Airdata Measurement and Calibration

NationalAeronauticsandSpaceAdministrationNASATechnicalMemorandum104316AirdataMeasurementandCalibrationEdwardA.Haering,Jr.December1995NationalAeronauticsandSpaceAdministrationDrydenFlightResearchCenterEdwards,California93523-02731995NASATechnicalMemorandum104316AirdataMeasurementandCalibrationEdwardA.Haering,Jr.NASADrydenFlightResearchCenterEdwards,California1ABSTRACTThismemorandumprovidesabriefintroductiontoairdatameasurementandcalibration.Readerswilllearnabouttypicaltestobjectives,quantitiestomeasure,andflightmaneuversandoperationsforcalibration.Thememoranduminformsreadersabouttower-flyby,trailingcone,pacer,radar-tracking,anddynamicairdatacalibrationmaneuvers.Readerswillalsobegintounderstandhowsomedataanalysisconsiderationsandspecialairdatacases,includinghigh-angle-of-attackflight,high-speedflight,andnonobtrusivesensorsarehandled.Thismemorandumisnotintendedtobeallinclusive;thispapercontainsextensivereferenceandbibliographysections.INTRODUCTIONAirdataarevitaltosuccessfullycompleteanaircraft'smissionandarederivedfromtheairsurroundingtheaircraft.References1–4supplypertinentinformationregardingairdatameasurementandcalibration.Theseairdataencompassindicatedandtrueairspeed,pressurealtitude,ambientairtemperature,anglesofattackandsideslip,Machnumber,andrateofclimb.Typically,pitotandstaticpressuresaresensedandconverted(bymechanicalmeansintheinstrumentsthemselves)intoindicationsonthealtimeter,verticalspeedindicator,airspeedindicator,andMachmeter.Similarly,measuredlocalflowanglesestablishanglesofattackandsideslip,andtheoutsideairtemperatureismeasuredandindicatedinthecockpit.(Instrumentsthatcanperformtheconversion,suchasairspeedindicators,altimeters,andMachmeters,donotcorrectforerrorsintheinputvalues.)Thesemeasuredparametersarecommonlyinputtotheairdatacomputer,which,usingappropriatealgorithmsandcorrectionfactors(orcalibrations,asdiscussedlater),canprovideotherparameters,suchastrueairspeed,requiredbytheaircraft'savionicsorflightcontrolsystem.Thepresenceoftheaircraftintheairstreamcausesinputerrorstothemeasuringinstruments—theaircraftdisturbstheairthatitfliesthrough,therebyalsodisturbingtheairdatameasurements.Figure1showstheairflowaroundanairplanewing.Theairabovethewinghaslowerpressurethantheambientair,whilethepressurebelowthewingishigherthantheambientair.Compressibilityandshockwavesalsodisturbtheairandaffectthemeasurements.CompressibilityeffectsbecomeimportantaboveapproximatelyMachnumber0.3.Asaresult,thestaticpressurearoundanairplanevariesconsiderablywithlocation.Localflowanglesalsodifferfromthefree-streamflowdirection.Instraight-and-levelflight,theairflowrisestothewingleadingedgeandfallsbelowthetrailingedge,causingerrorsinflowdirectionmeasurements.Tosomeextenttheseerrorscanbestudiedinwindtunnels,butwind-tunnelmeasurementscannotreplacein-flightmeasurements.Accurateairdataarenecessaryformanypurposesandapplications.Obviously,thepilotcannotsafelyflytheaircraftwithoutknowingairspeedandpressurealtitude.Incivilaviation,thesmallverticalseparationbetweenflightlevelsassignedbyairtrafficcontrollersisbasedonaccurateknowledgeofpressurealtitude.Numeroussystems,suchasautoflightcontrols,enginecontrols,cockpitandcabinenvironmentalcontrol,weaponsdelivery,navigation,andairtrafficcontrol,dependonaccurateairdata.Whenanunprovenairplaneundergoesenvelopeexpansion,2carefulattentionmustbepaidtoflightlimitsofequivalentairspeedtoensureflightsafety.Inflightresearch,mostmeasurandsarereferencedtoairdataquantities,andmanyparametersarenormalizedtodynamicpressure.Theaccuracyneededforaparticularapplicationdictateshowairdatashouldbemeasuredanddictatestheamountofcalibrationeffortrequired.References5and6specifytheaccuracylevelsrequiredofthepitot-staticsystembycivilandmilitaryorganizations.Flightresearchactivitiesmayrequirehigheraccuracies.TESTOBJECTIVESIfthelocationofthestaticportshasnotalreadybeenidentified,thenthefirstobjectivemustbetodeterminethebestlocationforthestaticports,thatis,wherethesmallestormostconstantpositionerrorsoccur.Oncethislocationisestablished,thecalibrationsofthetotalandstaticpressures,anglesofattackandsideslip,andairtemperatureshouldbedeterminedtoaccountforthedisturbingpresenceoftheaircraftintheflowfield.Thecalibrationsmustbeperformedundervariousflightconditionsofairspeedsandaltitudeaswellasaircraftattitudesandconfigurations(combinationsofflaps,gear,andexternalstores).MEASUREMENTANDCALIBRATIONOFAIRDATAQUANTITIESPitotPressuresThepitot,ortotal,pressureisthesumofthestaticpressureandthepressureriseresultingfromstagnationoftheairflow(dynamicpressure)inthepitottube.Totalpressureisgenerallyeasytomeasureaccurately;thelocationisnotcriticalaslongasthetubeopeningisoutsidetheaircraft'sboundarylayerandisorientedtotheincomingflow.Forwell-sitedoralignedprobes,thetotalpressureerrorisusuallynegligible.(Thisassumptioncanbecheckedbycomparisonwithaventuripitotorbyflyinginformationwithacalibratedpaceraircraft.)Theshapeofthepitottubeopeningdictatestheflowanglesatwhichthepitottubeworkswell.ForsupersonicflightFigure1.Airflowaroundanaircraftwing.9402263pitott