Inclination Effects on Lift - NASA

The angle between the chord line and the flight direction is called the angle of attack and has a large effect on the lift generated by a wing. + TextOnlySite +Non-FlashVersion + ContactGlenn Asawingmovesthroughtheair,thewingisinclinedtothe flightdirectionatsomeangle. Theanglebetweenthechordline andtheflightdirectioniscalledtheangleof attackandhasalargeeffectonthe lift generatedbyawing. Whenanairplanetakesoff,thepilotappliesasmuch thrust aspossibletomaketheairplanerollalongtherunway.But justbeforeliftingoff,thepilot "rotates" theaircraft.Thenose oftheairplanerises,increasingtheangleofattackandproducing theincreasedliftneededfortakeoff. Themagnitudeoftheliftgeneratedby anobjectdependsonthe shape oftheobjectandhowitmovesthrough theair.Forthinairfoils,theliftisdirectly proportionaltotheangleofattack forsmallangles(within+/-10degrees).For higherangles,however,thedependenceisquitecomplex.Asan objectmovesthroughtheair,airmolecules stick tothesurface. Thiscreatesalayerofairnearthesurfacecalleda boundarylayer that,ineffect,changestheshapeoftheobject.The flowturning reactstotheedgeoftheboundarylayerjustasitwouldtothe physicalsurfaceoftheobject.Tomakethingsmoreconfusing,the boundarylayermayliftoffor"separate"fromthebodyandcreatean effectiveshapemuchdifferentfromthephysicalshape.The separationoftheboundarylayerexplainswhyaircraftwingswill abruptlyloseliftathighanglestotheflow.Thisconditionis calledawingstall. Ontheslideshownabove,theflowconditionsfortwoairfoilsare shownontheleft.Theshapeofthetwofoilsisthesame.Thelower foilisinclinedattendegreestotheincomingflow,whiletheupper foilisinclinedattwentydegrees.Ontheupperfoil,theboundary layerhasseparatedandthewingisstalled.Predictingthestall point(theangleatwhichthewingstalls)isverydifficult mathematically.Engineersusuallyrelyonwind tunnelteststodeterminethestallpoint.Butthetestmustbedone verycarefully,matchingalltheimportantsimilarity parametersoftheactualflighthardware. Theplotattherightofthefigureshowshowtheliftvarieswith angleofattackforatypicalthinairfoil.Atlowangles,theliftis nearlylinear.Noticeonthisplotthatatzeroangleasmallamount ofliftisgeneratedbecauseoftheairfoilshape.Iftheairfoilhad beensymmetric,theliftwouldbezeroatzeroangleofattack.At therightofthecurve,theliftchangesratherabruptlyandthe curvestops.Inreality,youcansettheairfoilatanyangleyou want.However,oncethewingstalls,theflowbecomeshighly unsteady,andthevalueoftheliftcanchangerapidlywithtime. Becauseitissohardtomeasuresuchflowconditions,engineers usuallyleavetheplotblankbeyondwingstall. Sincetheamountofliftgeneratedatzeroangleandthelocation ofthestallpointmustusuallybedeterminedexperimentally, aerodynamicistsincludetheeffectsofinclination intheliftcoefficient. Forsomesimpleexamples,theliftcoefficientcan bedeterminedmathematically.Forthinairfoilsat subsonicspeed,andsmallangleofattack, theliftcoefficientClisgivenby: Cl=2*pi*a wherepiis3.1415,anda istheangleofattackexpressedinradians: piradians=180degrees Aerodynamicistsrelyonwindtunnel testingandverysophisticatedcomputeranalysistodeterminethe liftcoefficient. Let'sinvestigatethedependenceofliftonangleofattackusingaJava simulatorwhichsolvesthe fluidequations ofmotion. DuetoIT securityconcerns,manyusersarecurrentlyexperiencingproblemsrunningNASAGlenn educationalapplets.Theappletsareslowlybeingupdated,butitisalengthyprocess. IfyouarefamiliarwithJavaRuntimeEnvironments(JRE),youmaywanttotrydownloading theappletandrunningitonanIntegratedDevelopmentEnvironment(IDE)suchasNetbeansorEclipse. ThefollowingaretutorialsforrunningJavaappletsoneitherIDE: Netbeans Eclipse Asanexperiment,settheangleto5.0degreesandnotetheamountoflift. Nowincreasetheangleto10degrees.Didtheliftincreaseordecrease? Increasetheangleagainto15degrees.Whatdoyounoticeintheviewwindow? Settheangleto0degrees.Isthereanylift?Whatdoesthistellyou abouttheshapeoftheairfoil?Findtheangleforwhichthereis nolift. Youcandownloadyourowncopyoftheprogramtorunoff-linebyclickingonthisbutton: Youcanfurtherinvestigatetheeffectofangleofattackandtheother factorsaffectingliftbyusingthe FoilSimIIIJavaApplet. Youcanalso download yourowncopyofFoilSimtoplaywith forfree. Activities: GuidedTours FactorsthatAffectLift: Stabilators: Navigation.. Beginner'sGuideHomePage        +InspectorGeneralHotline +EqualEmploymentOpportunityDataPostedPursuanttotheNoFearAct +Budgets,StrategicPlansandAccountabilityReports +FreedomofInformationAct +ThePresident'sManagementAgenda +NASAPrivacyStatement,Disclaimer, andAccessibilityCertification       Editor:NancyHall NASAOfficial:NancyHall LastUpdated:May132021 + ContactGlenn