The`tumble'departuremodeinweightshift-controlledmicrolightaircraftGGra - PDF document

The`tumble'departuremodeinweightshift-controlledmicrolightaircraftGGratton1andSNewman2*1BritishMicrolightAircraftAssociation,Banbury,Oxfordshire,UK2DepartmentofAerospaceEngineering,UniversityofSouthampton,UKAbstract:Thecostofprivateorrecreational¯yingishighformostconventionalaircrafttypes.Duringthelast25years,however,analternativehasbecomeavailableintheformof VDF¯ighttestairspeedlimit(velocitydesign¯ighttest)VNEnormaloperationalairspeedlimit(velocityneverexceeded)VSOstallingspeedatMTOWinthelandingcon®gurationVMCvisualmeteorologicalconditionsWweightWTweightoftrikeWWweightofwingWXweatherxradialdistanceoftheaerofoilaboutthecentreofrotationduringtumbleXCGTperpendiculardistanceofthetrikeCGforwardsofthemonopoleychordwisedistanceofanaerofoilsectionforwardfromthetangentfromthechordlinethatinterceptsthecentreofrotationZCGTdistancebelowthehangpointofthetrikeCG(intheaxisparalleltothemonopole)ZDTdistancefromthehangpointtotheinterceptbetweenthemonopoleandthelineofactionofdrag(assumedtobethecentroidofareainthefrontview)ZTdistancefromthehangpointtotheinterceptbetweenthemonopoleandthethrustlinealocalangleofattackyanglebetweenlocalair¯owandaforwardperpendicularlinefromthehangpointˆa¡fWyTangleofactionofthrustfganglebetweenthemonopoleandtheearthZaxesfWwingcontrolangle(0placesthewingperpendiculartothemonopole)cazimuthangleduringsustainedtumbleorotationalvelocityduringtumble$rotationalvelocity1INTRODUCTIONSincetheirappearanceinthelate1970s,weightshift-controlledmicrolightaircraft[1]haveenjoyedaremarkablegrowthtobecomealargepartofrecrea-tionalaviation[2].Thishasinpartbeenduetotheirlowcostandinpartduetoanexcellentsafetyrecord[3],consistentlybelow30fatalaccidentspermillion¯yinghours.References[4]to[12]reportanumberofaccidentstoweightshift-controlledmicrolightaircraft.Withafewexceptionsonspeci®cpoints,thesereportsshowanumberofcommonfactors:1.Adeparturefromcontrolled¯ighteitherfollowinggrossmishandling,¯ighttothestallorduring¯ightinpotentiallyhighlyturbulentconditions.2.Inmostcases,theaircraftwasbeing¯ownatacomparativelylowweight.3.Damagetotheaircraftisconsistentwithverylargenegativegoverloadofthewing(usuallyfailureofthetopwiresandalsofailuredownwardsofthewingtips).Evidencesuggeststhatthewingisbeingforcedbythetotalpitchingmomenttoaveryhighnose-upattituderelativetothetrike(impactofthebasebarwiththefrontstrutwillthenfollow,usuallyresultinginafailureofoneofthesetwocomponents,causingthepropellersubsequentlytoimpactthekeeltube).Wherepilotshavesurvivedthedepartureitisnormalforthemtohavereportedthatthebasebarwas`snatchedfromtheirhands'witharateandforcebeyondtheirabilitytoholdit.(Notethattheterm`trike'describesalloftheaircraftthatisnotthewingorthehangbolt.Thewingandtrikearehingedinpitchandrollatthehangpoint,ofwhichthehangboltisthecentralcomponent,whoseremovalallowsthetwotobeseparatedforderigging.)4.Autorotationoftheaircraftinnose-downpitch,atarapidrate(inexcessof3608/s),followedby5.Break-upoftheaircraftin¯ight,preventingitfromsustaining¯ightandusuallyresultinginafatality.Therehasbeenonepreviousattempttoanalysethetumble,inthatcaseforhang-gliders,describedinreference[13].Thispaperdoesnotcontradictthose®ndings,butdoesprogresstheanalysisfurtherthanthepreviouswork,brie¯yintroducingaeroelasticandtransientaerodynamiceffectsandanalysinginducedcambereffects[14].TheBritishMicrolightAircraftAssociation(BMAA),incooperationwiththeUKAirAccidentsInvestigationBranch,theUKMicrolightmanufacturingindustryandAerospaceEngineeringattheUniversityofSouth-ampton,hasbeenfollowingaprogrammeofinvestiga-tionintothesesimilaraccidentssince1997.Conclusionshavebeendrawnconcerning,®rstly,thetumblemechan-ismand,secondly,severalmechanismsbywhichanaircraftcanenterthetumble.2THEMECHANISMOFTHEESTABLISHEDTUMBLEThetumblebehaviourofatwo-pieceairframelikeaweightshiftmicrolightcontainswhatinitiallyappearstobeaparadox.Thetumblerotationisknowntobenosedownwhilethebasebarisknowntobeonthefrontstrut,acontrolpositionassociatedwithanose-uppitchingmotioninnormal¯ight.TheremustthereforeGGRATTONANDSNEWMAN150Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 besomemechanismthatsustainstheseapparentlycontradictoryconditions.Figure2showsthesituationwiththemicrolightinnormalattitudeandwhenthewingispositionedfullynose-uprelativetothepilot.Thecentreofgravity(CG)ofthecompleteaircraftisalsoshown.ThetumblewillthereforebearotationaboutalateralaxisactingthroughapointontheaircraftclosetotheCG,andtheincidentair¯owoverthewingwillbeasshowninFig.3a(A).Thistypeofair¯owisexperiencedwithapitchingwinginnormal¯ight,whichgivesrisetounsteadyaerodynamicphenomena.Inparticular,theair¯owovertheleadingandtrailingedgesofthewingareappropriatetotheincident¯owoverasharplycamberedwing,asshowninFig.3a(B).Thiseffectisknownasinducedcamber.Figure3bshowsaphotographofanactualwingtip.Astheaircrafttumblesnose-down,theinertialeffectsuponthewingtiptrailingedgecomponentswilltendtoforcetheminadirectionfromtheuppertothelowersurface.Thesetrailingedgecomponentsareunlikethoseattheleadingedgeastheyarenotconstrainedbyaspar(althoughFig.3bshowsthe`tipstick'extendingfromtheleadingedge,whichisintendedtolimitthismovement).Tipsticks,alsoknownasminimumwashoutrods,arecantileverrodsprotrudingperpendicularlytothelead-ingedgeofthewingbeneath(oroccasionallywithin)thesail.Thesepreventthewashoutatthetipsreducingbelowapresetvalue(usuallyabout38)atlowornegativeanglesofattack.Asaconsequenceofthenose-downrotationoftheaircraft,theinertialloadingwilltendtodeformthetrailingedgestructuretowardsthelowersurfaceandthereforeproducealocalizedpositivecamber.Thiswillgenerateanadditionalliftinthetrailingedgeregionwhichwill,inturn,increasethenose-downpitchingmoment.ThisisillustratedinFig.4;itisalsoworthyofnotethatthewreckageofmostaircraftthathavesufferedatumble-relatedstructuralfailurehaveshownfailureofthewingtips,inthesenseofthetipbendingtowardsitslowersurface.Thereisthereforeasituationwhereawing±trikecombinationislockedintoacon®gurationwiththewingfullynose-up.Commencementofthetumblerotationcausesthetrailingedgepanelstode¯ectdownwards,formingsomeadditionallocalizedtrailingedgecamberthroughaeroelasticeffects.Thiscamberwillgenerateaerodynamicforces,which,inturn,increasethenose-downmoment.Thismoment,whenconsideredwiththemicrolight'sCGlocation,causesthewingtorotatewhiletranslating.Thewingseestheair¯owasaneffectivecamber,whichthereforegenerateadownwardliftforce.Figure4showsthecombinationoftheseaerodynamiceffects,whichexplainsthephenomenonandtheapparentparadox.Thecommentsaboutunsteadyaerodynamiceffectsarebasedonexistingknowledge[15]ofsuchpheno-mena.However,undernormalcircumstances(conven-tionallevelforward¯ight)awingwillsensetheseeffectsasasmallverticalwindperturbationsuperimposedonessentiallyforwardincidentair¯ow.Withthemicrolightwinginatumbleasituationexistswherethewingwilltranslateandrotatebutwithbothmotionsofequivalentmagnitude.Theaerodynamicsofsuchawingmotionismostunusualandwillrequiredetailedteststobeundertakentoestablishthe¯owpatternsandhenceanaccuratepredictionoftheaerodynamicforces.Withthesedataascienti®canalysisofthetumbleinstabilitycanbeachieved.Itisintendedtobuildawindtunneltestmodelofamicrolightwingandtousestate-of-the-artlaser-based¯ow-measuringtechniquesinordertodeterminethewing¯owinatumble.Asa®nalcomment,theequationsofmotionforthetumblingaircraftarenotdif®culttoderive.Theproblemoccurswhenthemotionisobtainedbysolvingtheseequations.Withtheadditionaldif®cultyoftheveryunusualaerodynamicsituationofthewingthesolutionwillalmostcertainlyrequireanumericalprocesstobeused. Fig.1Signconventions(two-axissystem,lateralaxisisnotusedinthispaper) Fig.2Exaggeratedillustrationofthemassdistributionoftheaircraft(a)innormal¯ightand(b)withthebasebarfurthestforward,showingtheapproximatepositionofthewholeaircraftCGTHE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT151G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering 3ESTIMATIONOFTHEMAGNITUDEOFINDUCEDCAMBERDURINGTHETUMBLEInordertoassessthedegreeofinducedcamberashortanalysisispresented.Awingsectionis®xedtoanaxisrotatingaboutapointwhichisdescendingvertically.WithreferencetoFig.5,anelementofthewingisconsidered,whichisdistanceytowardstheleadingedgefromareferencepointatwhicharadialline(lengthx)fromtheCGoftheaircraftmeetsthewingchordlineat908.ThiselementisrotatingabouttheCGatarateoandatanygivenmomentthelinebetweentheCGisperpendiculartothewingchord;theentiresystemdirectionofmovementiscˆot.Therateofvertical,translationalmovementisV.ThemotionofthiselementmaythereforebeexpressedbythevariouscomponentsoftranslationandrotationasindicatedinFig.6.Using Fig.4Aerodynamicforcessustainingthetumble Fig.3(a)Illustrationofinducedcamberduringtumble.(b)Photographoftheactualwingtipfrombelow(PegasusQ1wing,notunder¯ightloads),alsoshowinglocationontheaircraftGGRATTONANDSNEWMAN152Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 thisinformation,thelocalangleofattack(AoA)canbedeterminedat…x,y†,asshowninFig.7.Fromtheseresults,thefollowingexpressionmaybewrittenforthelocalAoA:aˆtan¡1 oy‡Vcoscox‡Vsinc³´…1†oraˆtan¡1 yx‡ Voxcosc1‡ Voxsinc0B@1CA…2†Thiscanbeexpressedintermsoftimeasaˆtan¡1 yx‡ Voxcosot1‡ Voxsinot0B@1CA…3†Fromequation(2)atypicalincident¯owanglevariationacrossthewingchordforthedatainTable1isgiveninTable2.Thismaybeillustratedgraphically,asshowninFig.8.Thusasigni®cantinducedcambereffectmaybeseenthroughoutthesustainedtumble.The75percentchordresultishighlightedinFig.8asitisapertinentresult.Analysisofapitchingandplungingaerofoilusingthinaerofoiltheory[14]resultsinaliftforceactingatthe25percentchordbasedonanincidencedeterminedbyconditionsatthe75percentchord.Inaddition,apitchingmomentisgeneratedinoppositiontothepitchingrateandthusactsasaviscousaerodynamicdamper.Figure8showsthatforapproxi-matelythree-quartersoftherotationcycletherotationimpartsanegativeincidence,givinganegativelift.Thisnegativeliftforcesustainsthenose-downtumblingmotionoftheaircraft.Variationofthe75percentincidenceshowsthatthepitchingmomentisnotconstantanda`pulsingtype'ofrotationratewouldbeappropriate.(ObservationofthetumblingincidentdescribedintheAppendixcon®rmsthisbehaviour.)Theaerodynamicdampingwilltendtolimitthetumblingrateoftheaircraft.4LONGITUDINALSTATICSTABILITYOFAWEIGHTSHIFTMICROLIGHT,AMODELDEVELOPEDWITHTHEINTENTIONOFUNDERSTANDINGTHEMECHANISMOFTUMBLEINITIATIONAweightshift-controlledmicrolightaeroplaneisbelievedtohavetwoseparateanddistinctlongitudinalstabilitymodes:thatofthewingandthatofthetrike.Innormal¯ight,whenthebasebarispositionedbetweenthepilotandfrontstrut,buttouchingneither,theseareseparate.Whenpitchingmomentsaretakenaboutthehangpoint,theaerodynamicpitchingmomentofthewingisbalancedbythewing'sownweight.Similarly,thepitchingmomentsofthetrikeaboutthehangpoint(duetoweight,dragandthrust)mustsumtozero.Notethatthismodel,whichisonlyestablishedforthepurposeofanalysingthetumble,disregardsforcesappliedbythepilottothebasebar.Thisisjusti®edbecauseinnormal(includingclimbingordescending) Fig.6Localtranslationalandrotationalvelocitycomponentsofthewingelement Fig.5Coordinatesusedfortumbleanalysis Fig.7VelocitycomponentsformingthelocalangleofattackTHE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT153G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering ¯ightpilotinputtothebarwillbenegligibleandwhateverairspeedtheaircraft`wishes'toadoptwillnormallybeacceptedbythepilot.Henceinmostmodesof¯ight,althoughthepilotwillbeholdingthebar,littleornolongitudinalforcewillbeapplied.Itisbelieved,mostlyfromtheevidenceofaccidentinvestigations,thatthetumbleoccurswhensomecombinationofconditionscausesthebasebartobepushedagainstthefrontstrut(thiswouldcausethecontrolinputusedbyapilottoapplythemaximumnose-uppitchingmoment)whilethesumofpitchingmomentsupontheaircraftarestronglynose-down.WingaerodynamicdataareavailablefromtestsusingtheBritishHang-GlidingandParaglidingAssociation(BHPA)testfacilityatRufforth,Yorkshire(seeFig.9).However,atheoreticalmodelisrequiredforthewholeaircraft,whichpredictsthepitchingmomentCMasafunctionofaircraftattitude.Itshouldthenbepossibletocombinethedataforboththewingandtrikeinordertoindicateatwhatcombinationofconditionstheaircraftmaycontinuetorotatenose-down,initiatingthetumble.Thismaythenbeusedtodeterminewhetheranaircraftdesignoffersanysigni®cantriskoftumbleentry,givenknowledgeofthewing'saerodynamiccharacteristics,andthedesiredorexisting¯ightandmanoeuvreenvelope.Considerthefollowingmodelofaweightshiftmicro-lightinsideview(seeFig.10),disregardingforthetimebeingtheaerodynamicpitchingmomentofthewing.Allpitchingmomentswillbetakenaboutthehangpoint.ThismodelisshowninFig.11andmakesuseofthefollowingde®nitionsandassumptions.4.1De®nitions1.WTistheweightofthetrike.AsshowninFig.12,itactsthroughthetrikeCG,whichislocatedZCGTbelowthehangpoint(inadirectionparalleltothemonopole)andXCGTforwardofthemonopoleaxis(inadirectionperpendiculartothemonopole).Theweightactsatananglefgrelativetothemonopoleaxis;fgˆ0whenthemonopoleisperpendiculartothesurfaceoftheearth,increasingwiththeaircraft'snose-upattitude.2.DTisthedragduetothetrike.ItactsthroughthemonopoleatapointZDTbelowthehangpointandatanangleyrelativetoaperpendiculartothemonopolesuchthatifyˆ0themonopoleisperpendiculartotherelativeair¯owandifyispositivethemonopoleismorenose-up.3.Tisthethrustduetotheengine.ItactsthroughthemonopoleatapointZTbelowthehangpointandatanangleyTrelativetoaperpendiculartothemonopolesuchthatifyTˆ0thethrustlineisperpendiculartothemonopoleandifyTispositivethethrustlinebecomesmorenose-up.4.WWistheweightofthewing.ItactsthroughthewingCGwhichisonthewingkeeladistanceLCGWbehindthehangpoint.ThewingitselfisatananglefWnose-upcomparedtoaperpendiculartothemonopole.Theweightactsatananglefgrelativetothemonopoleaxis;fgˆ0whenthemonopoleisperpendiculartothesurfaceoftheearth,becomingmorepositivewiththeaircraft'sattitudeincreasingnose-up. Table1Dataforatypical¯owanglevariationTumblerotationrate4008/sTumbletranslationspeed5m/sWingchord3mPerpendiculardistancefromtheCGtothewingchord2.0m,interceptingat0.6smc Table2Localangleofattack(deg)Chordwisestation(%)Azimuthangle(deg)020406080100045.8236.0923.227.35¡9.71¡25.223036.6827.6516.864.70¡7.90¡19.786030.7622.3012.682.28¡8.27¡18.299027.6719.279.920.00¡9.92¡19.2712027.2818.298.27¡2.28¡12.68¡22.3015030.1319.787.90¡4.70¡16.86¡27.6518037.6425.229.71¡7.35¡23.22¡36.0921050.8537.2616.35¡9.87¡32.67¡47.9524065.5654.6631.81¡9.63¡43.82¡60.2527072.5064.6946.590.00¡46.59¡64.6930068.5160.2543.829.63¡31.81¡54.6633057.6047.9532.679.87¡16.35¡37.2636045.8236.0923.227.35¡9.71¡25.22GGRATTONANDSNEWMAN154Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 5.Listheliftduetothewing.Itactsthroughthehangpointatanangleperpendiculartothewingandispositivewhenactingtowardstheuppersurfaceofthewing.6.DWisthedragduetothewing.Itactsthroughthehangpointinadirectionparalleltothewingkeelandispositivewhenactingtowardsthetrailingedge.4.2Assumptions1.Theaircraftisinanunacceleratedstate.2.Trikedragactsinthewindaxis.3.Wingliftanddragactatthehangpoint.4.Thepilothaslostcontrolofthewingandthereforeprovidesnoinput. Fig.8Localangleofattackvariation Fig.9BHPAhang-glidertestfacilityatRufforth,YorkshireTHE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT155G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering Themomentsinpitchaboutthehangboltactingonthewholeaircraftareasfollows(exceptforthewingaerodynamicpitchingmoments):1.Themomentduetothewingaerodynamicpitchingmomentisdisregardedinthisanalysis,forreasonsexplainedabove.2.Themomentduetotheeffectofwingliftistakentobezero,sinceliftisconsideredtoactthroughthehangpoint.3.Themomentduetotheeffectofwingdragistakentobezero,sincewingdragisconsideredtoactthroughthehangpoint.4.ThemomentduetotheeffectofwingweightisWWLCGWcosfW¡fg±²…4†5.Themomentduetothedragofthetrikeis¡DTZDTcosyˆ¡KDTV2ZDTcosy…5†6.ThemomentduetothrustisTZTcosfT…6†7.Themomentduetotheweightofthetrike(seeFig.12)isWT¡XCGTcosfg‡ZCGTsinfg±²…7†Summingthesecomponents,thetotalpitchingmomentactingupontheaircraftisgivenbyMTOTALˆWWLCGWcos…fW¡fg†¡KDTV2ZDTcosy‡TZTcosfT‡WT…¡XCGTcosfg‡ZCGTsinfg†…8†However,sinceitisdif®culttopredictthevalueof Fig.10(a)Forces,distancesandanglesrelevanttothelongitudinalstabilitymodel.(b)Dimensionsandanglesofthetheoreticalmodel Fig.11IllustrationofPegasusQuantum15-912inatypicalorientation(illustratedinFig.10) Fig.12CoordinatesofthetrikeCGrelativetothehangpointGGRATTONANDSNEWMAN156Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 thrustduringadeparturefromcontrolled¯ight,andalsotheeffectofthrustistopitchtheaircraftnose-up,whenconsideringtheriskofanose-downdeparture,zerothrustwillberegardedastheworstcase.Therefore,itisconservativeandappropriatetodisregarditfromtheaboveformula,whichissimpli®edfurthertoMTOTALˆWWLCGWcos…fW¡fg†¡KDTV2ZDTcosy‡WT…¡XCGTcosfg‡ZCGTsinfg†…9†Thisformulamaythenbeusedtoestimatethetotalpitchingmomentonthetrike.TheinputvariablesrequiredtodetermineMTOTALareasfollows:1.WWandWTarebasicdesignvaluesoftheaircraft.However,itshouldbeborneinmindthatwhileforaspeci®ctypeWWis®xed,WTwillvaryaccordingtooccupancyandfuelstate.However,foranygiventype,theminimumandmaximumpermittedloadingsarepublished.2.ZDT,yT,XCGT,ZCGTandKDTarefunctionsofaircraftgeometryandmaybeobtainedfromdesigndata.3.fW,fgandyare¯ightvariables.Itisknownthattypicallyinthecruisefg&¡15¯andfW&30¯.TherangeofvaluesoffWwillbeapproximately+10¯comparedwiththisvalue.Also,normal(andusuallyplacarded)operatinglimitationsforanaircraftinthisclassare+30¯pitchattitude,comparedtothenormallevel¯ightattitude.Therefore,itmaybeconsideredthatduring¯ightwithinthenormalenvelope,¡45¯fg15¯and20¯fW40¯.Forthepurposesofmodelling,itisappropriatetoexamineawiderrangeofvaluesoffgthanmightbeexperiencedwithinthenormalenvelope,sovaluesof¡105¯fg75¯willbeconsidered(equatingtoattitudesbetweenverticallyupwardsandverticallydownwards,asseenbythepilot).Thesigni®cantcaseistheonewherethepilotwouldnotbeabletopreventanose-downdeparture.Assumingthenafullnose-uppitchinceptorinput,itcanbefurtherassumedthatfWˆ40¯.4.y,thetrikeangleofattack,isrelevantinsofarasthedragduetothetrikeactsinanose-downdirection.Thereforeitwillbeconsideredtobe08,againsincethisistheworstcaseforanose-downpitchingdeparture.Insuchacase,apolarisrequired:MTOTALˆWWLCGWcos40¯cosfg¡KDTV2ZDTcos0¯¡WTX2CGT‡Z2CGTq6sinfg¡sin¡1 XCGTZCGT³´µ¶…10†However,forthepurposesofconsideringthetrikealone,the®rsttermofthisequationisomitted.Figure13showsthevalueofMTOTALasafunctionoffgforaMainairGeminitrike(omittingtheWWterm)forbothitsmaximumandminimumpermittedloadings.Visassumedtobe43knots,sincethisisatypicalcruisingairspeed,andalsothespeedaroundwhichthebestqualitywingaerodynamictestdataareavailable.ThismaybecomparedtothegraphinFig.14foracorrectlyadjustedMainairFlash2alphawing,whichmighttypicallybe®ttedtothistrike.Ifthewingispushedthroughthestallingangleofattacktoabout258AoA,thentheaerodynamicpitchingmomentwillbeabout600Nmnose-up,asseeninFig.14.However,ifthewingisconsidered,thispitchingmomentisreachedatabout208nose-up,regardlessofweight(whichequatesapproximatelyto358nose-upasseenbythepilot).Iftheaircraftwasstalledatagreaternose-uppitchattitudeof,forexample,308nose-up(458asseenbythepilot)thenwhilethewingpitchingmomentwillremainatabout600Nmthetrike,dependinguponweight,willhaveapitchingmomentof1000±1500Nmnose-down.Thiswill,oncethebasebarhasbeentouchedbythefrontstrut(creatingarigidsystem),forcethewholeaircraft,inarigidstate,nose-down,rotatingaboutthewholeaircraftCG,whichwill,duetotherelativemasses,beclosetothetrikeCG.Theeffectofthis,aspreviouslydiscussed,istoinduceanapparentreversecamberatthewing(seeFig.15).Thisinducedreversecamberislikelytocauseareversalinpitchstabilityandthusatendencytofurtherpitchdown.Anegativeliftforcewillalso`lock'thetriketotheaircraft,maintainingtherigidsystem.TheBMAA,inassociationwiththeUKCivilAviationAuthorityandtheUKmicrolightmanufactur-ingindustry,iscurrentlystudyingtheapproachdetailedabovewiththeintentofdevelopingpass/failcriteriaforneworsuspectaircraftdesigns.Oncetestedinsamplecerti®cationprogrammes,itisintendedthatthiswillultimatelybeincorporatedintothesafetyrequirementsforthisclassofaircraft[16].5AVOIDINGTHETUMBLETheanalysisaboveindicatesthatatumblecanpotentiallyoccuriftheaircraftentersa¯ightconditionwherethenose-downpitchingmomentduetotheweightofthetrikeisgreaterthanthatofthepitchingmomentofthewing,lockingthetriketothewingandtherebyforcingtheentireaircrafttopitchnose-downasarigidbody.Thismaybeenteredwiththeaidofenginethrust,creatingthissituationwhenthrustislosteitherdeliberately(throughthrottleclosure)orinadvertently(throughenginefailure).THE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT157G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering Usingthesimplemodelofthetrikelongitudinalstabilitygivenabove,addedtotheaerodynamiccharacteristicsofthetrike,itbecomespossibletopredicttheconditionsatwhichthetumblemightoccur.Itshouldbeborneinmindthatthetumblemightnotnecessarilyoccur,sincetherateofpitchingmustbesuf®cienttocausetheinversecamberonthewingthatisassociatedwiththesustainedtumble.Fromthisanalysis,thetumbleappearstobeafunctionofboththewingandtrikecharacteristics.Atrikewithhighweightoralongmonopolewillhaveagreaterpitchingmomentatasteepnose-upattitude,andthereforeagreatertendencytotumble.6THEEFFECTOFWEIGHTONTHETUMBLEItisknownfromthehistoryoftumbleaccidentsthatthemorehighlyloadedthetrikeisthelesstheaircraftwilltendtotumble.At®rstsightofthegraphinFig.13,thisdoesnotmakesense.However,oncethenose-down Fig.14CharacteristicsofMainairFlash2alphawing Fig.13Pitchingmomentofthetrikeaboutthehangpoint,zerothrustGGRATTONANDSNEWMAN158Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 pitchdepartureoccurs,amorelightlyloadedtrikewillresultinahigherCGandthusagreaterangleofin¯owintothewing.Therefore,foralowertrikemass,theinducedcamberatthewingwillbegreatersincethepointofrotationwillbeclosertothewing.Alsoatalowertrikemass,therotationalinertiainpitchwillbeless.Forexample,amicrolightaircraftwingmassof50kg,atrikemassof150kgandadistancebetweentheseCGsof1.5mgivesatotalrotationalinertiaof87.5kgm2.Thewingis1mfromtheaircraftCG.Withatrikemassof300kg,therotationalinertiais96.5kgm2andthewingis1.3mfromtheaircraftCG.Thus,withthesamepitchingmomentapplied,anddisregardingaerody-namicdamping(atpresent,dataisnotofsuf®cientquality),theeffectofanetnose-downpitchingmomentof500NmcanbeascertainedbythesimpleanalysisgiveninTable3.Itisthusdemonstratedthatareducedtrikeweightwillresultinasigni®cantlygreaterinducedcamberfollowinganaircraftstallatahighnose-upattitude.Therefore,theriskofthesustainedtumbleoccurringfollowinganose-upstallisconsiderablygreater.7EFFECTOFWINGSETTINGSExperiencehasalsoshownthatamis-riggedwing,particularlyoneinwhichtheluf¯inetensionisinsuf®cient,willdisplayagreatertendencytotumble.Figure16showsfourcurvesfortheMainairFlash2alphawing(seeFig.14)alreadydiscussed,foravarietyofconditions.Theseplotsvaryinthreeways,noneofwhichimpactsigni®cantlyuponthediscussionabove.Thewingswithinsuf®cientluf¯inetensiondisplaya¯atcurve(indicatingverylowapparentlongitudinalstaticstability)aroundthetrim.Withouteithertipsticksorcorrectluf¯inetension,thewingdisplaysapitchstabilityminimumaboutzeroAoA.Itisbelieved,frompreviousworkbyKilkenny[17]onhang-gliderstabilityandfromdiscussionswithmicrolightwingdesigners,thatthisisrelatedtotheluf®ngdive(amodeof¯ightwhereunsatisfactorylongitudinalstabilitycharacteris-ticscauseaconstantspeedoracceleratingdescentwhichisusuallyunrecoverable)andnottothetumble.Whileallwingsdisplayanapparenttendencytowardsanose-downpitchingmomentatverylowanglesofattack(wellbelowanythinglikelytobeexperiencedwithinthepermittedmanoeuvreenvelope),thisoccursatahigherangleofattackforawingwithouttipsticksandwithincorrectluf¯inetension.(NotethatthelackofdataatloweranglesofattackthanthoseshowninFig.16isduetoaphysicallimitationoftheBHPAtestrig.Theonlyotherknownfacilityintheworld(locatedinGermany)isofsimilardesignandthusatpresentthereisnomeansof®ndingoutexactlywhathappensattheseanglesofattack.)Itisproposedthatthislastcharacteristicissigni®canttotumbleinitiation.Whileitis,withthecurrentstateofknowledge,onlypossibletoconjectureastoexactlywhathappenstotheforcesandmomentsactinguponthewingduringtheinitialpitch-downoftumbleinitiation,itisareasonableassumptionthatthemis-riggedwingshownbycurve(4)inFig.16willshowagreatertendencytopitch-downasthereductioninAoAoccursthanthecorrectlyriggedwing(irrespectiveofanyinducedreversecamber).Insurveyingtheseplots,itappearsthatthecorrectluf¯inetensionisimportantinpreventingthetumble,butthepresenceoftipsticks Fig.15Illustrationofinduced¯owsuperimposedupontheaircraftimage Table3Effectoftrikemassuponin¯owangleforaconstantpitchingmomentLighteraircraftHeavieraircraftTotalrotationalinertia(kgm2)87.596.5Rotationalacceleration,assuminga500Nmnose-downpitchingmomentandnoaerodynamicdamping(rad/s)5.735.18Resultantrotationalvelocityafter1s,o(deg/s)328296Downwardvelocityofthenose,assumingitisanominal1minfrontofthehangpoint(m/s)9.387.23Nominalaircraftstallingspeedatthisweight&VSOMTOW=Wp…m=s†1518Approximateangleofresultant¯ow,atthewingleadingedge,atstallingspeed(deg)3221THE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT159G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering providesavaluablebackupÐifluf¯inetensionceasestobecorrect.Inthissituation,thetipsticksappearlikelytomaintainalargemarginbetweenthenormalcruiseconditionandthenormal¯yingrangeofpositiveanglesofattack.Figure16givesreasonablegroundstobelievethatpitchstabilityreversalwilloccuratanglesofattackabove¡20¯,whichthepreviousanalysisindicatesmightpotentiallyoccurwithsuf®cientmishandlinginalight-weightaircraft.8INITIATIONOFTHETUMBLEThedescriptionofthetumbleinitiationaboveshowsthatforthetumbletooccur,theaircraftmustbesteeplynose-upwithalowthrottlesettingorfailedengine.Thereareseveralwaysinwhichthismightoccur,whicharediscussedbelow:-8.1Cause1:the`whip-stall'Thewhip-stalliscausedbyanaggressiveentry(atahighdecelerationrate,wellinexcessofthe1kn/snormallyrecommended)totheaerodynamicstall,followedbyanequallyaggressiverecoveryinitiationbythepilot(pullinginthecontrolbarrapidly).Thisisamanoeuvrewhichmaybeusedbytestpilots(withgreatcare)toallowthemtodemonstrateVNEorVDFinthisclassofaircraft[18],whichareotherwisecontrollimitedandunabletodemonstratehigh-speed¯ightforcerti®cationpurposes.However,thereisabsolutelynoneedforaprivatepilottoevercarryoutthismanoeuvreinnormal¯ight;thewhip-stallisspeci®callyprohibitedbyallMicrolightmanufacturersandbytheUKpilottrainingsyllabus[19].Itisconsideredlikely(andseveraleyewitnessreportsoffatalaccidentsbearthisoutÐmostrecentlytheOctober2000fatalitytoaPegasusQuantum)thatthismechanismcanleadtothetumble.Thesequenceofactionsinthewhipstallisdetailedbelow:1.Thepilotplacestheaircraftinaclimbingattitudeandpushesthecontrolbaroutrapidlytoachieveahighdecelerationrate.Atthesteepestpossiblenose-upattitude,thethrottleisclosed.2.Theairspeeddecreasesrapidly,withunsteadyaerodynamiccharacteristicsdelayingtheonsetofaerodynamicstalluntilalowerairspeedisachievedthanwouldnormallybeexperienced.3.Atthepointofstall,thewing'saerodynamicpitchingmomentbecomesstronglynose-down.Duetothelowairspeed,thisislikelytobelessthanifthestallingangleofattackisreachedinalessdynamicmanoeuvre.4.Thetrikepitchesdownandpushesagainstthewing(withthefrontstrutagainstthebasebar),creatingarigidsystemuponwhichanetnose-downpitchingmomentisacting.8.2Cause2:combinedspiralinstabilitycombinedwithalossofhorizonWeightshiftmicrolightaircraftareapprovedinallcountriesofwhichtheauthorshaveknowledge,butonlyfor¯ightinvisualmeteorologicalconditions(VMC).Thisimpliesaguaranteedvisualhorizonwhichthepilotmayuseasareferencewhencorrectingsmall Fig.16EffectofdifferentriggingconditionsuponMainairFlash2alphawingGGRATTONANDSNEWMAN160Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 rollingdepartures(suchasmaybecausedbytemporaryinattentionorbyturbulence).However,itispossiblethroughill-luckorpoorjudgementforanaircrafttoenterinstrumentmeteorologicalconditions(IMC),whereade®nedhorizoncannotbeguaranteed(suchasincloud).Ifthishappens,anypilotwillattempttoremovethemselvesandtheiraircraftfromthisasquicklyaspossible;however,ifthepilotisunabletoextracttheaircraftfromthissituationitisalmostinevitablethatsomecause(mostlikelytheturbulencecommonlyfoundinsideorneartomostclouds)willinitiateanunde-mandedrollingmanoeuvre.Unlikemostconventionalaeroplanes,mostweightshiftmicrolightaircraftarespirallyunstable;thus,aninitialsmallbankangleislikelytoincreasewithout(unlessahorizonreferenceisavailable)thepilot'sknowledgeorabilitytocontrolit.Theaircraftwouldthenenteraslowroll,potentiallythrough908ofbanktoaconditionwherethependulumstabilitythatkeepsthetrikebelowthewingceasestoactÐinevitablycausingsomelossofcontrol.Itisthenpossiblethattheaircraftwill®nditselfinanunsustain-ablysteepnose-upattitude.Itisnoticeablethatsometumbleaccidentreports,particularlythoserelatingtotheG-MVEP,haveoccurredinconditionswherethehorizonwasknowntobepoorandwherethesubsequentdamagetotheaircraftshowedthatthebasebarhadfractured(incontactwiththefrontstrut)attheend,ratherthaninthecentreaswouldbeimpliedbyasymmetricmanoeuvre.Thisimpliesarollingcomponenttothedeparturefromcontrolled¯ight,whichwouldbeconsistentwiththismechanism.Table4showstheresultsofabrieftestcarriedouttodemonstratethespiralinstabilityofaweightshiftmicrolightaircraft.ARaven-Xweightshiftmicrolight,¯ownsolo,wastrimmedinmoderatelyturbulentairconditionsandthecontrolsreleased.Theresultantbankanglewasestimatedbaseduponavisualhorizon,andthetimetoreachthesegivenbankangleswasmeasured.Thisdemonstratesthatfollowing¯ightintoIMCsuchadeparturecouldreadilyhappenwithin60s(obviously,thepresenceofspiralinstabilitywillvarybetweenaircrafttypesanddifferentpowersettings).ThetestaircraftisillustratedinFig.17.InthecaseoftheG-MVEP,referredtoabove,itwouldbeareasonabledeductionthathavinglostthevisualhorizonthepilot(whowasstillundertraining)mighthaverolledbeyondpermissiblelimitsinunder60s.8.3Cause3:failedloopmanoeuvreWhileweightshiftmicrolightaircraftareneitherapproved,norshouldbe,foraerobatics,itisoccasion-allyknownforapilottoattemptaerobaticmanoeuvres.Thereareseveralreportedinstancesofpilotsattemptingtoconductaloopinsuchanaircraft.Ifpositivenormalaccelerationismaintainedthroughoutthismanoeuvreitcanbeexecutedassafelyasinanyotheraircraft.However,aswithanyotheraircraft,iftheaircraftrunsoutofenergyneartothetopoftheloop,thenthepilot®ndshimselfinvertedwithoutsuf®cientairspeedtocompletethemanoeuvre.Inthiscase,theinevitableconsequencewillbeanegativeangleofattack,leadingtoatumble.TheAppendixshowsasequenceofframesfroma®lmtakenofaFrenchCosmosaircraft.Theaircraftwas¯yinganairdisplaysequencethatincludedaloop,whichfailed.Theresultwasatumbleresultingintheaircraft'sdestructionanddeathofthepilotoncollisionwiththeground.8.4Cause4:¯ightthroughownwakevortexItiswellknownthataminimumsafeseparationshouldbeensuredbetweenlandingaircraft,particularlylargeraircraft,whichtendtogenerateverylargevortexwakesthatcannormallybeexpectedtoremainforupto80s[20,21]innormalconditions,ratherlongerinverystillair.Theweightshiftmicrolight,usingasitdoesadeltawing,tendstogenerateaparticularlylargewakevortexforthesizeoftheaircraftandiscapableofgeneratingaconsiderableupset[22].Forthisreason,pilotsof Table4Testtodemonstrateweightshiftspiralinstability(a)aircraftandatmosphericconditionsAircraftSouthdownRaven-X(Rotax447engine)‡60inch3-bladeIvoproppropellerat98pitch(propellerapprovedbyMAAN1076)RegistrationG-MNKZCrewGratton(solo)ConditionsCAVOK,lightturbulence,nilWx,OAT‡58C,No.3fromfronthangpointsettinggiving48mile/hIAStrimDate13February2001TestAircraft¯owninlightbutperceptibleturbulenceoverwoodland,nominal1000ftonQFE1024hPa(b)TestresultsPower(r/min)Timeat308bank(s)Timeat458bank(s)Timeat608bank(s)3000(lightidle)2540Testabandonedduetogroundproximity5000(PLF)1020256500(MCP)101520THE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT161G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering weightshiftaircraftaretaughtthatlevelturnsshouldneverbecontinuedbeyond2708andpreferablynotbeyond1808withoutclimbingordescendingduringtheturn.Consideringatypicalturningmanoeuvreat45knCAS,608bank,2000ft,itcanbeshownthattheturnratewillbe408/s.Hence,ifthepilotwereto¯yacontinuoustightbalancedturn,theaircraft'sownwakevortexwouldbemetinlessthan9sÐscarcelytimeforthevortextohavesigni®cantlydispersedinevenmoderatelydisturbedair¯owair.Itisknownthataircraft¯yingthroughthewakevortexofanothercansufferalarge-magnitudeundemandedroll.Itisthere-forereasonabletoassumethatthesamemechanismaswasdescribedaboveforalossofvisualhorizonmayalsooccur,althoughitislikelythattheonsetwillbemorerapid.ThefatalaccidenttoG-MVJUin1992wascon-sideredbytheAirAccidentInvestigationBranch(AAIB)reporttohavebeenatumbleandin-¯ightbreak-upfollowingapilot¯yingwhatwasobservedfromthegroundtohavebeenextremelytightturnsof3608ormore.9CONCLUSIONSThispaperhasexplainedthatthetumblemodeinaweightshift-controlledmicrolightissustainedbyinduced¯owasanaircraftrotatesinpitchaboutitsCG.Ithasdemonstratedthatthemodemaybeinitiatedbyalossofpowerinasteeplynose-uppitchattitude,causingtherotationofthetrikeaboutthehangpointtopushthewingnose-down,viacontactofthefrontstrutandbasebar.Usingthismodel,asimplemethodisshownwherebytheconditionsoftumbleentrycanbepredicted.Fourpossiblemethodsofentryhavebeenexplained,throughawhip-stall,rollingdeparture,afailedaero-baticmanoeuvreor¯ightthroughtheaircraft'sownwakevortex.Alloftheseoccasionsareshowntobeavoidablethroughgoodjudgementonthepartofthepilot.Thetumbleisapotential`killer'modeinthisclassofaircraft,ashasbeendemonstratedbyhistory.However,itisshownthat,througheducationandtheuseoftheapproachdevelopedinthispaper,tumbleentrycondi-tionscanbepredictedandthemanoeuvreslikelytocauseadeparturefromcontrolled¯ightavoided.ACKNOWLEDGEMENTSTheauthorswouldliketothankthefollowingfortheirinvaluableassistanceinconductingthestudiesreportedinthispaper.DrBillBrooks,TechnicalDirector,CycloneAir-sportsMrStuartCulling,SeniorEngineeringInspector,UKAAIB(nowretired)MrMarkDale,TechnicalOf®cer,BHPALtCol(Rtd)ChrisFinnigan,ChiefExecutive,BMAAMrJohnHamer,TestPilot,BMAAMrRogerPattrick,ChiefDesigner,MainairSportsMrsJoanWalsh,EngineerandMicrolightPilotDrPaulWelsh,AirworthinessEngineer,FlylightAirsports Fig.17SouthdownRaven-XG-MNKZGGRATTONANDSNEWMAN162Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 REFERENCES1Gratton,G.B.Theweightshift-controlledmicrolightaero-plane.Proc.InstnMech.Engrs,PartG:J.AerospaceEngineering,2001,215(93),147±154.2Gratton,G.B.Lessweightmorefun.AerospaceInt.,February2000,30±32.3AviationSafetyReview1990±1999,CAP701,1999(UKCivilAviationAuthority).4Brooks,W.G.ReportonanaccidenttoaPegasusQuantumSupersport503,inHolland,Michigan,USA,on16October2000;http://www.pegasusaviation.co.uk/pdf/Qtmacc.pdf.5MainairGeminiFlash2aG-MVEP;FatalAccident.Bulletin5/2000:EW/C97/10/5,UKAirAccidentsInvesti-gationBranch,2000.6MainairGeminiFlash2aG-MTLA;FatalAccident.Bulletin1/97:EW/C96/7/8,UKAirAccidentsInvestiga-tionBranch,1997.7PegasusXL-Q,G-MVCU;FatalAccident.Bulletin8/92:EW/C92/5/1,UKAirAccidentsInvestigationBranch,1992.8Thomas,J.ReportofanincidentthatoccurredonSunday21JulyatBedfordMicrolightCentre(unreferencedreportofarecoveredin-¯ightdeparturesubmittedtoBMAAÐMainairGeminiFlash2G-MNNU).9MainairGeminiFlash2aG-MTUW,FatalAccident.Bulletin9/89:EW/C1108,UKAirAccidentsInvestigationBranch,1989.10MainairGeminiFlash2aG-MNEJ;FatalAccident.Bulletin4/86:EW/C918,UKAirAccidentsInvestigationBranch,1986.11MainairSportsLimited,AccidentInvestigation,Bulletin20,1September1986(MainairGeminiFlash2G-MNNF,survivedin-¯ightbreak-upduring¯ighttesting).12MainairSportsLimited,AccidentReport,Bulletin21,15September1986(PegasusFlash2,G-MNYY,survivedin-¯ightbreak-upduringinitialtestingofanewlybuiltaircraft).13Morris,S.J.Asimpli®edanalysisoftumblingmotion.HangGliding,November1994,38±44.14Fung,Y.C.AnIntroductiontotheTheoryofAeroelasticity,1955(JohnWiley,Chichester).15Bisplinghoff,R.L.,Ashley,A.andHalfman,R.L.Aeroelasticity,1955(Addison-Wesley,London).16BritishCivilAirworthinessRequirements,SectionS,SmallLightAeroplanes,Issue2,CAP482(UKCivilAviationAuthority).17Kilkenny,E.A.Anexperimentalstudyintothelongitudinalaerodynamicandstaticstabilitycharacteristicsofhanggliders.PhDThesis,Cran®eldInstituteofTechnology,September1986.18Brooks,W.G.Flighttestingof¯exwingaircraft;http://www.raes.org.uk/light-av/brooks_p_1.htm.19MicrolightTrainingSyllabus,PPL(A),BritishMicrolightAircraftAssociation.20WakeTurbulence,GeneralAviationSafetySenseLea¯et15A(UKCivilAviationAuthority).21WakeTurbulence,AIC178/1993(Pink95)(UKCivilAviationAuthority).22Cosgrove,B.PilotsWeather,pp.45±46SBN1-84037-027-0.APPENDIXTimehistoryofafataltumbleincident,fromafailedloopNotethattheoriginofthispieceofvideoisnotentirelyclear.ItisbelievedtohavebeentakenatanairshowinEurope,theaircraftbeingidenti®ableasaFrench`Cosmos'type.Theexactdata,locationandsourcecannotbeveri®ed.THE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT163G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering Fig.18VideoofafataltumbleincidentGGRATTONANDSNEWMAN164Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003 Fig.18(continued)THE`TUMBLE'DEPARTUREMODEINWEIGHTSHIFT-CONTROLLEDMICROLIGHTAIRCRAFT165G01602#IMechE2003Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineering Fig.18(continued)GGRATTONANDSNEWMAN166Proc.InstnMech.EngrsVol.217PartG:J.AerospaceEngineeringG01602#IMechE2003