ou

.

NATIONALADVISORYCOMMITTEEFORAERONAUTICS

DESCRIPTION &ND

TECHNICALNOTE 3252

PRELIMINARY FLIGHT INVESTIGATION OJ?AN

INSTRUMENT FOR DETECTING SUBNORMAL ACCELERATION

DURING TAKE -OFF

By GarlandJ.MorrisandLindsayJ.Lina

LangleyAeronauticalLaboratoryLangleyField,Va

W ashhgton

Novembr 1954

-—_ .-. ._ .__. - ____ ._ .- ..__ ,. ____ . .. .. . ... ., -.,. - .-. —- -......5.____ _ ,-.... . ..-,

lZNATIONALADVISORYCOMMITTEEFORAERONAUTICS

fitiiFifiii00LL285

TECHNICALNOTE3252

DESCRDTIONANDPRELIMINARYFLIGHTINVESTIGATIONOFAN

INSTRUMENTFOR

By Garknd

DETECTINGSUBNORMALACCELEMTION

DURINGTAKE-OFF

J.MorrisandLindsayJ.Lina

An evaluationhasbeenmadeofa protoQpeinstrumentdesiweiitogivean immediateindicationof lossinairplaneaccelerationdueto “powerdeficiencyor increasedresistanceatanytimeduringtake-offatwhichthepilotstill.hasa choiceofconttiuingor stopping.Theprin-cipalcomponentsofthisinstrumentarea linearaccelerometerandapressurediaphragmcoupledtogethersothatthenormaldecreaseinaccel-erationwithincreasingvelociQduringtake-offis compensatedby theincreaseindynamicpressureinorderto givea constantpredictableindicatorreadingaslongas thethrustandresistancearenormal.

Satisfactoryoperationoftheinstrumentrequiresthatno substan-tialvariationinattitudeoftheairplaneoccursup to thespeedbeyondwhichthepilotcanno longersafelystopthetake-off.Measurementsmadeofattitudeangleandlongitudinalaccelerationsduringthetake-offonthreewidelydifferentQ_pesof tricycle-gearedairplanesindicatedthatatleastforthesecasesthevariationofattitudeanglewaswithintolerablelimits.A simplifiedproto@peoftheproposedinstrumentwastestedina tricycle-gearedjettrainer.Thetestsrevealeda low-frequencyoscillationintheindicationwhich,althoughundesirable,wouldnotundulyinterferewithreadingtheinstrument.Theindicationremainedessentiallyconstantthroughoutthetake-offup tonose-wheellift-offwhenfullpowerwasmaintained.Responseoftheindicationto simulatedpartialpowerlosswasimmediateandtheindicationwasconsistentforgivenpowersettingsindifferenttake-offs.

INTRODUCTION

Abilityofthepilottorecognizequicklyanyappreciabledeficiencyinairplameaccelerationduringtake-offisbecomingincreasinglyimpor-tantasperformanceduringtake-offbecomesmorecritical.Theuseofhighwingloadingsandwingswithlowermaximum-liftcapabilities,par-ticularlyonthenewer-jetairplanes,hasresultedin smallertake-off

.— — ——— ...— -—— —

2 NACATN 3252

performancemqginsonexistingrunways.Crasheshaveoccurredintake-offbecauseofthepilotfsapparentinabilitytorecognizethefactthattheairplaneperformancewaslessthanthatpredictedby theuseofavail-ablemeteorologicaldatiandtake-offcharts.Lossesinairplanetake-offperformancecsaoccurfroma lossinthrust,anincreaseinrollingoraerodynamicresistance,ormeteorologicalconditionsdifferentfromthoseusedinthetake-offcalculations.

An instrumentdesignedtoaidthepilotindetectingmalfunctionoftheairplaneduringtake-offhasbeenproposedintheLangleyFlightResearchDivision.Someprelhinaryflighttestsweremadetoverifyassumptionsonwhichthedesignoftheinstrumentisbased.A simplifiedversionoftheinstrumentwasthenconstructedandtistalledinan air-planeforevaluationintake-off.A descriptionoftheinstrumentandtheresultsoftheflighttestsarereportedherein.

A

ah

ax

b

CD

CL

F

Fst

dJ?/dq

‘De

$

sYMBc&s

effectivediaphragaarea

airplanelongitudinalaccelerationreferredtohorizontalplane,g units

longitudinalaccelerationreferredto airplaneaxes(accel-erometerresponse),g units

momentarmofdiaphragmaction

dragcoefficient

liftcoefficient

thrust

staticthrust

rateofchange

effectivedrag

ofthrustwithdynamicpressure

areaofairplaneincludlngfactorstotakeaccountofdecrementinthrustandfrictionwithincreasingspeedaswellasaerodynamicdrag, CDS- ~CLS+ ~

accelerationdueto gravi@.

. ..—. .—— — —.—

NACATN 3252 3

PT

K

z

P

q.

%

s

w

w

v

v

5

e

P

off,

totalpressure,~ + p

springconstantofdiaphragm

momentarmofaccelerometermass

staticpressure

-c pressure,&

impactpressure(approxM3telyequalto q forspeedsattainedduringtake-off)

wingarea

airplaneweight

accelerometer-elementweight

airspeed

coefficientoffriction

deflectionofdiaphragm

betweenwheelsandground

attitudeangleof

massdensity

BASIC

accelerometerunitfromhorizontal

DESIGNCONSIDERATIONS

Theimmdiaterecognitionofa malfunctionofanairplaneintake-whethercausedby lossofthrustor increasedresistance,isdepend-

entonthesensingof-achangeinacceleration.However,a simplelon-gitudinalaccelerometeralonewouldnotbe suitableas a detectorsincetheaccelerationofanairplanenormallydecreasesintake-offas aresultof increasingaerodynamicdraganddecreasingnetthrust.Thepilotmight,therefore,be unabletodistinguishbetweena deficiencyinaccelerationduetomalfunctionandthenormaldecreaseinaccelera-tionafterthetake-offisunderway.

However,by incorporatinga dynamic-pressuresensingelementwiththelongitudinalaccelerometer,thedecreaseinaccelerationwithincreasingspeedcouldbecompensatedfortogivea constantpredictablereadingaslongastheairplanefunctionsproperlywithinthelimits

— —.. .—z —— —

4 NACATN 3252

prescribedherein.A lossinaccele=tionatthestartofandduringthet&e-offwouldthereforebe quicklydetectableas a departureoftheindicationfromtheexpectedconstantvalue.An obviousshortcomingintheuseofanaccelerometerelement,ifit isfixedwithrespecttotheairplane,isthatitwouldrespondnotonlytohorizontalaccelera-tion,asdesired,butalsotovariationsinattitudeangleoftheair-plane.Itwasthought,however,thatwithmny modernairplanestheattitudeanglewhiletheairplaneisonthegroundisconstrainedwithinSW ldmitsby thearrangementof thetricycle-orbicycle-typegearssothatvariationsinattitudeangleduringthepartofthetake-offofprimaryconcern(uptothecriticalspeed,thatis,thespeedbeyondwhichthepilotcannolongerstopthetake-off)mightnotbe a seriousproblem.Thevalidityofthisassumptionisdiscussedlater.

Equationofmtion oftheairplane.-‘Themotionofanairplaneacceleratinginthe”take-offata constantattitudeanglemaybe expressedas

Wah=F6t-~q-dq

C&q - V(W- C!Lsq)

whichmaybe rearranged

Fstah.~-~

( )-p@-pc!.&+!g

andforsmll angles

ah.~-e”

where 6 ismeasuredinradians.Now,ifa constantinthetake-off,a constantfrictioncoefficient,and

(1)

(2)

angleofattacka constantvalue

of dF/dqsreassumed,theterm C@ - ~CLS+ ~ canbe replacedby a—

constantwhichforconveniencemaybe designatedeffectivedragarea ‘Deoftheairplane.Assume,inaddition,thattheaccelerometerunitisinitiallydinedwiththehorizontal(9= O)andthattheattituderemainsconstant,then

-— —

NACATN 3252 5

andfora giventake-offcondition(fixedvalueofgrossweightandstaticthrust)

~+#fDe = constant

Theterms“ineqyation(3)areillustrated

(4)

h figure1,whichshows

thevariationsof ~ and ~ De thathavebeencalculatedforthe~f

take-offofa multienginedjetbonber.Resultsareincludedtorepresentbotha properlyfunctiordngtake-offanda take-offinwhichanenginefailureoccursat a dynamicpressureof19poundspersquarefoot(indi-catedairspeedofabout75knots).

Equationsofforcesintheinstrum-ent.-Thebasicdesignoftheinstrumentisillustratedschematicallyinfigure2. As shownintheillustration,a constantreadingonthescaleismaintainedina normaltake-offby anequilibriumofmomentsdueto forcesresultingfromlon-gitudinalacceleration,fromimpactpressureactingonthediaphragm,andfromthespringforceofthediaphr~. A constantdialreadhgismaintainedina normaltske-offwithinlimitspresentedbecausethedecreaseinthemomentoftheaccelerometerelementiscompensatedforduringthetake-offby an increasedmomentfromtheairspeeddiaphragmresultingfromincreaseddynamicpressure.Thisprincipleofcompen-

satingforcechangesmustbe relatedto theadditionof ~ and ~ Deif

as,forexample,showninfigure1. Theequilibriumofmomentsintheinstrumentisgivenbythefollowingequation:

(Momentfromaccelerometerelement)= (Momentfromairspeeddiaphragm)

~Z = -b(Aq- K5) (5)

or

.

Inorderthat 5tion(6)forthefortheairplane

bAq bK5++T=T (6)

maybe independentofforwardspeed,thetermsinequa-instrumentMUStbe relatedtothetermsinequation(3)inthefollowingmanner:

. .——. ,-—.. —..—— — z —.——

6 NACATN 32>2

bA fDe—= —W2 w

or

(7)

z -WAb‘De

(8)

Then,thetermfortheinstrument,ontheleft-handsideoftheequation,mustbe relatedto thetermfortheairplane,ontheright-handsideoftheequation,asfollows:

cotiiningequations(8)

Equation(8)showsthat

FstbKb—=WV (9)W7,

and(9) .-

(b=~F~t

)- pw (lo)

~De

themomentarmoftheaccelerometerelementintheinstrmentmustbe adjustableto allowforvariationsingrossweightandeffectivedragareaoftheairplane,andecyxation(10)indicatesthatwiththisadjustmentthedeflectionoftheneedleonthedialoftheinstrument‘willbe independentofforwardspeedandproportionaltothezero-speedexcessthrustsincetheotiertermsareassumedconstant.Itcanbe seenthata givendecrementinaccelerationsuchasmightresultfroma lossofthrustwouldbe indicatedquantitativelyasa comparablepercentagechangein diald.eflection~

Additionalconsiderationsinthedesign.- TheadjustmentofthearmlengthZ (fig.2)canbemadea functionofweightchangealoneiftheassmptionismadethatfora givenairplanetheeffectivedragareawillbe fixed,thatis,thatthedragconfiguration,includingflappositionandexternalstoreinstallations,willnotbe changed.Otherwise,therelationbetweenarmlengthandairplaneweightwouldhavetobe changedinaccordancewithequation(8).Theassumptionofconstanteffectindragarea ‘De fora givenairplanealsoimpliesthatthevalueof dF/dqorrateof changeofthrustwithdynamicpressureincludedinthisfactorisindependentof speednot&ue. However,the

andatmosphericcontributionof

conditions. ThisimplicationisdF/dq tothevalueof fDe is

.

—.

NACATN3252 7

generallysmallandtheeffectofvariationsinthisfactorontheindi-cationsoftheinstrumentshouldbe sufficientlysmalllto Justifythisassumption.

Thediscussionofthetake-offindicatorsofarhasbeenbasedonthepremisethattheweightoftheairplaneattake-offwillalwaysbelmownwithincloselimits(1or2 percefi).Iftheweightestimateisinerror,thenthetake-offdistancepredictedfromtheregularairplanetake-offchartswillalsobe inerror.Iftheweightsettingofthetake-offindicatorinvolvesthesameerror,thenthedeflectionoftheneedleonthedialwillbe lessorgreaterthanthepredeterminedvaluedependingonwhethertheweightisunderestimatedoroverestimated,respectively.,Theindicationwouldthereforebe intherightdirectionto showwhethertheactualtake-offwouldbe longeror shorterthanpre-dictedbutitwouldnotbe quantitativelycorrect.Thatis,a givenpercentagedifferenceineitherinitialexcessthrustorweight(forafixedweightsettingoftheinstrument)wouldresultinthesamepercetiagechangein indicatorreadingbutintermsoftake-offdistancewouldcauseapproximatelythessmepercentagechangeinonecase(thrust)andapprox-imatelytwicetbepercentagechangeintheothercase(weight).Undercertaincircumstancestheinstrumentmightindicatea readinginthewrongdirectionwithrespecttothedifferenceintake-offdistance;forexample,iftheinitialexcessthrustwere7 percentinexcessoftheexpectedvalueandtheweightwereunderestimatedby 5 percent,theindi-catorreadingwouldshowa favomblemarginfromwhichitwouldbe expectedthatthetake-offwouldbe about3 percentshorterthanpredicted,whereasitwouldactua~ybe 3 percentlonger.Forthesuccessfuluseofthisinstrument,therefore,theweightoftheairphnemustbe knownaccurately.

Illustrativeexampleofthetake-offinstrument.- ~ orderto showa possiblegeneralarrangementofthetake-offindicatorincludingexternaladjustmentknobs,an illustrativeschematicsketchispresentedinfigure3. Manylibertieshavebeentaken,forconvenienceof illus-tration,withthesizesandarrangementofelementsofthemechanism;forexample,theaxisofrotationoftheaccelerometerelement,whichisshownhorizontal,wouldbe verticalinanactualinstrumentto avoidtheinfluenceofverticalacceleration.

Adjustmentoftheleverarmoftheaccelerometermassisaccomplishedby meansofan externalknobanda geartrain.Pushingintheexternalweightknobsimultaneouslyengagesthegearsystemsforadjustmentoftheleverarmandmovementoftheindicatorfortheairplaneweightscale.

Withtheadjusbentforairplaneweightproperlysetin,theposi-tionoftheindicatorneedleduringtake-offwillthenbea functiononlyof theinitialexcessthrust(fora fixedaerodynamicconfiguration),thatis,thedifferenceof thewheels.Because

betweenthestaticthrustandtherollingfrictiontheexpectedfull-throttlestaticthrustofthe

—

8 NACATN 3252

enginesisa lmownfumtionofalamsphericpressureandtemperature,thisinformationcanbe incorporatedina simplechart,accountbeingtakenoftheeffectofweightonroldingfriction(thecoefficientofroU@ frictionp can generallybe takenas 0.02fordryconcreterun~ys)andthecalibrationconstantsoftheinstrument.Fromthischartthereadingthattheinstrumentshouldhaveduringtake-off,iftheairplaneisfunctioningproperly,canbe determined.An exampleofthischartisgiveninfigure4. Theprocedureforreadingthechartisshownfora temperatureof @ F, a pressurealtitudeof3,000feet,anda weightof1~,000pounds;thesevaluesgivea dial-settingreading.of27.9.Thechartreadingcanthenbesettitotheinstrumenttopro-videa referencefortheactualreading.

Thepredictedpositionoftheindicabrneedleintal&-off,asdeter-minedfromthechart,canbe settitotheinstrumentby meansofanexternalknobgearedtoa rotatabledialonwhicharemarkedan arrowanda suitablescalearoundtherimstartingwithzeroatthearrow.Thedialisrotateduntilthemniberonthedialcorrespondingto thechartreadingisinlinewiththeindicatorneedle.Thissettingismadewiththeairplanestandingstillandwiththeenginesidldngor stoppedtoavoida nose-downnmmentduetobrakinganda possiblechsmgeinattitudeanglewhichisnotpresentwiththeairplanerunningwithbrakesreleasedinthetake-off.Becausetheinstrumentissensitiwto changesinattitudeangle,asdiscussedpreviously,settingthe&Ml_withrespecttothestaticpositionofthetidicatorneedleshouldautomaticallycom-pensateforsmallvariationsingroundattitudeanglecausedby variationsinairplaneloading.

Ina take-offtheindicatorneedle~ swingquicklyfrom“itsstaticpositiontm alinementwiththearrowonthe’dialiftheairplaneisfunctioningaccordingtoexpectations.fithethrustisbelownormalorthereisexcessiveresistance,theneedlepositionwillfallshortofthereferencepointer;thusa longertake-offrunthanexpectedis indicate&

- orderthatanindicationof stinormalperfomnancemighthavesomesignificancetothepilotintermsoftheincreaseintake-offrunthatwillbe required,someprovisiontomeetthisneedshouldbe incorporatedintheinstrument.Itwasfoundfromcalculationsthat,forallcondi-tionsunderwhichtake-offsarelikelytobe rode,a givenpercentagechangeininitialexcessthrust(ortake-offindicatorreading)willresultinnearlya fixedpercentagechangeintake-offdistance.Itappearsthereforethat,ifa secondaryreferenceisprovidedontheinstru-mentdialwithprovisiontomaintainitatan intervalbelowtheprimaryreferencearrowwhichisa constantpercentageofthereferencearrowordialsetting,thisintervalcouldbe consideredasrepresenting,closelyenough,a constantpercentageincreaseintake-offrunforallconditions. (A@’picalvalueoftheintervalwouldbe about7 percentofthedialsettingfora 10-percentincreaseintake-offrun.) ~ the

2ZNACATN 3252 9

.

arrangementshowninfigure3,thissecondarymeansofa subdialgearedtothedial-settingthanthemainlllal.A segmentof contrastingdialispaintedonthesubdialandexposedbydialadjacenttotheprimEryreferencearrow.

referenceisobtainedbyknobata lowergearratiocolorto thatofthemaina windowcutinthemainTheextentofexposed

coloredregionistherebyautomaticallyadjustedtomaintainitat theproperpercentageoftheprimryreferencesettingwhentheprimaryref-erenceordialsettingisadjusted.Forapplicationswheretheexpectedrangeofdialsettingsisnotexpectedtobe large,itmy be sufficienttoprovidea fixedsecondaryreferenceonthemaindial.

Ifeliminationoftheuseofa chartisconsidereddesirable,arefinementoftheinstrumentispossiblethatwillallowthepilottomakethenecessarypreflightadjustmentsby meansofknobshavingscalesforairplaneweight,atmospherictemperature,andbarometricpressure.Theinstrumentwouldrequirea computingmechanismthatwouldaccountforthevariationsof staticthrustwithatmosphericpressureandtem-peratureandthevariationOfrollingresistancewithweight.

FLIGETTESTS

Measurementsto.ver~ basicassumptions.-Beforeconstructionofan instrument,f~ghttestswerermdetodeterminetheextentof atti-tudechangeduringthetake-offrunaswellasthelineari~ofthevari-ationofaccelerationwithimpactpressure.Measurementswerewde ofimpactpressureandlongitudinalaccelerationduringtake-offsofamultienginedpropeller-drivenboniberequippedwitha tricyclelandinggear.Extensivetistrmentationofthisairp- foranotherinvestiga-tionalloweddetailedmeasurementsof strutandtiredeflectionsandthuspermittedan accwatedeterminationofattitudeangle.Theresultsofthemeasurementsareshowninfigure5. Theattitudeangleremained

constantwithin0.004radian()to

throughoutthetake-offuntilaction

wastakento Hft thenosewheeljustbeforelift-off.Thisvariationinattitudeanglewouldcausean errorofonlyO.O@tgorabout1 per-centinthehorizontalaccelerationasmeasuredinthisa~~e ~th alongitudinalaccelerometerand,hence,is sufficientlysmdlto permittheuseoftheproposedInstrument.Thevariationofaccelerationwithimpactpressureis showninfigure5 forseveraltake-offs.Theseresultsindicatethattherelationbetweenaccelerationand@act pressureisessentiallylineqrandconsistent.Datapointsatthestartofthetake-offhavebeeneliminatedfromtheplotbecausethepilotwasusingbrakesto maintain heading.

Measurementsofalsoobtainedduringwithanafterburner.

longitudinalaccelerationandimpactpressureweretake-offofa tricycle-gearedjetfighterequipped!l?heresultsareshowninfigure6. Hereagainthe

10

variationinaccelerationwithdynamicpressureis

NACATN 3252

apparentlylinear.Thefluctuationofabout*0.Olgintheaccelerometerreadingaboutthelinearvaro~tionprobablyreflectsa rockingmotionoftheairplane/

(about* ~)which,althoughlargerthantheattitudevariationofthe

boriber,isstillwithinthel~ts reqpiredforsatisfactoryuseoftheproposedtake-offindicator.

Testsofprelfrnlnq instrument.-lTiththeevidencethatatleastforsomeairplanesthevariationof attitudeangleissmallduringasubstantialpartofthetake-off,thedesignandconstructionofa pre-liminaryinstrumentwasundertakenforevaluationofthepracticabilityofthedevice.l!bisinstrumentincorporatesthemainprinciplesoutlinedabove.Inasmuchastheinstrumentwastitendedonlyfortestpurposes,itdidnotincludeprovisionsforexternaladjustmentoftheaccelerometersensitivityto compensateforairplaneweightchangesormeansforsettinga referencepointerattheexpectedreding. Theindicatordialissimplymsrkedindegreesof anguh.rdisplacementoftheindicatorneedle.Aphotographoftheinstrumentis shownasfigure7.

Theinstrumentwiththeaccelerometer-elementsensittvi~adjustedaccordingto equation(8)fortake-offweightandestimatedeffectivedragareawasinstalledina jettraherequippedwithtricyclelandinggear.A preliminary-checkofthesettingoftheinstrumentwasobtainedby havingthepilotmonitoritduringa take-off.Provisionwasthenmadeforobtainingdetailedobservationsof itsperformanceby photo-graphingtheinstrumenttogetherwiththeengine-speedaridairspeedindic-ators. A 16-mi~imeterguncamerasetat 24framespersecondwasused.

Threetake-offsweremadewithinaboutl~hoursduringwhichtimeatmos-

phericconditionsremainedessentiallyconstmt.Theplanewasrefueledaftereachtslce-offh ordertomaintaintheweightasnearlyconstantaspossible.

Theresultsofthethreetestsareshowninfigure8 astimehis-toriesofthereadingsofthetake-offindicator,engine-speedindicator,andairspeedindicator.Airspeedbelow~ lmotswasnotplottedbecausetheairspeedindicatorwasnotdesignedtobe readbelowthisspeed.

Thefirsttestwasa normaltake-offinwhichabout99percentnormalenginespeedwasmaintainedup to anairspeedof ~ hots. Thetake-off-indicatorreadingremdmedatabout3800whichfromthecalibra-tionoftheinstrumentrepresentedan initialexcessthrustof

3,935P~ds. An oscillationofasmch astlOOorabout@Z percentof

fuld--scaledeflectionwassuperimposedonthemeanlevelofthereading.Thefrequencyoftheoscillationdidnotcorrespondto thenaturalfre-quencyoftheinstrument.Above~ hots thenosewheelwasapparently

.

—

NACATN-3252 IL

beginningtorise;thisrisegavea spuriousindicationof increasingaccel-erationorthrustasa resultoftheincreasingattitudeangle.

A partialpowerfailureduringtike-offwasshilatedforthesecondtest.Enginespeedwaskeptat 99percentas intheprevioustestup toan airspeedof 55knots.Theindicatorreadingwasverynearlythesameasbefore(within50ontheaverageorabout1 percentofthepreviousindication)andthusindicatesthattheattitudeangleoftheairplaneremainedessentiallythesameforsuccessivetake-offs.Whentheenginespeedwasreducedabruptlyfrom99to 90percentof ratedp-peed,thetake-offindicatorrespondedquickly;thatis,itchangedfroma meanlevelreadingof37Y to 2920. Thisbhsmgerepresentsa reductionininitialexcessthrustof1)050pounds.Thechangeh staticthrustfortheindicatedchangeinenginespeedisshownbytheenginemanualtobe1,150pounds.Whentheenginespeedwasrestoredto 99percentatabout80knots,thetake-offindicatorreadingreturnedto itsinitialvalue.Inthiscase,thenose-wheellift-offwiththeassociatedfalseindica-tionof increasedthrustapparentlystartedatabout85to 90knots.

A partialpowerlossthroughoutthetake-offwass-ted inthethirdtest.Enginespeedofapproximately94percentnormalwasmain-tained.Thistimethecamerawasturnedon intimetorecordthewindupofthetake-offindicatorneedle.Afterreleaseofthebrakes,theneedleqyicklyrotatedtoandremainedataround330°up toa veloci@ofabout60hots. ‘I!hen,apparentlyasa resultofa slightengine-speedincrease,”theneedlemovedto335°andremainedinthispositionuntila veloci~of 80hots wasreached.!llhenosebeganliftingatabout80lmots.Thereadingof330°onthetake-offindicatorrepresentsan Mtial excessthrustof 3,315poundsor a reductionof 575poundsfromthe99-percentengine-speedcondition.3brcomparison,thereductioninstaticthrustasesthatedfromtheenginemanualwas700pounds.

Thel-cycle-per-secondfluctuationintheinstrumentreadingwhichoccurredinalltestsisundesirablebuttolerdble.Thefalseindicationpresentedbytheinstrumentafternose-wheellift-offis startedisnotbelievedtobe a seriou#fault,particularlyffnose-wheellift-offisdelayeduntilafterreachingthecriticalspeed.

CONCLUDINGREMARKS

An instrumentisproposedwhichisdesi~edtopresenttothepilota constantpredictableindicationduringtake-offaslongastheairplaneisfunctioningnormallyaswe~ asto showa rapidproportionatechangein indicationifa malfunctionoftheairplanecausesa reductioninnetacceleratingforcebelownormal.

— — — —-- ——

12 .

Properfunctioningoftheinstrument,erometerunitasa basicelement,requirestheairplaneshouldnotvsxysubstantially

NACATN 3252

whichincorporatesan accel-thattheattitudeangleofduringthepartofthetake-

offofinterest,thatis,& to criticalspeed.“l%ke-offmeasurementswiththreetricycle-gesredairplanesincludinga largebomber,a jetfighter,anda jettrainerindicatedthat,forthesecases,atleast,thisrequirementwassatisfiedup tonose-heellift-offspeed.

A preliminarytestinstrument,lacldngtheexternaladjustmentandpresetting,provisionsreqtiedforan operationalversionofthedevice,wastestedina jettraineranditsperformanceappearedtobe satisfac-tory. Theindicatorrespondedqticklyto changesinthrustshulatingpartialpowerfailure.The‘=adingwasessentiallyconstantfora givenpowersettingup tothestartofnose-wheellift-off and was consistentforsuccessivetake-offs.A fluctuationoftheindicationoftheinstru-ment(1cyclepersecond)ofasmuchas3 percetiofthemeanreadingoccurredinalltests;al-thoughthisvariationisundesirable,itwasnotconsidereda seriousdefect.

~ viewofthesimplici~ofthisinstrument,itshouldbereliableandfreeof servicemaintenanceproblems.Itisfeltthata take-offindicatoroftheproposedtypemeritsconsiderationforimprovingsafetyinthetake-off,particularlyincaseswherethetake-offperformance mmaybewginal.

LangleyAerofiuticalIaboratoryiNational.AdvisoryComnittieforAeronautics,

IangleyField,Vs.,July22,1954.

-—— ——— —. — ____

,

{

I

I

I

I

.—— —— — ; IIe

/--/“

//~//”

////--

//

// /“r I I I I I I

0 10 20 30 40 m 60 70 80

Dynamic pressure, q , lb/sqft

uz

Figure l.- Calculated variations

and the term Qf~ De for the

with dynamic pressure of the acceleration

take-off of a multienginedjet Ixmher.

..D)rect)on of motion —

— r--w

1

Figme 2.- Schematic

).

o

b

J

sketch Of ths b=iC deSig Of take-Off ilmt?nlmnt.

t.

i“

@ wdgM+.j@nEcat MlcaIOr ‘ .

d9

Figure3.-L-85617

Illu6trativs arrangement of take-off indicator.

16 NACATN 5252

.

0’

c

.++’v

Weight, lb

180,0001/ 16aOO0

Y /40,000120,000,

20 22 24 26 28 30 32 34 36

Dial setting

Figure4.-Ts.ke-offchartforpositioninginstrument.

dialsettingofproposed

II

I

.35–

~5.30 - vb.G, o

.25-

.20 I I I I I

!.:=/23456

q, , in. FfzO

Figure ~.- Variation of longitudinal acceleration with impact pressurefor several ‘calw-offs,and variation of attlb.uieauk with imactpressure duri~ one &e-off of a multienglne prope~zwlriwn-b ember..

30 r 00~

o

0

,201 I I I I I I I I I I I I I0/23456 789/0///2/3

qt, In. HI O

Figure 6.- V.a+tion of longitudinal acceleration with impact pressureduring the take-off of a jet fighter.

I

I

I

I1

i

1

i

i

1

,.

.

1

,

*

I

I

0

440r,,,Arst tQke-off

400 – Third

360 – I

320 –

/

.-[

Toke-off indicotor

280b Fmt take-off% /-

Time, Sec

Fig-me 8.- TiuE history of read.lwy of thespeed indicator, and airspeed indicatortrtiner.

/6 f8 20 22 24

take-off indicator, engine-of three take-offs of a Jet G