Angle of Attack - Code 7700

The angle of attack is the angle between the chord line and the relative wind. AOA Impact on Lift. images. The coefficient of lift is a measure of how much lift ... AngleofAttack Aeronautical Eddiesez: AngleOfAttackisastapleofmilitaryaviationthatseemstohavebeenignoredbymuchofthecivilianworld.Toobad,AOAcansaveyourlifewhenthechipsaredown.Mostpilotsknowthattheirwingsstallataparticularangleofattack,dependingonflapandotherhigh-liftdevicesettings.ButyearsofsimulatortraininghasputthefocusonairspeedwhichisonlyabyproductofAOA. WhatfollowsisadiscussionofwhatAOAactuallyis,howitaffectsthecoefficientoflift,andwhathappenstoairflowoverthewingasitisincreasedtoacriticalvalue.Thenwe'llseehowtheAOAindicationsystemworksonaT-38andaG450.YoumightenjoyEverybodyHasanAngle(ofAttack),astoryabouthowallthiswastrainedtoAirForcepilotsmanyyearsago. Photo:Wingsweepeffectonliftcurves(fromEddie'snotes) Clickphotoforalargerimage Everythinghereisfromthereferencesshownbelow,withafewcommentsinanalternatecolor. Lastrevision: 20150318 TheAerodynamicsofAOA Photo:AngleofAttack(fromEddie'snotes) Clickphotoforalargerimage Asanairfoilcutsthroughtherelativewind,anaerodynamicforceisproduced.Thisforcecanbebrokendownintotwocomponents,liftanddrag.Theliftproducedbyanairfoilisthenetforceproducedperpendiculartotherelativewind.Thedragincurredbyanairfoilisthenetforceproducedparalleltotherelativewind. Theangleofattackistheanglebetweenthechordlineandtherelativewind. AOAImpactonLift Thecoefficientofliftisameasureofhowmuchliftthewingcanproduceandcanonlybechangedbychangingtheshapeofthewingortheangleofattackatwhichitcutsthroughtherelativewind.Wecanchangetheshapeofthewingusingflaps,slats,orothersimilarleadingandtrailingedgedevices.Wechangetheangleofattackusingourflightcontrolsand,insomecases,powersettings. Foranygivenwing,anincreaseinangleofattackleadstoanincreaseinthecoefficientofliftupuntilthepointitdoesn't.Inthegraphshown,weseethatatypicalcamberedwingproducesahighercoefficientofliftbutwhenitgetstoitscriticalangleofattack,theliftdropsoffquickly.Asymmetricalwing,ontheotherhand,produceslesstotalliftbutwhenitdropsoff,itdropsoffslowly. [Dole,pg.36]TheimportanceofAOAindeterminingaircraftperformancecannotbeoveremphasized.WehavediscussedstallAOA,butthesefactsareofequalimportance:anaircrafthasitsmaximumclimbangleatacertainAOA,willachievemaximumrateofclimbatanotherAOA,andwillgetmaximumrangeatstillanotherAOA. BoundaryLayer Figure:DevelopmentofaBoundaryLayeronaSmoothFlatPlate,from[ATCM51-3,pg.53]. Tounderstandairfoilperformanceathighanglesofattack,onemustfirstconsidertheairflowatjustaboutanyangleofattack.Whenanairfoilpassesthroughanairstream,theparticlesofairrightnexttotheskinoftheairfoilarepulledalongatthesamespeedoftheairfoil.Asyougetfurtherawayfromtheskinoftheairfoil,theparticlesarelessaptto"grabon"tothewingandatacertaindistancetheydonot"grabon"atall.Thislayerofair,theparticlesthatgrabontothewingcompletelytothosethatdon't,isknownastheboundarylayer. Thebehavioroftheboundarylayerdetermines,ingreatmeasure,themaximumliftcoefficientandthestallingcharacteristicsoftheairfoil. [Dole,pg.39]Thebeginningofairflowattheleadingedgeofasmoothairfoilsurfaceproducesaverythinlayerofsmoothairflow.Thistypeofairflowiscalledlaminarflowandischaracterizedbysmoothregularstreamlines.Fluidparticlesinthisregiondonotintermingle.Astheairflowmovesbackfromtheleadingedge,theboundarylayerthickensandbecomesunstable.Smallpressuredisturbancecausetheunstableairflowtotumble,andintermixingoftheairparticlestakesplaces.Thistypeofairflowiscalledturbulentflow. AdversePressureGradient Aswesawinourdiscussionoflift,thepressureoverandunderthewingdecreasesasthevelocityoftheairincreases.Thepressuredifferentialoverthetopandunderthebottomofthewinggeneratetheaerodynamicforcethatbecomesliftanddrag. Thepointofminimumpressuredividestheairfoilintotwo.Forwardofthatpointthepressuregradientishelpingtoproduceliftand,toalesserdegree,apullingforceforward.Aftofthatpointwehavewhatiscalledtheadversepressuregradient.Thepressureherecontributestodragandaseparationoftheairflowingoverthewing.Moreonthatbelow. Astheangleofattackisincreased,thepointofminimumpressuremovesforwardandthesizeoftheadversepressuregradientincreases.Threethingshappenasaresult: Theliftcomponentofaerodynamicforceincreases,uptoapoint. Thedragcomponentofaerodynamicforceincreases. Theturbulentflowareaincreases,encouragingseparationoftheboundarylayer. AirflowSeparation Thefrictionalongtheairfoiltendstoreducethevelocityoftheairparticlesrightnexttothesurfacetozero,formingathinboundarylayerofair.Theadversepressuregradienttendstoexpandtheboundarylayertothepointtheairparticlesseparateandareneitherflowingwiththerelativewindorstickingtotheairfoil. [Dole,pg.43]Theresultoftheairslowingcreatesastagnatedregionclosetothesurfaceoftheobject.Airflowfromoutsidetheboundarylayerwilloverrunthepointofstagnationandcausethetheboundarylayertoseparatefromthesurface.Aflowreversalresults,andtheairflowmovesforward,liftisdestroyed,anddragbecomesexcessivelyhigh. Stall Theairflowseparationcanhappenunderconditionsofslowspeedflight,high-Gmaneuvering,orhighspeedshockwaves.AllthreewereofconcerntousinthesupersonicT-38,thefirsttwoareconcernsforalljetaircraft.MoreaboutthisinthesectionsLowSpeedFlight,andOperatingFlightStrength(V-g/V-nDiagrams). WhileweareonthesubjectofAOA,however,itmaybehelpfultolookattwoaircraftsystemsfordetectinganddisplayingAOAtothepilot. ExampleAOASystem:T-38 Wearen'tinthebusinessofflyingsupersoniconrazorthinwings,butthisillustrationfromtheT-38manualgivesanexcellentprimeronAOAversusairspeed. [1T-38A-1,pg.4-18]The[ARU-26/AAOA]indicatorpresentsAOAasapercentageofmaximumliftAOA.Thedialiscalibratedinunitesof.1counterclockwisefrom0to1.1.Eachunitrepresentsapproximately10%ofaircraftlift,from0%and0indicationto100%at1.0indication.Threepresetfixedindicesandtwocoloredarcsonthedialindicatethefollowing: .18WhiteIndex—MaximumRange(1-Gflight) .3WhiteIndex—MaximumEndurance(1-Gflight) .6WhiteIndex—OptimumFinalApproachat3-o'clockPosition(1-Gflight) .9to1.0YellowArc-BuffetWarning 1.0to1.1RedArc-StallWarning NormalizedAngleofAttack Gulfstreamcallstheirdisplayedangleofattackvaluetobea"normalizedangleofattack,"whichisjustawaytoconfusethingsabit.ThenormalizedAOAissimplythewayeveryoneelsereferstoAOAonthepilot'smeter:1.0isstall,0.0iszeroG.Theyexplaineditthuslyinamishapreport: Figure:G650pitchlimitindicator,fromNTSBAAR-12/02,figure2. [NTSBAAR-12/03,¶1.1]NormalizedAOAisameasureoftheusableAOArangeofanairplane,withanormalizedAOAof1.0correspondingtothereferencestallAOAinfreeairandanormalizedAOAof0.0correspondingtothezero-liftAOAinfreeair.Thisfigureispresentedforinformationpurposesonlyandisnotintendedtodepicttheflightconditionsonthedayoftheaccident. ExampleAOASystem:G450 NotallmodernaircraftincludeanAOAdisplayforpilotuseinflight,onlyusingtheinformationforstallwarningprotection.TheCL-604,forexample,hasanAOAindicatormarked"Notforuseinflight."TheG450,ontheotherhand,doesincludeanAOAindicatoronthePilot'sFlightDisplay.Themanual,however,issilentonthesubjectof"normalizedAOA."ItappearstobetheirtermconvertingtheAOAindegreestothe1.0scale. Figure:G450NormalizedAOADigitalReadout,from[G450AOM,§2B-05-00,pg.25]. HowtoUseAOA MostcivilianaircraftseemtobesilentonthesubjectofactuallyflyingbyreferencetoanAOAindicator,soIcertainlycan'trecommendyoudothat.ButIdorecommendyoukeepaneyeonitandseewhatitnormallyindicatesforvariousphasesofflight.I'vedonethatfortheG450andalsopresentfindingsforCessnaCitations. GulfstreamG450 TakeoffRotation:0.2to0.3 Cruise(0.80Mach):0.15to0.20 Cruise(LRC):0.30 Pattern:0.40 VREF:0.50 CessnaCitations AreaderwithconsiderableexperiencewiththeentirelineofCessnaCitationsoffersthefollowing: V2≈0.7 VREF,bestglide,VX,maximumendurance≈0.6 MaximumRange≈0.35 Armedwiththisinformation,youcanhaveabackupplaniftheairspeedindicatorsquitorifthereisadisparityyoucannototherwiseexplain. SeeAlso: PortionsofthispagecanbefoundinthebookFlightLessons1:BasicFlight,Chapter12. References: 14CFR25,Title14:AeronauticsandSpace,FederalAviationAdministration,DepartmentofTransportation AirTrainingCommandManual51-3,AerodynamicsforPilots,15November1963 Connolly,ThomasF.,Dommasch,Daniel0.,andSheryby,SydneyS.,AirplaneAerodynamics,PitmanPublishingCorporation,NewYork,NY,1951. Davies,D.P.,HandlingtheBigJets,CivilAviationAuthority,Kingsway,London,1985. Dole,CharlesE.,FlightTheoryandAerodynamics,1981,JohnWiley&Sons,Inc,NewYork,NY,1981. FAA-H-8083-15,InstrumentFlyingHandbook,U.S.DepartmentofTransportation,FlightStandardsService,2001. GulfstreamG450AirplaneFlightManual,Revision35,April18,2013. GulfstreamG450AircraftOperatingManual,Revision35,April30,2013. Hage,RobertE.andPerkins,CourtlandD.,AirplanePeformanceStabilityandControl,JohnWiley&Sons,Inc.,1949. Hurt,H.H.,Jr.,AerodynamicsforNavalAviators,SkyhorsePublishing,Inc.,NewYorkNY,2012. NTSBAircraftAccidentReport,AAR-12/03,CrashDuringExperimentalTestFlight,GulfstreamAerospaceCorporationGVI(G650),N652GD,Roswell,NewMexico,April2,2011 TechnicalOrder1T-38A-1,T-38A/BFlightManual,USAFSeries,1July1978. Learningneverstops AlwaysrememberthatEddie,whenyougetrightdowntoit,isjustapilot.Hetriestogiveyouthefactsfromthesourcematerialsbutmaybehegotitwrong,maybeheisoutofdate.Sure,hewarnsyouwhenheisgivingyouhispersonaltechniques,butyoushouldalwaysfollowyourprimaryguidance(Aircraftmanuals,governmentregulations,etc.)beforelisteningtoEddie. Pleasenote:GulfstreamAerospaceCorporationhasnoaffiliationorconnectionwhatsoeverwiththiswebsite,andGulfstreamdoesnotreview,endorse,orapproveanyofthecontentincludedonthesite.Asaresult,Gulfstreamisnotresponsibleorliableforyouruseofanymaterialsorinformationobtainedfromthissite. Copyright2019.Code7700LLC.AllRightsReserved.