Stabilizer flutter investigated by flight test

STABILIZER FLUTTER INVESTIGATED BY FLIGHT TEST E. F. Baird, N. S. Sinder, R. B. Wittman- Grumman Aircraft Engineering Corp., Bethpage, New York Abstract coverage of important parameters for all configura- tions over the complete speed and maneuver en- Flight flutter tests were conducted on an experi- velope. The recorded data from such tests must be mental airplane which resulted in the successful pre- adequate to enable prediction of incipient flutter at diction of a limited amplitude stabilizer flutter at subcritical speeds, and to indicate the source and supersonic speeds. The flutter obtained was unusual nature of any existent flutter. These requirements in that fore and aft bending of the stabilizer carry- are especially important in the testing of current through structure contributed to the flutter condition.

aircraft that are designed to probe into high Mach During flight tests the impending flutter conditionwas number, temperature, and dynamic pressure regimes observed from force per unit amplitude, damping where many unknown parameters must be defined.

coeiilcmnt, ana Irequency measurements. Adescrip- tion is given of the physical and operational charac- The testing procedures that were used in in- teristics of the test equipment and telemetering vestigating the flutter problems encountered in the facilities. A flutter analysis using measured modes transonic speed transition of our F9F-6 and FgF-8 and incompressible two-dimensional strip air forces airplanes were direct and simple. The excitation yielded a conservative flutter speed. Sled tests of a medium consisted of transient inputs of rudder pedal similar stabilizer configuration had lead to the con- kicks and control stick lateral and longitudinal jabs clusion that flutter would not be encountered. Certain to force primary surface oscillations. An airborne overall conclusions are reached regarding this par- oscillograph was used to record the data. The re- sults obtained from these tests were also direct and ticular flightfluttertesting program and the need for a concerted research effort in this field.

simple and merely served to show the absence or presence of flutter without indicating the build-up to INTROD UC TION or margin from the critical speeds. These tests also showed that considerable refinement of the flutter Development of higher performance aircraft with flight test program would be necessary to permit a reduced flutter margins has increased the need for rapid, yet safe, evaluation of future aircraft through- early and accurate determination of an aircraft's out speed envelopes that were expected to be almost flutter characteristics and establishment of its safe twice the ranges previously investigated. Considera- flight envelope. The resulting emphasis on flutter tion of these refinements along with the experience investigation during the initial stages of the flight gained in the production and flight testing of previous test schedule has produced considerable advances in aircraft played an important part in the design of the flutter testing equipment and techniques. Grumman FIlF-1 Navy supersonic fighter that has been demonstrated to be flutter free to its maximum EAS of 848 knots, a dynamic pressure of 2456 PSF.

Two apparently contradictory requirements are paramount in the flutter flight testing of present day aircraft. The tests must be conducted with full These refinements and the present methods, assurance of maximum safety, yet must be satis- techniques, and philosophies applied to the flutter factorily completed in the minimum time. Further, flight testing of the Grumman FllF-1F high per- the test program must provide a comprehensiVe formance airplane are discussed in the following text.

These procedures a r e familiar to all flutter special-

ists, however, there is little evidence to establish

their accuracy, validity, and scope. The primary

objective of this paper will be to examine an applica-

tion of these techniques in the investigation of stabil-

izer flutter.

The techniques and equipment used for the

F11F-1F flutter tests were essentially the same as

those developed on the F11F-1 airplane. The data

link used in the conduct of the flight tests is shown in

Figure 1, Accelerometers were appropriately mount-

ed on the wing, stabilizer, fin, and fuselage to sense

the excitation from an unbalanced mass shaker mount-

were

ed in the afterbody. The accelerometer outputs

relayed to an FM/FM telemeter package, amplified, and transmitted to the ground station, a mobile van.

The signal received at the van was appropriately dis-

criminated and sent to several simultaneously dis-

playing mediums for immediate and rapid analysis

by flight test and flutter engineers. These flight data

were compared to calculations and model test results.

Figure 1. Airplane-Telemeter Data Link

Through the use of appropriate charts and overlays,

frequency, amplitude, and decay rates were plotted as

fin, fuselage and wing modes. The shaker consists of

functions of airspeed and Mach number and the results

a rotating unbalanced mass which is driven by a hy-

relayed to the pilot along with recommendations for

continuance of the flight. draulic motor. An unbalance of 2.5 in. lbs. was used

for the flight tests. This weight was the minimum

The compact shaker assembly shown in Figure that could satisfactorily excite the required modes and 2 is approximately eight inches high, seven inches the maximum excitement that the pilot wanted to

tolerate. Motor speed and consequently excitation

wide, and fifteen inches long. This unit was designed

frequency is governed by a cam positioned flowvalve.

to fit into the tail skid compartment of the FllF-1F

and consequently was ideally suited to excite all of The cam and valve are integrally designed to provide the critical stabilizer modes as well as many of the optimum frequency sweep characteristics for the

Figure 2. Eccentric M a s s Shaker

A data recording system revolving around tele-

particular phase under consideration. For the subject

metry has been successfully developed for the high

tests the frequency programming started at 10 cps, linearly progressed to 28 cps in 30 seconds, more risk flutter flight testing of the F11F-1 and F11F-1F

a maximum of 35 cps within5 aircraft. A small twelve channel, self calibrating

rapidly advanced to

telemeter package translates the D.C. voltage outputs

seconds, and returned to the initial frequency inabout

of eleven data transducers and one communications

7 seconds where the cam motor was automatically

channel into a frequency modulated signal which is

stopped preparatory to another start signal from the

amplified and transmitted on an FM carrier. For the

pilot. The pilot was also able to stop or reverse the

cam motor at any point of the cycle and thus maintain flight testing of the F11F-1F at Edwards A i r Force

Base, the signal was received in the Grumman de-

constant frequency shaker operation. This was usually

done at a resonate mode of particular interest. A set signed and build telemetering van. An interior view

of the van is shown in Figure 3. This van was de-

of hydraulically actuated brakes within the shaker

signed with special attention to the incorporation of

could also be used by the pilot to stop the mass rota-

tion within 3/4 of a cycle a t 60 cps and even faster at features that would optimize the data recording and lower frequencies. Brake actuation also closed two analysis. Particular emphasis was placed upon the

rigid requirements of flutter flight testing. The

solenoid valves which trapped the hydraulic fluid with-

present system includes:

in the shaker motor thus increasing braking effective-

ness. This was followed by cutoff of hydraulic pres-

sure to the shaker system. Thepilotwas thus able to: a) T w o receivers

b) Complete signal monitoring equipment to

Sweep the unbalanced mass through a pre-

a)

insure the validity of the data

scribed frequency cycle.

Select and maintain a specified shaker fre- c) Two tape recorders

b)

quency.

d) An automatic sequencer

Rapidly start and stop the shaker at anyre-

c)

Analog computer for direct and immediate

quired frequency.

e)

data processing such a s addition, subtrac-

These several shaker operations, surprisingly tion, multiplication, integration, filtering, enough, required a minimum of pilot attention and and other applications.

effort to accomplish. Throughout the design and de-

velopment of the shaker components considerable

coordination between design engineers and flight test

pilots evolved a rather simple operations system.

Pilot requirements ultimately resulted in:

Pressing a thumb button on the side of the

a)

control stick to start the shaker and program

it through one frequency sweep.

Pressing another button on top of the control

b)

stick to stop the shaker immediately.

Actuating a switch near the throttle quadrant

c)

to maintain constant frequency or reverse

cam rotation.

The pilot was informed of shaker frequency by

a dial gage located on the instrument console adjacent

to the airspeed, Mach number, and altitude gages.

A unique component of the shaker system, and

one that sometimes worked too effectively, was the

shaker controller. This unit was an automatic safety

device that stopped the shaker through actuation of the

shaker brakes when and if the wing, stabilizer, or fin

oscillations exceeded pre -determined accelerations.

When so stopped, the shaker could be restartedby the

pilot's depressing the start button. However, the shak-

er would operate only i f the surface oscillations were

Figure 3. Interior of 'Telemeter Van

below the controller cut-off limits.

THE G P U N W (llF IF AIPpu\Nf

f ) A single channel long persistance oscil-

loscope for x-y data presentations

g) A 50 channel oscillograph

h) Two banks of Sanborn recorders of eight

channels each for immediate anddirect time

history display of vital parameters

A special two speed feature, ten to one in ratio

was built into the Sanborn recorders to permit accu-

rate recording of higher frequency flutter data. Paper

speed could be controlled either by ground personnel

or remotely by the pilot through the telemeter link.

Another feature added to these recorders consisted of

two tables, seven feet in length, especially constructed

to permit viewing and analysis of a large quantity of

data. Special take-up reels allowed stopping of the

paper while the pens continued to transcribe the tele- S T A 8 I L I I f R FROM THE F l i F l F AIRPLANE meter signal at the proper paper speed.

The validity of the techniques developed f o r

flutter flight testing with the shaker and telemetering

was determined by a flight investigation of the F11F-

1F stabilizer flutter problem. The second aero-

dynamic prototype F11F-1F airplane is shown in

Figure 4. This airplane is a modification of its pre-

decessor, the F11F-1 Tiger, and has the same wings,

fin, and fuselage center section. The stabilizer plan-

form, which is also shown in Figure 4, is also un-

changed but the airfoil section was decreased from a

varying 6-4% section to a constant 3% thickness and

the weight increased by 45%. The major changes were

necessitated by the installation of a more powerful

5-79 engine in place of the 5-65. The larger diameter

of the 5-79 required increasing the afterbody cross-

section and in turn the breadth of the stabilizer yoke.

Figure 4. The Grumman F11F-1F Airplane Stabilizer

These revisions to the stabilizer and its yoke have

from the F11F-1F Airplane

changed the surface's vibration characteristics by

lowering the first symmetric mode, primarily verti-

cal bending, from 20.0 to 12.7 cps and the second

symmetric mode, primarily yaw, from 25.4 to 17.8

cps to produce a limited amplitude stabilizer oscil-

1ST FUSELAGE

lation that has been encountered in flight throughout

V . B . MODE

a wide Mach number and altitude range. The node

(14.8 CPS)

lines for this revised stabilizer -yoke combination are

shown in Figure 5. No structural damage has result-

ed from the oscillations and the mild onset of the vi-

2ND SYM STAB

bration permitted an investigation of this flutter

MODE (17.8 CPS) through flight test with relative safety.

Prior to the first flutter incident transient in-

puts of aft stick jabs had been made from 200 to 510

MODE (56 CPS)

knots as a cursory check of the overall stability

characteristics. These test failed to indicate any in-

1ST SYM STAB

cipient flutter and in some instances actually showed

MODE (12.7 CPS)

increased damping. The results of these tests led u s

to delay the planned flutter flight test program until

after a flight evaluation of the airplane had been com-

pleted and to extend the initial restrictions of 450

MODE (56 CPS)

knots to speeds in excess of 500knots. During the ex-

tension of these restrictions the flutter condition that

had been predicted by theoretical calculations but had Figure 5. Stabilizer Vibration Node Lines not been totally substantiated by model or sled tests Shortly after the completion of the flight evalu- was first encountered at a speed of approximately ation program the delayed formal flight flutter pro- gram was conducted with extremely encouraging re- 530 knots, 80 knots in excess of the initial restrictions sults. Within five flights, through use of the unbalanced and within about 10 percent of the predicted critical mass shaker and the telemetering, we were able to de- speed. Appropriate restrictions of 475 knots below fine the problem area, extrapolate the test results to 35,000 feet were imposed.

the critical speeds, and define the flutter modes.

The speed capabilities of the FllF-1F air- This flight test investigation was conducted over plane permitted these restrictions to be exceeded an area of .45 to 1.52 Mach number and 200 to 500 easily. As a result stabilizer flutter on the FllF-1F has been encountered a total of nine times, twice by knots at the altitudes of 35,000, 27,500, and 20,000 feet. The test points that were attained are shown in Grumman pilots, and the remainder by evaluation Figure 6. The initial flights started at the highest pilots. In all cases the onset of the vibrations were altitude and scheduled shaker sweeps from approx- noted on telemetering records and the pilots were told imately 200 knots to the maximum safe speed based to decrease speed. They all did so immediately.

upon flutter considerations. The results of these The oscillations have occurred in a narrow air- sweeps served to define the critical resonate fre- quencies and their variation with air speed and to speed band, 500 to 580 knots EAS from 5000 to 38,000 feet and from Mach .95 to 1.80 as shown in Figure 6. indicate the regions of decreasing stability. A more accurate definition of the decay rates was accomplished In only three instances were they of sufficient mag- by having the pilot attain a given speed and Mach num- nitude to be felt by the pilots. These particular os- ber and operate the shaker at the prescribed frequen- cillations imposed a maximum acceleration of about cy by referring to the cockpit indicator. The indicator i15 g normally and _5 g fore andaft on the stabilizer.

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0 IOO 200 EOUIVALENT AIRSPEED KNOTS Figure 6. Flight Flutter Points accuracy of about _I cps, however, was inadequate for Identification of the critical modes from flight the tests and the actual resonate frequency was at- data was made through use of accelerometers mounted tained by having a ground observer guide the pilot in in the fuselage as well as the stabilizer tips. The re- selecting the true resonance. This was accomplished sults of a theoretical flutter analysis for the FllF-1F rather simply by comparing the frequency and ampli- stabilizer as shown in Figure V indicated a possible tude of a particular telemetered data channel with a coupling between the stabilizer first symmetric mode, preset oscillator frequency on a dual channel scope. primarily vertical bending, and either the stabilizer After a bit of practice with the airplane on the ground second symmetric mode, fore and aft bending, or the the actual resonance could be attained in flight within fuselage first vertical bending mode with either of the five to ten seconds. Once the resonate frequency was latter modes increasing their frequency withincreas- attained the pilot stopped the shaker then restarted it ing airspeed. The telemetered data, however, as in- at the same frequency to define the decay rates three dicated in Figure 8, showed that the fuselage mode frequency remained relatively invariant with airspeed times. The damping characteristics of three modes were investigated by the technique, bit stick jabs whereas the stabilizer first symmetric mode frequency were made on the last flight to show the trends that increased with airspeed from 12.7 cps on the ground to could be determined by this transient input method. 1V cps at 500 knots to couple with the second symmet- ric mode which itself varied but little with airspeed.

These results along with the marked increase in am- In these five flights a total of 31 shaker sweeps, plitude of these modes at speeds in excess of 400 37 resonate stops, and 11 stick jabs were made to de- knots focused our attention on the stabilizer modes as fine rather completely the mechanism of the flutter problem. Through the use of telemetering and im- the fundamental problem.

mediate data evaluation the airplane was tested to 95% of the critical speed at the three altitudes in- In this method of testing with forced harmonic vestigated where in each case the tests were discon- excitation the amount of damping in the modes can be tinued when the monitored data indicated marginal examined in two ways. First, the loss of damping may damping.

be evidenced by the sharpening of the resonant peak

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SECOND SYM STABILIZER _ FIRST SYM STAB MODE FIRST FUSELAGE VERT I , *%. BENDING MODE

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ALTITUDE 20,000 FT.

3O u 2o i o Z (7 IO I00 200 300 400 500 EQUIVALENT AIRSPEED - KNOTS Figure 7. Results of Theoretical Flutter Analysis accompanied by amplitude increases. A sharp peak tions of problem areas are certainly evident from is defined as one which has a pronounced amplitude both plots and the fact that an adventuresome extra- peak occurring over a narrow frequency range. The polation would yield a predicted flutter speed of about amount of damping may thus be expressedbythe ratio 520 kts., in excellent agreement with the flutter that of the incremental frequency that defines equal am- was actually experienced, is quite encouraging. In plitude boundaries of the resonate mode. Qualitative- fact monitored telemeter data of shaker sweeps made ly this is a good indication of damping but because of on the first flight were used to limit the speed of the the sensitivity of the ratio to frequency inaccuracies flutter program well below the maximum capabilities it becomes impractical for use in the analysis of of the airplane.

flight data. The second method permits a rapid and direct indication of the damping in the modes by de- Post flight analysis of these data was conducted termination of the amplitude of surface vibration at a to determine a more precise indication of the critical particular resonance. The data accumulated from speeds. The ratio of shaker force inputto unit veloc- shaker sweeps is summarized in Figure 8. These data ity of the stabilizer oscillation was plotted as a function of airspeed and Mach number. The results of this show a rapid reduction in the magnitude of the recip- rocal of the stabilizer tip vibrational amplitude, l/A, analysis are presented in Figure 9. Since the numer- for both stabilizer modes as the flutter speed is ical values of the test data for both modes closely approached. This reduction of 1/A for both modes is coincided the individual test points are omitted and in good agreement with the theory which predicts that the resulting faired curves are separated by applying the damping of both modes will decrease at higher an appropriate weighing factor. These results agree speed. relatively well with the 1/A data and a mathematical extrapolation yields a critical speed of 560 knots.

Extrapolation of these frequency and 1/A data to predict incipient flutter and the critical speed may The measured values of damping coefficient not be done with absolute certainty. Definite indica- which were obtained once the modal frequencies had 3.0 _]"_] Znd SYM. STABILIZER MODE z.0

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600 700 100 200 _00 400 500 EQUIVALENT AIRSPEED - KNOTS Figure 9. Results of Flutter Flight Tests Frequency Response Measurements been defined reflect the gradual deterioration of damp- over a 300 knot span of airspeeds while the damping ing with airspeed as shown in Figure 10. The reduc- coefficients from decay measurements only decreased tion near the flutter speed, however, does not seem to by one-half. This variance of data may in part be ex- plained by the changes in mode shape with increases be compatible with the 1/A and the F/A curves where in airspeed which effect the output of stabilizer tip the stability had decreased to one-fourth its value .O9 O FIRST SYM'.

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_u M .O3 M I00 ZOO 300 400 500 600 700 EQUIVALENT AIRSPEED KNOTS Figure 10. Results of Flutter Flight Tests Decay Measurements contains integral cylinders which are hydraulically accelerometers and by the possibility that the reson- actuated by an electro-hydraulic valve. The oscil- ate Peaks and frequencies were not always attained lation of the mass can be controlled both in frequency during the decay maneuvers.

and amplitude and programmed to any desired fre- quency sweep. In addition, a theoretical development A comparison of damping coefficient data ob- program has been undertaken using an analogue com- tained from aft stick jabs with the shaker excited puter that is set up to describe a discrete mass repre- decays showed that transient inputs were unable to sentation of an aircraft wing. Some objectives of this excite adequately the critical modes, yielded a great deal of random scatter, and allowed no proper pre- program are: diction of the critical speed. Even a rough extra-

1) To determine stability criterion which can

polation of the damping coefficients excited by the be applied to subcritical response data and transient induced oscillations would show a flutter extrapolated to predict critical speeds.

speed 50% higher than that predicted from shaker de- cays. In fact, a series of aft stick jabs was made To examine physical behavior of a surface

2)

prior to the initiation of this flutter program at 20,000 ft. and at 15 knot increments from 300 to 520 knots. in the vicinity of critical speeds in order to understand more fully the reasons for the These tests showed absolutely no evidence of impend- sudden decrease in damping for small ing flutter.

speed increases.

The results of this FllF-1F flutter program 3) To evaluate the effects of configuration corroborated the theoretical calculations and iden- changes.

tified as well as partially explained the mechanism of mode coupling. Since the restrictions imposed by Our experience from the FllF-1F and other the stabilizer oscillations do not hinder the FllF-1F flutter programs has indicated that: flight test program, no major effort has been under- taken to eliminate the problem. However, a simple A controlled well defined excitation force

1)

change to the stabilizer yoke which increased the fore is necessary to permit athorough evaluation and aft stiffness and raised the second symmetric mode of all pertinent modes.

frequency to 24.5 cps was flight tested. The results from this second series of flutter tests indicated that

2) Incipient flutter may be predicted at sub-

the critical speed of this configuration was substan- critical speeds from the results of flight tially increased.

tests.

_._rtain limitations in the testing techniques and By tne use of shaker vxcltAtlor, three __I_,_A data analysis were quite evident at the conclusion of indications of incipient flutter are readily this flutter program. First, the means of determining available for rapid analysis. The first, the stability criteria are far from adequate and may be reduction of frequency ratio, andthe second, classed as being part of the current state of the art; the decrease of I/A and F/A, proved to he second, the methods of establishing adequate margins more effective than the third, the deteriora- from incipient flutter and predicting critical speeds tion of damping coefficient.

are rather difficult to define; third, the mechanics of exciting a structure at a desired resonate frequency

4) Telemetering flight data for analysis by

needs improvement; and fourth, a single tail shaker ground personnel greatly reduces the time does not excite all of the wing modes required for required to complete the tests, increases complete definition of the flutter spectrum.

the safety of the program, and permits a wide latitude of data processing techniques.

To overcome some of these limitations, we, at Grumman Aircraft, have developed a resonance de- The limitations and problems in the testing tector to obtain, automatically, excitation cut-offs at techniques and equipment realized at the conclusion of resonances that are determined during shaker fre- the program are currently being investigated. Appro- quency sweeps. This device will shut off the shaker priate modifications to future flight flutter tests will for a predetermined interval at a prescribed resonate be made based upon our findings, the experience of mode then will allow the shaker to continue the sweep others, and the information acquired at this sym- until a new resonance is excited. To excite wing modes posium.

more adequately a reciprocating mass shaker, three inches in depth has beendeveloped. The shaker weight