed-Bound Patient - How To Decrease the Cost of Health Care - Eating B'dang B'dang - How To Read a Model Plane Review - 12 Precent of Something - A Swiss Tourist Guide - Warning - An History of the Yarmulke - Pshtwar B'dang Some Unpublished Works - Effectiveness of Mathematics - The Soft Sell on Hard Sails - The Piper Cub Offense - Usborne Medieval Port - Widgets of the Week - Math and Science Book Reviews - The Old Slipstick - Next Time, It Can Be Worse

© 2001 Jef Raskin. Send comments or suggestions to Jef or webmaster Aza.


GIF Collection, Airfoil Misconception

UP to SCIENCE HOBBYIST | UP to LIFTING FORCE MISCONCEPTIONS

AIRFOIL DIAGRAMS

fig. 1 Diagrams in grade K-6 textbooks. The air in the lefthand diagram approaches the wing horizontally and also leaves the wing horizontally. This violates Newton's laws, since by F=ma there cannot be a lifting force unless air is accelerated downwards. The wing must deflect the horizontally-moving air downwards, as shown in the righthand diagram.

http://www.eskimo.com/~billb/wing/airgif.html (1 of 5)24-1-2004 18:14:12


fig. 2 Actual windtunnel photograph of air flowing around a wing. Pulsed smoke streams illustrate that parcels of air which are divided by the leading edge DO NOT recombine at the trailing edge. Therefor the "wing shape" explanation of lifting force falls apart.



fig. 3 A flim clip of windtunnel experiments shows a single "plane" of air as it approaches an airfoil and is sliced into upper and lower portions. Note that the air flowing above the wing quickly outraces the air flowing below. The air flowing above and below the wing never rejoin again. The real reason for the rapid flow of air above the wing is never explained in "bernoulli"-based textbooks.



fig. 4 The confusing aspects of "airfoil shape" shown here can totally obscure the true nature of aerodynamic lift. Many authors point out that airfoils give positive lift even if the attack angle is zero (so presumably the explanation of choice should be "wing shape", and not "attack angle".) But there is a problem here. To determine if an airfoil is tilted, we cannot rely on construction of the geometrical attack angle. Geometrical attack angle is very sensitive to tiny bumps on the wing's leading edge, since tiny bumps can change where we draw the main 'chord.' Yet tiny bumps on the leading edge can have little effect on deflection of air, while the tilting of the airfoil shown in the fourth section can have an enormous effect upon the deflection of air and upon lifting force. SMALL FEATURES ON THE LEADING EDGE CAN CAUSE US TO TILT THE ENTIRE WING, WHILE WE DENY THAT WE HAVE DONE SO. To determine the effective attack angle, we cannot trust the simple geometrical rules. To determine whether an asymmetrical wing is REALLY set to zero attack angle, we instead must inspect the trailing edge of the airfoil to see if it directs air downwards more than the leading edge pulls air upwards.

fig. 5 Fluid simulation from SAAB Aircraft shows phase lag between upper and lower air parcels after an airfoil has passed. Air travels much faster over the top of the airfoil, and then it never rejoins the air which has travelled below. Note that the airfoil has deflected the air downwards.



fig. 6 An air flow simulation from J. S. Denker's HOW IT FLYS, in the Airfoils chapter. Note that in the top diagram, the asymmetrical (cambered) wing has been adjusted to produce zero lifting force. There is no "slip" or "phase delay" between upper and lower airflows. In the middle and bottom diagrams, the angle of attack is progressively increased, which creates an increasing lifting force. Increasing the angle of attack also increases the phase delay between upper and lower air flows. So not only is the common "wingshape / Bernoulli" explanation wrong, but it even covers up one of the most interesting phenomena in airfoil physics: the fact that the time delay between upper and lower airflows is proportional to the attack angle and the lifting force!

Maintained by Bill Beaty. Mail me at: [email protected]. If you are using Lynx, type "c" to email.


The Airfoil Misconception in K-6 Textbooks

AIRFOIL MISCONCEPTIONS | SCIENCE HOBBYIST

AIRPLANE FLIGHT ANALOGY

1997

William Beaty

(See also J. Denker's critique and my response, 8/99)

The controversy about wings and the lifting force has a definite origin. It arises because we are taught about the flow patterns surrounding two-dimensional airfoil crossections... and then we apply those concepts to 3D wings. This is a major mistake. The behavior of 3D wings is fundamentally different than the behavior of 2D airfoils. In a 3D world, an airplane produces a downstream wake with net downwash, but in a 2D wind-tunnel there is no such wake, and the 2D upwash must always be equal to the 2D downwash. Even more important, 3D wings have finite area, while 2D wings act as if they are infinitely long. An infinite wing gives some strange results which finite 3D wings never produce. For example, if a 2D infinite wing should ever deflect even a tiny portion of the oncoming air downwards, it would deflect an infinite amount of air and produce an infinite lifting force. As a result, a 2D infinite airfoil does an odd thing: it applies a sensible, FINITE force to an infinite mass of air, and yet a net amount of air does NOT move downwards. It acts like a reaction engine, but where the "exhaust" has zero velocity and infinite mass. This strange effect only applies to 2D airfoils, and is never seen with 3D airplanes flying through 3D air. The controversy about "Bernoulli versus Newton" is really a controversy about two-D versus three-D. It's a controversy over the physics of airfoils in two-dimensional "flatland" worlds, versus the more ordinary physics of short 3D wings in a 3D world. I could attempt to explain the problem in words, but words are easily misunderstood (especially when emotions run high.) A visual analogy works much better. Below is my explanation for how a three-dimensional airplane flys through 3D space. It is very different than the typical 2D explanations found in most textboooks. My "circulation" is flipped ninety degrees! http://www.amasci.com/wing/rotbal.html (1 of 8)24-1-2004 18:16:35


Imagine a huge, disk-shaped helium balloon floating in the air. The disk stands on edge. It is weighted for neutral buoyancy. A small platform sticks out of its rim. (If you feel the need, you should imagine a counterweight on the opposite rim to the platform, so the balloon hovers without rotating.) See fig. 1 below _____ _--

--_

/ __| |

\ .

fig. 1

\_

| |

DISK-BALLOON WITH A SMALL PLATFORM

_/ --_____--

Now suppose I were to leap from the top of a ladder and onto the balloon's small platform. The balloon would move downwards. It would also rotate rapidly counterclockwise, and I would be dumped off. Next, suppose we have TWO giant disk-shaped balloons stacked adjacent to each other like pancakes standing on edge. ____ _-- _____ / _---_ __| / \ __| . | | | \_ _/ --_____--

fig. 2 TWO DISK-BALLOONS, STACKED ADJACENTLY

They do not touch each other. Both have platforms. If I jump onto the first platform, but then I immediately leap onto the next platform, I can stay up there for a tiny bit longer. Next, suppose we have a row of these disk-balloons one KM long. It looks like fig. 2 above, but with hundreds of hovering balloons. Now I can run from platform to platform, and I will stay aloft until I run out of balloons. Behind me I leave a trail of rotating, http://www.amasci.com/wing/rotbal.html (2 of 8)24-1-2004 18:16:35


downward-moving balloons. I can remain suspended against gravity because I am flinging mass downwards. The mass takes the form of helium mass trapped inside the balloons. I am also doing much more work than necessary, since the energy I expend in rotating the balloons does not contribute to my fight against gravity. (In truth, all my work is really not necessary, I could simply walk along the Earth's surface with no need to move any massive gasbags!) To make the situation more symmetrical, let me add a second row of platform-bearing balloons in parallel to the first row: _____ _--

_____ --_

/

_--

TWO LONG | BALLOONS |

\

/

.

|__ |

\_

--_

_/ --_____--

fig. 3 END VIEW OF

__|

\ .

|

|

ROWS OF DISK-

| \_

_/ --_____--

There's one platform for each of my feet. I can run forwards, leaving a trail of "wake turbulence" behind me. The "wake" is composed of rotating, descending balloons. Fig. 4 below show an animated GIF of this process.

http://www.amasci.com/wing/rotbal.html (3 of 8)24-1-2004 18:16:35


Fig. 4 Forcing the balloons downwards Also see: Smoke Ring animation "DISK BALLOONS" BEHIND AIRPLANES An aircraft does much the same thing as me and my balloons: it remains aloft by throwing down a spinning region of mass. This mass consists of two long, thin, vortex-threads and the tubular regions of air which are constrained to circulate around them. The balloons crudely represent the separatrix of a vortex-pair: the cylindrical parcels of air which must move with closed streamlines. How do airplanes fly? Real aircraft use "invisible disk-balloons" to stay aloft. The two rows of "invisible balloons" form a single, very long, downwards-moving cylinder of air. This single cylinder has significant mass and carries a large momentum downwards.

_____ _--

_____ --_

_--

/ \ | / AIRCRAFT, W/ | ___ | | | ___ ROTATED BY THE | ---____/ \____--http://www.amasci.com/wing/rotbal.html (4 of 8)24-1-2004 18:16:35

--_

fig. 5 FRONT VIEW OF

\ |

AIR MASSES

|

WINGS'


PRESSURE DIFFERENCE \_ _/ --_____--

\_/

\_

_/ --_____--

\

| | | |

\ ______ / ___ \ SECTION OF AN / / \ \ HAS STREAMLINES | | o | | PERFECTLY \ \___/ / PAIR OF ROTATING \_______/ THE BALLOONS ARE

| |

/ / | |

______ / ___ \

|

|

|

/

|

|

|

|

|

|

|

|

|

/

|

|

\

|


|

\ o

|

\___/ \_______/

\

\

ACTUAL WAKE

|

WHICH DO NOT

/

RESEMBLE A BALLOONS. A CRUDE

ANALOGY. /

/

fig. 6 THE CROSS-

\


Fig. 7 Actual downwash made visible (See efluids.com photo gallery and another photo

FLY FASTER FOR LESS DRAG My forward speed makes a difference in how much work I perform. If I walk slowly along my rows of balloons, each platform sinks downwards significantly. I must always leap upwards to the next platform, and each balloon is thrown violently downward as I leap. I tire quickly. On the other hand, if I run very fast, my feet touch each platform briefly, the balloons barely move, and the situation resembles my running along the solid ground. Similarly, if a real aircraft flys slowly, it must fling the vortex-pairs violently downward. It performs extra work and experiences a very large "induced drag." If it flys fast, it spreads out the necessary momentum-changes, and therefore it needs only to barely touch each parcel of mass (each "balloon.") Hence, faster flight is desirable because it requires far less work to be performed in moving the air downwards. And if a slow-flying, heavilyloaded aircraft should fly very low over you, its powerful wake vortices will blow you over and put dust in your eyes. http://www.amasci.com/wing/rotbal.html (6 of 8)24-1-2004 18:16:35


All of my reasoning implies that modern aircraft actually remain aloft by launching "smoke rings" downwards. Imagine one of the flying cars in the old 'Jetsons' cartoon, the ones with those little white rings shooting down out of the underside. But rather than launching a great number of individual rings, modern aircraft throw just one very long ring downwards, and they are lifted by the upward reaction force. A CRUDE PREDICTION How well does the "disk balloons" model correspond to the real world? Well, we can pull an equation out of the motions of the balloons, and use it to predict both aircraft energy use and induced drag. If the equation is at all similar to the actual aerodynamics of a realworld airplane, then the "disk balloons" are a useful model. If the equation is faulty, then the model only has weak ties with reality. Suppose the "disk-balloons" contain air which rotates as a solid object, (or imagine radial membranes in the balloons.) If I add together the work done in creating the circulatory flow, plus the work done in projecting the constrained air downwards, I arrive at a predicted aircraft power expenditure of: Power = 8 * (M * g)^2 / [ pi * span^2 * V * density ] M * g being aircraft weight, V is velocity of horizontal flight, and "density" is the density of air. Induced drag should then be power/V: Induced Drag = 8 * (M * g)^2 / [ pi * span^2 * V^2 * density ] What happens if I assume that the air within the disk-balloons is not "solid", but instead it is made to whirl faster near the center of the balloon, such that the tangential velocity of the air is constant regardless of its distance from the center of the balloon? (Imagine a wing which produces a downward velocity of net downwash which is constant at each point along the whole span of the wing.) If the "downwash" is constant across the wingspan, then the modified "balloon equation" predicts a power expenditure of 2x that above. How does this match reality? I'm looking for information on this at the moment. I'm told that these two equations are identical to the equations of real aircraft, except that the http://www.amasci.com/wing/rotbal.html (7 of 8)24-1-2004 18:16:35


number "8" is replaced by a factor which is dependent upon the particular geometry of the wing. Pretty good for an "amateur aerodynamicist", eh? One final note. The downwash of real airplanes contains rapidly rotating air. This represents wasted energy, since only the "shell" of each "balloon" needs to rotate as the air moves downwards. Is there a wing which can produce a downwash vortex-pair without any spinning cores? Maybe it would use less fuel than modern wings.

LINKS ● ● ● ●

Water Striders fling underwater vortices (Nature 8/03) Harvard U: Fish Gotta Swim Wakes in flapping flight Applet: flow around doublet/vortex etc.

● ●

Created and maintained by Bill Beaty. Mail me at: [email protected]. If you are using Lynx, type "c" to email.


BAD PHYSICS: Misconceptions spread by K-6 textbooks

up to SCI. MISCONCEPTION COMMENTS GOOD STUFF NEW STUFF SEARCH | | | |

RECURRING SCIENCE MISCONCEPTIONS IN K-6 TEXTBOOKS William J. Beaty

ALWAYS UNDER CONSTRUCTION WARNING: This file is currently being written, edited, corrected, etc. It does still contain some mistakes of its own. I placed it online as a sort of 'trial by fire' in order to hear readers' responses so I could target weak or unclear sections for improvement. (And, as my site points out, NOBODY is perfect so we should always practice critical thinking. Take all information with a grain of salt, including everything here!) Please feel free to send public comments to me with the COMMENT BOOK. If you prefer that nobody else sees your comments, send private comments to me via this form. ● ● ● ● ● ●

THE MISCONCEPTIONS MAIN MISCONCEPTIONS PAGE Jump to ELECTRICITY MISCONCEPTION PAGE COMMENT BOOK, publicly express your opinions on all this Suggest Your Own K-6 Textbook Miscon Try "SCIENCE MYTHS" SPREAD BY K-6 TEXTBOOKS

AM I JUST A NITPICKER? "Errors, like straws, upon the surface flow; He who would search for pearls must dive below." - John Dryden

THE MISCONCEPTIONS: ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● ●

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SCIENTISTS USE THE SCIENTIFIC METHOD? not quite. CLOUDS REMAIN ALOFT BECAUSE WATER DROPLETS ARE TINY? Wrong! THE SKY IS BLUE BECAUSE OF COMPLICATED PHYSICS No, it's simple. A LEMON-BATTERY CAN LIGHT A FLASHLIGHT BULB? doesn't work! SOUND TRAVELS BETTER THROUGH SOLIDS & LIQUIDS? No it doesn't. GRAVITY IN SPACE IS ZERO? It's actuallly strong. FILLED AND EMPTY BALLOONS DEMONSTRATE THE WEIGHT OF AIR? Misleading. GASES ALWAYS EXPAND TO FILL THEIR CONTAINERS? Not quite. FRICTION IS CAUSED BY SURFACE ROUGHNESS? Obsolete idea! ICE SKATES FUNCTION BY MELTING ICE VIA PRESSURE? nope. THE EARTH HAS 92 CHEMICAL ELEMENTS? LIGHT FROM THE SUN IS PARALLEL LIGHT? A WING'S LIFTING FORCE IS CAUSED BY ITS SHAPE? FOR EVERY ACTION, THERE IS AN EQUAL AND OPPOSITE REACTION? BEN FRANKLIN'S KITE WAS STRUCK BY LIGHTNING? THE MAIN LENS OF YOUR EYE IS INSIDE THE EYE? WHEN ONE PRISM SPLITS LIGHT INTO COLORS, A SECOND IDENTICAL PRISM CAN RECOMBINE THEM? CLOUDS, FOG, AND SHOWER-ROOM MIST ARE MADE OF WATER VAPOR?

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RAINDROPS HAVE POINTS? AIR IS ALMOST ENTIRELY WEIGHTLESS? SHADOWS VANISH ON CLOUDY DAYS BECAUSE THE SUN ISN'T BRIGHT ENOUGH? INFRARED LIGHT IS A FORM OF HEAT? THERE ARE SEVEN COLORS IN THE RAINBOW? THE EARTH'S NORTH AND SOUTH MAGNETIC POLES RESIDE JUST BELOW THE SURFACE? LASER LIGHT IS "IN PHASE" LIGHT? LASER LIGHT IS PARALLEL LIGHT? LASERS ARE COHERENT BECAUSE ATOMS EMIT IN PHASE? IRON AND STEEL ARE THE ONLY STRONGLY MAGNETIC MATERIALS? RE-ENTERING SPACECRAFT ARE HEATED BY AIR FRICTION? CARS AND AIRPLANES ARE SLOWED DOWN BY AIR FRICTION? THE NORTH MAGNETIC POLE OF THE EARTH IS IN THE NORTH? SALT WATER IS FULL OF SODIUM CHLORIDE MOLECULES? LIGHT AND RADIO WAVES ALWAYS TRAVEL AT "THE SPEED OFLIGHT"?

That's the way all the books were: They said things that were useless, mixed-up, ambiguous, confusing, and partially incorrect. How anybody can learn science from these books, I don't know, because it's not science. - Dr. Richard Feynman, in "Surely you're Joking, Mr. Feynman"



CORRECT: THERE IS NO SINGLE LIST CALLED "THE SCIENTIFIC METHOD." IT IS A MYTH The rules of a science-fair typically require that students follow THE SCIENTIFIC METHOD, or in other words, hypothesis-experiment-conclusion. The students must propose a hypothesis and test it by experiment. This supposedly is "The Scientific Method" used by all scientists. Supposedly, if you don't follow "The Scientific Method" listed in K-6 texts, then you're not doing science. (Some science fairs even ban astronomy and paleontology projects. Where's the experiment in these?) Unfortunately this is wrong, and there is no single "Scientific Method" as such. Scientists don't follow a rigid procedurelist called "The Scientific Method" in their daily work. The procedure-list is a myth spread by K-6 texts. It is an extremely widespread myth, but this doesn't make it any more real. "The Scientific Method" is part of school and school books, and is not how real science is done. Real scientists use a large variety of methods (perhaps call them methods of science rather than "The Scientific Method.") Hypothesis/experiment/conclusion is one of these, but it certainly is not the only one, and it would be a mistake to elevate it above all others. We shouldn't force children to memorize it. And we shouldn't use it to exclude certain types of projects from science fairs! If "The Scientific Method" proves that Astronomy is not a science, then it's "The Scientific Method" which is unscientific, not Astronomy. "Ask a scientist what he conceives the scientific method to be and he adopts an expression that is at once solemn and shifty-eyed: solemn, because he feels he ought to declare an opinion; shifty-eyed because he is wondering how to conceal the fact that he has no opinion to declare." - Sir Peter Medawar There are many parts of science that cannot easily be forced into the "hypothesis/experiment/conclusion" mold. Astronomy is not an experimental science, and Paleontologists don't perform Paleontology experiments... so studying dinosaurs or stars must not be science? Or, if a scientist has a good idea for designing a new kind of measurment instrument (e.g. a telescope), that certainly is "doing science." But where is The Hypothesis? Where is The Experiment? The Atomic Force Microscope (STM/AFM) revolutionized science. Yet wouldn't building such a device be rejected from many science fairs? After all, it's not an experiment. So, were the creators of the STM not doing science when they came up with that device? The Nobel prize committee disagrees. Forcing kids to follow a caricature of scientific research distorts science, and it really isn't necessary in the first place. Another example: great discoveries often come about when scientists notice anomalies. Isaac Asimov said it well: "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny...' " This suggests that lots of important science comes NOT from proposing hypotheses or even from performing experiments, but instead comes from learning to see what nobody else can see. Scientific discovery comes from something resembling "informed messing around," or unguided play. Yet "The Scientific Method" listed in textbooks says nothing about this. As a result, educators treat science as deadly serious business, and "messing around" is sometimes dealt with harshly.

ARTICLES: http://www.amasci.com/miscon/miscon4.html (3 of 32)24-1-2004 18:18:25


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Ten Science Myths (McComas) Scientific Methods (Denker) The Scientific Method (Simanek) On Scientific Method (Bridgman) Theories DON'T become laws Dispelling Common Science Myths "Why should there be the method of science? There is not just one way to build a house, or even to grow tomatoes. We should not expect something as motley as the growth of knowledge to be strapped to one methodology." -Ian Hacking

CORRECT: THE SKY IS BLUE BECAUSE AIR IS BLUE. This one isn't purely an error. Still, it involves misconceptions on the part of authors. Why is the sky blue? Usually the books start going on about wavelengths of light, Tyndall effect, and Rayleigh scattering. They teach some complicated physics first, then use it to explain blue sky and sunsets. Their explanations are correct. But what if you don't understand the physics? DOesn't this make their explanation useless? And do you just give up? They're wrong: you don't need complicated physics to understand this. The sky is blue for a very simple reason: AIR IS NOT A TRANSPARENT MATERIAL. INSTEAD IT IS BLUE! The sky is blue for much the same reason that a cloud of powder is white. Powder isn't invisible. Throw some dust into the air on a sunny day and you'll see a visible cloud. But what happens if you could throw some AIR? You might think that a cloud of air would be invisible. You'd be wrong. Air isn't invisible, instead it's a powdery-blue substance. True, small amounts of air are almost perfectly transparent. So are small amounts of water. Go to an opaque muddy river or pond and use a glass to dip out a cup of water. The water looks clear, no? Yet the river is opaque brown. When you try



to look through ten cups of water, or a hundred cups, the water seems to turn into brown paint. Air behaves like this too. A mile of air looks clear, but ten miles of air looks misty blue, and a thousand miles of air looks opaque white. The air is acting like the dirty river water, where a thin layer looks clear but a thick layer doesn't. The sky is blue because air is a powdery blue material, and when the sun shines on it, you can see this blue color. Stare upwards on a sunny day, and you're looking into a thick cloud of air. There really is no "sky" up there. You're not looking at a blue surface. Instead you're just seeing the Earth's layer of blue air against the blackness of outer space. Suppose you could go far out into space away from the Earth, then build yourself a thin hollow glass bubble a thousand miles wide. Viewed from the Earth, your thin glass bubble would be almost invisible. OK, now fill your bubble with air. It won't be invisible any more. It will look like a giant droplet of bright blue paint. It might even look whitish in the middle, since very thick layers of air seem as white as milk. What if you let your giant glass bubble crash into the moon? The air inside would pour out over the moon's surface and form a thick layer of atmosphere. The moon wouldn't look white anymore. It would turn blue. OK, now here's a question. Smoke is white, milk is white, and powder is white. A big cloud of particles should look like white smoke, not like a blue dye. Why is air blue instead of white? And even more important, why are sunsets red? (Does this mean that air is a red substance?!!) Ah, if you start wondering about such things, then *now* you finally need the advanced physics explanations.

CORRECT: CLOUDS ACTUALLY REMAIN ALOFT BECAUSE THEY ARE WARM INSIDE. Clouds are heavy. Evaporated water (H2O gas) is less dense than air, so moist air rises, but when the H2O gas condenses to form clouds, it contracts by about 1000 times and turns into very dense liquid water. (Imagine that the helium in a balloon condensed into a liquid. Would it still be buoyant?) Even a small cloud contains many tons of liquid water. How can clouds remain aloft? Many sources claim that clouds remain aloft because the water droplets are so small and widely separated that gravity has less effect on them. This is wrong. It doesn't matter if you break up a body of water into tiny droplets; its weight remains the same. You can't fool gravity. If a cloud contains tons of water, it will be pulled down to the Earth's surface with the http://www.amasci.com/miscon/miscon4.html (5 of 32)24-1-2004 18:18:25


same force whether the water forms a cloud or whether it forms raindrops. The answer lies elsewhere. Some sources claim that clouds remain aloft because of updrafts: because the air had been rising, and the rising air blows the cloud droplets upwards. Wrong again: it would take an upwards hurricane-wind to keep so many tons of water suspended. [WRONG! G. Beaulieu points out that cloud-stuff is only 1/10 percent more dense than air.] An updraft should be instantly halted as soon as the low-density water vapor turns into a dense liquid. [Correct. The excess weight will slow the updraft, stop it, then reverse it.] Still other sources claim that clouds stay up there because the droplets are very tiny, so they settle through the air very slowly. This is true, but it doesn't explain how weighty water can remain aloft. Stop and think a bit... if we have hundreds of tons of water, will its weight disappear simply because it has been divided into tiny droplets? No, instead the heavy droplets drag the air downwards as they fall. Air that contains water droplets is denser than normal air (its weight is increased by almost exactly the weight of the suspended water droplets, which works out to around 1/10 percent of the weight of the air in a particular volume.) Dense air falls fast! In other words, the tiny droplets will still race downwards because they form dense cloud-matter, and both the droplets and the air between them will be dragged downwards by gravity. Anyone playing with humidifier fog knows this. Yet even some professional meteorologists are saying these things. They should know better. So why *DO* clouds stay up there? Why don't they pour downwards to form a ground-hugging fog? The answer is simple: the weight of the water droplets is countered by the buoyancy of heated air between the droplets. Clouds are like hot air balloons. Whenever liquid water condenses from H2O gas, it releases thermal energy. When moist air turns into droplet-filled air, the droplets are hot, and they warm the air too. Clouds stay up there because they're less dense on average than surrounding air. In fact, if the water droplets should fall out of the cloud as rain, then the remaining hot air is no longer weighed down by tons and tons of water, and it races upwards. This rising hot air is the "engine" which drives the violent updrafts in thunderstorms and hurricanes. Hot air with its water removed no longer floats serenely along as clouds, instead it forms upward jets with hurricane velocity. Try making this "Touch The Clouds" device and you'll discover that droplet-filled air can be very dense. You can easily pour it from a pitcher and fill some cups. But we also know that hot air is less dense that cool air of the same pressure, so hot must rise through cooler air. Mix the two ideas together: dense air which is full of water droplets becomes less dense when heated, and at a certain high temperature it should be buoyed upwards by the atmosphere even though it's full of heavy water droplets. More thinking: helium gas rises in air, but liquid helium does not. Liquid helium is heavy like liquid water (though not quite as heavy as an equal quantity of water.) This is because each gram of liquid helium occupies a certain small volume, while each gram of helium gas occupies a much large volume, and the is bouyed upwards by the surrounding air. So, what happens when helium condenses into liquid? It shrinks greatly, becoming more dense than the surrounding air, then it dribbles downwards. It falls downwards even if it's a large blob of liquid, and it falls downward even if it takes the form of tiny droplets. THE SAME IS TRUE OF WATER. Water vapor (h2o gas) like helium is lighter than air, and it will rise. However, if that vapor should condense into droplets, it greatly contracts in size and greatly increases in density. A cloud of water droplets is heavy, and it SHOULD fall downwards. Even if the droplets are so tiny that they individually settle slowly, the droplet together have significant weight, so the droplets should drag the air downwards as they go. The dense, droplet-filled air can fall very fast, even though the individual droplets remain "stuck in the air" because of forces of viscosity. Whenever vapor condenses to form droplets, it releases "heat of condensation" which causes the remaining air to expand. The warm air can even expand MORE than the volume left empty by the condensing vapor, causing the average density to fall and causing clouds to rise upwards rather than just float. When clouds form, they usually pour upwards, not downwards. They are a bit too hot, so they try to rise to a higher level. http://www.amasci.com/miscon/miscon4.html (6 of 32)24-1-2004 18:18:25


Wrong: Scientific American "Ask an Expert" Tell them to calculate the heat released by condensation of cloud water, the temperature of resulting air, and the weight of a 1KM cloud compared to 1KM of nearby air which is cooler yet droplet-free. ● Wrong: New Scientist "Last Word" ● Wrong: National Geographic Kids ● Wrong: UK ScienceLine ● Wrong: Star Tribune: kid's weather questions ● Wrong: Madsci: ask an expert ● Wrong: U. Indiana Moment in Science ● Wrong: U. Corp. Atmos Research ● Wrong: NASA p.u.m.a.s. (they even mention "lighter than air"... then deny it! ● Wrong: ● Wrong: Starbase outreach pgm ●

Correct: ● Steve's Weather FAQ ● n

CORRECTED: A SINGLE LEMON BATTERY CANNOT LIGHT A FLASHLIGHT BULB Gradeschool science books sometimes contain "experiments" which do not work. The prism experiment below is one of them. Another is the "lemon battery" or "potato battery" used to run a light bulb. Stick some copper and zinc into a single lemon, and this "battery" does create a small voltage. Touch your lemon-cell to the wires of a loudspeaker or headphones and you'll hear a clicking sound. Connect it to an old-style panel meter (voltmeter or milliamp-meter, the kind with the moving needle,) and your lemon can make the meter needle move. Three or four lemon-cells connected in series can run an LCD digital clock or light a red Light Emitting Diode LED. (If you try the digital clock or LED, remember that polarity is important, and if it doesn't work, try reversing the connections.) However, the lemon's electrical output is far too feeble to light a standard flashlight bulb. Same with motors, buzzers, etc. The lemon battery is too weak. The experiment described in the books doesn't work. Example: stick a fairly wide copper strip and a similar zinc strip into a lemon. (This works much better than copper pennies or zinc nails.) First use the strips to tear up the inside of the lemon, then insert the metal strips very close together to give best results. The area of each "battery plate" is around 1 inch square. Measured voltage: 0.91V. Measured shorthttp://www.amasci.com/miscon/miscon4.html (7 of 32)24-1-2004 18:18:25


circuit current: two milliamps (0.002 Amps) immediately decreasing to a constant half a milliam (0.0005 amps.) What does this mean? Well, a typical flashlight bulb draws an ENTIRE 1/4 AMPERE when lit. Not half a milliamp, but 250 milliamps or 0.250 Amps. You'd need 500 lemons wired in parallel! 0.2500amps / 0.0005amps = 500 lemons. However, there are specialized light bulbs which draw very tiny currents. From Radio Shack we can get a #272-1139 incandescent bulb which only draws around fifteen milliams (0.015 amps) at 0.7 volts when lit very dimly in a darkened room. To light this bulb we only need 0.0150A/0.0005A = 30 lemons wired in parallel. But wasn't the lemon's electric current higher at the start? 0.002 amps, not 0.0005 amps? Yes, so with only TEN lemons wired in parallel, maybe we could cause a special hyper-sensitive light bulb to blink on for a second or two before going dark. This still translates into "the experiment doesn't work." One single lemon cannot light up any sort of incandescent bulb. At best we can use several lemons to light an LED. If a textbook contains the bulb-lightning experiment, it means that the author never performed the experiment to see if it works. LOTS of books and websites say that a lemon can light a flashlight bulb. Every single one of these is wrong. The mistake is like a kind of infection. If you aren't careful, then your science website can catch a disease! Can't we build a larger lemon-juice battery in a jar which will light a small bulb? Yes, but your battery needs to be fairly large; much larger than a couple of metal parts stuck into a lemon. At the very least you'll need a jar for the juice, plus some sheets of copper and zinc several inches wide. If you don't have that special Radio Shack bulb, then you'll need more than one lemon-juice jar hooked in series to make the 1.5 volts needed by a standard flashlight bulb. If you really want to light up a small lightbulb, why not build an ELECTRIC GENERATOR instead? How to cheat! There is a secret way to make a lemon-cell light up an incandescent bulb. You have to cheat! Buy yourself a "super capacitor" or "memory backup capacitor" via mail-order surplus. They cost a few dollars. You want a value between 0.1 farad and 0.5 farads. Try one of these suppliers: ● ● ●

All Electronics Electronics Goldmine Jameco Electronics

Build a lemon battery and connect it to the terminals of the super capacitor. (Me, I use alligator clip-leads bought from Radio Shack.) Wait for a few minutes. Now connect your flashlight bulb to the supercapacitor terminals and it should light brightly for a few seconds. (If not, then remove the bulb and try connecting your lemon cell to the capacitor for 15 minutes to make sure the capacitor gathers enough energy.) The capacitor slowly collects electrical energy from the lemon battery, then it dumps that energy into the flashlight bulb over a very short time. You can even use this trick to let your lemon battery run a low-voltage buzzer or turn a small motor (look for "solar cell motors" from various mail order suppliers or Radio Shack.) As with the bulb, you must charge up the capacitor for many minutes, then use it to run your bulb or motor for a few seconds. It's not an ideal experiment, and it's hard to explain how capacitors work. But it's easier than trying to connect thirty lemon-cells in parallel! ● ● ● ●

Four lemons light an LED Note about Lemon energy Tongue tingle, but no lightbulb Digital watch yes, lightbulb no



CORRECTED: ICE SKATES DO NOT FUNCTION BY MELTING ICE VIA PRESSURE It is commonly stated that ice skates have low friction because ice melts when pressure is applied to it. This is not quite correct. A demonstration using an ice cube, a wire, and two weights is often provided to illustrate the phenomena. However, while pressure does affect the melting point of ice, the pressure provided by the skates is not enough to melt ice except when the temperature is a fraction of a degree below 0C. Also, the icecube and wire demonstration is very misleading because it is always performed in a heated room, and the wire doesn't melt ice entirely by pressure, it melts the ice by thermal conduction of warm room temperature along the wire. (Also, narrow gaps in ice always freeze closed because the simultaneous melt/freeze process at water/ice boundary acts to flatten points and fill crevices) Another point: the weight of small objects is too low to create high pressure, yet small objects do experience low friction when on ice. The low friction of ice is probably caused by a layer of liquid water a few hundred molecules thick which always spontaneously develops on the surface of ice. Also, melting from frictional heating can provide liquid water as lubrication. Here's more on this whole debate, and also a bit from BAD CHEMISTRY

CORRECTED: THERE ARE NOT 92 ELEMENTS ON EARTH Uranium has the highest atomic number of the elements commonly found in the environment, and some books will tell you that there are 92 elements found on earth: atomic numbers 1 through 92 (hydrogen through uranium). This is wrong. Unfortunately there are two elements below Uranium which are radioactive and have extremely short half lives. These are http://www.amasci.com/miscon/miscon4.html (9 of 32)24-1-2004 18:18:25


Technetium and Promethium. These two elements do not occur naturally on Earth, and this reduces the total number of elements found in the environment to 90. However, in the 1970s a natural uranium reactor was found in an ancient streambed in Africa, and the mineral deposits at the site contained traces of a long-lived Plutonium isotope (atomic number 94.) This brings the total number of elements on the Earth back up to 91. (Note: Technicium, though not found naturally on Earth, is present in some stars, detected by spectral analysis.) See THE PHYSICS TEACHER, Vol.27 No.4 p282

LIGHT FROM THE SUN IS PARALLEL? NOPE. Some books state that because the sun is so far away, sunlight arriving at the Earth is almost perfectly parallel. This is incorrect. The books reason that the more distant the object, the more parallel the light, and since the sun is so far away, sunlight is perfectly parallel. They make a mistake. While it is true that light from *each tiny point* on the sun's surface is just about perfectly parallel by the time it reaches our eyes, light from the sun as a whole is not. This is because the sun, though very distant, is very large. A similar situation exists with light from the sky. We wouldn't say that the blue sky emits parallel light. Yet light from the sky comes from many miles away. If sunlight were perfectly parallel, there would be some interesting effects which are usually smeared out by the sun's disklike image. First of all, if the sun were tiny, then to us it would look like a very bright point, like an intensely bright star or a welding arc. Also, shadows on the ground would lack penumbras and be almost perfectly sharp. Without the penumbras, diffraction of waves would be revealed, and parallel dark and bright lines would appear at the edges of shadows. At nightfall the advancing shadows of distant mountains would be seen to race across the ground. During sunset the sun wouldn't gradually sink below the horizon, instead it would wink out. During the day the variations in air density would cause the ground to be covered by moving patterns of light; patterns similar to those seen on the bottom of a swimming pool but in this case made by "waves" in the sky. Solar and lunar eclipses would lack penumbrae. Looking at the sun might burn your retina, since the parallel light would be focused to a tiny point. And if sunlight were perfectly parallel, a large convex lens could concentrate sunlight into an intense pinpoint rather than into a small disk. Also, a if a small concave lens were placed near the focus of a large convex lens, the pair lenses could be used to concentrate sunlight and form it into a thin, dangerously powerful parallel beam. Try to do this with the real sun, and all you get is a large, projected image of the sun's disk.



CORRECTED: WITH AN AIRCRAFT WING, THE LIFTING FORCE DOES NOT COME FROM THE DIFFERENCE IN CURVATURE BETWEEN THE TOP AND BOTTOM SURFACES Also: wings/lift webpage Some books explain that the lifting force on an aircraft wing is created because the upper wing surface is longer and more curved than the lower. They state that air dividing at the leading edge of the wing must rejoin at the trailing edge, therefore the upper air stream must move faster, and so the wing is pulled upwards by the Bernoulli effect. This is not correct: the air divided by the leading edge does NOT rejoin at the trailing edge, and there is no "race" to catch up. The same books often contain a misleading diagram showing a flat-bottomed wing with flow lines of the surrounding air. (see below.) This diagram actually shows a zero-lift condition. In order to create lift, a wing must deflect air downwards.

Both the explanation and the diagram have serious problems. They wrongly imply that inverted flight is impossible and that an aircraft with equal pathlengths of upper and lower wing surfaces will not fly. They also wrongly suggest that aircraft can violate conservation of momentum by remaining aloft without reacting against the air, and without causing a downward motion of the air. Yet upside-down flight is far from impossible; it is a common aerobatic move. And many wings have equal pathlengths, including even the thin cloth wings of the Wright Brothers' flyer! And anyone standing under a slow, low-flying plane or below the thin, fast wings of a helicopter will know that there is a very great downward flow of air below the wings. All of this indicates that there is a serious problem with the "curved top, flat bottom" explanation. Below is an alternative. As a plane flies, its wings cut through the air at an angle. This "angle of attack" causes the wing to apply a downward force to the air. Or rather than being tilted, the wings can be curved or "cambered", and this makes the trailing edge of the wing tilt down at an angle. As a result, the moving air streams downwards at an angle. As a result, the wing is pushed upwards and backwards. (These two forces are called "lift" and "induced drag.") The lower surface of the wing causes air to move down, but that's not the only important effect. Because the flowing air adheres to the TOP of the wing, the tilt of the wing also causes the upper surface of the wing to pull downwards upon the air above it. The air ABOVE the wing moves down and the wing is forced upwards. As any plane flies, a stream of air is sent diagonally downwards by its wings, and the wing acts like a 'reaction engine' much like a jet engine or a rocket. Unless a wing is either tilted or cambered, it cannot force the air downwards and http://www.amasci.com/miscon/miscon4.html (11 of 32)24-1-2004 18:18:25


cannot generate any "lift." It may help to imagine a hovering helicopter: a helicopter can hover because its rotor applies a downward force to the air, and the air applies an upward force to the rotor. As a result, the air flows downwards and the upward force supports the craft. But like any airplane, a helicopter rotor is a moving wing, and it's this small wing which sends the air downwards. Like any wing, helicopter rotors are reaction engines, they push air downwards, and the air pushes them upwards. They are not "sucked upwards," and neither are airplanes. You may have seen a plane's downwash of air in movies: a "cropduster" plane sends out a trail of fertilizer mist, and the trail of mist does not float, instead it moves immediately down into the crops, driven downward by the moving air. Air from wings can even be dangerous: if a plane flies too low, the downwash from its wings can knock people over. The "Bernoulli effect" is still true. It explains how the top of the wing is able to "pull downwards" on the air flowing over it. And the Bernoulli Effect proves extremely useful in calculations of the lifting force during classes in airplane physics and during experimental work in aerodynamics. But airplanes also obey Newton's laws: accelerate some air downwards, and you'll experience an upwards force. ● ● ●

WEBSITE: Airfoil misconceptions in K-6 textbooks SOME EMAIL DEBATE My improved explanation:DISK BALLOONS

SOUND TRAVELS BETTER THROUGH SOLIDS? NO. Many elementary textbooks say that sound travels better through solids and liquids than through air, but they are incorrect. In fact, air, solids, and liquids are nearly transparent to sound waves. Some authors use an experiment to convince us differently: place a solid ruler so it touches both a ticking watch and your ear, and the sound becomes louder. Doesn't this prove that wood is better than air at conducting sound? Not really, because sound has an interesting property not usually mentioned in the books: waves of sound traveling inside a solid will bounce off the air outside the solid. The http://www.amasci.com/miscon/miscon4.html (12 of 32)24-1-2004 18:18:25


experiment with the ruler merely proves that a wooden rod can act as a sort of "tube," and it will guide sounds to your head which would otherwise spread in all directions in the air. A hollow pipe can also be used to guide the ticking sounds to your head, thus illustrating that air is a good conductor after all. Sound in a solid has difficulty getting past a crack in the solid, just as sound in the air has difficulty getting past a wall. Solids, liquids, and air are nearly equal as sound conductors. It's true that the speed of sound differs in each material, but this does not affect how well they conduct. "Faster" doesn't mean "better." It is true that their transparency is not exactly the same, but this only is important when sound travels a relatively great distance through each material. It's also true that complex combinations of materials conduct sound differently and may act as sound absorbers (examples: water with clouds of bubbles, mixtures of various solids, air filled with rain or snow.) And last: when you strike one object with another, the sound created inside the solid object is louder than the sound created in the surrounding air. So, before we try to prove that solids are better conductors, we had better make sure that we aren't accidentally putting louder sound into the solids in the first place.

GRAVITY IN SPACE IS ZERO? WRONG. Everyone knows that the gravity in outer space is zero. Everyone is wrong. Gravity in space is not zero, it can actually be fairly strong. Suppose you climbed to the top of a ladder that's 300 miles tall. You would be up in the vacuum of space, but you would not be weightless at all. You'd only weigh about fifteen percent less than you do on the ground. While 300 miles out in space, a 115lb person would weigh 100lb. Yet a spacecraft can orbit 'weightlessly' at the height of your ladder! While you're up there, you might see the Space Shuttle zip right by you. The people inside it would seem as weightless as always. Yet on your tall ladder, you'd feel nearly normal weight. What's going on? The reason that the shuttle astronauts act weightless is that they're inside a container which is FALLING! If the shuttle were to sit unmoving on top of your ladder (it's a strong ladder,) the shuttle would no longer be falling, and its occupants would feel nearly normal weight. And if you were to leap from your ladder, you would feel just as weightless as an astronaut (at least you'd feel weightless until you hit the ground!) So, if the orbiting shuttle is really falling, why doesn't it hit the earth? It's because the shuttle is not only falling down, it is moving very fast sideways as it falls, so it falls in a curve. It moves so fast that the curved path of its fall is the same as the curve of the earth, so the Shuttle falls and falls and never comes down. Gravity strongly affects the astronauts in a spacecraft: the Earth is strongly pulling on them so they fall towards it. But they are moving sideways so fast that they http://www.amasci.com/miscon/miscon4.html (13 of 32)24-1-2004 18:18:25


continually miss the Earth. This process is called "orbiting," and the proper word for the seeming lack of gravity is called "Free Fall." You shouldn't say that astronauts are "weightless," because if you do, then anyone and anything that is falling would also be "weightless." When you jump out of an airplane, do you become weightless? And if you drop a book, does gravity stop affecting it; should you say it becomes weightless? If so, then why does it fall? If "weight" is the force which pulls objects towards the Earth, then this force is still there even when objects fall. So, to experience GENUINE free fall just like the astronauts, simply jump into the air! Better yet, jump off a diving board at the pool, or bounce on a trampoline, or go skydiving. Bungee-jumpers know what the astronauts experience. Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. --Fred Hoyle

CORRECTED: FOR EVERY ACTION, THERE IS NOT AN EQUAL AND OPPOSITE REACTION Newton originally published his laws of motion in Latin, and in the English translation, the word "action" was used in a different way than it's usually used today. It was not used to suggest motion. Instead it was used to mean "an acting upon." It was used in much the same way that the word "force" is used today. What Newton's third law of motion means is this: For every "acting upon", there must be an equal "acting upon" in the opposite direction. Or in modern terms... For every FORCE applied, there must be an equal FORCE in the opposite direction. So while it's true that a skateboard does fly backwards when the rider steps off it, these MOTIONS of "action" and "reaction" are not what Newton was investigating. Newton was actually referring to the fact that when you push on something, it pushes back upon you equally, EVEN IF IT DOES NOT MOVE. When a bowling ball pushes down on the Earth, the Earth pushes up on the bowling ball by the same amount. That is a good illustration of Newton's third Law. Newton's Third Law can be rewritten to say: FOR EVERY FORCE THERE IS AN EQUAL AND OPPOSITE FORCE. Or "you cannot touch without being touched."



Or even simpler: Forces always exist in pairs.

CORRECTED: BEN FRANKLIN'S KITE WAS NEVER STRUCK BY LIGHTNING Many people believe that Ben Franklin's kite was hit by a lightning bolt, and this is how he proved that lightning is electrical. A number of books and even some encyclopedias say the same thing. They are wrong. When lightning strikes a kite, the electric current in the string is so high that just the spreading electric currents in the ground can kill anyone standing nearby, to say nothing of the person holding the string! What Franklin actually did was to show that a kite would collect a tiny bit of electrical charge-imbalance out of the sky during a thunderstorm. Air is not a perfect insulator. The charges in a thunderstorm are constantly leaking downwards through the air and into the ground. Electric leakage through the air caused Franklin's kite and string to become charged, and the hairs on the twine stood outwards. The twine was then used to charge a metal key, and tiny sparks could then be drawn from the key. Those tiny sparks were the only "lightning" in his experiment. (He used a metal object because sparks cannot be directly drawn from the twine; it's conductive, but not conductive enough to make sparks.) His experiment told Franlkin that some stormclouds carry strong electrical charges, and it IMPLIED that lightning was just a large electric spark. The common belief that Franklin easily survived a lightning strike is not just wrong, it is dangerous: it may convince kids that it's OK to duplicate the kite experiment as long as they "protect" themselves by holding a silk ribbon and employing a metal key. Make no mistake, Franklin's experiment was extremely dangerous. Lightning goes through miles of insulating air, and will not be stopped by a piece of ribbon. If lightning had actually hit his kite, he would have been gravely injured, and most probably would have died instantly. See LIGHTNING SURVIVOR RESOURCES



THE MAIN LENS OF YOUR EYE IS INSIDE THE EYE? NOT QUITE. Some textbooks assume that the small lens found deep within the eyeball is the eye's main lens, and the cornea of the eye is simply a protective window. The textbook diagrams even depict light rays passing into the eye and only bending as they pass through this internal lens. But in the human eye, the small lens found within the eyeball is not the main imaging lens. The cornea is actually the main lens; it is the strongly curved transparent front surface of the eye. Most of the bending of the light occurs at the place where the light enters the surface of the cornea. When you look at your eye in the mirror, you are looking directly at the eye's main lens. When you want to change the focusing power of your eye, you apply "contact lenses" to the cornea surface, or you undergo surgery which re-sculpts the cornea's curvature. The smaller lens inside the eye acts only to alter the focus of the eye as a whole. Muscles change its shape in order to correct the focus for near and far viewing. Without this small internal lens, human vision would be blurry, and vision would be unable to accommodate for near and far views. But without the cornea lens, [the human eye would be blind] IMPROVED VERSION: witthout the cornea lens, human vision would rely upon the pinhole-camera effect of the eye's pupil, and vision would be incredibly blurry. Open your eyes underwater in dimly-lit conditions to see what vision would be like without a cornea.



CORRECTED: WHEN ONE PRISM SPLITS LIGHT INTO COLORS, A SECOND IDENTICAL PRISM CANNOT RECOMBINE THEM A single prism can split a sunbeam into a rainbow. Many children's science books show how a second similar prism can be used to recombine the colors. This is incorrect, two prisms do not work as shown. Prisms of two DIFFERENT sizes can split and then focus the colors into momentary recombination at a particular distance. With THREE prisms in a special arrangement, the splitting and complete recombining of colors can be accomplished. But books which depict one prism splitting the colors and a second identical prism recombining the colors into a single white beam are in error, and are no doubt the source of endless frustration for those of us who try to duplicate the effect with real prisms. The "rainbows" can also be recombined by placing a screen at just the right place, and by bouncing the colors off many small mirrors so the colored beams converge upon a screen. Recombination can also be done with a convex lens or a concave mirror and a screen. I hope that very few students will attempt to perform the color recombination experiment depicted in their books, for disappointment awaits. (MORE)

CLOUDS, FOG, AND SHOWER-ROOM MIST ARE WATER VAPOR? NO. All three things are made of small droplets of liquid water hanging in the air. When water evaporates, it turns into a transparent gas called "water vapor." When it condenses again, it can take the form of rain, snow, rivers, and oceans, but it also can take the form of clouds, mist, fog, etc. Fog can make surfaces wet, but not because of condensation. Instead, the fog droplets collide with the solid surface. Fog is liquid water, not a vapor. Fly an ultralight aircraft slowly through a large dense cloud, and you'll become damp. To look for water vapor, look at the bubbles in rapidly boiling water. Look at the small empty space at the spout of a boiling teakettle. Look at the far end of the teakettle's plume of mist, where the mist seems to vanish into the air. Look at the empty air above a wet surface. In these situations you see nothing, and that's where the vapor is. Water vapor seems invisible because it is transparent. Clouds and fog are not transparent. They are http://www.amasci.com/miscon/miscon4.html (17 of 32)24-1-2004 18:18:25


composed of liquid droplets.

CORRECTED: RAINDROPS DON'T HAVE POINTS!! Nearly every drawing of raindrops depicts them as having a sharp upper point. This is wrong. Surface tension of water acts like a stretched "bag" around the water, and unless some other force is acting, it pulls the water into a spherical shape. Our eyes do see tiny droplets as a blur, but a flash photograph reveals that small raindrops are nearly spherical. The larger ones are distorted by the pressure of moving air, but this doesn't make points, it makes them somewhat flattened. Think of it this way: underwater bubbles are not pointed as they rise, just as falling water drops are not pointed as they fall. And while it's true that the SYMBOL for water is a droplet with a point, REAL water droplets look nothing like the symbol. And when water drips from a faucet, it never actually has a point. Instead it has a narrow neck, and after the neck has snapped, it is yanked back into the falling ball of water. See Dr. Fraser's BAD SCIENCE for lots more about this.

AIR IS WEIGHTLESS? NO. http://www.amasci.com/miscon/miscon4.html (18 of 32)24-1-2004 18:18:25


We are not conscious of air's weight because we are immersed within it. In the same way, even a large bag of water seems weightless when it is immersed in a water tank. The bag of water in the tank is supported by buoyancy. In a similar way, buoyancy from the atmosphere makes a bag of air seem weightless when it's surrounded by air. One way to discover the real weight of air would be to take a bag of air into a vacuum chamber. Another way is to weigh a pressurized and an unpressurized football. A cubic meter of air at sea-level pressure and 0C temperature has a mass of 1.2KG. The nonmetric rule of thumb says that the air that would fill a bathtub weighs about one pound. Here's a simple way to detect the mass of air even though the air seems weightless: open an umbrella, wiggle it slightly forwards and back, then close it and wiggle it again. When you wiggle it when open, you can feel its increased mass because of the air the umbrella must carry with it. (Ah, but then we must explain the difference between weight and mass!)

CORRECTED: FILLED AND EMPTY BALLOONS DO NOT DEMONSTRATE THE WEIGHT OF AIR Many books contain a incorrect experiment which purports to directly demonstrate that air has weight. A crude beambalance is constructed using a meter stick. Deflated rubber balloons are attached to the ends, and the balance is adjusted. One balloon is then inflated, and that end of the balance-beam is supposed to sag downwards. A large amount of air supposedly weighs more than a small amount of air. Unfortunately this experiment lies. When immersed in atmosphere, buoyancy causes full and empty balloons to weigh the same. But then why does the above experiment work? It doesn't! The experiment will fail unless you know the trick: blow the balloon up near to bursting. It secretly relies on the fact that the air within a high-pressure balloon is denser than air within a low pressure balloon. Obviously this does not DIRECTLY demonstration anything about the weigh of air, and it's dishonest to tell students that it does. To illustrate the problem, try this instead: attach two opened paper bags to the balance, adjust it, then crush one bag so it contains little air. The balance WILL NOT MOVE. What does this teach your class; that air is... weightless? Yet air does have significant weight. We just can't detect this weight directly by using balloons or paper bags. Here's a way to make the experiment more honest. Perform the balance-beam experiment again, but blow one balloon REALLY full so the rubber feels hard and the balloon is about to pop. Blow up the second balloon so it is ALMOST full, but still a bit stretchy. Try to keep the balloons the same size. Now the balance will show that, even though the balloons are nearly the same size, the "hard" balloon is heavier. Does this teach misleading things to your class? No, instead it exposes the dishonesty of the original demonstration. In truth, balloons full of air do not weigh more than empty ones. However, COMPRESSED air does weigh more than UNCOMPRESSED air. http://www.amasci.com/miscon/miscon4.html (19 of 32)24-1-2004 18:18:25


What if we lived underwater, how could we use the balance-beam to measure the weight of water directly? The answer is that we cannot. If a water-filled balloon and an uninflated balloon were compared underwater, the experiment would show that they weigh the same, which seems to prove that water is weightless. When underwater, a bag full of water weighs just the same as a flattened bag which contains nothing. The situation with air is identical: if we live our lives immersed within a sea of air, we cannot use a balance to easily detect the actual weight of the air. (In fact, a bathtub full of air weighs about a half kilogram, but we cannot sense this weight while living in an atmosphere.) It's hard to teach the weight of water to the fishes, and hard to teach the weight of air to human grade-schoolers. These experiments could only work if performed in a vacuum environment (say, on the moon's surface.) We humans are like fish underwater: we're not aware that our ocean of air has any weight. To better demonstrate the weight of air directly, hook a heavy bottle to a vacuum pump, pump all the air out, seal it, then weigh the bottle. Break the seal and let the air in, then weigh it again. The difference in weight is the weight of the air contained in the bottle. Another: use a balance to compare the weight of two vacuum-containing bottles, then open one of them so it becomes filled with air. The bottles will then weigh differently, and the difference is the true weight of the air in one bottle. Or another: build a balance using upside-down paper bags, then place a candle below one of them, then remove the candle again. That bag rises, indicating that a volume of warm air weighs slightly less than a volume of cool air. (Don't set the bag on fire!!) But note that this candle experiment says nothing simple and direct about the actual weight of a volume of unheated air.

CORRECTED: IN THE EVERYDAY WORLD, GASES DO NOT EXPAND TO FILL THEIR CONTAINERS What is the difference between a liquid and a gas? Both are "fluids", both can flow. Gases are USUALLY less dense than liquids, although gases under fiercely high pressure can approach the density of liquids, so that's not a good criterion. The main difference is that gases are a different phase of matter: a gas can be made to condense into a liquid form, and a liquid can be made to evaporate into gas. Another major characteristic: because there are bonds between its particles, when a liquid IS PLACED INTO A VACUUM ENVIRONMENT, it will not expand continuously, while a gas in a vacuum chamber will expand continuously until it hits the walls. This is very different than the oft-quoted rule that "gases always expand to fill their containers." This rule only works correctly if the container is totally empty: the container must "contain" a good vacuum beforehand. However, we all live in a gas-filled environment. All our containers are pre-filled with air. In our environment, any new quantity of gas will not expand, it will just sit there. If you squirt some carbon dioxide out of a CO2 fire extinguisher, it will not instantly expand to fill the room. Instead it will pour downwards like an invisible fluid and form a pool on the floor. It behaves similarly to dense sugar-water which was injected into a tank of water: it pours downwards, and only after a very long http://www.amasci.com/miscon/miscon4.html (20 of 32)24-1-2004 18:18:25


time it will mix with the rest of the water. "Mixing" is very different than "expanding to fill!" The rule about gases does not involve mixing, instead it involves compressibility and instant expansion into a vacuum. In an air-filled room, dense gases act much like liquids; they can be poured into a cup or bowl, poured out out onto a tabletop, and then they run off the edge onto the floor where they form an invisible mess. :) Less dense gases will stay where they are put, like smoke or like food coloring which has just been injected into a fishtank. Gas of even lesser density rises and forms a pool on the ceiling. Only in the world of the physicist, where "empty container" always implies a vacuum, does the rule about gasses work properly.

CORRECTED: SHADOWS DO VANISH ON CLOUDY DAYS, BUT NOT BECAUSE THE SUN ISN'T BRIGHT ENOUGH Shadows appear when an object blocks a light source. The shape of the shadow is created by the shape of the opaque object AND by the shape of the light source. On a cloudy day the whole sky acts as a light source, and a person's shadow spreads out and becomes a dim fuzzy patch which surrounds the person on the ground on all sides. The shadow is so spread-out that it seems absent entirely. When the sun is visible, the same shadow is concentrated in one specific place and becomes easy to see. But even the shadows made by sunlight will have fuzzy borders, since the sun is a small disk rather than a tiny dot. On cloudy days, the fuzzy borders of your body's shadow become much much larger than the shadow itself, so that the shadow seems to vanish.



CORRECTED: FRICTION IS NOT CAUSED BY SURFACE ROUGHNESS Some books point to surface roughness as the explanation of sliding friction. Surface roughness merely makes the moving surfaces bounce up and down as they move, and any energy lost in pushing the surfaces apart is regained when they fall together again. Friction is mostly caused by chemical bonding between the moving surfaces; it is caused by stickyness. Even scientists once believed this misconception, and they explained friction as being caused by "interlocking asperites", the "asperites" being microscopic bumps on surfaces. But the modern sciences of surfaces, of abrasion, and of lubrication explain sliding friction in terms of chemical bonding and "stick & slip" processes. The subject is still full of unknowns, and new discoveries await those who make surface science their profession When thinking about friction, don't think about grains of sand on sandpaper. Instead think about sticky adhesive tape being dragged along a surface.

CORRECTED: NO, INFRARED LIGHT IS NOT A KIND OF HEAT Infrared light is invisible light. When any type of light is absorbed by an object, that object will be heated. The infrared light from an electric heater feels hot because the light is EXTREMELY BRIGHT LIGHT. Just because human eyes cannot see the light which causes the heating does not mean that the light is made of some mysterious entity called "heat radiation." When bright light shines on an absorbtive surface, that surface heats up. And this is no benign misconception. Those who fall under its sway may also come to believe that *visible* light cannot heat surfaces (after all, visible light is not "heat radiation.") Misguided science students may wrongly believe that warm objects emit no microwaves (since only IR light is "heat radiation"), even though hot objects actually do emit microwaves. Or they may believe that the glow of red hot objects is somehow different than the infrared glow of cooler objects. Or they may believe that IR light is a form of "heat," and is therefore fundamentally different than any other type of electromagnetic radiation. In his book "Clouds in a Glass of Beer," Physicist C. Bohren points out that this "heat" misconception may have been started long ago, when early physicists believed in the existence of three separate types of radiation: heat radiation, light, and actinic radiation. Eventually they discovered that all three were actually the same stuff: light. "Heat radiation" and "actinic radiation" are simply invisible light of various frequencies. Today we say "UV light" rather than "actinic radiation." Yet the obsolete term "heat radiation" still lingers. Since human beings can only see certain frequencies of light, it's easy to see how this sort of confusion got started. Invisible light seems bizarre and mysterious when compared to visible light. But "invisibility" is caused by the human eye, and is not a property carried by the light. If humans could http://www.amasci.com/miscon/miscon4.html (22 of 32)24-1-2004 18:18:25


see all the light in the infrared spectrum, we would say things like this: "of COURSE the electric heater makes things hot at a distance, it is intensely BRIGHT, and bright light can heat up any surface which absorbs it." PS, if you're interested in physical science misconceptions, Bohren's Book is an excellent resource. He's like me, and complai ns about several specific misconceptions which keep his students from understanding science.

CORRECTED: THERE ARE NOT SEVEN COLORS IN THE RAINBOW Actually here is a very large number of distinct colors in any rainbow. And neither are there sharp divisions between the bands of color, yet numerous textbooks depict them. In reality, between yellow and green we find yellow-green, and between green and yellowgreen is GREENISH yellowgreen, and on and on. How many colors are in a rainbow? Thirty? Sixty? It's not easy to say, for it depends on the particular eye, and the particular rainbow. What of the teachers and students who look in vain for the yellow-green in their textbook's depiction of rainbows? They've crashed into a longrunning textbook misconception: the strange idea that rainbows have exactly seven distinct bands of color and no more, and with nothing in between those uniform bands of 'official' color.



CORRECTED: ACTUALLY, THE EARTH'S NORTH AND SOUTH MAGNETIC POLES RESIDE DEEP WITHIN THE EARTH'S CORE Many textbooks have an erroneous diagram of the earth which shows a bar magnet within it, and the ends of this bar magnet extend to just beneath the earth's surface. These diagrams depict the magnet's field lines as radiating from spots on the earth's surface. This is very misleading. The earth's magnetic poles actually behave as if they're deep within the earth, down inside the core. The Earth's magnetic field does not come from a giant bar magnet, but if we IMAGINE that it does, then the imaginary "bar magnet" inside the earth is short, stubby, disk-shaped, and part of the iron core deep inside the planet. The typical textbook diagram is incorrect, and there are NO INTENSE MAGNETIC FIELDS at the land surface near the earth's "north pole" and "south pole." If you stand at the Earth's south magnetic pole, metals aren't attracted to the ground more strongly than anywhere else. The Geomagnetic "poles" on the earth's surface are not places where the field is strong. They are simply the points on the landscape where the field lines are perfectly vertical. Proper diagrams should instead show the field lines to be radiating from poles inside the earth's core. They should show the field lines around the northern and southern areas of the earth's surface as being approximately vertical and parallel, not "radial" like a spiderweb and not concentrated into special points on the surface. Another error associated with the above: some books claim that the earth's field at the magnetic poles is much stronger than elsewhere. This is untrue. The field strength at the north magnetic pole above Canada is about the same as the field strength in Virginia! And the strongest field in the Earth's northern hemisphere does not appear at the north magnetic pole at all, the north pole actually has a weaker field than elsewhere. The strongest fields in the northern hemisphere are not in one but in two places: west of Hudson bay in Canada, and in Siberia. LINKS ● ● ● ●

Correct diagram of Earth's field NOAA questions about Earth's field Field strength map The Great Magnet, the Earth



LASER LIGHT IS "IN PHASE" LIGHT? WRONG. It's incorrect to say that "in laser light the waves are all in phase." When two light waves travelling in the same direction combine, they inextricably add together, they do not travel as two independent "in-phase" waves. The photons in laser light are in phase, but the WAVES are not. Instead, ideal laser light acts like a single, perfect wave. When the light wave within a laser causes atoms to emit smaller, in phase light waves, the result is not "in phase" light. Instead the result is a single, more intense, amplified wave of light. In-phase emission leads to amplification, not to multiple in-phase waves. If the atoms' emissions weren't in phase, the result would NOT be light that's out of phase. Instead the the atoms would absorb light rather than amplifying it. Each atom in a laser contributes a tiny bit of light, but their light vanishes into the main traveling wave. The light from each atom strengthens the main beam, but loses its individuality in the process. 99 plus 1 equals 100, but if someone gives us 100, we cannot know if it is made from 99 plus 1, or 98 plus 2, or 50 plus 50, etc. All the *PHOTONS* in a single wave of light are in phase. This might be one reason that people say that laser light is "in phase" light. However, in-phase photons are nothing unique, and they don't really explain coherence. Any EM spherewave or plane-wave is made of in-phase photons. For example, all the photons radiated from a radio broadcast antenna are also in phase, but we don't say that these are special "in phase" radio waves, instead we just say that they are waves with a spherical wavefront. Even if all the photons in laser light are in phase, it is still incorrect to say "all the WAVES are in phase." Photons are not waves. They are quanta, they are particles, and they do not behave as small, individual "waves." Yes, all the photons are in phase, but only because they are part of a single plane-waves. The light from a laser is basically a single, very powerful light wave. Single waves are always in phase with themselves, but it's misleading to imply that a single plane-wave or sphere-wave is something called an "in phase" wave. Laser light could more accurately be called "pointsource" light. Sphere waves or plane waves behave as if they were emitted from a single tiny point. The physics term for this is "spatially coherent" light. Light from light bulbs, flames, the sun, etc. are the opposite, and are called "extended-source" light. Extended-source light comes from a wide source, not from a pointsource, and the waves coming from different parts of the source will cross each other. Starlight and the light from arc welders is "point-source" light and is quite similar to laser light. Light from arc-welders and from distant stars has a higher spatial coherence than light from most everyday light sources. (Note: the sun is a star, correctly implying that light becomes more and more spatially coherent as it moves far from its source. This is a clue as to the REAL reason that lasers give spatially coherent light! (See below)



CORRECTED: LASER LIGHT IS NOT PARALLEL LIGHT Light from most lasers is not parallel light. However, if laser light is passed through the correct lenses, it can be formed into a tight, parallel beam. The same is not true for light from an ordinary light bulb. If light from a light bulb were passed through the same lenses, it would form a spreading beam, and an image of the lightbulb would be projected into the distance. Laser light can form beams because a laser is a pointsource, and when you project the image of a pointsource into the distance, you form a narrow parallel beam! However, it is simply wrong to state that laser light is inherently parallel light. Laser light can be FORMED INTO parallel light, while the light from ordinary sources cannot. Most types of lasers actually emit spreading, non-parallel light. Lasers in CD players and in "laser pointers" are semiconductor diode lasers. They create cone-shaped light beams, and if a parallel beam is desired, they require a focusing lens. The same is true for the lasers in inexpensive "laser pointers." Take apart an old laser-pointer, and you'll find the plastic lens in front of the diode laser inside. Classroom "HeNe" lasers also create spreading light. The laser tube within a typical classroom laser contains at least one curved mirror (called a "confocal" arrangement,) and it creates light in the form of a spreading cone. It's a little-known fact that manufacturers of classroom lasers traditionally place a convex lens on the end of their laser tubes in order to shape the spreading light into a parallel beam. While it's true that a narrow beam is convenient, I suspect that part of their reason is to force the laser to fit our stereotype that all lasers produce thin, narrow light beams. The manufacturers could save money by selling "real" lensless laser tubes having spreading beams. But customers would complain, wouldn't they? We have been brought up to believe that laser light is parallel light.



CORRECTED: LASERS EMIT COHERENT LIGHT, BUT NOT BECAUSE THE ATOMS EMIT IN-PHASE LIGHT WAVES In-phase emission causes the AMPLIFICATION of light, it doesn't cause coherent light. Because the atoms emit light in phase with incoming light, they will amplify the light, but they amplify incoherent light too, and they don't make it coherent. The coherence of laser light has another source... Laser light has two main characteristics: it is "monochromatic" or very pure in frequency (this also is called "temporally coherent.") Laser light also has a point-source character of sphere waves and plane waves (also called "spatially coherent.") Even fairly advanced textbooks fail to give the real reason why laser light is spatially coherent. They usually point out that the laser's atoms all emit their light in phase, and pretend that this leads to spacial coherence. Wrong. It is true that the fluorescing atoms in a laser all emit light that's in-phase with the waves already traveling between the mirrors. But the in-phase emission only creates amplification of the traveling waves, it does not create spatially coherent light. For example, if you were to feed incoherent light into a HeNe laser tube, the atoms would emit in-phase waves, and the laser would amplify the light. But the brighter light would still be incoherent! Lasers certainly can amplify the COHERENT wave which is trapped between their mirrors. But how did the light within the laser get to be coherent in the first place? Lasers create coherent light because of their mirrors. The mirrors in a laser form a resonant cavity which preserves coherent light while rejecting incoherent light. How does it work? Imagine a simplified laser having flat, parallel mirrors. As light bounces between the mirrors, the light "thinks" that it's traveling down an infinitely long "virtual tunnel". (Have you ever held up two mirrors facing each other? Then you've seen this infinite tunnel.) When a laser is first turned on, it fluoresces; it emits light which is NOT coherent. Different random light waves start out from different parts of the laser. After a few thousand mirror bounces, all the waves have added and subtracted to form just one single wave. In the case of flat-mirror lasers, this wave is a nearly perfect plane wave. A single plane wave is coherent (to be incoherent, you must have at least two different waves.) This can be a bit confusing. After all, the individual atoms each emit a wave. Don't all these waves add up to messy incoherent light? No. The in-phase emission preserves coherence as it amplifies. It's true that each atom emits light waves in all directions. However, these sideways waves cancel each other out, and only the waves that travel in the same direction as the incoming light will be preserved. It's as if the atoms "know" which direction to send out a beam. But in reality, the atoms don't know this. Instead, they just emit a light wave which is in phase with the incoming light, and for this reason the wave from the atom will cancel out everywhere except in a line with the incoming light. If the light in a laser were ALREADY coherent, then the atoms will amplify it but won't make it more coherent. The coherence comes from the great distance that the light has travelled as it bounced between the mirrors. A similar thing happens with starlight: starlight is coherent! Starlight travels far from its original source and all the waves add up to form a wave with a single wavefront. Light from distant stars is spatially coherent, even though sunlight is not, yet the sun is a star. The farther the light travels from its source, the more it approaches the shape of a perfect plane wave. And a perfect plane wave is perfectly coherent. Laser light is spatially coherent because, among other things, the bouncing light has traveled millions of miles between mirrors, and all the various competing waves have melded together to form a single pure plane-wave or sphere-wave. P.S. The pure color (monochrome) laser light is ALSO created by the mirrors. Huh? Yes, but the reason for this is not totally straightforward (and it's quite a bit beyond the K-6 level of these webpages!) The two mirrors of a laser can trap a standing wave of light. The space between the mirrors is like the string of a guitar: there can be a fundamental wave, or overtone waves, or complicated waves which are a mixture of these. But waves of non-overtone frequencies cannot exist between the mirrors. Since the distance between the crests of a lightwave is very small, LOTS of different overtones can fit between the mirrors, and each overtone is a slightly-different pure color of light. Light from a neon sign is reddish, but it doesn't have the extreme purity of laser light. Now for the weird part: when http://www.amasci.com/miscon/miscon4.html (27 of 32)24-1-2004 18:18:25


a Helium-Neon laser first operates, many different overtones of red light are amplified and the beam contains many slightly-different colors of red at the same time. It's not yet monochromatic. As time goes on, some of these colors are amplified a bit more than others, and this uses up the available energy coming from the power supply. In other words, the different waves start competing for limited resources! Just one wave "wins" in the end, and all of the other overtones drop out of the running. The laser light is not just red light. Instead it is a SINGLE PURE OVERTONE-WAVE, a pure frequency where the string of waves just perfectly fits in the space between the two mirrors. Change the spacing of the laser's mirrors, and you change the frequency of the light.

CORRECTED: IRON AND STEEL ARE NOT THE ONLY STRONGLY MAGNETIC MATERIALS There are numerous others. Nickel and Cobalt metals are very magnetic. (U.S. "nickel" coins contain copper which spoils the effect, so try Canadian nickels made before 1985.) Most other materials are "diamagnetic," and are repelled visibly by very strong magnets, although some materials are "paramagnetic" and are attracted. Supercold liquid oxygen is attracted by magnets. Some but not all types of stainless steel are nonmagnetic. There are even some metals which are individually nonmagnetic, but which become strongly magnetic when mixed together, chromium and platinum for example, and compounds of manganese and bismuth.



CORRECTED: RE-ENTERING SPACE CAPSULES ARE NOT HEATED BY AIR FRICTION They are heated as they plow into the atmosphere and compress the air ahead of them. Ever pump up a bicycle tire and discover that the pump and the tire have become hot? The same effect causes spacecraft and supersonic aircraft to heat up as they compress the air at their leading edges. The heat doesn't come from *rubbing* upon the air, it comes from *squeezing* the air. This applies mostly to blunt objects such as Apollo reentry vehicles. It does not apply as much to the Space Shuttle: with wings oriented mostly edge-on to the moving air, the surfaces of the Shuttle ARE heated by friction. But when the Shuttle first reenters the atmosphere, the bottom of the craft faces forwards, and in that case the Shuttle is heated by air compression, NOT by friction.

CORRECTED: CARS AND AIRPLANES ARE NOT SLOWED DOWN BY AIR FRICTION They are slowed because it takes energy to stir the air. While direct friction between the air and the car's surface does play a part, the work done in stirring the air far exceeds the work done in direct frictional heating. If vehicles did not send air swirls and vortices spinning off as they moved, they would barely be slowed by the air at all. Eventually the swirling air is slowed by friction and ends up warmer, but this occurs long after the vehicle has passed.



CORRECTED: THE NORTH MAGNETIC POLE OF THE EARTH IS NOT IN THE NORTH Opposite poles attract. If we hold two bar magnets near each other, the "N" pole of one magnet is attracted by the "S" pole of another. If we suspend a bar magnet by a thread, the "N" pole of that magnet will point... toward's Earth's north! Something is wrong here. Shouldn't the "N" pole of a magnet point towards the "S" of the Earth? Alike poles should not attract. Either the "N" and "S" printed on all bar magnets is reversed, or the "N" and "S" on the Earth is backwards. Which is it? Physics defines "N-type" magnetic poles as being the north-pointing ends of compasses and magnets. Wind an electromagnet coil, see which end points towards the north, and that end is the N pole of the electromagnet. Therefore, the magnetic pole inside the northern hemisphere of the Earth is a south-type magnetic pole. The Earth's northern magnetic pole is an S! It has to be this way, otherwise it would not attract the N-pole of a compass. This is a long-standing but arbitrary physical standard, much the same as defining electrons as being negative. Like it or not, we are stuck with negative electrons, and seconds which last about 1/100,000 of a day, with backwards Earth poles, with centimeters which are about as wide as a small finger, etc. Interesting email msgs on magnetic polarity Also see Dexter Magnetics for more on this.



CORRECTED: ACTUALLY THERE ARE NO SODIUM CHLORIDE MOLECULES IN SALT WATER Salt is not made of NaCl molecules. Salt is made of a three-dimensional checkerboard of oppositely charged atoms of sodium and chlorine. A salt crystal is like a single gigantic molecule of ClNaClNaClNaClNaClNaClNa. When salt dissolves, it turns into independent atoms. Salt water is not full of "sodium chloride." Instead it is full of sodium and chlorine! The atoms are not poisonous and reactive like sodium metal and chlorine gas because they are electrically charged atoms called "ions." The sodium atoms are missing their outer electron. Because of this, the remaining electrons behave as a filled electron shell, so they cannot easily react and form chemical bonds with other atoms except by electrical attraction. The chlorine has one extra electron and its outer electron shell is complete, so like sodium it too cannot bond with other atoms. These oppositely charged atoms can attract each other and form a salt crystal, but when that crystal dissolves in water, the electrified atoms are pulled away from each other as the water molecules surround them, and they float through the water separately.

CORRECTED: LIGHT AND RADIO WAVES DO NOT ALWAYS TRAVEL AT "THE SPEED OF LIGHT" They only travel at the "speed of light" (186,000 miles per second) while moving through a perfect vacuum. Light waves travel a bit slower in the air, and they travel LOTS slower when moving through glass. Why does light bend when it enters glass at an angle? Because the waves SLOW DOWN. Why can a prism split white light into a spectrum? Because within the glass THE SPEED OF LIGHT WAVES IS DIFFERENT FOR DIFFERENT WAVELENGTHS. And while the numerical value for the speed of light in a vacuum, "c," is very important in all facets of physics, as far as light waves are concerned there is no single unique speed called "The Speed Of Light." [note for advanced students: ok ok, I'll add this: light *waves* within a transparent medium are slow, even though the wave's photons are thought to jump from atom to atom always at a speed of c.]



Created and maintained by Bill Beaty. Mail me at: [email protected]. If you are using Lynx, type C to email.


Hoe vliegt een vliegtuig?

De virtuele windtunnel Onder deze tekst ziet u de virtuele windtunnel (al dan niet geladen), ontworpen door de NASA. Het is niet zo heel moeilijk de windtunnel te bedienen, maar u zult misschien enige uitleg nodig hebben voor het optimaal benutten ervan: Linksboven ziet u de weergave van het object dat zich momenteel in de windtunnel bevindt, rechtsboven is een schermpje met enkele grafieken. Daaronder is een gedeelte met knoppen om de vorm van het voorwerp in te stellen, en helemaal onderaan bevindt zich een scherm om opgeroepen data in te tonen. In het eerste schermpje staat zoals al gezegd een afbeelding van het voorwerp; met de bovenste knoppen in dat schermpje, Edge, Top, Side-3D, Find en Zoom kan men instellen vanaf welke positie men het voorwerp in de windtunnel bekijkt. 'Edge' betekend dat het voorwerp vanaf de zijkant wordt getoond, hierbij is de luchtstroom (die zo van links naar rechts stroomt) ideaal zichtbaar. Als men op 'Top' klikt zal de bovenkant zichtbaar worden, hierdoor zal de luchtstroom echter wel stoppen. Als u op Side-3D klikt zal het voorwerp in 3D worden weergegeven, de luchtstroom gaat nu wel door (mits u niet eerst op 'Top' heeft geklikt). Let op: de luchtstroom zal exact door het midden van het 3D voorwerp gaan en dus niet om het witte gedeelte heen. Met de knop 'Find' kunt u de ideale uitvergroting (waarop het meeste te zien is) bekijken, en met de knop 'Zoom' kan man zelf de vergroting kiezen, door de schuif onder de knop te bewegen. Onder in het schermpje staan nog een aantal knoppen: Streamlines, Moving, Frozen en Geometry. 'Sreamlines' betekend stroomlijnen, als deze knop geactiveerd is kunt u zien hoe de lucht langs het voorwerp beweegt, als u echter op 'Moving' klikt kunt u niet alleen dat zien maar ook hoe snel de lucht zich op verschillende plaatsen beweegt en hoe groot de luchtdichtheid, die mede de lift bepaald, daar is. Wanneer de knop 'Frozen' aan staan wordt het beeld dat vlak voor het activeren van die functie bij 'Moving' te zien was stilgelegd, zodat u dat moment op uw gemak kan bekijken. Door het activeren van de knop 'Geometry' worden meer inputgegevens zichtbaar alsmede andere gegevens zoals de Chord line en de Mean Chamber Line. In het volgende scherm is waarschijnlijk een grafiek te zien. Met de knop 'Rescale' kunt u de schaalverdeling opnieuw idealiseren, dit is handig wanneer u de instellingen heeft veranderd. Onder deze twee schermpjes is het belangrijkste bedieningspaneel, met daarop de volgende knoppen: Met de eerste knop kan men switchen tussen 'Lift' en 'Cl-no units'. Met de tweede kan bepaald worden of er een ideale of meer realistische luchtstroom optreed. 'Show Geom' laat de eigenschappen van het voorwerp dat zich in de windtunnel bevindt en die van de omgeving in het onderste gedeelte zien. http://members.chello.nl/~j.hummelink1/anw_vw.htm (1 of 3)24-1-2004 18:20:25


'Show Data' toont de standaard of later ingestelde gegevens van het voorwerp en omgeving. Met de knop 'Reset' kunnen de gegevens weer naar hun standaardvorm worden teruggebracht (dat is de vleugel die u bij het opstarten van de virtuele windtunnel ziet). Het zwarte schermpje met de groene cijfers boven 'Input' laat de lift of het aantal Cl-no units zien. Onder 'input': Als met de knop 'Flight Test' indrukt kan men met de schuifjes die er naast zullen verschijnen de windsnelheid en hoogte waarop het voorwerp zich bevindt instellen, ook kan men met het menu'tje boven de schuifjes de omgeving bepalen. In het menu daarboven kunt u bovendien de omgeving bepalen: Earth - Average Day = de aarde op een gemiddelde dag Mars - Average Day = de planeet Mars op een gemiddelde dag Water-Const Density = Water met constante dichtheid Specify Air T & P = Lucht met zelf in te stellen temperatuur en druk. Specify Fluid Density = vloeistof met zelf te bepalen dichtheid Onder de schuifjes zijn drie vakjes met daarin de eigenschappen van de omgeving: de druk in kPa (= Press-kPa), de temperatuur in graden Celcius (= Temp-C) en de dichtheid in Kgom-3 (= Density…kg/cu m) Met de knop 'Shape' de vorm van het voorwerp bepalen, in het menu kan het soort object worden bepaald; van boven naar beneden: 'Airfoil' is de vleugel zoals wij die kennen, 'Ellipse' betekend Ellips, een 'Plate' is natuurlijk een gewone plaat, 'Cylinder' is in het Nederlands cilinder en een 'Ball' is een bal. Met de schuifjes eronder kan de vorm van het gekozen voorwerp worden en bij sommige voorwerpen zoals de bal en cilinder kan in plaats van de hoek waaronder het voorwerp geplaatst is (= Angle-deg = de hoek in graden) de rotatiesnelheid vastgesteld worden (= Spin rpm = aantal rotaties per minuut) en in plaats van de kromming (= Chamber-%c) kan de diameter in meter (= Radius m) worden ingesteld. Bij de cilinder is het mogelijk de breedte (= Span m = breedte in meters) vast te stellen in plaats van de dikte (= Thick-%crd), bij een bal is die natuurlijk gelijk aan de diameter. Wanneer men 'Size' heeft aangeklikt kan men de grootte van de vleugel bepalen, dit geldt niet als met een bal of cilinder als voorwerp heeft gekozen omdat de grootte dan al bij 'Shape' is ingesteld, u zal hier dus dezelfde instellingen te zien krijgen als bij 'Shape'. Met het schuifje 'Chord-m' kan de lengte van de Chord line in meters aangepast worden, 'Span-m' staat voor de spanwijdte in meters en 'Area-sq m' voor de oppervlakte van de vleugel in m2. Bij 'Aspect Rat' is de grootte van de Chord line ten opzichte van de spanwijdte weergegeven (Aspect Rat = Span-m/Chord-m). Met het menu'tje boven 'Output' kan geswitcht worden tussen de lift in Newtons of in Pounds. Wanneer u de lift in kg wilt weten moet u de lift in Newtons delen door 9,81. Onder 'Output':

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Met de knop 'Plots' veranderd het scherm met de grafieken in een vlak met verschillende grafiekopties, u kunt hier uit een diagram met oppervlaktedruk of oppervlaktesnelheid kiezen. En wanneer de omgeving op 'Mars/Earth - Average Day' ingesteld staat en het voorwerp niet een bal of cilinder is, is het ook nog mogelijk grafieken te verkrijgen met lift of Cl-no units uitgezet tegen snelheid (= Speed), hoogte (= Altitude), dichtheid (= Density), vleugeloppervlakte (= Wing Area), hoek (= Angle), Kromming (= Chamber) en dikte (= Thickness). Als men op de knop 'Probe' klikt zal er een violet puntje op het zwarte scherm komen dat met de schuifbalken in het vak rechtsboven geregeld kan worden, hier vanuit kan informatie worden gewonnen over die specifieke plek met de knoppen 'Velocity' en 'Pressure' (= snelheid en druk). Met de knop 'Smoke' kan een klein straaltje rook gegenereerd worden uit het paarse punt. Op de 'Lift Meter' is te zien hoeveel lift er ontstaat door de gecreëerde vleugel. Bovenstaande handleiding hebben we zelf gemaakt, de windtunnel hebben we niet zelf gemaakt. Niets van deze pagina mag zonder toestemming van de makers gekopieerd worden.

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De foute vleugeltheorie de meest gebruikte vleugeltheorie en waarom hij fout is. Zeiltheorie. inleiding

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