Unsteady Velocity Measurements Taken Behind a Mode

NationalAeronauticsandSpaceAdministrationLangleyResearchCenter•Hampton,Virginia23681-0001NASATechnicalMemorandum4738ATCOMTechnicalReport97-A-001UnsteadyVelocityMeasurementsTakenBehindaModelHelicopterRotorHubinForwardFlightJohnD.BerryJointResearchProgramOfficeAeroflightdynamicsDirectorateU.S.ArmyAviationandTroopCommandLangleyResearchCenter•Hampton,VirginiaMarch1997Printedcopiesavailablefromthefollowing:NASACenterforAeroSpaceInformationNationalTechnicalInformationService(NTIS)800ElkridgeLandingRoad5285PortRoyalRoadLinthicumHeights,MD21090-2934Springfield,VA22161-2171(301)621-0390(703)487-4650AvailableelectronicallyatthefollowingURLaddress:(ref.1).Becausethehubcontributessuchasignificantportionofoverallvehi-cledrag,itisimportanttounderstandandaccuratelypre-dictthesourcesandeffectsofhubdrag.Anotheradverseaerodynamiceffectoccursduringflightconditionswhenthedownstreamturbulentflowfromthehubinteractswiththeempennageandtailrotorinstandardhelicopterconfigurations.Thiseffectdegradesyawcontrolduringtheseflightconditions.Modelingthehelicopterconfigurationaccuratelyisimportantforpreventingsignificantdelaysduringvehi-cledevelopment.Thecomplexityusedinmodelingtheaerodynamicsofhelicopterfuselagesvariesgreatly.Parametersmaybedeterminedfrombothpoweredandunpoweredscalemodelwindtunneltestsandfromanal-ysesranginggreatlyincomplexity.Itisimportanttounderstandthatfewanalysesfullyaccountforthediffer-encesinfuselagedragcausedbytheeffectoftherotorwakeonthefuselage,aneffectstudiedduringwindtun-nelexperiments(ref.2).Currently,onlyexperimentalmethodsusingpoweredrotorcraftfuselagemodelscangiveinsightintotheadverseeffectsofthehubregiondescribedinthepreviousparagraph.Aerodynamicinvestigationswereconductedtomea-surethevelocityfieldsinthevicinityofamodelhelicop-terconfiguration(ref.3).Themodelconfigurationconsistedofagenericrotorandfuselagerepresentativeofawiderangeofrealistichelicopters.Comparisonsofthemeasuredvelocityfieldwithanalyticalpredictionsweremade(refs.4and5)tohelpunderstandandvalidaterotorinflowvelocitypredictions.Theeffectoftheheli-copterfuselageontheseinflowvelocitymeasurementswasstudied(ref.6).However,theeffectsoftheregionofacknowledgedseparationbehindtherotorhubwerenotassessed.Thisinvestigationwasconductedtoprovideandanalyzedataneededtoformthebasisformodelingtheregionbehindthehub.Theuseofawell-documentedhelicoptermodelconfigurationusedforpreviousaerody-namicconfigurationstudiesaddstothevalueofthisspe-cificexperimentaldata.SymbolsFigure1showsthecoordinatesystemusedinthisstudy.bnumberofblades,4CTrotorthrustcoefficient,0.0064(nominal)cbladechord,2.61in.FFouriertransformofonce-per-revolutionvelocityFumagnitudeofharmoniccomponentsofuvelocityFwmagnitudeofharmoniccomponentsofwvelocityNunumberofpointsineachazimuthalbinusedforcomputation(representedbyNuinfigures)Nwmeasurementcountforverticalvelocitymea-surement(representedbyNwinfigures)PMTphotomultipliertubeRrotorradius,2.82ftrradialdistancefromhubcenter,ftTthrust,lbfT2Dturbulenceintensity,uvelocityinxdirection,ft/secumeanvelocityinxdirection,ft/secTρπR2ΩR()2------------------------------,12u′2w′2+()⁄u∞-----------------------------------------2velocityfluctuationinxdirection,ft/secaveragesquareoffluctuatingvelocityinxdirectionu∞freestreamvelocity,ft/secwvelocityinzdirection,ft/secwmeanvelocityinzdirection,ft/secvelocityfluctuationinzdirection,ft/secaveragesquareoffluctuatingvelocityinzdirectionwfvelocityinducedinforwardflight,ft/secwhvelocityinducedinhover,ft/secxdownstreamdistancefromhubcenter,ftylateraldistancefromhubcenter,ftzverticaldistancefromhubcenter,