航空专英1教案第5章ForcesinFlight

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Ch.5Stabilityandcontrol5.1Balanceandtrim1,balanceinstraightandlevelflightbalance
see-sawᩨᩚbean᫡woodenbeanᩈ᫡suspendedbean᫡fulcrumtrimequilibrium⊝controlcolumn᫃rudderpedalᔣalign,alignmentᡂFigure2.2-1See-saw.Figure2.2-2UnbalancedFigure2.2-3Rebalanced

1Figure2.2-4RebalancedBalanceconsistsoftwoelements-thetotalforcesactingtheaircraftandthealignmentoftheseforces.Whentheforcesarebalancedandalignedtheaircraftissaidtobeinequilibrium.Therearetwotypesofforces-staticforces,dynamicforces.

2Weightisastaticforceitcanbeconsideredconstantatanytime.Thrustvarieswithenginepower,propellerrpmandairspeedbutcanbesetataconstantvaluebythepilot.Liftisaerodynamicforcewhichchangeswithairspeedandflapextensionbutwhichcanbecontrolleddirectlybythepilotchangingtheangleofattack.Dragchangeswithangleofattack,configurationandairspeed.♦LLift(2,000units)T:hrust(200units)
""brag(200units)WWeight(2,000units)Figure2.2-7Liftbalancesweight.3ndthrustbalancesdrag,instraightandlevelflight.Instraightandlevelflight,liftopposesweightL=WthrustopposesdragT=D.Theliftandweightwillonlydecreasegraduallyastheweightdecreaseswithfuelburn-off.Thethrustanddragwillvaryconsiderablydependingonangleofattackandthereforeairspeed.2,pitchingmoment

3Lift-Nose-downCGCPWeightyLiftAkWeightyFigure2.2-8Lift-weightcoupleproducesapitchingmoment.Nose-upThrustWNose-downFigure2.2-9Thrust-dragcoupleproducesapitchingmomentUndermostconditionsofflighttheCPandCGarenotcoincident,i.e.arenotattheonepoint,causingnose-downpitchingmomentoranose-uppitchingmoment.Thedifferentlinesofactionofthethrustforceanddragforce

4produceanothercouple,causinganose-downpitchingmomentoranose-uppitchingmoment.IdeallythepitchingmomentfromthetwocouplesshouldneutralizeeachotherinlevelflightsothatthereisnoresultantmomenttendingtorotatetheaircraftALiftThrust-dragcouplenose-upLift-weightFigure2.2-10coupleLift-weightcoupleandthrinose-downdragcoupleinequilibrium.Weight▼Figure2.2-11VerticallyoffsetthrustlineALiftThrust-dragThrusl-dragcoupleIcouple(Reduced(nose-upnose-upmoment)moment)▼ThrustLift-weigh!couple(nose-downLift-weightmoment)couple(nose-downWeight▼moment)Figure2.2-12Followingalossofthrust,thelift-weightcouple,

53,thetailplanehorizontalstabilizer☢function!"ᵨcounteract⊝!$%residualᒕ'Ḅ,neutralize*+coincident,ThrustBalancingaerodynamicforceWeight''059EPSFigure2.2-13Thetailplaneprovidesthefinalbalancingmoment.Thefunctionoftailplane(orhorizontalstabilizer)istocounteracttheseresidualpitchingmomentsfromthetwomaincouplesandtodampedanyoscillationinpitch,i.e.ithasastabilizingfunction.Thetailplaneusuallyhasasymmetricaloranegativelycamberedaerofoil.Themomentproducedbythetailplanecanbevariedeitherbymovingtheelevatororbymovingtheentiretailplane.Themomentarmoftailplaneisquitelong,andtheaerodynamicforceprovidedbythetailplaneneedsonlytobesmalltohavea

6significantpitchingeffect.Theareaofthetailplaneissmallcomparedwiththemainplanes(mainwings).5.2StabilityTherearetwoelementsofstability,staticanddynamic,andforanaircraft,itisusualtoseparatethemodesintothethreeaxesofmovement.Thereislongitudinalstability,lateralstabilityanddirectionalstability.Thereisaninseparablerelationshipbetweenlateralanddirectionalstability.5.2.1Staticstabilityanddynamicstability1,StaticstabilityOscillationDisturbedReleasedFigure2.2-14Staticanddynamicstability

7Thestaticstabilityoftheaeroplanedescribesitstendencytoreturnitsoriginalcondition(angleofattack)afterbeingdisturbedandwithoutanyactionbeingtakenbythepilot.Thestrengthofthetendencyisthemeasureofitsstability.2,Dynamicstabilityremove
◀deadbeatᕜḄ₩Ḅindefinitely▲ᙢdivergentᦣshortperiodᕜlongperiod,phugoidᕜDynamicstabilityisconcernedwiththemotionofthebodyafterthedisturbingforcehasbeenremoved.Itisanoscillationwhichmaystopimmediately(well-dampedordeadbeat),continuebutreduceslowly(slightlyorlightlydamped),continueindefinitely(undamped)orgetworse(dynamicallyunstableordivergentoscillation).Imaginetheaircraftistrimmedinstraightandlevelflight,theaircraftwillrespondtotheverticalgustbypitchingdowntomaintainitstrimmedangleofattack.Thisoscillationisgenerallywell-dampedandreducestozeroin1or2oscillation.Thedampingisprovidedbytheaironthehorizontalareaoftheaircraft.Thepitchingoscillationisknownastheshortperiodpitchingoscillation(SPPO).

8Theairspeedoftheaircraftmayalsochangeandthiscancauseaslower(longperiod)oscillationwheretheaircraftleisurely(᠒᠒ᙢ)followsapathwheretheairspeedandaltitudeareexchanged.Theslowmotioniscalledthephugoid.3,thethreereferenceaxesCentreofgravity(CG)Normal(verticalaxis)WAEFSFigure2.2-15Angularmotioncanoccuraboutthreeaxes.Werefer஺)themotionoftheaircrafttomotionabouteachofthreeaxes-eachpassingthroughthecentreofgravityandeachmutuallyperpendicular(at90°toeachother).Thesearesometimescalledbodyaxes.

9Figure2.2-16Rollingaboutthelongitudinalaxis.Figure2.2-17Pitchingaboutthelateralaxis.

10YawingplaneofmotionFigure2.2-18Yawingaboutthenormalaxis.Stabilityaroundthelongitudinalaxisisknownaslateralstability.Stabilityaroundthelateralaxisisknownaslongitudinalstability.Stabilityaroundthenormalaxisisknownasdirectionalstability.Rotationaroundapointoraxisiscalledangularmotion;thenumberofdegreesofrotationiscalledangulardisplacement,andthespeedwithwhichitoccurs,angularvelocity.Themotionofanaircraftisbestconsideredineachoftheplanes

11separately,althoughtheactualmotionoftheaircraftisalittlemorecomplex.Forexamplepollingintoalevelturntheaircraftwillnotonlyrollbutalsopitchandyaw.Wewillconsiderlongitudinalstabilityfirst,thendirectionalstabilityandlateralstability.Rollandyawarecloselyconnected.5.2.2LongitudinalstabilityinbuiltᨵḄᑁᙠḄdart╔arrowTobelongitudinalstable,anaircraftmusthaveanaturalorinbuilttendencytoreturntothesameangleofattackafteranydisturbancewithoutanycontrolinputbythepilot.Iftheangleofattackissuddenlyincreasedbyadisturbance,thenforcewillbeproducedthatwilllowerthenoseanddecreasetheangleofattack.1,thetailplaneandlongitudinalstabilityRestoringNose-downaerodynamicpitchingmomentFigure2.2-20Longitudinalstabilityfollowingan'uninvited'nose-uppitch.

12GustNose5downFigure2.2-21Longitudinalstabilityfollowingan,unmvited,nose-downpitch.Ifadisturbance,suchasagust,changestheattitudeoftheaircraftbypitchingitnoseup,thetailplanewillbepresentedtotherelativeairflowatagreaterangleofattack.Thiswillcausethetailplanetoproduceupward,ordecreased,aerodynamicforce,whichisdifferenttothatbeforethedisturbance.Thealteredaerodynamicforcegivesanose-downpitchingmoment,tendingtoreturntheaeroplanetoitsoriginaltrimmedcondition.Example:thetailfinofadartoranarrow.Figure2222longitudinalstabilityisprovidedbythe(ailfinsofadan2,theCGandlongitudinalstability

13Figure2.2-23AtforwardCG-greaterlongitudinalstability-longermomentarm.ThefurtherforwardtheCGoftheaircraft,thegreaterthemomentarmforthetailplane,andthereforethegreatertheturningeffectofthetailplaneliftforce.AforwardCGleadstoincreasedlongitudinalstabilityandaftmovementoftheCGleadstoreducedlongitudinalstability.Themorestabletheairplane,thegreaterthecontrolforcethatyoumustexerttocontrolormovetheairplaneinmanoeuvers,whichcanbecometiring.Thetailplaneprovidesstaticlongitudinalstability.3,designconsiderationTailplanedesignfeaturesalsocontributegreatlytolongitudinalstability-tailplanearea,distancefromthecentreofgravity,aspectratio,angleofincidenceandlongitudinaldihedralareconsideredbythedesigner.Athighangleofattackthemainplanemayshieldthetailplaneorcausetheairflowoverittobeturbulent.Thiswilldecreaselongitudinalstability.

145.2.3DirectionalstabilityDirectionalstabilityofanaeroplaneisitsnaturalorinbuiltabilitytorecoverfromadisturbanceinyawingplanewithoutanycontrolinputbythepilotIftheaircraftisdisturbedfromitsstraightpathbythenoseortailbeingpushedtooneside(i.e.yaw).Theverticalfin(ortailorverticalstabilizer)issimplyasymmetricalaerofoil.Asitisnowexperiencinganangleofattack,itwillgenerateasidewaysaerodynamicforcewhichtendstotakethefinbacktoitsoriginalposition.UninvitedyawFigure2.2*24DirectionalstabilityfollowingaskidThepowerfulmoment(turningeffect)oftheverticalfin,duetoitslargeareaandthelengthofitsmomentarmbetweenitandcentreofgravity,iswhatrestoresthenosetoitsoriginalposition.ThegreaterthefinareaandkeelsurfaceareabehindtheCG,and

15thegreaterthemomentarm,thegreaterthedirectionalstabilityoftheaeroplane.Thefinprovidesdirectionalstaticstability.Astheyawcausesrollingmomentsothatbehavioroftheaircraftwithyawandsideslipinvolvesbothitsdirectionalstabilityanditslateralstability.5.2.4LateralstabilityLateralstabilityisthenaturalorinbuiltabilityoftheaeroplanetorecoverfromadisturbanceinthelateralplane,i.e.rollingaboutthelongitudinalaxiswithoutanycontrolinputbythepilot.Adisturbanceinrollwillcauseonewingtodropandtheothertorise.Whentheaeroplaneisbanked,theliftvectorisinclinedandproducesasideslipintotheturn.Asaresultofthissideslip,theaeroplaneissubjectedtoasidewayscomponentofrelativeairflow.Thisgeneratesforcesthatproducesarollingmomenttorestoretheaeroplanetoitsoriginal

16wings-levelposition.1,wingdihedralFigure2.2-27Positivedihedralcorrectsuninvitedbank.Eachwingisinclinedupwardsfromthefuselagetothewingtip,andaddstothelateralstabilitycharacteristicsoftheaeroplane.Positivewingdihedralincreaseslateralstability.

17Astheaircraftsideslips,thelowerwing,duetoitsdihedral,willmeettheupcomingrelativeairflowatagreaterangleofattackandwillproduceincreasedlift.Theupperwingwillmeettherelativeairflowatalowerangleofattackandwillthereforeproducelesslift.Itmaybeshieldedsomewhatbythefuselage,causinganevenlowerlifttobegenerated.Therollingmomentsoproducedwilltendtoreturntheaircrafttoitsoriginalwings-levelposition.Figure2.2-28Anhedralonahigh-mountedsweptwing.Negativedihedral,oranhedralhasadestabilizingeffect.Insomeaircraftwithahigh-mountedsweepwing,anhedralisusedtocompensateforexcessivelateralstability.

182,wingsweepbackThewingcanaddtolateralstabilityifithassweepback.Astheaircraftsideslipsfollowingadisturbanceinroll,thelowersweepbackwinggeneratesmoreliftthantheupperwing.Thisisbecauseinthesideslipthelowerwingpresentsmoreofitsspantotheairflowandhighervelocitythantheupperwingandthereforethelowerwinggeneratesmoreliftandtendstorestoretheaeroplanetoawing-levelposition.3,highkeelsurfacesandlowCG

19Figure1-31(RighUFinandrudderFigure1-40KeelsurfaceareachangeswithCGposition.

20Inthesideslipthatfollowsadisturbanceinroll,ahighsidewaysdraglinecausedbyhighkeelsurfaces(highfin,aT-tailhighonthefin,highwings,etc.)andalowCGwillgivearestoringmomenttendingtoraisethelowerwingandreturntheaircrafttotheoriginalwings-levelposition.Figure2.2-30HighkeelsurfacesandalowCGcorrectuninvitedbank.4,high-wingaroplaneIfagustcausesawingtodrop,theliftforceistilted.Theresultantforceswillcausetheaircrafttosideslip.Theairflowstrikingtheupperkeelsurfaceswilltendtoreturntheaircrafttothewings-levelcondition.Ahigh-wingaroplaneincreaseslateralstability,ithaslessdihedral

21comparedtoamid-or-lowwingdesign.Figure2.2-31Pendulumstabilitytends5,lateralanddirectionalstabilitytogether1)rollfollowedbyyaw!"#$%Forlateralstability,itisessentialtohavethesideslipthatthedisturbanceinrollcauses.Figure2.2-32Rollcausessideslip,whichcausesyaw.Thesideslipexertsaforceonthesideorkeelsurfacesofthe

22aircraft,which,iftheaircraftisdirectionallystable,willcauseittoyawitsnoseintotherelativeairflow.Therollhascausedayawinthedirectionofthesideslipandtheaeroplanewillturnfurtheroffitsoriginalheadinginthedirectionofthelowerwing.Thelateralstabilitycharacteristicsoftheaeroplane,suchasdihedral,causethelowerwingtoproduceincreasedliftandtoreturntheaircrafttothewings-levelposition.Therearetwoeffectsinconflicthere:Thedirectionallystablecharacteristics(largefin)wanttosteepentheturnanddropthenosefurther.Thelaterallystablecharacteristics(dihedral)wanttolevelthewing.spiralmode&'(᝱Dutchroll₹ᐲright,-best᩽/wallow᤯᤮Ifthefirsteffectwinsout,i.e.strongdirectionalstabilityandweaklateralstability(largefinandnodihedral),thentheaircraftwilltendtobankfurtherintothesideslip,towardsthelowerwingwithnosecontinuingtodrop,untiltheaeroplaneisinaspiraldive.Thisiscalledspiralinstability,orthespiralmode.Ifthelateralstability(dihedral)isstronger,theaircraftwillrightitselftowings-level,andifthedirectionalstabilityisweak(smallfin)theaircraftmayshownotendencytoturninthedirectionof

23sideslip,andcausingthewallowingeffect,Dutchroll,whichisbestavoided.2)yawfollowedbyroll$%"#!Figure2.233YawcausesrollandsideslipandfurtherrollIftheaircraftisdisplacedinyaw,itiscancausesideslip.Thissideslipwillcausethelateralstabilitycharacteristicsoftheaircraft'swing,suchasdihedral,sweepbackorhigh-wing.Thiscausesarollingmomentthatwilltendtoraisetheforwardwing,resultingintheaircraftrollingtowardsthetrailingwingandawayfromthesideslip.Theaircraft'sinherentdirectionalstability(fromthefin)willtendtoweathercockoryawtheaircraftinthedirectionofsideslip.3)stabilitycharacteristicsandaeroplanecontrolIfthedirectionalstabilityispoor(smallfin)andthelateralstabilityisgood(dihedral)itcancauseDutchroll(rolling/yawing

24oscillation).Oftentheaircraftisfittedwithayawand/orrolldamper(asmallcontrolsurfacedrivenbyarategyro)tostoptheoscillation.Itisuncomfortableforthepilotandpassengers.Ifthedirectionalstabilityisdominant(largefin)andthelateralstabilitynotsostrong,itcancausespiralinstabilityorspiralmode.rategyro23▨&dominantᓰ67ᓰ89ᙢ:6,Stabilityonthegroundtipover;᎔taxing=>groundloopᙢ!skid?!brakeᑷAwheelABrunwayC⍝Thecentreofgravity(CG)mustliesomewhereintheareabetweenthewheelsatalltimesontheground,otherwisetheCentreofgravityFigure2.2-34TheCGmustremainwiihintheareaboundedbythewheels.aeroplnewilltipover-forwardsorbackwards.

25TrackApplyleftrudderWeathercockingtocounleraclthistendencycrosswindandmaintainthedesiredtrackalongtherunway.WindRudderdeflectedu/tocounteractyawFigure2.2-35Weathercocking.5.3ControlThecontrolsurfacesarethemeansbywhichthepilotovercomethestaticstabilityoftheaircraftandcausesachangeinflightpathorachangeintrimmedconditions.Figure2.2-36Theprimarycontrolsurfaces-elevator,aileronsandrudder.Usuallytherearethreesetsofprimarycontrolsystemandthreesetsofcontrolsurfaces:

26•theelevatorforlongitudinalcontrolandbalanceinpitch,operatedbyforeandaftmovementofthecontrolwheelorcolumn;•theaileronsforlateralcontrolandbalanceinroll,operatedbyrotationofthecontrolwheelorsidewaysmovementofthecontrolcolumn;•therudderfordirectionalcontrolandbalanceinyaw,operatedbytherudderpedals.Ideallyeachsetofcontrolsurfacesshouldproduceamomentaboutonlyoneaxisbut,inpractice,momentsaboutotheraxesareoftenproducedaswell,e.g.ailerondeflectiontostartarollmayalsocauseadverseyaw.Thedeflectionofthecontrolsurfaceschangestheairflowandthepressuredistributionoverthewholeaerofoilandnotjustoverthecontrolsurfaceitself.Theeffectistochangetheliftproducedbythetotalaerofoil@controlsurfacecombination.Anaeroplanewithtoomuchstabilitydesignedintoithaslimitedcontrollability.Thedesignermustachieveareasonablebalancebetweenstabilityandcontrollability.Forinstance,apassengeraircraftwouldrequiremorestability,

27whereasafighterwouldbenefitfromgreatercontrollabilityandmanoeuvrability.5.3.1Pitchcontrol1,ElevatorUpwardaerodynamic,ControlcolumnControlcolumnforce/—jbackUpforward/TelevatorNoseNose%_~^75^downDownelevatorDownwarda029AFigure2.2-37TheelevatoristheprimarypitchingcontrolTheprimarycontrolofangleofattackistheelevator.Thepilotmovestheelevatorbyfore-and-aftmovementofthecontrolcolumn.Whenthecontrolcolumnismovedforward,theelevatorsmovedownwards,changingtheoverallshapeofthetailplane-elevatoraerofoilsectionsothatitprovidesanalteredaerodynamicforce.TheeffectistocreateapitchingmomentabouttheCGoftheaircraftthatmovesthenosedown.Whenthecontrolcolumnispulledback,theelevatormovesupandanalteredforceisproducedbytailplane-elevatoraerofoil,causingthenoseoftheaircrafttopitchup.Thestrengthofthetailmomentdependsontheforceitproduces

28andthelengthofthearmbetweenitandtheCG.Theforcegeneratedbythetailplane-elevatorcombinationdependsontheirrelativesizeandshape,thetailplanebasicallycontributingtostabilityandtheelevatortocontrol.Thelargertherelativesizeoftheelevator,themorethecontrol.Toretainsatisfactoryhandlingcharacteristicsandelevatoreffectivenessthroughoutthedesiredspeedrange,thepositionoftheCGmustbekeptwithintheprescribedrange.TheforwardallowablelimitoftheCGisdeterminedbytheamountofpitchcontrolavailablefromtheelevator.TheaftlimitoftheCGisdeterminedbytherequirementofadequatelongitudinalstability.Steadyflightatalowspeedandahighangleofattackwillrequiresignificantup-elevator,andbackwardpressureonthecontrolcolumn,tokeepthenoseup.

29Steadyelevatordeflectionatdifferentspeeds.ஹAtahighcruisespeedtherewillneedtobeasteadydowndeflectionoftheelevatortokeepthenosedownandmaintainalowangleofattack,henceasteadyforwardpressureonthecontrolcolumn.2,Thestabilatororall-flyingtailFigure2.2-40Separatetailplaneplusmovingelevator(left),andstabilatororall-flymgtail(right).Somedesignerschoosetocombinethetailplaneandelevatorintotheonesurfaceandhavethewholetail-planemovable-knownastheallmovingtail,theflyingtailortheslabtail.

30Whenthecontrolcolumnismovedtheentire'slab'moves.5.3.2Rollcontrol1,AileronsTheprimarycontrolinrollistheailerons.Theaileronsareusuallypositionedontheoutboardtrailingedgeofthemainplanes.Theaileronsactinopposingsenses,onegoesupastheothergoesdown,sothattheliftgeneratedbyonewingincreasesandtheliftgeneratedbytheotherwingdecreases.Aresultantrollingmomentisexertedontheaeroplane.Themagnitudeofthisrollingmomentdependsonthemomentarmandthemagnitudeofthedifferingliftforces.•Thedowngoingaileronisontheupgoingwing.•Theupgoingaileronisonthedowngoingwing.2,AdverseaileronyawDeflectinganailerondowncausesaneffectiveincreaseincamberofthatwingandanincreaseintheeffectiveangleofattack.Thelift

31fromthatwingincreases,butunfortunatelysodoesthedrag.Astheotheraileronrises,theeffectivecamberofthatwingisdecreasedanditsangleofattackisless,thereforeliftfromthatwingdecreases,asdoesthedrag.RollrightSameamountofailerondeflectionRearviewFigure2.2-42Downwardaileronhasincreaseddrag-adverseyaw.Thedifferingliftforcecausestheaircrafttobankoneway,butthedifferentialailerondragcausesittoyawtheotherway./'◞I'.^jIncreaseddeflectionofupgoingaileronDifferentialaileronsFigure2.2-43Differentialaileronsequaliseailerondrag,reducingadverseyaw.Adverseaileronyawcanbereducedbygooddesignincorporating

32differentialailerons,Friseailerons,orcouplingtheruddertotheailerons.LiftFigure2.2-44Frise-typeaileronsequaliseailerondragandreduceadverseyawDifferentialaileronsGHIJaredesignedtominimizeadverseaileronyawbyincreasingthedragonthedowngoingwingontheinsideoftheturn.Thisisachievedbydeflectingtheupwardaileronthroughagreateranglethanthedownwardaileron.Friseaileronsincreasethedragofthedescendingwingontheinsideoftheturn.Astheailerongoesup,itsnoseprotrudesintotheairstreambeneaththewingcausingincreaseddragonthedowngoingwing.Ontheotherway,thewingisrising,thenoseofthedowngoingailerondoesnotprotrudeintotheairstream,socausenoextradrag.Frise-typeaileronsmayalsobedesignedtooperatedifferentially,toincorporatethebenefitofdifferentialailerons.•Coupledaileronsandruddercausetheruddertomoveautomaticallyandyawtheaeroplaneintobank,opposingtheadverseyawfromtheailerons.

33Theprimaryeffectofrudderistoyawtheaeroplane,andthesecondaryeffectistorollit.Theprimaryeffectofaileronsistorolltheaeroplane,andthesecondaryeffectistoyawit.3,Rollisfollowedbyyaw!"#$%Thesecondaryeffectofaileronsistocauseyaw.Whentheaeroplaneisbankedusingtheailerons,theaeroplanewillslip.Asaresultofthesideslip,theairflowwillstrikethesideoftheaeroplaneandthelargekeelsurface(rearfuselageandfin),whicharemainlybehindtheCG,causethenoseoftheaeroplanetoyawinthedirectionofbank.

34Figure2.2-46Rollisfollowedbyyaw.5.3.3Yawcontrol1,RudderFigure2.2-55Leftrudderpressure-noseyawsleft.Theprimarycontrolintheyawingistherudder.Therudderishingedtotherearofthefin(orverticalstabilizer).Itiscontrolledfromthecockpitbytherudderpedalstotherudderbar.Bypushingtheleftpedal,therudderwillmoveleft.Thisaltersthefin-rudderaerofoilsection,andsidewaysliftiscreatedthatsendsthetailtotherightandyawstheaeroplanetotheleftaboutthe

35normalaxis.Withleftrudderapplied,theaeroplaneyawsleft,hinge67!cockpit8‧2,Yawisfollowedbyroll:;<=>?Thesecondaryeffectofrudderisroll.Theprimaryeffectofrudderistoyawtheaeroplane.Havingyawedtheaeroplane,thefurthereffectofrudderistocausearoll.3,SlipstreameffectAstheslipstreamcorkscrews@ABCDEFaroundthefuselage,itstrikesonesideofthefln/rudderatadifferentangletotheother.Leftyawtendency/------*<.■5.ClockwiseRightrudderrequiredtobalanceslipstreameffectFigure2.2-57Theslipstreamstrikesonesideoftherudder.Iftheslipstreamoverthefinandrudderchanges,thentherudder

36deflectionmustbechangedtobalanceit.4,Rudderincrosswindtake-offandlandingGHI=J'Ḽ▮$Ḅᔣtake-off=J!landingḼ▮!approachEM!crab:;!crab-wise:;ᔣ!touchdown7ᙢ!OᙢcrosswindHIFigure2.2-58Crosswindtake-offIngroundoperation,anycrosswindwillhitthesideoffinandtendtoweathercocktheaircraftintowind.Theruddermustbeusedtostoptheaircraftyawingintowindandkeepittrackingstraightalongtherunway.

37WindAligntheaeroplanewiththerunwaycentrelineusingtherudderjustpriortotouchdownWind0B3AFCSFigure2.2-59Crosswindlanding.Onapproachtoland,themostcommontechniqueistocrabtheaircraftintowindsothatitisflyinginbalance(i.e.directlyintotherelativewindandwiththerudderballcentered)andtrackingsomewhat'crab-wise'alongextendedcenterlineoftherunway.Justpriortotouchdowntheaircraftisyawedwiththerudder,sothatwhenthewheelstouch,theyarealignedinthedirectionoftherunway.

38Anothertechniqueinacrosswindlandingisthesideslippingapproach.Neartheground,youwouldyawtheaeroplanestraight(withtherudder)sothatitisalignedwiththecenterline.Unlessthewheelstouchalmostimmediately,thewindwillcausetheaeroplanetodrifttowardsthesideoftherunway.Toavoidthis,youwouldlower(usingtheailerons)theinto-windwingsufficientlytostoptheaircraftdriftingoffthecenterlinepriortotouchdown.Theaeroplaneisnowsideslippingandflyalitterbitoutofbalance.5,RudderpowerWhiletheruddermustbesufficientlypowerfultohandletheaboverequirementssatisfactorily,itmustnotbetoopowerful.Givenmaximumdeflectionbythepilotitshouldnotcausestructuraldamage.

395.4OthercontroldevicesOtherscontroldevicesinclude:•modifiedailerondesignPQRSTU•spoilers᡾Wᱏ!YZ[•slots\•leadingedgestripsD]ᩩspeedbrakes_`[vortexgeneratorsaWbcᘤ•strakesH[•tabse᦮ᱏ1,ModifiedailerondesignandspoilersToreduceadverseaileronyaw,thereareseveraltechniquesemployed:Figure2.2-47Leftailerondown.Figure2.2-48Rightaileronupfurtherthandown.•differentialaileronsFriseaileronsspoilers,whichareplate-likesurfacethatareraisedfromtheupper

40surfaceofthewingtoincreasedragontheside.Theymaybeusedinconjunctionwiththeraisedaileronorinsteadofit.Thustheailerongoesdownononesideandthespoilerisraisedontheother.aileron/rudderinterconnect•manualrudderinput2,SlotsTheyareusuallyincorporatedwithinthewingleadingedgeaheadoftheailerontoretainaileroneffectivenessevenatthepointofstallMostarefixedandexposedtotheairflowathighangleofattack.Somearecreatedbyamovableplatewhichpopsout@ghFathighangleofattackduetothereducedstatepressure.3,Leadingedgestrips/slatsFigure2.2-49Leading-edgestrip.Stripsandothershapes,includingrope@ijF,areaddtotheleadingedgeofthewingtoencourageattachmentoftheairflowathighangleofattack.Inthisway,theaileronsremaineffective.4,Speedbrakes

41Figure2.2-50Speedbrake(retracted)onaMooneyaircraft.Speedbrakesareflatplatesurfacewhichslideoutorhingeupwardfromtheuppersurfaceofthewing.Someaircraftshavecombinedspoilersandspeedbrakes.5,VortexgeneratorsVortexgenerators(VGs)arefinger-likeprotrusionsontheuppersurfaceofthewing.Theyaresmallplatessetatanangletotheflowwhichgenerateasmallvortex.Theymayevenbeaerofoilshaped.Figure2.2-51VortexgeneratorsTheintentistousethevortextostirtheboundarylayerandpromoteanearliertransitiontoturbulentmixedflow.Turbulentflowremainsattachedforagreaterdistanceovertheaerofoil,and

42turbulentattachedflowisbetterthanturbulentseparatedflow.TheVGsstirtheboundarylayerandeffectivelyreducethehighdragofseparatedflow.Figure2.2-52VentralfinonaRobin2160.6,StrakesQuitecommononaircraftareextensionstolowerleadingedgeoffintoincreasethechordandstrengthofthefin(dorsalfin),underthefuselage(ventralfin).Figure2.2-53StrakesunderaTBM700.Allofthesedesignadjustmentsaremadetoimprovethecontrolandstabilityatvariouspartsoftheflightenvelope.

43dorsalkdorsalfinlm!ventralfinῳm7,Wingtipshaping,fences,endplatesandwingletsQoCp!Qᑐ!r[%QosQThesedevicesimproveliftandreduceinduceddragbyreducingthespanwiseflowoftheair.Figure2.2-54WingtiptankonaCessna310.8,Tabs1)fixedtabte᦮ᱏAfixedtabisasmallmetaltabthatcanbendtoasetpositiononthegroundonly.Itisfixedinflight.Itisusualontheaileronsandruddersofsmallaircraft.2)trimtabse᦮ᱏ

44Figure2.2-66Elevatortrimtab.Anaircraftisintriminpitch,rolloryaw,whenitmaintainsasteadyattitudewithoutthepilothavingtoexertanypressureonthecontrolcolumn.Thefunctionofthetrimtabistoreducethecontrolsurfaceforcetozeroforthatconditionofflight,sothattheaeroplanewillmaintainit'hands-off'.Inmostlightaircrafttrimsystemaremechanicallyoperatedbyatrimwheel.3)balancetab⊡ve᦮ᱏ1.Backpressureoncontrol,3.Balancetabcolumn஻goesdownMainaerodynamic4.Createssmallaerodynamicforcefromhorizontalforcewhichhelpsholdstabiliserandelevatorelevatorup«•“”|Figure2.2-67ThebalancetabOnconventionaltailplaneitisquitecommontohaveabalancetabincorporatedaspartoftheelevator.

45Ifthepilotexertsbackpressureonthecontrolcolumn,theelevatorisraisedandthebalancetabgoesdown.Theelevatorbalancetabunitgeneratesasmallupwardaerodynamicforcethatactstoholdtheelevatorup,therebyreducingthecontrolloadrequiredofthepilot.Thebalancetabactsautomaticallyastheelevatormoves.4)anti-balancetab}⊡ve᦮ᱏFigure2.2-68Anti-balancetab.Theanti-balancetabmovesinthesamedirectionasthetailplanebutmovesfurther.Thedeflectionatthetrailingedgeofthesurfaceincreasesthestickforce.Theincreasedload,thatthepilothastoapply,tomanoeurvetheaircraft,preventsoverstressingthestructure.

465)servotab~e᦮ᱏAservotabisvariationofthebalancetabwherethepilotcontrolisconnected,nottothemaincontrolsurface,buttothetab.Asthecontrolinputmovestheservotabintotheairflow,theaerodynamicforcesgenerateddrivethemaincontrolsurfaceintheoppositedirection,causingthedesiredmanoeurve.Servotabswereusedonlargetransportaircraftastheforcerequiredtomovethesurfacewasbeyondthereasonablestrengthofthepilot.5.5Stabilityversuscontrolversus...trade-offᢚ⊭!ᩗ⊝hand-off!!1,StabilityversuscontrolDonotconfusestabilitywithcontrollabilityStabilityisthetendencyoftheaeroplanetoreturnitsoriginalconditionafterbeingdisturbedandwithoutanyactionbeingtakenbythepilot.

47Controllabilityreferstotheeasewithwhichthepilotcanmanoeuvretheaircraftandhenceovercomethestability.Thereisasignificanttrade-offbetweenstabilityandcontrollability.Ahighdegreeofstabilitymakestheaircraftresistanttochangeandtherebytendstoreducethecontrollabilityi.e.goodstabilitymakesitharderforthepilottocontrolandmanoeuvretheaeroplane.Anaeroplaneisinastateofequilibriumwhenthesumofalltheforcesonitiszeroandthesumofalltheturningmomentsonitiszero.Theaircraftisintrimifallthemomentsinpitch,rollandyawarezero.Anunstableaircraftisdifficulttoflybecausethepilotmustcontinuallyinterferebyapplyingcontrolforces.Astableaircraftcanalmostfly“hand-off'andrequireonlyguidanceGᑴ9Jratherthansecond-to-secondGᶌMNOUOU
controlinputs.2,ControlresponsePQRSThesizeandshapeofthecontrolsurfaceanditsmomentaboutthecentreofgravityareofgreatimportanceinitseffectiveness.SincethesizeandshapearefixedbythedesignerandCGonlymovessmalldistances,thesecanbeconsideredconstant.Thevariablesincontroleffectivenessareairspeedandcontrolsurface

48deflectionangle(V,5).Theaerodynamicforcesvarywiththedynamicpressure(l/2pV2).veryeffectivenotveryeffective84“sFigure2.2-61Controlsaremoreeffectivewithincreasedairflow.Doublingtheairspeedquadruplestheeffectofthesamecontrolsurfacedeflection.Atlowairspeed,achievingadesiredchangeinattituderequiresamuchgreatcontrolsurfacedeflection,athigherairspeed,controlsaremoreeffective.1)slipstreamincreasesrudderandtailplaneresponseAtlowairspeed,butwithhighpowerset,theslipstreammayflowstronglyoverthetailsection,makingtheelevatorandtheruddermoreeffectivethanatthesamespeedwithnopoweron.Theaileronsarenotaffectedbytheslipstream.oeSAEPSFigure2.2-62Propwash(slipstream).2)controlloadsfeltbythepilot

49Hingemomentatthecontrolsurface.Whenacontrolsurfaceisdeflected,theaerodynamicforcesproducedbythemovingcontrolsurfaceitselfopposesitsdeflection.Thiscausesamomenttoactonthecontrolsurfaceaboutitshingelinetryingtoreturnthecontrolsurfacetoitsoriginalfairedposition,andthepilotmustovercomethistomaintainthedesiredposition.Thepilotfeelsthisasstickforce.Figure2.2-64Insethingebalance(left)andhornbalance(right)aerodynamicbalancevoverbalancevinsethingeᑁ6hornbalance⊡vbalancetab⊡ve᦮ᱏstickforceᩚZAnaerodynamicbalanceonacontrolreducesstickloadonthepilot.Thedesignerprovidesaninsethinge,ahornbalanceorabalancetabtousetheaerodynamicforcesproducedbythedeflectedcontrol

50surfacetopartiallybalanceorreducethemoment,i.e.aerodynamicbalanceofacontrolsurfaceisdesignedtoreducethecontrolforcesrequiredfromthepilot.Thedesigner,however,mustbecarefulnottooverbalancethecontrols,otherwisethepilotwillloseallsenseoffeel.3,Massbalancing⊝@FCentreofgravityFigure2.2-65Externalmassbalancetoavoidcontrolflutter.Amassbalancepreventsflutter.Ahighspeedcontrolsurfaceshaveatendencytoflutter.Flutterisavibrationoroscillationthatresultsfromthechangesinpressuredistributionoverthesurfaceasitsangleofattackisalteredandwhenthecentreofgravityisbehindthehingeline.Toavoidthistendencytoflutter,thedesignerneedstoalterthemassdistributionofthesurfacetobalancethesurfacesothatthecentreofgravityison,orclosetothehingeline.Oftencontrolsurfacesarefabric@4Fcoveredonanotherwiseall-metalaircrafttoreducemassandhenceflutter.4,Summaryofcontrol

51SummaryofControlsTheprimaryaerodynamiccontrolsaretheelevator,aileronsandrudder.Theotherpri@marycontrolisthethrottleorthrustlevers.ControlPlaneAxisDirectEffectIndirectEffectelevatorpitchlateralpitchattitude/angleofattackairspeed/altitudechangeaileronsrolllongitudinalrollyawrudderyawnormalyawrollthrottlepitch/yawlateral/normalrate-ofclimb/descent/airspeedflightpathchangepitch/yawairspeed/altitudeFigure2.2-70Aileroncontrolsystem

52Figure2.2-71Typicallightaircraftflightcontrolsystems.