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B!OLL-RATE STABILIZATION OF AMISSILE CONFIGURATION WITH WRAP-AROUNDFINS IN INCOMPRESSIBLE FLOW

PETER DANI LLS

SAMUEL R. HARDY

Warfare Analysis Department

DECEMBER 5976

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.Ij3.ATE.§5TABILIZATION OF A -ISSILE -C-NFIGURATION .4,~- WIT .WRAP-AROUND FINS IN__ ___________________

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This report contains the results of a subsonic wind tunnel test conducted to study the steady-stat(ro~l-ratu characteristics of various wrap-around fin (WAF) missile configurations at angles of attack from 0 t90 The tests show that fin slots combined with wing tip fe.ices and roll tabs eliminate the excessiv4rolI-rzte characteristics exhibited by WAF configurations at all angles of attack.

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FOREWORD

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The results of a study to eliminate the excessive roll rates at high angles ofattack exhibited by missile configurations with wrap-around fin stabilizers arepresented. The purpose of this study was to experimentally determine a method ofroll-rate stabilization for this type of configuration to be used as an air-launchedweapon.

This work was performed under the Inde Exploratory DevelopmentProgram sponsored by the Director of Naval Labor. .

I This report was reviewed by H. P. Caster, Head, Exterior Ballistics Division.

Released by:

I R. A. NIEMANNHead, Warfare Analysis Department

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CONTENTS

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FOREWORD ................................... ..........INTRODUCTION ....... ................................... .1WIND TUNNEL *TESTS ........................................ 1

DISCUSSION OF RESULTS ............................. 2CONCLUSIONS .................................... 7COM M ENT .. .. . . . .. .. . . . .. . . . . . . . . . . . . . . . . . . .. . .. 8REFERENCES ..................................... 8

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INTRODUCION

This work was undertaken as an initial phase of a research program to

determine the potential of wrap-around fins (WAFs) as stabilizers for air-launchedweapons. As a result of a previous investigation concerned with rocket flightdynamics,' it appeared that WAF configurations of proper design may avoid rollresonance and roll-yaw coupling, thus alleviating two problems plaguing current

general-purpose bombs. However, a problem that is' characteristic of WAF missileconfigurations is high roll rate at high angles of attack. The elimination of high rollrates is an important factor in controlling Magnus instability of air-launchedweapons, particularly bombs. 2 -4

The purpose of this study was to determine if a fin modification could befound for WAF configuration, that would provide roll-rate stabilization at all anglesof attack.

WIND TUNNEL TESTS A

Free-rolling tests were conducted in the 28- by 40-in. subsonic wind tunnellocated at Edgewood Arsenal. 5 The test specimen consisted of a free-rolling modelwith an ogival nose and a cylindrical afterbody, sting-mounted on ball bearings. Themodel was designed so that various fin configurations could be easily interchanged.A schematic of the basic configuration with fins extended (open) is presented iFigure 1. Steady-state roll rates were measured with a magnetic tachometer forangles of attack from 0 to 90".

VON KARMAN WRAP-AROUND-FIN 1.5"

1 69.61.AFT VIEW

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Figure 1. Schematic of Wind Tunnel Model (Basic Configuration)

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DISCUSSION OF RESULTS I

The basic WAF configuration was tested, and a plot of the reduced frequency(Pd/2V) versus the model angle of attack (ot) is shown in Figure 2.

where

P = steady-state roll rate, rad/secd = reference diameter, 0.133 ft.V = velocity of free stream, ft/sec

cc = total angle of attack,0

'This configuration exhibited large negative steady-state roll rates at high angles of

attack and a small positive, steady-state roll rate at zero angle of attack. The barindicates a regioa of roll lock-in, e.g., where the motion, if stopped, will remainstopped.

1 A straight fin configuration with the same planform as the basic WAF

configuration was also tested (see Figure 2). As expected, the configuration exhibitedroll speed-up in both the positive and negative directions. 2 The unsymmetrical

character of the roll rates exhibited by this configuration is probably due to strutinterference. Interference is inferred since models with longer sting-to-strut distancesexhibit more-symmetrical roll characteristics.

It had previously been shown that fin slots eliminated roll speed-up of straightfinned missiles.2, 3 Consequently, fin slots were tested on the basic WAFconfiguration in the hope that the roll rates at high angles of attack would bereduced. The fin slot, centrally located, had an area of about 30 percent of thetotal fin area. The slot reduced roll rates at high angles of attack approximately50 percent. Data for the slotted wraparound fin are presented in Figure 2.

At this point, it was obvious that an additional modification to the fin wasrequired in order to provide roll-rate stabilization.

Since WAF configurations are not symmetrical in crossflow (see Figure 3), itwas expected that a part of the high-angle-of-attack roll rate might be produced by

differential drag. Consequently, fin fences might tend to equalize the dragdifferential between retreating and advancing fins. Figure 4 shows the effects of finfences on the basic WAF configuration without slots. With fences at the fin tips,the roll rate is reversed except at very high angles of attack where the configurationcan roll in either direction.

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ROLL DIRECTIONAT

HIGH ANGLE OF ATTACK -i

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SCROSS-FLOW

Figure 3. Wrap-Around Fins in Crossflow

:, An inboard movement of the fence (Figure 4) gives the WAF configuration theg:! characteristics of a straight-fin configuration; e.g., the missile rolls equally well in

i i both directions at high angles of attack,.i

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Using a combination of slots and fences, the WAF configuration was then 5.roll-rate-stabilized, as shown in Figure 5. The addition of a toll tab provided the ,

slots, fences, and roll tabs are given in Figure 6. It should be noted that furtheri refinement of the slot geometry and fence location was not attempted.

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005 INCHES FROM FIN TIP* 0.11 INCHES FROM FIN TIP*12

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ANGLE OF ATTACK DEGREES ANGLE OF ATTACK - DEGREES

Figure 4. Steady-State Roll Rate vs Angle of Attack forWrap-Around Fin Configuration With Outboard Fences

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WITH SLOT AND OUTBOARD FENCE WITH SLOT, ROLL TAB, AND OUTBOARD0.06 INCHES FROM FIN TIP FENCE 0.08 INCHES FROM FIN TIP*

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ANGLE OF ATTACK -- DEGREES ANGLE OF ATTACK - DEGRtEES :

Figure 5. Steady-State Roll Rate vs Angle of Attack for.WrapAround Fin Configuration

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ANGL OFATTCK - DEREE ANGE O ATACK DEREE

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SIDE VIEW REAR VIEW

Figure 6. Roll Stabilized Wrap-Around Fin Configuration WithSlot, 100 Roll Tab, and Outboard Fence0.08 In. From Fin Tip

CONCLUSIONS RERVE

The following conclusions are made based on the results of this study:

F. In incompressible flow, a missile conf igth rap-around fins can

be roll-rte-stabilized using fences and slots. FencRoll rates of the basic WAF configuration at high angles of attack are

The fprobably due to vortices shed from the fins and e differential drag of

I iSurprisingly, th eie f position can cause the basic WAF configuration to n

+ behave like its straight fin counterpart, probably by equalizing the drag in crossflow.The slots eliminate the roll rates caused by vortex shedding on the wrap-around

~fins. Thus, the combination of fences and slots eliminates the large roll rates2.exhibited by WAF missile configurations at high anges of attack.are

puifnd i

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COMMENT

The method of roll-rate stabilization presented in this report for wrap-around-finconfigurations is relatively simple and practical. Consequently, additional high-speedtests are strongly recommended.

REFERENCES

AA1. F. L. Stevens, T. J. On, and T. A. Clare, Wraparound Versus Cruciformn

Fins: Effects on Rocket Right Performance, AIAA Paper, No. 74-777, August1974.

2. J. D. Nicolaides, Missile Flight and Astrodynamics, TN 100-A, 1959-1961,Bureau of Naval Weapons, Washington, D. C.

3. P. Daniels, A Study of the Nonlinear Rolling Motion of a Four-Finned Missiles,Journal of Spacecraft and Roc:ets, Vol. 7, No. 4, April 1970, pp. 510-512.

4. P. Daniels, Fin Slots Versus Roll Lock-in and Roll Speed-up, Journal of

Spacecraft and Rockets, Vol. 4, No. 3, March 1967, pp. 410-412.

5. A. Flatau, Facilities and Capabilities of Aerodynamic Group ResearchLaboratories, EASP 100-79 of June 1970, Edgewood Arsenal, Edgewood,Maryland.

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