Flight testing air-to-air missiles for flutter

FLIGHT TESTING AIR-TO-AIR MISSILES FOR FLUTTER

C. R. K1aschinski, -- Hughes Aircraft Co.,

Culver City, California

Abstract rence of flutter if total prevention of flutter results in a large increase in size and weight. Another The philosophy of the design of air-to-air important consideration is the tactical use of the missiles and hence of flight testing them for flutter missile and its speed-altitude profile. A salvo-type differs from that of manned aircraft. Hughes Aircraft missile, for instance, need not have as high an Company puts primary emphasis on analytical and individual reliability as that of a singly launched laboratory evaluation of missile susceptibility to missile.

aeroelastic and aero-servo-elastic instabilities and uses flight testing for confirmation of the absence It is clear then, that in designing air-to-air of such instabilities. Flight testing for flutter is missiles, flutter has to be kept in view right from the accomplished by using specially instrumented pro- initial stages of design and has to be given its rightful grammed missiles, air or ground launched with a place within the overall weapons system.

booster to reach the extreme flight conditions of tactical use, or by using guided missiles with tele- We at Hughes put primary effort on analytical metered performance data. The instrumentation and and laboratory evaluation of missile susceptibility to aeroelastic and aero-servo-elastic instabilities and testing techniques are discussed along with the success of recent flight tests.

use flight testing for confirmation of the absence of such instabilities. As is common practice, previous INTRODUCTION AND DESIGN PHILOSOPHY experience on successful designs and parametric studies of the type given in Reference 1 can be used The philosophy of the design o; air-to-air to advantage in the preliminary design stage of a missiles and hence of flight testing them for flutter missile. By the time the missile development reaches differs from that of manned aircraft. The primary the flight test stage, considerable confidence can be consideration in piloted military or civil aircraft gained in the structural integrity of the missile through is safety of crew and passengers. Elimination of classical studies or through analog studies and wind- the occupant from a missile, however, does not elimi- tunnel testing of designs with unusual features. How- nate the need for a flutter-free vehicle but a different ever, effects of aerodynamic heating and stabilities at philosophy prevails. The emphasis is shifted from large angles of attack and large control-surface de- personnel safety to weapon reliability. Weight and flections can, at present, be evaluated only through size are extremely importantparameters in the desigu flight tests under actual flight conditions and time of an air-to-air missile, even more so than in other histories.

types of missiles; therefore, reliability must be com- promised and an overdesigned structure cannot be tolerated. Flutter margins have to be decided upon in the light of reliability of other components of the INSTRUMENTATION AND FLIGHT TESTING FOR FLUTTER system. For example, if the system failure is one in ten, the missile need not be designed for a failure due to flutter of one in a thousand. Thus, it may Flight testing for flutter of air-to-air missiles even be found advisable to permit occasional occur- may be divided into three phases, namely,

(1) Captive flight A number of experimental missiles are equipped

with special instrumentation for monitoring perform-

(2) Specially instrumented programmed flight

ance and flutter data, and their guidance units are

(3) Monitored guided flight replaced by program control timers. The instrumen-

tation can thereby be optimized to measure the re-

Captive Flight sponse of predetermined missile maneuvers at pre-

scribed launch altitudes and speeds. The missile-

Transonic speeds are usually one of the critical booster combination is carried aloft by a suitable speed regimes for the incidence of flutter. Captive aircraft and released by it when the attitude, speed, flights can be used to detect any flutter tendencies at and altitude of the aircraft a r e such that after transonic speeds even though such flights are only booster rocket-engine burnout the combination would partially representative of free flights due to support be at the desired flight angle and the maximum characteristics. This can be done simply as visual critical design launch speed, or slightly in excess of inspection of the missile after a captive flight or more it. Timers and acceleration switches carried in the thoroughly with the use of strain gauges, recorders, booster delay its ignition by a preselected drop time or telemetry. Using conventional methods of airplane and ignite the missile rocket-engine after booster

The missile then carries aut programmed

flutter flight testing, one can also add shakers or burnout.

impulse devices and measure the decay rates. This maneuvers.

phase of flight testing for flutter can be carried out

at relatively low cost and yields spot checks of the Three types of flutter instrumentation have analytic work early enough to add confidence in the been used successfully in flight tests using the are as follows:

structural design. booster technique. They

Instrumented Programmed Flight The first consists of apair of aft-looking 16 mm.

modified GSAP* Fairchild cameras mounted in a

special recoverable nose section. These cameras

Normally, the missile structure and its control

all four control surfaces in their view (see

system are available long in advance of the aircraft have

which is to carry the missile as a part of the weapon Figure 1) and photograph them in flight. This optical system. Flutter flight testing can then be carried instrumentation was used in early flight tests of

missiles ground launched with a booster to observe

out either in the speed and altitude capabilities of an

control-surface flutter, if any, and separation of

existing aircraft which may not meet the critical de-

booster from the missile.

sign conditions of tactical use, or it has to be delayed

In order to

until the availability of tactical aircraft.

bridge this gap, we, in cooperation with the Lockheed

Aircraft Corporation, have developed a booster tech-

*Gun Sight Aiming Point

nique for our missiles which has proved very success-

ful.

Figure 1.

MAGNETIC PICKUP

X 160081 - MAGNET

16-801-

TRANSDUCER

Figure 2.

The second type of instrumentation is a motional absence of flutter in the tactical speed-altitudeprofile pickup developed at Hughes. This consists of a small of a missile.

horseshoe permanent magnet installed in the foot of the control-surface and a coil wound on a horseshoe Monitored Guided Flight core mounted opposite this magnet and in the foot of the stabilizer or wing (see Figure 2). Relative motion For missiles designed with very low flutter caused by vibrations generates an AC signal whose margins, a continuous monitoring of experimental, magnitude depends on the frequency and amplitude of prototype, and production missiles is necessary in vibration, and control-surface deflection. This signal order to maintain a check on manufacturing toler- is suitably filtered to flatten itsfrequency response ances and fabrication techniques. This can be ac- and is fed into the coder of a telemeter unit having complished by regular telemetering of control-surface 2000 sample per second pulse duration modulation.

position and the three body angular rates. Addition The frequency, the amplitude, and the rate of sub- of pitch and yaw accelerometers is useful in deter- sidence or divergence of any buzz or vibration can be mining proper aerodynamic performance, thereby obtained by this type of instrumentation.

assuring the absence of instabilities which might impair the guided flight of a missile and reduce the The third type of instrumentation is a self- overall weapon realiability considerably.

generating type vibration pickup mounted in the aft end of the missile. The output of this pickup is fed In closing, we are happy to say, in all humility, into the same type of telemeter unit as mentioned that all the Falcon series air-to-air guided missiles above. Destructive flutter can also be detected by designed so far have not experienced a single case simply looping a wire into the control-surface in of flutter, and hope that we shall continue to design series with the pickup. Loss of a control-surface is them that way.

then indicated by a step change in telemeter level.

Further verification of flutter of a destructive nature can be made by regular 30 sample per second REFERENCES telemetering of control-surface position and missile response in body angular velocities and accelerations.

1. Chawla, J. P., Aeroelastic instability at High Mach The above three types of instrumentation have Number, journal of the Aeronautical Sciences, Vol.

been used successfully by us at Hughes to confirm the 25, No. 4, April 1958, pp. 246-258.