Copy No. 102

RM No. L7A08.! T

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RESEARCH MEMORANDUM-

EFFECT OF NUMBER OF FINS ON THE DRAG OF A POINTED

OF REVOLUTION AT LOW SUPERSONIC

By

N. Mastrocola

Langley Memorial AeronauticalLangley Field, Va.

VELOCITIES

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April 7, 1947 .—

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RESEARCHMEMORANDUM

EFFECTOF NUMBEROF FINS CN THE DRAG OF A P03NTEDBODY

OF REVOLUTIONAT LOW SUPERSONICVELOCITIES

By N .Mas.trocola

Resultsof flighttestsconductedat the test stationof theLangleyPtlotlessAircraft Research.2iviE!ionat WallopsIsl.amd,Va’.,to detemine the effectof numkerof fins on the drag of a pointedbody moving” at 10V supersonicvelocities,axe presetitedc

The test data indicate that the interferencedrag increa~edwith increasednumberof fins up-toa Mach numberof 1.35;above “,thisvaluethe effectis reversedto the end of the testMach‘numberrange. The ma~itude of interferenceeffects,for the,blufffinsectionsused h thesetests,is such as to make theseeffectsimportantin Ostimating“the.drag ,of a, rnultffh tail @oup c me f~.dragwas fouridto @e comparativelyIergeand was attiibu~d to theblunt.‘leadingeitge’and equaretratlingedge of’the fin airfoilsection, ,Flight tkst&ta ‘ofa fifiessbody”are neededtothe interference effec,ts.” ~~.

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INTRODUCTION

The PilotlessAirc~”aft:iesearchDivisionofAeronauticalLaboratoryhas undertakena @neralto providedata pertainingto the drag of bodies

furthereyaltite,.. . . . —

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the LangleyMemorialflightinvestigationat supersonic

velocIties. As part of this investigationsome testsweremade toevaluatethe effectof numberof fins on the zero-~ft drag of apdnted body of revolutionat supersonicvelocities.

The test datareportedhereinare results from flighttestsofthree-and five-finbodiesat speedsup to a Mach numberof approxi-mately1.4. Data for a four-finbody, previously reportedinreference1, were reworkerland are also presentedfor comparisont

Theseflighttestswere conductedat the test stationof theLangleyPilotlessAircraftResearchDivisionat WallopsIsland,Va.

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BODIESAND T!!ISTS

Bodies.- Thotographeof the testbodieswith the three-,four-,and five-fin tail@ouys are presentedae figure1. The principaldimensionsof the body are givenfl,nfigure2 alongwith somedetaKlsof thefins. The simplifiedairfoilsectionof the finswas dictatedby theneed for a reflectorstripon thefin trailingedgeand bythe requirementfor an easilyfabricatedshape.

Each bot!y.was propelledby a 3.25-inch-diameterMk. 7 aircraftrocketmotorenclosedwithinthebody. At a preignitiontemperatureof 69oF therocketmotorsprovidedabout2200 poundsof thrust forapproximately0.87 second.

Tests.-Four testflightsweremade: twowith a three-fintdlend two with a five-fintail.

The three-and five-finbodies,as well as thelodieainreference1, were launchedat an elevatiw angleof 75° to thehorizontal.The trajectoryof thebodieswas approximatelya straightline duringthe coastingflight,afterthe propeK&nt was expended,becauseof the hi@ elevationangleand the shortlnumingdurationofthe rocketmotor. The flightvelocitywas measuredduringthe coastingflightby means of a CW Dopplerradarset @N/TPS-~)locatednear thepointof launching.

RESULTSAND DLgCUSSION

A discussionof the procedureemjloyedin the reducticmof dataobtainedby the CW Dopplerradarset to yield the variationof dragcoefficientwithMach numberis givenin reference1.

The variationof dragcoefficientwithMach numberfor the finarrangements‘tested”is givenIn figure3. Data for the four-finmodelwhich were previouslyreported(reference1) havebeen reworkedyieldingmore accuratepointsand sllghtlyalteringthemean curve.Thesedataare pres&ted in ffgure3 for c@pariscm. The dragcoefficientsare basedon the exposedarea of one ffn (0.237sq ft).Each of the presenttestcurvesis a mean throughthetest pointsobtainedfrom flighttestsof two identicalm6dels,

Becauseof the relativelylargescatterof the data showninfigure3, it appe4rsthatno clearquantitativeconclusionsas to the

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variaticmin interferenceMach numbercan he drawn,tie data can be made.

due to numberof fins or with changingbut some qualitativeobservationsof

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The incrementsbetweenthe curvesof figure3 are anindicationof the drag of one fin plus the changein interferencedrag (effectof %ody on fine and effectof fins on body end oneach other)causedby the changein numberof fins. These incre-ments plottedagainstMach number (fig. 41, ‘showthe variationof fti dragplus the additionalinterfermcedragresultlngfrmthe additionof a fin to a three-f3nand a four-fintailgroup.The differencebetweenthesecurves (fig.h),is a measureof theincreasein interferencedrag causedby an increasein numberoffins from four to five and the resultingrearrangementof the fine.These data indicatethat.tl.interferencedra~ increasedwithincreasednudberot fins up to a Mch nuuiberof approximately1.35where the curvescrossand the effectis reversed,ta the end ofthe Mach numberrangetested.

Becausethe interferencedrag increase is due to the changeof interferenceeffectson five fins causedby the rearrangementof the fins,ally 1/5 of the drag incrementcan be attributedtoone fin. Then, tie increaseof interferencedrad of a singlefinin a five-fintailgroupis approximately8 yercentat M = 1.0and O percentat M = 1.35 based on”thedrag of a a5nglefinplus the interferencedrag in a four-fin~oup. The magnitudeoftiheseeffects,for thebluff fti sectionused, is largeenoughtomake them imyortentin estimatingthe drag of a multifintail group.

The lowercurveof fi~urek may alsobe consideredta be ameasureof the drag of one fin if the interferencedrag of a four-fin grou~ is assumedto be,very small. This curve,then,can beused in conjunctionwith the three-findata of figure3 to obtainthebasic‘bodydrag. The variationof body dragwith Mach number,computedin the abovemanner,is shornin figure5. The body dragcurveis differentfrom the curves for similaibodiesat supersonicspeedsapparentlybecausethe assumptionof negligibleinterferenceeffectson a four-fintailgroupIs false, It may be concludedthen,that the interferencedrag of a four-fintail group,with abluntfin section,is not smalland thatthemagnitude of the effectscannot be determinedunlegsdrag datafor a finlessbody are madeavailable. Such datawouldallow the separationof fin and bodydragand so permitthe evaluationof the effectsof nuniberof finson the interferencedrag. The inherentdifficultiesinvolvedinflyinga finlessbody,however,have delayed.attemptsto obtainsuch databy means of bilotlessaircraftflights.

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NACA RM No. L7A08

The dragcoefficientfor a ‘singlefin (fig.J+)is of theorderof O,04or 0.05. Thesevaluesare wmnpa&htZVe~liighforthe aspectratioand angleof sweepbackemployedcm the fin andmust be attributedto the fin airfoilsection, The blunt leadingedgeand Wlicksquaretratlingedge incorporatedIn the sectionmake high values of d~ag seem veryprobable. It Is advisable,therefore,to use a fin sectionmore suitedto supersonicflightif otherfln requirementscan alsobe fulfilled.

CONCLUDINGREMARKS

The flighttestsreportedherein,alt.boughnot sufficientforthe quantitativeevaluationrf fin-interferenceeffects,do ~ovidesomequalitativeinformation:

1. The magpitudeof fin and body interferenceeffects,for ablufffin section,appearshigh enoughto make theseeffectsimportantin estimatingthe dragof a nmltifintail group.

2, The ~terfel*ence drag Wcreased with increasednrmberof‘fins up to a Mach numberof.1.35; abovethisvaluethe effectis ~reversedto the end of the testMach numberrange.

3. The fin dragwas found to.becomparativelylargeand wasprobablycausedby thebluntleadingedge ad thicksquare&ail.ingedge of the fin airfoilsection,

4, Data, @mKLar to those of the present tests~body are necessaryto separatefin and body dragandevaluateinterferenceeffects..It@ alsoadvisabledrag datafor fin sectionsmore suitedto siipetisonicthoseof the presenttests. .

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1. Alexpnd~r,Sidney R., and Katz,Ellis: FlightTeststo Determinethe Effectof L@n.g’i&ofa ConicalWindshieldon the Drag of aBluffBody at SupersonicSpeeds, NACARM NO. L&l&, 1.946.

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rararonao(!aKi«?) Mastrocola, 1.

AUTHOR(S)

C-22-12-10 DIVISION! Ordnance and Armament (22) SECTION. Ballistics (12) CROSS REFERENCES. Projectiles, Fin-stabilized - Aero- dynamics (75416.2); Projectiles - Drag (75415.6); Interference effects - Aerodynamics (52501)

AUOQ 4056 BRIG. AGENCY NUMBED

RM-L7A08

AMER. TITLE. Effect of number of fins on the drag of a pointed body of revolution at low

Fo«yN.Tmf?up6r80nlcV8locitles

ORIGINATING AGENCY. National Advisory Committee for Aeronautics, Washington, D.C. TRANSLATION.

COUNTRY u.

LANGUAGE [FORG'N.CLAS.1 JLSJCIASS_L DATE UonioAS ""[Apr' 47 Eng.

PAGES 9

IU.US. 7

FEATURES photo3, diagrs, graphs

OGSTOACT Test data indicate that interference drag increased with increased number of fins up to

a .Mach number of 1.35; above this value the effect is reversed to the end of the test Mach number range. Large fin drag is attributed to blunt leading edge and square trailing edge of fin-airfoil section; However, magnitude of these interference effects will be important in future studies of drag of a multifln tail group. Flight test data of a finless body are needed to evaluate further the interference effects.

NOTE: Requests for copies of this report must be addressed to: 'N.A.C.A., Washington, D.C.

T-2. HQ.. AIR MATERIEL COMMAND ^SLECJHNJCALJNDEX WRIGHT FIELD. OHIO, USAAF

NOVEMBER 1340