Rotating Shake Test and Modal Analysis of a Model

NationalAeronauticsandSpaceAdministrationLangleyResearchCenter•Hampton,Virginia23681-0001NASATechnicalMemorandum4760ARLTechnicalReport1389RotatingShakeTestandModalAnalysisofaModelHelicopterRotorBladeW.KeatsWilkie,PaulH.Mirick,andChesterW.LangstonVehicleTechnologyCenterU.S.ArmyResearchLaboratoryLangleyResearchCenter•Hampton,VirginiaJune1997Printedcopiesavailablefromthefollowing:NASACenterforAeroSpaceInformationNationalTechnicalInformationService(NTIS)800ElkridgeLandingRoad5285PortRoyalRoadLinthicumHeights,MD21090-2934Springfield,VA22161-2171(301)621-0390(703)487-4650AvailableelectronicallyatthefollowingURLaddress:(ARES)test-bedintheHelicopterHoverFacility(HHF)atLangleyResearchCenter.ThemeasureddatawerecomparedwithpretestanalyticalpredictionsoftherotatingbladefrequenciesmadeusingtheMSC/NASTRANfinite-elementcomputercode.TheMSC/NASTRANsolutionsequencesusedtoanalyzethemodelweremodifiedtoaccountfordifferentialstiffeningeffectscausedbythecentrifugalforceactingonthebladeandrotatingsystemdynamiceffects.ThecorrelationoftheMSC/NASTRAN-derivedfrequencieswiththeexperimentaldatais,ingeneral,verygoodalthoughdiscrepanciesinthebladetorsionalfrequencytrendsandmagnitudeswereobserved.TheproceduresnecessarytoperformarotatingsystemmodalanalysisofahelicopterrotorbladewithMSC/NASTRANareoutlined,andcompletesam-pledatadecklistingsareprovided.IntroductionCalculationoftherotatingsystemmodalpropertiesofrotorbladeandhubassemblies,particularlyinthecaseofbearinglesshubdesigns,oftenrequirestheuseofmodernfinite-elementcomputercodes.Onewidelyusedfinite-elementcodeisthecommerciallyavailableMSC/NASTRANprogram(refs.1–4).Althougharotatingsys-temmodalanalysiscanbeperformedusingthestandardreleaseversionsofMSC/NASTRAN,somepotentiallysignificantdynamiceffectscausedbyrotationwillnotbeaccountedforproperly.BymodifyingthestandardMSC/NASTRANsolutionsequencetoincludetheadditionalrotationaleffects,amoreaccuratemodalanalysisofarotatingstructuremaybeperformed.Thisreportdocu-mentsanexperimentalevaluationoftheabilityofthismodifiedMSC/NASTRANproceduretoaccuratelypre-dicttherotatingbladefrequenciesofamodelarticulatedhelicopterrotorblade.ExperimentalApparatusandProceduresTestFacilityTestswereconductedintheLangleyHelicopterHoverFacility(HHF)showninfigure1.TheHHF,ahigh-bayfacilityenclosedbya30-ftby30-ftby20-ftcoarse-meshscreen,isusedforhovertestingandrotor-craftmodelbuildupandcheckoutpriortotestingintheLangleyTransonicDynamicsTunnel(TDT).ModelsaremountedontheteststandsuchthattherotorplaneofL-78-5962Figure1.HelicopterHoverFacility(HHF).2rotationiseffectivelyoutofgroundeffect(15ft,orapproximately1.6timestherotordiameter).AllhovertestingintheHHFisconductedatsealevelatmosphericconditions.ModelDescriptionAfour-bladedarticulatedrotorhub,withcoincidentlead-lagandflappinghinges,wasusedinthisexperi-ment.Thestructuralandinertialpropertiesofthemodelbladesarelistedintable1.Thebladeswererectangularinplanformandpossessednobuilt-intwist.AstandardNACA0012airfoilcontourwasusedovertheaerody-namicportionsoftheblade.Onebladewasinstrumentedwithflapwise,chordwise,andtorsional-directionstraingaugesmountedatthreeradiallocations.Thebladeplan-formgeometry,withstrain-gaugelocationsindicated,isillustratedinfigure2.ThetestbedforthisexperimentwastheNASA/U.S.ArmyAeroelasticRotorExperimentalSystem(ARES)modelshowninfigure3.TheARESmodelhasastream-linedfuselageshellthatenclosestherotorcontrolsanddrivesystem.Thefuselageshell,whichisnotusuallyinstalledwhentestingtheARESmodelintheHHF,wasomittedduringthistest.Themodelrotorispoweredbyavariable-frequency,synchronouselectricmotor(ratedat47-hpoutputat12000rpm)thatisconnectedtotherotorshaftthroughabelt-driven,two-stage,speed-reductionsystem.Collectivepitchandcyclicpitchinputsarepro-videdthroughaconventionalswashplatearrangement.Theswashplateispositionedbythreeelectricallycontrolledhydraulicactuators,whicharecontrolledremotelyfromtheHHFcontrolroom.Signalsfromthebladestraingauge,aswellasthesignalfromastrain-gauge-instrumentedpitchlinksignal,aretransferredfromtherotatingsystemtothefixedsystemthrougha30-channelslip-ringassembly.Table1.PropertiesofModelRotorBlade(a)StructuralpropertiesInboardsectionradius,in.Sectionspararea,in2Chordwiseareamomentofinertia,in4Flapwiseareamomentofinertia,in4Torsionalareamomentofinertia,in4Sectionmass,lb/in.Sectionmassmomentofinertia,in-lb2/in.Centerofmassoffsetforwardofelasticaxis,in.3.005.500.50000.50000.26320.42510.22000.06.870.3710.15000.05000.13160.19380.048260.08.870.3710.02500.00400.01050.040860.025050.010.6250.3710.02500.00400.01050.151130.056710.012.500.3770.03550.003940.03940.140480.055590.013.000.3860.02520.002490.009760.031340.027790.015.3750.3390.02520.002490.009760.043760.029610.017.850.2780.030400.002310.006740.041230.028880.023.750.2490.026360.001810.005650.04000.028100.028.250.2240.024470.00