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RESEARCH MEMORANDUMCIASSIF CATI N CHANGED

O.L+L#Q,~ h ~“ ‘........... ..........2..3-LBY AiJTHORIW OF ------- - / -a -W(d)

LONGITUDINAL-STABILITY CHARACTERISTICS OF THE

NORTHROP X-4 AIRPLANE (USAF NO. 46-677)

By Melvin Sadoff and Thomas R. Sisk

Ames Aeroxmxtical LaboratoryMoffett Field, Calif.

CLASSIFICATIONCHANGED

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NACA - Dryden/%@.Ey authorityof..c&?E8?..2A2----.-...-Da?.e..4:!2:?;..--

CLASSFISD INCllfrt3m

Thta matertal contains fnforxnation affecting b Mtlonal Defense of h Untt8d States wtti the meaningof tbe espionage laws, Title 18, U.S.C., Sscs. 79S and 704, Uw transmlmion or mvelatlon of whtch !.n anymanner to urmuthmfzed person ls prohibited by law,

NATIONAL ADVISORY COMMITTEEFOR AERONAUTICS

WASHINGTON

June 29, 1950.4

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I

i lIACARM A50D27

RATIONALADVTSORYCOMMITTEEFOR AERONAU1’ICS

RESEARCHMEMORANDUM

LONGITUDINAIAZABILIT’YCHARACTERISTICSOF THE

NomERoPx-4 AIRPiwwl (w MO● 4G677)

By Melvin Sadoff and ThoxmisR. Sisk

c

\ <

SUMMARY

The results obtainedfrom severalrecent flights on the T?orthropX-4No. 2 airplaneare presented. Informationis includedon the longitudinal–stabilitycharacteristicsin straightflight over a Mach number range of0.38 to about 0.63, the longitudinal-stabilitycharacteristicsin acceler-ated flight uver a Mach number range of 0.43 to about 0.79, and the short.-period longitudinal-oscillationcharacteristicsat Mach nurribersof 0.49ana 0.78.

It was shown that the stick–fixedand stick-freestatic longitudinalstability,as measured in straightflight,were positiveover the testspeed range with the center of gravity locatedat about 18.o percent ofthe mean aerodynamicchord.

During the longitudinal-stabilitytests in acceleratedflight aninadvertentpitch-p of the airplane occurredat a Mach nwiber of about0.79 and a normal-forcecoefficientof about 0.45 (normalacceleratimfactor,Az = 7), in which the accelerationbuilt up rapidly to Az = 6.2(whichwas in excess of the load factor, 5.2, required for dem~strationof the airplane)before recoverycould be initiated.

A comparisonof the experimentallydeterminedeleven angles requiredfor balance and the elevcm+mgle gradientswith values estimatedfromlimitedwind-tunneldata showedfairly good agreement. Wind–tmel tits,however,were not availablein the region where the pitch+zp occurredsothat an evaluationin this regard was not possible.

The shor&period oscillationwas lightly damped and did not meet theAir Force requirementsfor satisfactoryhandlingqualities. The pilot,however, did not object to the low damping characteristicsof this air-plane for small+mplitude oscillaticms. Theory predictedthe period ofthe short-periodlongitudinaloscillationfairly well; however,thedamping evaluatedfrom the theory indicatedcasiderably greater dampingthan was actuallymeasured in flight, especiallyat the higherMach numbers.

I

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INTRODUCTION

The X-k airplanewas constrwted as part of the JointAir Force -Navy - NACA researchairplaneprogram to provide research informationon the stabilityand control characteristicsof a 6emitaillessconfig–uration at high subsoniclkch numbers.

The airplane is currentlyundergoingdemonstrationflfght tests by theNorthropAircraftCorporationat Edwards Air Force Base, Muroc, California.During these tests NACA instrumentshave been installedfor the measurementof stabilityand control characteristics. Previousresults on the X-kairplane ue presentedin references1 through6. The present report pre–

sents some results of the measurementsof the longitudinal-stabilltycharacteristicsof the airplane, which were obtainedin flkhts 12. 1?.. 4.

and 15

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s

M.A.C.

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of the acceptancetests of the secondX--kairplane (USAFNo. 4&677).

snmoLs

indicatedairspeed,miles per hour

pressurealtitude,feet

normal accelerationfactor (theratio of the net aerodynamicforce along the airplaneZ axis to the weight of the air–plane)

lateralaccelerationfactor

longitudinalaccelerationfactor

Mach number

dynamic pressure, pounds per square foot

stick force, pounds

wing area, square feet

wing mean aerodynamicchord, feet

airplaneweight, pounds

eleven hinge moment, inch-pounds

rudder hinge moment, inch-pounds

NACA RMA50D27

I

3

L

R

pitchingvelocity,radians per second

rollingvelocity,radians per second

period of longitudinaloscillation,seconds

time to damp to one4alf amplitude,seconds

effectivelongitudinalcontrolangle(’% ; 5eR)~ ‘ePees

effectivelateralcontrolangle, degrees

rudder single,de~ees

sideslipangle, degrees

WAZ

()normal force coefficient —

qs

stick–forcefactor, feet squared

Subscripts

left eleven

right eleven

AIRPLANE

The NorthropX4 airplane is a semitailless.researchairplanehavinga vertical tail but no horizontal-tailsurfaces. It is poweredby twoWestinghouseJ–304E-7-9 enginesand is designedfor flight research inthe high subsonicspeed range. A three-viewdrawing of the airplaneisshown in figure 1 and photographsof the airplaneare presentedas figure 2.The physicalcharacteristicsof the airplaneare listed in table I.

INSTRUMENTATION

StandardNACA instrumentswere used to record altitude;airspeed;normal, longitudinal,and lateralaccelerations;right and left elevenpositions;rudder position;sideslipangle; pitchingand rollingangular velocities;stick force;pedal force;and elevenand rudder hingemoments. In addition,normal acceleration,altitude,airspeed,rightand left eleven positions,and rudder positionswere telemeteredto a ground

4 ~cA RM AL50D27

station. All the internalrecords were correlatedby a common timer.The hinge+nomentdata are included in this reTort to show only quali–tative changes since there is some uncertaintyregardingthe validityof the absolutemagnitudesof the measured hinge moments.

The airspeedand altituderecorder is connectedto the airspeedhead on the verticalfin. This installationhas not as yet beencalibrated.

TESTS, RESULTS,AND DISCUSSION

Longitudinal-StabilityCharacteristicsinStraightFlight

The static longitudiml stabilityof the X-h airplanewas measuredin straightflight by trimmingthe airplaneat approximately32> milesper hour and then making steady runs at 20-mile-per40ur incrementsovera speed range from about 220 to 400 miles per hour. Tests were conductedat 10,,000and 15,000 feet pressurealtitude. The results of these meas–urements are shown in figure 3 where the eleven control positionand elevenstick force are plottedas functionsof indicatedairspeed,and where theelevon<ontrol ~ositionand stick-forcefactor F/q are plottedas functionsof normal force coefficient. The data show that the airplane is staticallystable stick fixed and stick free as shown by the increasingup-elevencontrolrequiredas the speedwas reduced, and by the pull forces requiredbelow trim speed and push forces requiredabove trim speed. The positivestabilityis also indicatedby the stable slopes of the variationof beand F/q with CN.

LongitudinalStabilityCharacteristicsinAcceleratedFlight

The longitudinal-stabilitycharacteristicsof the X-4 airplaneinacceleratedflight were measured in steady turns and gradual pull-ups.Measurementswere made at steady incrementsof accelerationfrom trimmedconditionsat a Mach number of 0.44 at 10,000 feet and at severalMachnumbers from 0.5 to 0.79 at 20,000 feet. For the most part, data presentedfor values of normal+accelerationfactor less than 2 were obtainedinsteady turns while the data for values of normal accelerationfactor above2 were obtainedin gradual pull+ps.

Figure 4 gives severalrepresentativetime historiesof Mach number,elevo.~stick force, eleven position,normal accelerationfactor, andnormal-forcecoefficientduring typicalacceleratedstabilityruns.These data in time-historyform sho .~~.”terestingitem in connection

‘Ec&j&k~

HACA RM A50D27

I

5

with the booster control system. There are appreciablefriction (*5 lb)and inertiaforces inherentin the hydrauliccontrolsystem,with theresult that the eleven positiondoes not necessarilyfollow the appliedcontrolforces. This characteristicof the control system can be easilyseen in this figurewhere the eleven continuesto move in the upwarddirectionalthoughthe applied controlforce is being decreased. Thischaracteristicof tne control systemmakes the airplanevery difficultto trim for given flight conditionsand has been a source of annoyanceto the pilot. The data in figure 4 show, however, that the airplane,aerodynamicallyspeaking,has normal controlcharacteristics.

At normal-forcecoefficientshigher than those obtainedin the runsgiven in figure4 for a Mach number of 0.79 a longitudinalinstabilitywas encountered. A time history of this phenomenonis given in figure 5.In figure >(a), which gives the quantitiespertainingto the longitudinalcharacteristics,it can be seen that, althoughthe eleven-controlmotionwas stoppedat 0.4 secondwhen 5 AZ wa: reached,the airplanecontinuedto pitch upward. At 0.8 second,when 5.3 AZ was reached,the pilot abruptlydeflectedthe elevensdownwardbut the airplanedid not responduntil avalue of 6.2 AZ was reached at 1.3 seconds. The pilot reportedno warningsuch as buffetingbefore the airpl=e began to pitch upward,but did reportthat the right wing tended to drop as the pitchingbecame evident. Theaccelerometerrecords taken during this run showed that a slightbuffetingbegan at about 0.5 &econdprior to the lmgitudinal instabilityand coptinued well into the recovery. The wing heavinessreportedby the pilot,however, is evident in figure >(b) which gives the lateraland directionalcharacteristicsmeasured. The recoveryfrom this unstableconditionwasmarked by an oscillationabout all three sxes of the airplane,which thepilot probablyreinforcedby abrupt controlmotions. The objectionablelarg~+amplitudelongitudinaloscillationswhich were sustainedduring thelatter part of the recoveryapparentlyresultedfrom the poor damping-in-pitch characteristicsof the airplane. This point will be discussedmorefully in a subsequentsection. It shouldbe noted that, as in figure 4,the elevo~ontrol motion does not follow exactlythe controlforce. Themaximum value of normal+cceleration factorreached (6.2)was in excessof that requiredfor demonstrationof the airplane (>.2).

From the data given in figures 4 and 5 and similardata not presented,figure 6 was prepared,which gives the variationof elevo~ontrol anglewith normal-forcecoefficientand the variationof eleven stick force withnormal accelerationfor the Mach numbers tested. These data show that forthe Mach number range coveredand for values of normal force up to about0.45, the airplaneis longitudinallystable stick fixed and stick free.At a Mach number of 0.79, the airplane is shown to be unstableabove anormal-forcecoefficientof 0.45. The data illustratingthis longitudinalinstabilitywere tsken from the run presentedin figure > prior to theabrupt controlmotions,but because of the abruptnessof the pitchingmotion do not necessarilyData from earlierflights

show the exact eleven angles requiredfor balance.(reportedin reference6) show that at lower

6 NACA m A50D27

Mach numbers (M = 0.28) the airplanedoes not exhibita longitudinalinstabilityeven at values of normal<orce coefficientapproachingthosefor stall (CN = 0.85). Wind-tunneltests of en X-4 model at low Machnumbers (reference7) indicatedthat chordwisefences would be requiredto eliminatelongitudinalinstabilityat the stall. It is possiblethatthe fences are effectivein delayingthe longitudinalinstabilityat lowMach numbers,but that increasingthe flight Mach number decreasestheeffectivenessof the fences.

The apparentlongitudinalstabilityof the X-4 airplanein acceleratedflight is illustratedin figure 7 where values of dbe/~, as determined.from the data of figure 6, are plottedas a function of Mach number forvalues of normal-forcecoefficientup to 0.4. The stabilityof the air–plane at a normal<orce coefficientof 0.5was measuredonly at M = 0.28(datafrom reference6) and M = 0.79, and the values of d5e/~ underthese ccmiitionsare indicated. The data given in figure 7 show that thelongitudinalstabilityof the X-4 up to a ~ of 0.4 is essentiallycon–stant .rithMach number up to a Mach number of 0.79. At a nor?ml-forcecoefficientof 0.5, the airplane stabilityvaries from a positivevalueat M = 0.28 to a negativevalue at M = 0.79. The exact vsriationwithMach number is not knuwn since,as mentiunedabove, data were availableonly at two Mach numbers.

Comparisonof Experimentaland EstimatedData

—A comparisonof the experimentalelevenangles requiredfor balance

at severalvalues of CN and the elevon+ngle gradientswith valuesestimatedfrom the wind-tunneldata in reference8 is presentedin figure &The elevon+ngle data are comparedin figure 8(a),while the comparisonof the control-anglegradientsis shown in figure 8(b). The experimentaleleven-angledata were derivedas a cross plot of the data in figure 6and from other data not presented (fromreference6).

The agreementshown between the estimted and the experimentaleleven angles and elevon+ngle gradientsis consideredfairly good Inview of the fact that the wind-tunneldata, obtainedwith a center ofgravity at 21.5 percent of the M.A.C.,were correctedto an averageflightvalue of 18.5 percentof the M.A.C. Unfortunately,no wind-tunneldatawere availablein the CN range above 0.4 without a doubtfulextrapo-lation of the data, so no reliablecomparisoncould be made at the valuesof CN and M where the longitudinalinstabilitywas encounteredinflight.

IS=uiii&”

Dynamic Longitudinal-StabilityCharacteristics

RACA RMA50D27

I

7

A measure of the dynamiclongitudinalstabilityof the X-h airplanewas obtainedin longitudinaloscillationswhich were excitedby abruptlydeflectingthe eleven controland returningit to trim positionat Machnumbers of 0.49 and 0.78. Time historiesof thf3Be oscillationsare givenin figure 9. It can be seen from the data in this figure that the X-hairplanewill not meet the requirementsfor satisfactorydampingof thelongitudinalshort-periodoscillationwhich requiresthat the oscillationdamp to one–tenthamplitudein one cycle (reference9). The pilot did notconsiderthe dampingcharacteristicsof the airplaneobjectionableforthese small amplitudeoscillations. However,as was pointed out previ-ously, the poor dampingcharacteristicswere objectionablefor large ampli–tude oscillations. The period P and the time to damp to one-halfamplitudeT1/2 were determinedfrom these oscillationsand are presentedas func–tions of Mach number in figure 10. Also presentedin this figure are thevariationsof period and time to damp to one-hall?amplitudewith Machnumber as computedby the methods of reference10. The data in thisfigure show that the theory predictsthe period of the oscillationfairlywell but that it overestimatedthe damping,especiallyat high Mach numbers.

CONCLUSIOITS

The results of the longitudinal+tibilitymeasurementsobtainedcmthe secondX~airphe duringflights 12, 13, and 15 showedthe following:

1. wit~ the airptie center of gravity at approximately18.o percentof the M.A.C., the stickgixed and stick-freelongitudinalstabilityinstraightflight were positiveover a &ch number range of 0.38 to 0.63.

2. In acceleratedflight, the airplanewas stableup to values ofnormal<orce coefficientof 0.4 throughoutthe speed rmge from Mach num-bers of 0.h4to 0.79. At a Mach number of 0.79, the airplanebecameunstableat higher values of normal-forcecoefficientand a violentnose-up pitchingwas encountered.

3* In the run where the longitudinalinstabilityoccurred,the air-plane reached a normal accelerationfactor of 6.2 which is in excess ofthe load factor requiredfor demonstrationof the airplane (5.2).

4. The elevenangles requiredfor balance at severalvalues ofnormal<orce coefficientand the elevon+ngle gradientswere estimatedfairly well from availablewind-tunneldata over the Mach number smi n~l-force-coefficientrange considered.

5. The short-periodlongitudinaloscillaticmis lightlydamped anddoes not meet the Air Force requirementsfor satisfactoryhandlingqulities.

-.

●

8

The pilot, however,did not object to low dampingcharacteristicsof thisairplanefor small-amplitudeoscillations.

6. The theory estinatedthe period of the short-periodlongitudinaloscillationfairly well; huwever,it overestimatedthe damping,especiallyat high Mach numbers.

Ames AeronauticalLaboratory,NationalAdvisoryCommitteefor Aeronautics,

Moffett Field, Calif.

lamERENcEs

1. Drake, Hubert M.: Stabilityand ControlData ObtainedFrom FirstFlight of X-k Airplane. NACA RM L9A31, 1949.

2. Williams,Walter C.: Results ObtainedFrom SecondFlight of X-4Airplane (A.F.No. 4U76). HACA RM L91?21,1949.

3. Williams, Walter C.: Results ObtainedFrom Third Flight of NorthropX-4 Airplane (A.F.No. 46-676). NACA RM L5G20a, 1949.

4. Valentine,George M.: Stabilityand ControlData ObtainedFromFourth and Fifth Flights of the X-4 Airplane (A.F.No. 46-676).KACA RM L$1225a,1949.

5. Matthews,James T., Jr.: Results ObtainedDuring Flights 1 to 6 ofthe NorthropX-4 Airplane (A.F.Ho. 4&676). NACA RM L9K22, 1~0.

6. Sadoff,Melvin, and Sisk, Thomas R.: Stall CharacteristicsObtainedFrom Flight 10 of NorthropX-4 No. 2 Airplane (USAFNo. 46-677).NACARMA50A04, 19.

7* Pass.,H. R., and %yes, B. R.: Wind-TunnelTests of a l/4-ScaleModel of the NorthropXS-4 Airplane. Rep. Ho. A+fT~6, NorthropAviationCorp., 1947.

8. PreliminaryReport on HighApeed Wind–I’unnelTests of a l/&ScaleReflectionPlane Model of the Northrop~ Airplane. Rep. Ho. 69,Swthern CaliforniaCooperativeWind Tunnel,Calif. Inst. Tech.,Pasadena,Calif., Oct. 28, 1948.

9. Anon.: Stabilityand ControlCharacteristicsof Airplanes. AAFSpecificationNo. R–181X, A~. 7, 1945.

10 ● Greenberg,Harry, and Sternfield,Leonard: A TheoreticalInvesti-gation of LongitudinalStabilityof Airplaneswith Free ControlsIncludingEffect of Friction in Control System. NACA Rep. 791, 1944.

9

TABLE I. - PHYSICALCHARACTERISTICSOF X-4 AIRPLANE

Engines (two).. . . . . . . . . . . . . . . WestinghouseJ–30-WE-7-9

Rating (each)staticthrust at sea level, pounds . . . . . . . 16OO

Airplaneweight (averagefor flights 12, 13, and 15), pounds

Maximum (238galfuel) . . . . . . . . . . . . . . . . . . . . 7847Minimm(10 galtrappedfuel) . . . . . . . . . . . . . . . . 6477

Wing loading (averagefor flights 12, 13, and 15),pounds persquarefoot . . . . . . . . . . .

Maximum. . ● * . . . * ● ● ● ● ● . ● ● ● ● ● .* ● * ● 9 ● 9 39.2

Minimum. . . . . . . . . . . . . . ., . . . . ● ● . ● ● * ●32.4

Center+f~avity travel (averagefor flights 12, 13, and 15),percen’tiM.A.C.

Gearup,fullload. . . . . . . . . . . . . . . . . . ● . c ●19.10Gearup, post flight . . . . . . . . . . . . . . . . . . . . .17.10Gear down,fullload. . . . . . . . . . . . . . . ● . c ● ● ●19c40

Gearduwn, post flight . . . . . . . . . . . . . c ● ● o 0 ● . 17.50

Height, over-a~,feet. . . . . . . . . . . . . . . . . ● S 0 C 14.83

Length, over-all,feet . . . . . . . . . . . . . . . . . . . . .23.25

wing

Area, squarefeet... . . . . . . . . . . . . . . . . . . . . . 200Span,feet. . . . . . . . . . . . . . . . . . . . . . . . . .26.83Airfoil section. . . . . . . . . . . ., . . . . . , NACA ooltikMean aerodynamicchord, feet . . . . . . . . . . . . . . . . . 7.81Aspect ratio. . . . . . . . . . . . . . .. . . . . . . . . . . 3.6Rootchord,feet. . . . . . . . . . . . . . . . . . . . . . .10.25Tipchord,feet. . . . . . . . . . . . . . . . . .P . . . . .4.67Taper ratio. . . . . . . . . . . . . . . . . . . . . . . . . 2.2:1Sweepback (leadingedge), degrees . . . . . . . . . . . . . . 41.57Dihedral (chordplane), degrees . . . . . . . . . . . . . . . 0

Wing boundary-layerfences

Length, percentlocal chord . . . . . . . . . . . . . . . . . . 30.0Height, percentlocal chord . . . . . . . . . . . . . . . . . . 5.0Locationjpercent semispan . . . . . . . . . . . . . . . . . . ~.O

10

TABLE I. -CONCLUDED

l’iACARM A50D27

wing f laps (Wwt)

Area, squarefeet... . . . . . . . . . . . . . ●..*. 0 I-6.7Span, feet . . . . . . . . . . . . . . . . . . . . . . ...8.%Chord, percentwingchord . . . . . . . . . . . . . . . . . . 25Travel,degrees. . . . . . . . . . . . . . . . . . . . . . . 30

Dive+mdce dimensionsa8 flaps

Travel, degrees. . . . . . . . . . . . . . . . . . . . . . . *6o

Elevens

Area (total),squarefeet . . . . . . . . ..OO. . . ...17.20Spe31(2elevons), feet.. . . . . . . . . . . . . . . . ..15.45Chord, percentwingchord . . . . . . . . . . . . . . . . . . 20Movement,degreesUP. ● ● **. ● . . . ● ● ● . ● ● ● . ● . . . ● ● ● . . . ● 35Down. . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Operation . . . . . . . . . . Hydraulicwith electricalemrgency

Vertical tail

Area, squarefeet. . . . . . . . . . . . . . . . . . . . . . .16Heightjfeet. . . . . . . . . . . . . . . . . . . . . . . . 5.96

Rudder

Area, squarefeet. . . . . . . . . . . . . . . . . . . . . . 4.1Span,feet. . . . , . . . . . . . . . . . . . . . . . . . . 4.3Travel, degrees. . . . . . . . . . . . . . . . . . . . . . *3OOperation. . . . . . . . . . . . . . . . . . . . . . . . Direct

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