LessonOneBernouIli’sPrincipleWhyPlanesFlyAviationisdependentupontheprinciplediscoveredbyaneighteenthcenturyscientist，DanielBernoulli.①Bernoulli’sprinciplehelpstoexplainwhyanairplaneflies，howawingproduceslift，andhowfuelflowsinanaircraftcarburetor．Itexplainstherelationshipbetweenthepressure，thevo1umeandthevelocityofafluidinmotion．Simplystated，thevelocityofagivenvolumeofafluidinmotionisinverselyproportionaltoitspressure.Asvelocityincreases，pressuredecreases．Sincebothgasesandliquidsareclassifiedasfluids．We’lluseairtodemonstratethepropertiesofafluidinmotion．ThereareseveralsimpleexperimentsthatdemonstrateBernoulli’sprinciple．Thesimplestmaybeconductedwithonlyasheetofpaper．②Holdthepaperuptoyourlipsfindblowacrossthetopsurface．Noticethatthepaperrises．Thevelocityoftheairoverthetopofthepaperisgreaterthanthatunderthepaper．Alowpressureareaiscreatedabovethepaperandthedifferenceinpressurecausesthepapertorise．Perhapsyouwalkpastthevacuumcleanerdepartmentofdepartmentstoreandyou’llseeaping—pongballsuspendedintheairabovevacuumcleanerseeminglywithoutsport．Theairpassingaroundthecurvedsurfaceoftheballistravelingfasterthantheairjustbeonthesurface．③Again,thehigherpressureoutsidetheairflowcausestheballtoremaininthecenteroftheairflow．Anothersimpleexperimentusesapieceofpaper,astraightpinandasimplesewingspoo1．First，pushthepinthroughthepaperandinsertitintheholeinthespool，thepinsimplykeepthepaperfromdroppingtothefloor.Now,blowthroughtheotherendofthespool,alowpressureareaiscreatedasthevelocityoftheairincreasesbetweenthespoolandpaper．Thehigheratmosphericpressureonthebackofthepaperholdsthepaperclosetotheendofthespool．Theseexperimentsillustratethefactthatwhenthevelocityofthefluidincreases,thepressuredecreases．Conversely,whenthevelocitydecreases,thepressureIncreasesHowdoesthisapplytoaviation?Let'splacearestrictioninthepathofmovingairinthiswindtunnel．Smokeinjectedintotheairstreamindicatesthepaththeairtakesaroundandthroughtherestriction．Therestrictioniscalledaventuri，agivenvolumeofairtravelsthroughandaroundtheventuriinagivenperiodoftime．Theentirevolumemustpassal1pointsinthesystem.Let’sconsider3moleculesofairtobeourgivenvolume，all3moleculesmustpassaroundandthroughtheventuriandmeetattheotherendatthesametime.Noticethatthemoleculepassingthroughtheventurimustfollowalongercurvedpath,travelingatgreaterdistanceinthesameperiodoftimeasthemoleculethatfollowsastraightpath．Thatmeansthemoleculestravelingthroughtheventurimusttravelfaster.AccordingtoBernoulli’sprinciple，asthevelocityincreases,pressuredecreases，thiscausesareducedpressureinthecenteroftheventuri，whilepressureremainsthesameateitherend．Nowlet’slookatahalfsectionoftheventuri,doestheshaperemindyouanything?Youarelookingatthecrosssectionofanairfoil，suchasawingtheprimaryliftproducingsurfaceofanaircraft．Thesameprincipleapplieshereasintheventuri，the2moleculesofairrepresentacertainvolume，bothmoleculesmustreachthetrailingedgeofthewingsectioninthesameperiodoftime，butthemoleculetravelingovertheuppersurfacemusttravelfurtherthantheonetravelingunderthewingbecauseacurvedpathitmustfollow.Again，themoleculetravelingthe-1-curvedpathmusttraveltasterthantheonetravelingthestraightline.Thepressuredifferentialbetweentheupperandlowersurfacesofawingproducesliftthatkeepstheaircraftaloft④Wemayillustratesimplythetremendousamountofforcet...
