Survey of aircraft subcritical flight flutter testing methods

A , R , A . , P , REPORT N O , 218 SURVE f OF AIRCRAFT SUBCRITICAL FLIGHT FLUTTER TESTING METHODS Robert HosenLa-iii (NASA-CR-132479) SURVEY 02 A I R C R A F T 174-34468 SUBCRXTICAL PLIGET FLUTTER TESTING ClETHODS !Faronautical Research Associates of Princeton) 30 p HC $3.25 CSCL C 1 C Unclas 63/02 51024 Pro-pnrPcl under Contract No NAS1-11672 by Aeronautical Research Associates of Princetor,, Inc.

50 Washington Road, Princeton, New J e r s e y 08540 for NATIONAL AERONAUTICS AND SPACE ADMINISTRATION August 1974 FOREWORD A . R . A .P., under contract with Langley Research Center, N A S A , has been studying the problem of s u b c r i t i c a l f l i g h t f l u t t e r t e s t i n g o.? a i r c r a f t . As p a r t of t h i s e f f o r t , a survey was aade of a i r c r a f t i n d u s t r i e s t o e s t a b l i s h a summary of t e s t i n g techniques and data a n a l y s i s methods t h a t a r e c u r r e n t l y helng used, The survey was made by Mr. Robert Rosenbaum, and i s reported h e r e i n , This r e p o r t should be considered a companion t o a second report, NASA CR-132480, e n t i t l e d " S u b c r i t i c a l F l u t t e r Testing and System I d e n t i f i c a t i o n , I' by D r , John C , Houbolt, p r i n c i p a l Investigator of the contract e f f o r t , who also performed some e d i t i n g of t h i s survey summary.

SURVEY O F AIRCRAFT SUBCRITICAL FLIGHT FLUT'I%R TESTING M E T H O D S by Robert Rosenbaum* SUMMARY The r e s u l t s of a survey of U.S., British and French sub- c r i t i c a l aircraft f l i g h t f l u t t e r t e s t i n g methods are presentcd and evaluation of the a p p l i c a b i l i t y of these methods t o the t e s t i n g of the space s h u t t l e are discussed. Ten U.S. a i r c r a f t programs covering the l a r g e c i v i l t r a n s p o r t a i r c r a f t and a v a r i e t y of m i l i t a r y a i r c r a f t are reviewed. I n a d d i t i o n , t h r e e major French and B r i t i s h programs a r e covered by the survey.

The s i g n i f i c a n t d i f f e r e n c e s between the U.S., French and B r i t i s h p r a c t i c e s i n the a r e a s of methods of e x c i t a t i o n , data a c q u i s i t i o n , transmission and a n a l y s i s are reviewed. The e f f e c t of i n t e g r a t i n g the d i g i t a l computer i n t o t h e f l i g h t f l u t t e r test program i s discussed. S i g n i f i c a n t saving i n a n a l y s i s and f l i g h t t e s t time are shown t o r e s u l t from the use of special d i g i t a l computer r o u t i n e s and d i g i t a l f i l t e r s .

Computer techniques have been developed which minimize the e f f e c 6 of extraneous noise (such as caused by turbulence) i n the response s i g n a l , INTRODUCTION A survey of s u b c r i t i c a l f l i g h t f l u t t e r t e s t i n g methods employed by U.S. a i r c r a f t manufacturers was conducted during the latter half of 1972. The sur.vey was conducted by v i s i t i n g a number of companies and d i s c u s s i n g w i t h t h e i r r e p r e s e n t a t i v e s the methods c u r r e n t l y used i n the areas of: e x c i t a t i o n ; data a c q u i s i t i o n , r e t r i e v a l and a n a l y s i s ; and methods employed i n e x t r a p o l a t i n g from s u b c r i t i c a l conditions t o the f l u t t e r boundaries. The survey d i d not include manufacturers of general a v i a t i o n aircraft. I n a d d i t i o n t o d i r e c t discussions with Tepresentatives of U.S. inaustry, a l i m i t e d survey of Frencii and B r i t i s h p r a c t i c e s was made through personal c o n t a c t , correspondence and review of published m a t e r i a l . For the U.S.

as well as the Anglo-French surveys, emphasis was placed on r e c e n t l y completed programs and programs i n pi'ogress, as well as those pragrams.which, although t h e f l i g h t t e s t i n g had not as y e t started, were s u f f i c i e n t l y far advanced i n t h e planning * Aeronautical Consultant stage t o be considered c u r r e n t programs. Programs which were s t i l l i n the research o r preliminary development phase were not considered. The survey was intended t o e s t a b l i s h the c u r r e n t state of the art of s u b c r i t i c a l f l i g h t f l u t t e r t e s t i n g , The survey 3rought out the fact t h a t a number of survey-type r e p o r t s a J s o e x i s t , see references 1 througn 8.

Having determined the state of the art, an evaluation of the a p p l i c a b i l i t y of c u r r e n t methods t o t e s t i n g the space s h u t t l e was made.

SURVEY FINDINGS U.S. Industry The following material summarizes the information gathered during the survey of s u b c r i t i c a l f l i g h t f l u t t e r t e s t i n g methods. Not a l l the companies which are a c t i v e i n major f l i g h t f l u t t e r t e s t i n g programs were contacted. However, * those companies which were contacted were considered t o provide a broad c r o s s s e c t i o n of U.S. i n d u s t r y p r a c t i c e s and were r e p r e s e n t a t i v e of the state of the art i n regard t o such tests.

The companies which were contacted and the a i r c r a f t discussed covered t h e producers of t h e l e a d i n g commercial t r a n s p o r t s (Boeing 747 and SST, McDonnell-Douglas-West D C - 1 0 Zild Lockheed- C a l i f o r n i a L - l o l l ) as well as producers of a v a r i e t y of types of m i l i t a r y a i r c r a f t (Lockheed-Georgia C - 3 , Lockheed-California S - 3 , General Dynamics F-111, G r m a n F-14, McDonnell-Douglas- East F-15, and LTV A-7A).

A t t h e time of the survey of t h e l i s t e d a i r c r a f t , t h e 747, 1.-1011, DC-10, C - 3 , F '11 and A-7A had completed t h e i r t e s t s ; the F-14 and S-3A werr i n t h e m i d s t of t h e i r t e s t s , a n d the F-15 was j u s t ready t o s t t i c i t s test program. The SST program, prior. t o i t s cancellation,had been scheddled t o start i n A p r i l of 1973.

McDonnell-Douglas - L m g Beach, California: D C - 1 0 . - On t h c

early f l i ghts f o r %zeds UT, t o 3 0 m t s e x c i t a t i o n was achieved by p i l o t pulses a n d - by pilbt-induced forced o s c i l l a t i o n s .

Aerodynamic vancs were then ased as e x c i t e r s on a l l subsequent f l i g h t f l u t t e r tests. The vanes were l o c a t e d at the w i r , g t i p s and t i p s of the h o r i z o n t a l stabilizers and f l n . The vanes used on a l l of the surfaces were the same s i z e (12" span, 16" chord). The frequency range covered was mainly 1 t o l O H z i n 90 seconds, followicg the exponential sweep law. Some sweeps covered the range of 1 t o 20Hz. I n general, the frequency sweeps for wings, v e r t i c a l t a i l , and h o r i z o n t a l t a i l were separate. Some tests were conducted w i t h a l l vanes operating simultaneously, For each test condition, a f t e r sweeping throQgh the frequency range, 4 t o 6 resonant frequencies were selected f o r dwell and quick stop of the e x c i t e r s . Damping was then obtained from the decay records of each of the s e l e c t e d frequencies.

(accelerometer, O n the o r i e r of 300 t o 400 channels of data strain gage and p o s i t i o n i n d i c a t o r s ) were recorded on tape recorders on board the a i r c r a f t . The data were transmitted by PCM t o the grouild s t a t i o n . The demodulated PCM s i g n a l was fed t o a d i g i t a l computer as well as s t r i p c h a r t s providing twenty- four channels f o r d i r e c t monitoring. Broad band f i l t e r s were I n addition, used t o eliminate high frequencies above 6 0 ~ ~ .

tracking f i l t e r s of 2Hz bandwidth were used during sweeps.

Force input from the vanes was obtained from s t r a i n gages on the vane shaft. F l u t t e r i n d i c e s used i n evaluating s t a b i l i t y over the required operating envelope were the v a r i a t i o n i n amplitude p e r u n i t input f o r c e and damping as funccion of d y n a d c pressure and Mach number.

Lockheed-California Division: L-1011. - Aerodynamic vanes

i n s t a l l e d outboard of the wing t i p s and s t a b i l i z e r t i p s were used a s the source of s i n u s o i d a l e x c i t a t i o n . Both wing and the wing vane span s t a b i l i z e r vanes had a chord of 18 inches; was 28 inches, while the s t a b i l i z e r vane span was 20 inches.

For t h e wing sweep, the frequency range varied from 1 t o 18 Hz i n approximately 90 seconds, Lockheed's procedure on the L-1011 involved swesping up and down through the frequency range. During the sweep the period of the excihations f o r c e decreased l i n e a r 1 7 with time; i . e . , l i n e a r period sweep law.

The 90-second sweep time was the time t o sweep i n one d i r e c t i o n , For t h e s t a b i l i z e r , t h e frequency range covered was 3 t o 25 Hz i n approximately 30 seconds (time f o r sweep i n e i t h e r d i r e c t i o n ) .

Response data viere obtained from accelerometers, s t r u i n gages, and p o s i t i o n i n d i c a t o r s , w i t h approximately 75s obtained from s t r a i n gages and 25% from accelerometers and p o s i t i o n i n d i c a t o r s . One hundred and twenty channels of data were recorded on tape on board t h e a i r c r a f t , The data channels were arranged i n 6 banks of 20 channels each. Any s e t of 20 chahnels could be transmitted v i a F M telemetry a t any one time. Damping was obtained from the bandwidth of the response curves; resonant frequencies were obtained from the s h i f t of the frequency of peak response i n sweeping up and down. T h i s method i s covered i n the paper by E . Bartch ( r e f , 1 f o r the case of a l i n e a r v a r i a t i o n of frequency w i t h

time. 4 or the l i n e a r period sweep l a w used i n these t e s t s , a

modification of the sweep parameter i n the c h a r t s of reference 1 was employed. Most of the t e s t runs involved sweeping up and down the frequency range, However, t o a very l i m i t e d e x t e n t , damping was a l s o obtained from decay records after dwelling at a resonant frequency and quickly stopping the e x c i t a t i o n .

Lockheed-Californla Division: S 2 . - E x c i t a t i o n of the S-3A was by aerodynamic vanes i n s t a l l e d on each side of the f u s e l a e

Vane Eiize was 1 8 "

i n the v i c i n i t y of and below the s t a b i l i z e r , chord and 22'' span. The frequerxy range of i n t e r e s t was 1 . 5 t o 18 Hz f o r t h e wing a n d 3 t o 25 Hz f o r s t a b i l i z e r . Time t o sweep through the frequency range of i n t e r e s t was approximately 90 seconds i n both cases. The sweep frequency v a r i a t i o n followed t h e l i n e a r period law, I n c o n t r a s t w i t h the L-1011 t e s t , the S-3A sweep was up only ( i n c r e a s i n g frequency), Damping was obtained from response decay records after dwelling a t a resonant frequency and quickly stopping the e x c i t a t i o n . Pulse code modulation was used f o r t r a n s m i t t i n g the data from the a i r p l a n e t o the ground s t a t i o n .

Uoein . 747.- E a r l y i n t h e t e s t progran, t h e 747 was cleared

over 6 e m a or p o r t i o n of the f l i g h t envelope by p i l o t pulsing

of t h e c o n t r o l s . On the basis of these t e s t s , and on a n a l y t i c a l s t u d i e s and wind tunnel model t e s t i n g , i t was decided t h a t wing t i p e x c i t a t i o n only was appropriate f o r subsequent s u b c r i t i c a l f l i g h t f l u t t e r t e s t i n g . An aerodynanic vane, 2 f t 2 i n a r e a , was located outboard of each wing t i p . The vanes were e l e c t r o - h y d r a u l i c a l l y driven. Although the e x c i t a t i o n had the capa- b i l i t y t o operate over the range of .5 t o 20 Hz, most s i n e sweeps covered the range of 1 . 5 t o 7 Hz i n 90 seconds. The sweep frequency v a r i e d exponentially w i t h time.

Date. from accelerometers, straingage pickups and c o n t r o l p o s i t i o n i n d i c a t o r s were transmitted by F M telemetry t o a ground s t a t i o n f o r a n a l y s i s . The t e s t technique included frequency sweeps t o determine resonant frequencies. Quick at t h e resonant frequencies provided the decay records stops f o r which the damping values were obtained. The v a r i a t i o n of frequencies and damping w i t h dynamic pressure and Mach number w a s e s t a b l i s h e d , e s p e c i a l l y t o assure adequate damping over t h e required f l i g h t envelope. Figure 1 shows t'ie l o c a t i o n of t h e tLerodynamic vanes at the wing t i p s of t h e 747 wing.

Boeing: Supersonic Transport.- A t the time the SST program

was cancelled i n 19'(1 , plans for the f l i g h t f l u t t e r program were

well advanced. F l i g h t f l u t t e r t e s t s were scheduled t o s t a r t on A p r i l 1, 1973. Based on a n a l y s i s and f l u t t e r model t e s t i n g , there were i n d i c a t i o n s t h a t i n the transonic and low supersonic speed regime the required f l u t t e r margins could not be met.

Because of the ljmited funding f o r the phase I11 government- i n d u s t r y c o s t sharing program, it was of p a r t i c u l a r importance t o rejuce the time f o r f l u t t e r t e s t i n g . The technique of searching f o r resonant frequencies by r e l a t i v e l y slow sweeps, tuning these frequencies by hand, and then quickly stopping the e x c i t a t i o n t o determine damping (as i n t h e 747 t e s t s ) was t h e r e f o r e discarded.

Wing t i p aerodynamic vanes similar t o those used on the I n addition, a f t body i n e r t i a 747 were. planned for the SST.

shakers were planned t o provide l a t e r a l and v e r t i c a l body e x c i t a t i o n . Plans were t o use a fast sine sweep covering the range of 1 to 20 Hz f o r the wing vanes and sweeps from 5 t o 1 0 Hz f o r t h e i n e r t i a e x c i t e r s . Varia'Lion of frequency w i t h time was t o follow the exponentlal sweep law.

Response data were t o be obtained from accelerometers and c o n t r o l p o s i t i o n indica.tors, Vane input data were t o be obtained from s t r a i n gages on the torque = h a f t of t h s vanes.

The data were t o be recorded on tape on the a i r p l a n e and telemetered t u a ground s t a t i o n . After demodulation, the force input and response data were t o be recorded on a magnetic tape recorder and a step c h a r t recorder f o r d i r e c t observation of the analog signals, and was also t o be d i g i t i z e d and fed t o the d i g i t a l computer f o r f a s t Fo..rier transform a n a l y s i s . From the r a t i o of the Fourier transforms of response t o input, the frequency response informatir?p vas t o be obtained i n t h e form of Kennedy-Pancu vector p l r , t s ( r e f . 9). The frequencies and damping f o r each mode would '.hen be deduced; t h e v n r i a t i c n of damping w i t h Mach number was considered of prime i n t e r e s t .

Figure 2 shows the flow diagram f o r data a c q u i s i t i o n , t r a n s - mission and reduction t h a t was envisioned f o r the SST. Thew proposed f l i g h t f l u t t e r t e s t techniques were evaluated i n low speed wind tunnel stlidies and a r e reported i n reference 10.

Lockheed-Georgia Division: C-5A - The vane system f o r

e x c i t a t i o n of the C-5A a i r c r a f t d i f f e r e d s i g n i f i c a n t l y from the e x t e r n a l aerodynamic vane system used by Boeing, Douglas and t h e Lockheed-California Division. The system consisted of one r o t a t i n g vane on the top of each wing near t h e wing t i p and one r o t a t i n g vane on the top of each horizontal s t a b i l i z e r t i p . The vanes r o t a t e d through 360° w i t h two cycles of e x c i t a t i o n provided f o r each revolution. Each vane was supported between two pylons which were s t r u c t u r a l l y fastened t o t h e a i r c r a f t at the c l o s i n g r i b of t h e surface.

The vane chord was 1 4 inches. The span of the wing e x c i t e r could be varied from 13 t o 26 inches by telescoping two 13-inch s e c t i o n s . For the s t a b i l i z e r , a f i x e d 13-inch span e x c i t e r was used since l a y e r vane spans were found t o lead t o high s t r u c t u r a l l o a d s . Figure 3 shows the vane and pylon assembly and Figure 4 i s a. photograph of the assembly on t h e C-5A a i r p l a n e . Wind tunnel tests of the i n s t a l l a t i o n indicated t h a t the b e s t measure of o s c i l l a t i n g input force could be obtained from the drag measured on the vane supporting s t r u c t u r e . The vanes could be tuned manually t o any desired frequency w i t h a quick stop c a p a b i l i t y , o r t h e y could be programmed t o Eweep over tha range of .25 t o 12.5 revolutions per second, which r e s u l t e d i n e x c i t a t i h n varying from .5 t o 25 Hz. Frequency of e x c i t a t i o n varled expo- n e n t i a l l y w i t h time, The time t o sweep through the frequency range was 60 seconds f o r most f l i g h t s . However, f o r those f l i g h t s involving sus4,ained d i v e a t t i t u d e , the time t o sweep was reduced t o 30 seconds t o minimize a l t i t u d e v a r l a t i o n at the t e s t p o i n t . Both s e t s of vanes were synchronized t o permit either symmetric or antisymmetric e x c i t a t i o n . When not i n operation t h e vanes automat1.ca.lly returned t o a. zero l i f t strsamLined p o s i t i o n . , On-board instrumentation included accelerometers, s t r a i n gages and ccjntrol p o s i t i m i n d i c a t o r s . Eighty channels of data were recorded on an airborne tape recorder. Thirty channels of preselected data were telemetered t o the grow-d.

Any of the other channels could be selected i n f l i g h t f o r transmission t o t h e ground s t a t i o n .

The telemetered data were recorded on tape a t the ground receiving s t a t i o n and simultaneously monitored on pen recorders f o r visual r e a l time q u a l i t a t i v e evaluation, Twenty of the t h i r t y telemetered s i g n a l s were then transmitted from the receiving s t a t i m t o the h y b r i d computers. Nine of t h e signals i n analog f m m were then passed through 2 Hz bandwidth tracking f i l t e r s ccntimc?>.sl,ytuned t o p a s s only the ex- c i t a t i o n frequency. Tne t i m e of z e m crossings and peek amplitudes wel-e then u s e d by t h e d i g i t a l computer t o generate response envelopes from which frequences and amplitudes were obtained. Amplitudes were normalized through use of the concurrent amplitude of the drag force s i g n a l of the vane.

S i g n i f i c a n t frequencies and a m p l i t u d e were displayed approxi- mately 3 seconds s,fter the end of each sweep.

During t h e sweep t e s t s twc b a s i c s t r u c t u r a l modes were excited t o amplitude; approaching 8C$ of l i m i t load. The damping f o r these modes d h s obt.aii?ed by tuning t o the c r i t i c a l frequencies, dwelling at the fraquency arid then using the quick stop techhique t o obtain the decay records.

Some d i f f i c u l t i e s were encountered w i t h the tracking f i l t e r s which r e s u l t e d i n phase d r i f t during t h e sweep t e s t s .

This pr.evented a c m r a t e determination of damping values. The f l u t t e r index used i n the s u b c r i t i c a l t e s t s and f o r extrapo- l a t i o n beyond the operating envelope was i n the form of a p l o t of ncrmalized amplitude versus v e l o c i t y or Mach number.

Genersl Dynamics-Ft. Worth: F-111.- Wing e x c i t a t i o n was provided througn aerodynamic vanes located outboard of the almost f u l l span f l a p s , The vanes were within the normal contour of the wiag and were similar t o small pan a i l e r o n s .

The area cf each vane was approximately l . 5 f t 3 . The

hydraulic power supply used t o drive the vanes was completely independent of the normal a i r p l a n e hydraulic system. System frequency c a p a b i l i t y extended frcm very nearly z e r o t o 50 I I z .

Sweep v a r i a t i o n followed the zxponential sweep l a w , startirig from 35 Hz and endjng at 2 Hz f o r each run. Time t o sweep through t h i s range was approximately 45 seconds .

Horizontal and v e r t i c a l t a i l ex2:itatlon was provided by i n e r t i a shakers. Thess u n i t s were h y d r a u l i c a l l y driven the mass i n each u n i t moving i r ! pure t r a n s l a t i o n . A 15 peak force shaker was I n s t a l l e d on t h e v e r t i c a l tai1;a 300# peak force shaker was i n s t a l l e d a t each side of the rear fuselage f o r h o r i z o n t a l t a i l e x c i t a t i o n .

Response data from accelerometers, p o s i t i o n i n d i c a t o r s and f l i g h t parameters were recorded on tape i n the a i r p l a n e .

Twenty-four channels of data were transmitted by F M telemetry t o a ground stt?,tion f o r recording i n raw form (no f i l t e r i n g ) , Band pass and t r a c k i n g fiJters were then used t o provide analog records on a n x-,y p l o t t e r a t the grcund s t a t i o n . Most of the f l u t t e r data were obtained from the s e l e c t e d telemetered transmissions. On board tape recording was used i n post f l i g h t a n a l y s i s of random channels other than those telemetered t o the ground, Frequency sweeps were used t o i d e n t i f y resonant frequencies.

Damping of modes of i n t e r e s t was obtained from decay records r e s u l t i n g from dwelling a t the resonani frequencies and quickly stopping t h e excite. , Good r e s u l t s were claimed f o r t h e wing vanes and vertical.

t a l l i n e r t i a shaker. However, f o r $he h o r i z o n t a l t a i l , i n s u f f i c i e n t e x c i t a t i o n was obtained from the i n e r t i a shakers at the side of the fuselage. Adequate h o r i z o n t a l t a i l e x c i t a t i o n was obtained from the wing aerodynamic vane when the wing was i n the 7 2 O sweep p o s i t i o n . However, f o r the 50’ sweep p o s i t i o n , wing t i . p e x c i t a t i o n was not e f f e c t i v e i n e x c i t i n g t h e h o r i z o n t a l t a i l , An cxplosive shaped charge was t r i e d on the h o r i z o n t a l t a i l f o r the 50” sweep p o s i t i o n but was not successful. F i n a l l y , r e l i a n c e was placed on p i l o t pulsing as the s o u r c e of e x c i t a t i o n f o r the 5O* sweep p o s i t i o n , Pigure 5 shows the l o c a t i o n of the wing e x c i t e r , as vel1 as a flow char?; f o r data transmission.

McDonnell Douglas-St. Louis, Missouri: 1;’-15.- Th. F-15 program was planned s o that I t would f ollow the p a t t e r ? of the-most r e c e n t F-4 f l i g h t flutter t e s t w i t h the-major s i g n i f i c a n t modifi.Tation being the ut.l.llzation of d i g i t a l computing equipment i n c o n j u n c t i o n w i t h the analog equipment used.

previously Aerodynamic e x c i t a t i o n i s provided by introduc,ng s i n u s o i d a l l y varying e l e c t r i c a l s i g n a l s t o the servos cf t h e a i l e r o n and s t a b i l a t o r c o n t r o l systems. Linear frequency sweeps over the frequency 7 m g e of 2-16 Iiz were planned i n 100-200 seconds. Some sweeps i n t h e range of 5-10 Hz i n approximately 45 seconds wer? a l s o planned.

S t r a i n gages, accelerometers and p o s i t i o n i n d i c a t o r s w i l J be used t,o obtain response data.

Input f o r c e w i l l be obtained from‘the c o n t r o l surface Sctuator force l i n k s .

A l l of the response and inpu2 force da%a w i l l be recorded on tape by on-board recorders and 18 channels of s e l e c t e d data w i l l be transmitted b,v F M telemetry t o the ground s t a t i o n .

Data received a t the ground s t a t i o n w i l l pass through band pass and t r a c k i n g f i l t e r s and recorded 0- pen-type s t r i p c h a r t s f o r immediate v i s u a l on-line monitoring, and, i n addition, w i l l be recorded on tape. The tape w i l l be sent t o a hybrid computer where the analog data from t h e tape w i l l be d i g i t i z e d . The fast Fourier transform w i l l be used t o compute the Fourier transform of technique the response and input f o r c e . A p l o t of the t r a n s f e r function w i l l be obtained from the r a t i o of response and input transforms. Damping and resonant frequencies w i l l be obtained from Kennedy-Pancu vector r e p r e s e n t a t i o n of the t r a n s f e r function. The computer has a l s o been programmed t o compute the Zimmerman f l u t t e r margin (ref. 11) and t o e x t r a p o l a t e t o the predicted f l u t t e r speed f o r each combination at' two frequencies that m i t couple a t the

f l u t t e r speed or Mach number. Figure P shows a schematic

drawing of the fliat f l u t t e r system.

Although t h e basic program t o be followed i s the one outlined above, the e x c i t e r system can be c o n t r o l l e d so that the p i l o t can s e l e c t any frequency and d w e l l a t that frequency. I n a d d i t i o n t o the completely automatic l i n e a r frequency sweep, quick s t o p s of the e x c i t a t i o n are planned a t s e l e c t e d frequencies i n order t o obtain the damping from the r e s u l t i n g decay curves.

O r - : F-14.- E x c i t a t i o n of the wing is by aero- dynamic vanes l o c a t e d outboard of the wing flaps and within the normal ontour of the wings. The vane area is approxi-

mately 1 f t B . Each f i n is f i t t e d with an a u x i l i a r y aero-

dynamic vane. The f i n vane i s l o c a t e d at the t i p of each f i n , externcll t o f i n surface, and has an area of 30.7 i n O 2 .

A n i n e r t i a shaker is used on the h o r i z o n t a l t a i l (right side only). Most tmts have been conducted by sweeping through the frequency range of 5-50 Hz i n 50 seconds following t h e exponential sweep v a r i a t i o n . The system has the c a p a b i l i t y of sweeping through the range of 2-70 Hz i n 15 seconds and i n the l a t t e r p a r t of the p r o g r m , the upper end of the frequency range is t o be extended from 50-70 Hz. In a d d i t i o n t o the rapid sweep, the c a p a b i l i t y e x i s t s f o r manually s e t t i n g the frequency, dwellin a n d stopping the e x c i t a t i o n r a p i d l y . Approxlmtely 98$ of the data have bden obtained from r a p i d sweep tests.

Response data are obtained from accelerometers, v e l o c i t y pickups and c o n t r o l p o s i t i o n i n d i c a t o r s and the data transmitted t o the ground by a hybrid PCW/FM t r a n s - mitter. Demodulated F M and P C M data are recorded on magnet-ic tape on the ground. The analcg data are d i g i t i z e d f o r use i n data reduction by the high fipaed d i g i t a l computer.

I n p a r a l l e l with the d i g i t a l flow, 16 channels of the analog data are fed t o two banks of 8 channel Brush recorders. The r e s u l t i n g s t r i p c h a r t s are monitored f o r the evaluation of data u a l i t y which m i g h t influence the d i g i t a l computer pro-

gram s 8 . 4 , ) Celemetrg malfunction during sweep, etc.), as well

as for monitoring f l i g h t safety. A t each test point frequencies and damping a r e obtained from the d i g i t a l T u t e r by a tech- nique destfiBeU by fhmam*as 'bode1 hatching 1 (ref. ' 6).

An a n a l y t i c a l model of the motion of a multimode repre- sentbtion of t h e a i r c r a f t response t o an e x t e r n a l l y applied force i s programmed i n the form of difference equations i n the d i g i t a l computer. Constants a r e derived which cause the model response t o be I d e n t i c a l , within some prescribed degree of accuracy, t o the a i r p l a n e response t o an iit*L*:+ f o r c e . The constants a r e then used t o determine frequer,, L . ~ - d damping.

Matching i s done over a limiced frequency r .‘:de b) f)e of

d i g i t a l f i l t e r s . Each s i g n i f i c a n t frequency band 1 : analyzed

separately and a d i f f e r e n t model match i s obtained f o r each band. The system used by Grunman has the capabl.lity of computing a l l the resonant frequencies and assoc; ated dampings i n l e s s than 30 seconds a f t e r a 15-second shaker sweep from 5-50 Hz. A high storage capacity i s required.

The procedure a c t u a l l y followed is one i n which f i v e separate shakers sweeps may be made during a single f l i g h t run ( t y p i c a l l y involving wing symmetric and antisymmetric t a i l and f i n s ) .

e x c i t a t i o n , and e x c i t a t i o n s of the horizontal The response data from the f i v e sweeps are d i g i t i z e d and stored on a d i s k . The a i r p l a n e i s then slowed while a l l the frequencies and dampings are computed; t h i s computation takes l e s s than f i v e minutes.

Vought Aeronautics Division of LTV: A - 7 A . - The A-7A i s a close d e r i v a t i v e of t h e F-U a i r p l a n e which was a supersonic fighter. The F-8 has had a successful h i s t o r y of approxi- mately 15 years. It was f e l t t h a t on the basis of a n a l y s i s and wind tunnel f l u t t e r model t e s t i n g t h a t the wing s t o r e s problem was the only p o t e n t i a l l y c r i t i c a l f l u t t e r problem.

On the basis of the a n a l y t i c a l and wind tunnel evaluation, it was determfned that the c r i t i c a l frequencies were i n the range of 3 t o 9 Hz.

Based on t h e engineering evaluat.ion of the r e l a t i o n s h i p between the F-8 and the A-7A, as well a8 wind tunnel t e s t s and a n a l y t i c a l work, the decision was reached t h a t the complexity involved i n the i n s t a l l a t i o n and operation of harmonic e x c i t a t i o n was unwarranted. Excitation was provided by p i l o t pulsing on the c o n t r o l s . S t r a i n gage and data were accelerometer and p o s i t i o n i n d i c a t o r response recorded on board the a i r p l a n e and telemetered down t o %he ground s t a t i o n f o r recording and a n a l y s i s of the decay records. The decay records were used t o obtain damping at each s u b c r i t i c a l f l u t t e r p o i n t .

Anglo-French Programs Concorde SST.- Information on the Concorde f l i g h t f l u t t e r program was obtained during a v i s i t t o Societe National I n d u r t r e l Aerospatiale (SNLAS) a t ToulouEe, France e a r l y I n March 1970. Additional su lementary info?mation was obtained from P i a z z o l i c s paper k r o e l a s t i c Test Equip- ment f o r the Concorde SST” ( r e f . 12 and by correspondence with t k . a B r i t i s h A i r c r a f t Corp,, B r s t o l , England.

E x c i t a t i o n f o r the Concorde tests has varied, covering

both s i n u s o i d a l sweeps and transie. > e x c i t a t i o n , For the

French prototype ( a i r p l a n e O O l ) , harmonic e x c i t a t i o n was provided by electrodynamic shakers i n the e a r l y flights.

Each shaker c o n s i s t s of a c o i l which i s r i g i d l y attached t o t h e s t r u c t u r e and a permanent magnet which produces a magnetic f i e l d perpendicular t o the c o i l . An a r t i f i c i a l seismic p l a t - form i s created by suspending the permanent magnet f r o a the s t r u c t u r e by a " s o f t " suspenp,on system, The sus3ension system frequency f o r t h i s type of seismic e x c i t e r is chosen so t h a t its n a t u r a l frequency is a t most l/3 the value of the lowest n a t u r a l frequency of the s t r u c t u r e it is attached t o .

Ground v i b r a t i o n tests of the Concorde .*ndicated that the lowest frequencies of the wing were of the order of 2.5-3 Hz.

The s p r i n g suspension f o r the shakers W a i ; t h e r e f o r e t a k e n as approximately .75 Hz.

A s i n the case of slectrodynamic shakers used f o r ground v i b r a t i o n tests, simultaneous operation of numerous e x c i t e r s i n f l i g h t was employed. Input force which i s p r o p o r t i o n a l t o t h e c u r r e n t i n t h e shaker c o i l was fcund t o be e a s i l y c o n t r o l l a - ble f o r each shaker independently and was k e p t constant during a frequency sweep, Ten shakers were used on the wings ( 5 on each wing), f o u r on the f i n and f i v e on the fuselage ( 3 f o r v e r t i c a l e x c i t a t i o n and 2 for l a t e r a l e x c i t a t i o n ) . The f l i g h t t e s t was programmed t o i n s u r e proper scheduling of f o r c e for each mode under study. P r i o r t o each f l i g h t a t a b u l a r form was provided t o t h e f l i g h t engineer l i s t i n g 24 cect 31 fre- quencies t o be explored. Sweeps were then made ' 15s below 15% above it. The a .de and each c e n t r a l frequency t o phase d i s t r i b u t i o n of f o r c e s a p p r o p r i a t e t o eat d e was pre- programmed f o r each shaker. Ground v i b r a t i o n ' - 3 were used a6 t h e s t a r t i n g p o i n t and a n a l y s i s was used t o ,,edict t h e change i n s t r u c t u r a l mode shape due t o aerodynamic e f f e c t s , For each subsequent step up i n air speed o r Mach number, the d i s t r i b u t i o n s of f o r c e input were s u i t a b l y modified. The frequency range covered by t h e harmonic e x c i t a t i o n tests was from 2 t o 50 Hz. The time allowed f o r e x p l o r a t i o n of each c e n t r a l frequency of i n t e r e s t was a d j u s t a b l e t o i n s u r e t h a t n e g l i g i b l e d i s t o r t i o n of t h e response ;rould r e s u l t ; 1 .e . , slow sweeps were used approximating steady state c o n d i t i o n s , For the most p a r t the electrodynamic shakers worked w e l l .

However, d i f f i c u l t i e s were encountered under t u r b u l e n t f l i g h t c o n d i t i o n s and f o r flight i n t h e t r a n s o n i c speed regime.

Because of t h e very s o f t su6penslon, turbulence caused the permanent magnet t o bottom o u t and shut off the shaker. I n the t r a n s o n i c range, v o r t i c e s shed from the l e a d i n g edge of the wing were at a frequency c l o s e t o the n a t u r a l frequency of the suspended permanent magnet and r e s u l t e d i n i n t o l e r a b l e response of the m8s.

Harmonic e x c i t a t i o n of the f : n and rudder was provided on the 002 a i r p l a n e ( t h e B r i t i s h prototype) by feeding e l e c t r i c a l siiplals t o the power c o n t r o l s . The f r e q u e m y range wacI from 4 t o 36 Hz following e i t h e r t h e l i n e a r frequency 6 ’ 2 1 the l i n e a r period sweep law. The time t o sweep the e n t i r e range varied up t o 4 minutes, I n a d d i t i o n t o s j n u s o i d a l e x c i t a t i o n , t r a n s i e n t e x c i t a t i o n of s e v e r a l types were used. FOP c o n t r o l surface e x c i t a t i o n , Impulsive, explosive charges were employed. The combustion time chosen was .O3O seconds to match the medium frequency of t h e r e l e v a n t f r e q u e m y band of 10-20 Hz f o r the c o n t r o l surfaces, P i l o t e x c i t a t i o n of tLe c o n t r o l s w a s also usec;.

However, because of i n a b i l i t y t o adequately c o n t r o l the p i l o t puLse ( i n order t o e x c i t e higher frequencies), shaped e l e c t r i - C a l i n p u t s were ,Id t o t h e power c o n t r o l s which provided r e - peatable r e s u l t s . The e l e c t r i c a l i n p u t s provided t r i a n g u l a r shaped impulses of 50 milliseconds duretior and c o n t r o l s u r f a c e amplitude of up t o 2 degrees of r o t a t i o n .

Accelerometers and p o s i t i o n lndicatcrrs were used t o o b t a i n response data. ThE data were recorded on airborne tape re- corders. Selected channels of data were s v a i l a b l e i n t h e form of s t r i p c h a r t s f o r on-beard monitoring 0 2 the records. Data obtained i n f l i g h t were transposed on t h e ground t o d i g i t a l tape f o r a n a l y s i s after each flight.

For harxonic e x c i t a t i c n admittance cuyve.. were obtained and Kennedy-Pancu p l o t s were >sed t o obtain f r e q u e x i e s and damping. I n the case of analyals of the response records fron t r a n s i e n t e x c i t a t i o n , early techniques employed analog a p p l i c a t i o n s of the Mazet method. ?‘he o r i g i n a l record played in reverse i s p a s s e d through a t r a c k i n g f i l t e r t o i s o l a t e each mode and determine frequencies and damping ( r e f . 1 2 ) .

More r e c e n t l y , a d i g i t a l a p p l i c a t i o n of the Mazet approach has been used with 8 s i m i f i c a n t reduction i n a n a l y s i s time,

- . - The A-’;JOB used e x c i t a t i o n techniques

e s s e n t i a l l y simil ar t o those used on the Concorde 001 air-

plane . F i f t e e n electrodynamic shakers were d i s t r i b u t e d as’

follows: on each wing - 2 at the wing t i p , 2 at midspan and

1 on the n a c e l l e ; on t h e s t a b i l i z e r - 1 a t each t l p ; on the

f i n - 2 at t h e t i p e,nd 1 at*midspan. I n a d d i t i o n , e l e c t r o -

magnetic c o n t r o l of the a c t u a t i n g rod permitted symmetric and antisymmetric e x c i t a t i o n of the a i r c r a f t through the control systems. Symmetric e x c i t a t i o n was provided through t h e e l e v a t o r s and low speed a i l e r o n s , whereas antisymmetric excitation was provided through the rudder and low speed ailerons Impulsive e x c i t a t i o n was p r o v i d e d by 19 e x c i t e r s similar t o those used m the Concorde 001, Combustion time for the e x c i t e r s was i n the range of 25-30 milliseconds.

The impuls!.ve e x c i t e r s were d i s t r i b h t e d as follows: 3 on each rudder, 3 on each e l e v a t o r , 3 on rear span of each wing, 2 on each engine nacelle.

1 2 Accelerations of various poirlts of the s t r u c t u r e were recorded on magnetic t a p e . A t l e a s t 48 accelerometer records from accelerometers l o c a t e d on f i x e d and movable s u r f a c e s were recorded f o r each f l i g h t (13 l a t e r a l , 34 v e r t i c a l and 1 l o n g i t u d i n a l ) .

Data a n a l y s i s techniques were i d e n t i c a l t o those used on the Concorde 001. I n regard t o f l u t t e r i n d i c e s , Aerospatiale (SNIAS) c a r e f u l l y follows the v a r i a t i o n of damping as a function of speed ( o r Mach number) as the sole c r i t e r i o n f o r the a u t h o r i z a t i o n of each following f l i g h t . Gomparisons be- tween c a l c u l a t i o n s and t e s t r e s u l t s Pre c o n t i n u a l l y made t o provide a d d i t i o n a l assurance f o r t h e s a f e continuation of t h e tests, Jaguar.- Sine sweep, random n o i s e and impulsive excl.- t a t i o n was employed on t h i s m i l i t a r y a i r c r a f t . Sweep e x c i - t a t i o n was provided through the c o n t r o l surf-ces by i n j e c t i n g e l e c t r i c a l s i g n a l s i n t o the s e r v o system. Sxeeps through a f a c t o r of 7 from e i t h e r 3, 6 o r 8 Hz were used i n 100 seconds.

The l i n e a r period sweep l a w w a s used. The random noise was a pseudorandom binary sequence w i t h bandwidth of e i t h e r 50 o r 12.5 Hz w i t h a sequence p e r i o d of about 10 seconds, The binary sequence i n the form of an e l e c t r i c a l s i g n a l drove the c o n t r o l system through the servos. I m p u l s i v e e x c i t a t i o n was either by p i l o t input (used f o r q u a l i t a t i v e i n d i c a t i o n of damping but not f o r q u a n t i t a t i v e a n a l y s i s ) o r by f i r i n g ex- plosive charges mounted on t h e main s u r f a c e s t o e x c i t e important bending and t o r s i o n modes of the s u r f a c e s and fuselage.

Current data reduction techi-iques are based O I L the use of the fast Fourier transform method. Noise i s removed from the sweep e x c i t a t i o n records by c o r r e l a t i o n techniques.

D I S C U S S l O N A comprehensive survey of t h e methods and techniques of f l i g h t testirig p r a c t i c e d i n t h e U.S.A. and Great B r i t a i n during t h e 1950ls was presented a t a conference sponsored j o i n t l y by the A i r c r a f t I n d u s t r i e s Associat5or: and the Office held i n Washington, D.C. on May 15-16, of S c i e n t i f i c Research, 1958. The proceedings of that conference were published i n 0 ~ ~ - 9 - 0 2 6 9 (ref. 1). O f p a r t i c u l a r i n t e r e s t a t t h a t time was the development of aeroiynhmic vane e x c i t e r s f o r l a r g e air- c r a f t and shaped explosive charges t o provide c o n t r o l l e d pulse e x c i t a t i o n , p a r t i c u l a r l y f o r a p p l i c a t i o n t o smaller a i r - c r a f t , I n a d d i t i o n , the development of h y d r a u l i c a l l y driven i n e r t i a shakers and the i n t r o d u c t i o n of v a r i a b l e frequency e l e c t r i c a l s i g n a l s i n t o She servos of t h e autc,iilot system were advances i n e x c i t a t i o n techniques described by i n d u s t r y r e p r e s e n t a t i v e s . I n regard t o methods of obtaining resonant frequencies and a s s o c i a t e d damping from t h e response data, Of p a r t i c u l a r i n t e r e s t were the a p p l i c a t i o n of the Kennedy-Pancu vector p l o t t i n g method as reported by Broadbent of the RAE i n Great B r i t a i n and the technique developed by E. Bartch of Lockheed i n which the s h i f t of frequency and amplitude of peak response f o r a l i n e a r l y varying frequency sweep up and down the frequency range i s used t o determine the resonant frequencies and damping of l i g h t l y damped modes.

In tne i n l o r i a p ~ r l n r l from 1958 t o the present time, a numbtu cf pzpers on f l i g h t f l u t t e r tessiing have been presented at meetings of AGARD Technical P a n e l s and published either as AGARD r e p o r t s o r incorporated i n AGARD manuals ( r e f s . 2-6).

It is of' i n t e r e s t t o note t h a t for the most p a r t e x c i t a t i o n techniques developed during the 1950's and e a r l y 1960 period a r e s t i l l used today. P i l o t pulsing of the c o n t r o l s i s s t i l l used although generally the technique is reserved f o r checkout of instrumentation and q u a l i t a t i v e evaluation of t h e response of lower s t r u c t u r a l frequencies. Most companies t e n d t o use p i l o t pulsing of the c o n t r o l s j u s t before the start of a sinusoidal sweep of the frequency range. United S t a t e s industry relies almost exclusively on sinusoidal e x c i t a t i o n provided by aux- i l i a r y aerodynamic vanes, i n e r t i a shakers or the power cor,trol system, O f the t e n a i r c r a f t covered by the U,S. survey, e i g h t used s i n u s o i d a l l y driven aerodynamic vanes and one used t h e power c o n t r o l system f o r wSng e x c i t a t i o n . Several of the air- c r a f t which used aerodynamic e x c i t a t i o n for t h e wings used i n e r t i a shakers t o e x c i t e the empennage, I n the case of t h e 10th a i r c r a f t , p i l o t p u l s i r g of the c o n t r o l s was the only source of e x c i t a t i o n . This a i r c r a f t was a low speed d e r i v a t i v e of an e a r l i e r higher speed vehicle and t h e t e s t was aimed at am evaluation of possibly c r l t i c a . 1 s t o r e configurations i n t h e frequency range of 3-9 Hz.

For t h e harmonic e x c i t a t i o n t e s t programs, a fairly rapid sweep of the frequency range of i n t e r e s t was used i n almost a l l cases. The e f f e c t of sweep r a t e , although s u f f i c i e n t t o shift t h e steady stace resonant frequency and decrease the amplitude, was slow enough t o permit i d e n t i f i c a t i o n of the region of c r i t i c a l frequencies so t h a t p i l o t s could tune t o the resonant frequencies of i n t e r e s t s o that aSsGciated damping values could be e s t a b l i s h e d , Most c m p a n i e s used the expo- n e n t i a l v a r i a t i o n of frequency w i t h time i n the sweeps; one used t h e l i n e a r v a r i a t i o n cf irequency, and one used t h e l i n e a r period sweep v a r i a t i o n , Table 1 summarizes the e x c i t a t i o n metirods, frequency ranges, time t o sweep and sweep laws followed f o r the a i r c r a f t covered i n t h e survey of U.S.

practices. It may be noted that s i n g l e source e x c i t a t i o n on any one surface has been t h e practice i n t h e U.S. (one e x c i t e r on each wing t i p , one on each s t a b i l i z e r , one on t h e f i n ) The use of explosive charges f o r pulse e x c i t a t i o n has f o r a l l p r a c t i c a l purposes been abandoned by U.S. manufacturers.

Where they hsve been used i n recent years the r e s u l t s have been u n s a t i s f a c t o r y , Rancom e x c i t a t i o n techniques have not been 1 4 used for f l u t t e r t e s t i n g nor has programmed pulsed e x c i t a t i o n been used t o drlve the power c o n t r o l systems.

French and B r i t i s h p r a c t i c s i n regard t o e x c i t a t i o n , Table 11, although similar t o U.S. p r a c t i c e i n some r e s p e c t s , d i f f e r s quite s i g n i f i c a n t l y i n others. Thus, for Anglo-French Concorde, sinusoidal e x c i t a t i o n was provided by electromagnetic seismic exciters i n the wing, f i n and fuselage, I n addition, s i n u s o i d a l e x c i t a t i o n of t h e f i n and rudder was providsd through the c o n t r o l system. Transient e x c i t a t i o n was a l s o used i n t?ie Concorde t e s t s In the form of explosive charges (used mainly or. c o n t r o l surfaces and shaped e l e c t r i c a l i n p u t s t o t h e power c o n t r o l s were use h t o provide repea%able t r i m g u l a r shaped pulses.

Neither the B r i t i s h nor t h e French have used aerodynamic vanes i n f l u t t e r testing. I n c o n t r a s t with U,S. psacticc., the (5 on each wing, 4 on t h e f i n , Concorde used m u l t i p l e shakers 2 along t h e fuselage f o r l a t e r a l e x c i t a t i o n and 3 along t h e fuselage f o r v e r t i c a l e x c i t a t i o n ) . For the smaller m i l i t a r y air- Craft such as the Anglo-French Jaguar prograin, power c o n t r o l system e x c i t a t i o n has been used involving e i t h e r harmonic sweeps or pseudorandom binary input. In a d d i t i o n , groups of explosive t o provide pulse ex- charges have been used on main surfaces I n the case of the c i t a t i o n of wing and fuselage sodes.

Dassxslt Mercure, pseudorandom excitation was used t o d r i v e the norma1 power c o n t r o l system as w e l l as t o dr.ive h y d r a u l i c a l l y powered i n e r t i a shakers, I n regard t o data a c q u i s i t i o n and transmissioc, it i s t h e e n e r a l p r a c t i c e by U.S. industry t o record +,'ne response data

f obtained from accelerometers, s t r a i n gages and c o n t r o l

p o s i t i o n i n d i c a t o r s ) on magnetic tape on 'Soard tSe a i r p l a n e .

A f a i r l y large number of channels of data are telemetered t o a ground s t a t i o n ir, the form cf freqdercy modulated or pulse code modulated signals, After 8,emodulation on the ground, the signals a r e recorded on k p e , ApproKimately 20 channels of data ( v a r i e s w i t h t h e cor,ipanies) are recorded on pen-type s t r i p c h a r t s f o r on-line v i s u a l montoring. I n a a d i t i o n , a l l of t h e ground-recorded tape data are a v a i l a b l e f o r computer a n a l y s i s .

I n c o n t r a s t w i t h U.S. p r a c t i c e , the French and B r i t l s h transmit l i t t l e i f any f l i g h t t e s t data b y telemetry. D a t i z a r e .recorded and monitored by f l i g h t personnel on board the a i r p l a n e during the t e s t . The majority of the response deta are then analyzed after each f l i g h t , The g r e a t e s t ad-Jance i n the f i e l d of f l i g h t f l u t t e r t e s t i n g within the last few years has occurred i n the f i e l d of data reduction and analysis. This i s a t t r i b u t a b l e t o t h e use of high speed d i g i t a l computers. The more s o p k i s t i c a t a d a n a l y s i s methods a s s o c i a t e d with the use of the d i g i t a l com- p u t e r s (fas,t Fourier transfcrm, a,uto-and c r o s s - c o r r e l a t i o n the use of t r a n s i e n t e x c i t a t i o n i n the form r o u t i n e s ) permit of very rapid frequency sweeps or programed pseudorandom e x c l i a t i o c . The e f f e c t of extrane-ous noisz such as atmospheric turbulence can be minimized. Loss of t e s t time due t o the presence of atmospheric turbulence can thus be s i g n i f i c a n t l y reduced, The c o r r e l a t i o n technlques a r e based on the f a c t that if a system with a s i n g l e degree of freedom i s subjected t o ex- c i t a t i o n which hds a flat energy s p e c t r a l d e n s i t y over a band- width wider than the r e s a i a n t frequency, the response auto- c o r r e l a t i o n function is an exponentially decaying o s c i l l a t i o n whGse decay rate a n d frequency are i d e n t i c a l t o those of the impulsim responsa of the system. If more than one resonant re- sponse is present i n the system then the response a u t o c o r r e l a t i o n function ( f o r f l a t energy s p e c t r a l input is a superposttion

of the exponentially decaying cosines. A fnce fast frequency

sweeps, pseudorandom binary sequences and shaped charge i n p u t s provide wLde spcztrum e x c i t a t i o n , the a u t o c o r r e l a t i o n function of the response t o such e x c i t a t i o n w i l l be a decaying o s c i l - l a t i o n containing the frequencies and dam i n g of each of the

resonant modes of the system (refs. 13, 1 t , 15). The decaying

o s c i l l a t i o n can be analyzed by the d i g i t a l Mazet (ref. 12) technique by passing the a u t o c o r r e l a t i o n s i g n a l backwards through a narrow band p a s s recursive f i l t e r . X n a d d i t i o n t o the Mazet technique, the damping and frequencies of the resonant modes can be obtained by t a k i n g t h e Fourier transform of one Elde of the a u t o c o r r e l a t i o n function and using the Kennedy-Pancu vector p l o t s . I n order t o reduce the noise i n the s i g n a l due t o turbulence, Turner and Elkins (ref. 14) suggest the use of an e q o n e n t i a l weighting function before Fourier transforming. The the waighting f m c t i o n does n@t affect the fre- introcluction of quencies i n t h e t r a n s f e r f u n c t i o n and i n c r e a s e s the damping i n a aanner than can e a s i l y be corrected.

CURRENT STATE O F THE ART O F SUBCRITICAL FLIGHT FLUTTER TESTING U,S., English and French sub- A review of the most recant c r i t i c a l f l i g h t f l u t t e r test programs (F-14, F-15, SST, Concorde and Jaguar i n d i c a t e s that g r e a t strides have been made i n

advancipj i he state of t h e art of s g b c r i t i c a l f l i g h t f l u t t e r

t e s t i n g through t h e use of lai.ge, high speed d i g i t a l computers, I n t h e last f e w years there has been a steady t r e n d from analog processing t o d i g i t a l processing of response records. During tt.18 same timz, because of the greater and faster a n a l y s i s c a p a b i l i t y of t h e d i g i c a l computer, the t r e n d has been away from t h e use of slow frequency sweep which r e s u l t e d i n e s s e n t i s l l y Bteady state response conditions for t h e p l o t t i n g of a b i t t m c e curves, as well a8 away from t h e use of the technique of tuning of resonant frequencies, dwelllng and The quickly sto3ping of t h e e x c i t a t i a n t o c b t a i n decay curves.

developmeut of s p e c i a l digital computer r o u t i n e s and digital f i l t e r s have, i n addition t o iwduclng response a n a l y s i s time, opened the boor t o transient e x c i t a t i o n programs (such as fast frequency sweeps, programmed pulsed e x c i t a t i o n or random e x c l t a t i m ) n i t h an a s s o c i a t e d reduction i n t e s t t h e .

Grumman t s model natching technique McDomell-Douglas use sf the fast Fourier transform, vector p l o t t i n g and f l u t t e r margin determination by a fully svtom%ted d i g i t a l computer routine, and the Anglo-French use of fast Fourier transform r o u t i n e s i n conjunction with autocorrelation and c r o s s are examples of a p p l i c a t i o n s of the c o r r e l a t i o n techniques, latest techniques of s u b c r i t i c a l f l i g h t f l u t t e r data reduction .arid a n a l y s i s techn5ques e A f a i r l y new process (randomdec) , involving the a n a l y s i s of a random output system, has a l s o created much i n t e r e s t , reference 16., SPACE SHUTTLE FLIGHT FLUTTER TESTING F l i g h t f l u t t e r tests of a i r c r a f t are conducted under conditions that can e s s e n t l a l l y be described as steady state.

I n these t e s t s che a i r c r a f t i s flown at constant speed and a l t i t u d e , and the resenant modes are e x c i t e d by one or more methods of s x c l t a t i o n . The msponse i s recorded from t r a n s - ducers and analyzed on t h e ground for each steady state condition. The d e c x i o n of whether t o i n c r e a s e speed a t a given a l t i t u d e is based on t h e varLation of t h e damping and frequency ( f o r each of the modes) w i t h airspeed and Mach number that has been e s t a b l i s h e d , If adequate s t a b i l i t y appears t o e x i s t a t the highest speed tested, the test speed at the same a l t i t u d e i s incre.ctsed. In this manner the speed- a l t i t u d e or Mach-altitude range over which t h e a i r c r a f t is designed t o be operated is shown t o be safe or an i n c i p i e n t f l u t t e r condition i s i d e n t i f i e d , Thus, the clearance of the a i r c r a f t over i t s required operating range, or the approach t o a p o s s i b l e c r i t i c a l f l u t t e r condition, i s schieved through a f l i g h t - b y - f l i g h t cautious extensjar, of the f l i g h t envelope.

Based on preliminary mttlyses, the c r i t i c a l f l u t t e r region f o r t h e s h u t t l e will be i n t h e transonic, high dynamic pressure range during the launch phase or’ t h e operation.

Figure 7 shows nominal space s h u t t l e t r a j e c t o r y d u r i r g launch for the first two minutes of flig’.it. From this f i g u r e it can be seen t h a t a maximum dynamic pressure of 650 psf at a Mach number of 1.45 w i l l be reached i n approximately 80 seconds a f t e r launch. The time t o cover the Mach range from M = .95 t o 1-45 will be approximately 1 5 seconds. Off-nominal t r a j e c t o r i e s w i l l l e a d t o more severe dynamic pressures o r more c r i t i c a l rates of change i n the parameters.

Although cansideration has been given t o the use of a u x i l i a r y airbreathing engines an t h e s h u t t l e t o evaluate low- speed handling c h a r a c t e r i s t i c s , such a vehicle would not have the c a p a b i l i t y of achieving the c r i t i c a l Mach nun;?er and dynamic pressure simultaneously, It I s f a . i r l y c l e a r t h a t a cautious P l i g h t - b y - f l i g h t ex- tension of the flight envelope at stabilized flight conditione of speed and a l t i t u a e , i n a manner similar t o aircraft testing, is not feas-ible f o r the s h u t t l e , Techniques of the type described i n reference 17 a r e t h e r e f o r e of i n t e r e s t . Greater emphasis w i l l be required on a n a l y t i c a l work and wind tunnel t e s t i n g D f f l u t t e r mcdels of the launch vehicle and shuttle.

F l i g h t conditions least l i k e l y t o l e a d , t o f l u t t e r , &s based on should be model t e s t i n g and the b a s t malysis availabLe, carefully considered i n s e l e c t i n g the first launch t r a j e c t o r y .

As much response data as p o s s i b l e should be acquired during the lauiich f o r v e r i f i c a t i o n of the methods t h a t have been developed f o r evaluating f l u t t e r i n a t r a n s i e n t environment .

Preliminary analysis of the frequency range of i n t e r e s t i n regard t o p o s s i b l e f l u t t e r i n d i c a t e s that wing frequencies ' up t o 10 Hz ant! empennage frequencies from 10 t o 30 Hz m i g h t be invoived, For the evaluation of f l u t t e r , t r a n s i e n t ex- c i t a t i o n i n the form of square waves of approximately 16 millisecoml s .duration or pseudorandom white noise ) e x c i t a t i o n

o f adequate bandwidth could be impose 6 through the fly-by-

wire control system during the c r i t i c a l phase of the launch trsjedtory. The techniques employed i n date reduction sad a n a l y s i s of aircret s u b c r i t i c a l f l i g h t f l u t t e r t e s t records couldthen be used t o o b t a i n damping and frequencies of modes t h a t appear s i g n i f i c a n t with respect t o f l u t t e r .

REFERENCES Proceedings of the F l i g h t F l u t t e r Testing S posium, 1.

May 15-16, 1958, Washington, D,C. OSR-9-02 t ? 9.

Ferdman, S . , Schaffer, A.P., and Caporali, R.L.: Advances 2, in Flight F l u t t e r - T e s t i n g Through AutomatLC Data Acquisition and Processing Techniqces. AGARD F l i g h t Test Manual, Vol. IV, P a r t IVD, Dec. 1963.

DeVries, G. : U t i l i z a t i o n of Electrodynamic 'Vibrator i n 3.

Flight Measurement of the Response of an Aircraft Wing t o Harmonic Excitation. AGARDOGRAPH 56, P a r t 11.

Laidlaw, W.R, , Butterworth, W . T e : Some Recent Develop-

4.

ments i n the A r t of 1r.-Flight Vibration Testing.

AGARDOGRAPH 56, Part 1x1.

Piazzoli, G.: Aeroeleastic Test Methods, Experimental 5 .

Tecmiques. AGARD Report 5738 6, Baird, E.F., Clark, W.B.: Recent Developments i n F l i t

F l u t t e r Testing i n the United States. Presented at 3 &" th

Meeting af the AoARD Structures and Materials Panel, A p r i l 1972, A W D Report 596.

Piazzoli, Q , : F l i g h t F l u t t e r T e s t i n g Methods and Techniques. Presented at 34th Meetifig of the AQARD S t r u c t u r e s and Materials Panel, A p r i l 9-14, 1972.

8. Dat, R.: The Theoretical and Experimental Methods Used i n France f o r F l u t t e r Prediction. A I A A Paper 73-329 presented at A I A A Dynamics S p e c i a l i s t s Conference at Williamsburg, Va., March 19-20, 1973.

Kennedy, Charles C., Pancu, C.D.P.: Use of Vectors i n 9 .

Vibration Measurement and Analysis, J. of the Aeronautical Sciences, V O ~ . 14, NO. 11, NOV. 1947.

Ryneveld, A .D. : Transient Excitation Techniques f o r Wind 10 .

Tunnel and F l i g h t F l u t t e r Testing of SST Configurations -

SST Technology Follow-on Program - Phase 11. Rept. No.

FAA-55-73-14, Y ! . : r 1974.

Zimmerman, NOH,, Weissenburger, J.T.: Prediction of 1 1 .

F l u t t e r Onset Speed Based on Fli@-:t Testing at* S u b c r i t i c a l Speeds. Journal of A i r c r a f t , July-August 1964, Piazzoli, G.: Aeroelastic T e n t Equipment f o r the Concorde 12.

SST. Proceedings, 6 t h Symposium on Aerospace Instrumentation, Cranfield (U.K.), March 23-26, 19700 Baldock, J.C.A., Skingle, C,W.: F l u t t e r Technology i n the 13.

United Kingdom - A Survey. A I A A Paper 73-330, presented at AIAA Dynamics S p e c i a l i s t s Conference Williamsburg, V a . , March 29-20, 1973.

Turner, M . R . , Elkins, J . 4 . : Digital Analysis of F l i g h t 14.

F l u t t e r Tests. B r i f ; i s h A i r c r a f t Corporation F i l t o n Division Report C;EN/~74S-4/7893, May 1969.

White, R.G.: Evaluation of Dynamic C h a r a c t e r i s t i c s of 15.

Structures by Transient Testing. Journal of Sound and Vibration, March 1971.

16 . Cole, Henry A., Jr.: On-Line F a i l u r e Detection and Damping

Measurement of Aerospace Structures by Random Decrement Signatures. NASA CR-2205, March 1973.

Reed, W . H . , 111: E f f e c t of a Time Varying Test Envlron- 17.

ment on the Evaluation of Dynamic S t a b i l i t y with Appli- c a t i o n t o F l u t t e r Testing. Journal of t h e Aerospace; Sciences, July 1958.

APPENDIX The author wishes t o acknowledge and thank the following people who have provided assisr.ance f o r the survey of f l i g h t f l u t t e r t e s t i n g techniques c u r r e n t l y i n use by the U.S., French a n d B r i t i s h i n d u s t r i e s .

Boeing-Seattle: J. Turner, W . Bingham, J. Louie, P. Jennings, A . Reinweld, N . Olsen NeDonnell-Douglaa-West: A . Tracy, J. McOrew Lockheeh-California: H . Hassig, R, 0 'Connell, S . Hurley, A . MaEsena Lockheed-Georgia: W . Grosser, J. Crooks, D. Cone, W . Bensen, J. Bailey, J. McAvoy G r u m m a n : E . B a i r d , W . Clarke, G , N I C O S , P. Walsanen McDonnell-Douglas-East: C. Perisho, N, Z i m m e r m a n , H , Katz, M. Ferman General Dynamics-Ft. Worth: L. Wilson, N. Mitchell, R. Peloubet L.T;V.: L. Head, W , Brock, W. Storey, R . Hancock Aerospatiale (SNIAS): R . Rouges, E . Roustan, J. Wagner Drtssault: J. Czinezenheim

B r i t i s h A i r c r a f t Corp. - Commercial: N. Harpur

British A i r c r a f t Corp. - Military: D. K. P o t t e r

TABLE I S W R Y O F SUBCRITICAL FLUTTER TESTINO TECHNIQUES USED I N THE U . S .

( a ) Aerodynamic E x c i t a t i o n COMPANY AIRPLANE SURFACE LOCATION RANGE SECONDS E x t e r n a l vane8 1-20 Hz 90 Exponent la1 McDonnell- DC-10 Wings h o r i - at t i p s or and Douglar- z o n t a l main s u r f a c e s 1-10 HZ go Long Beach V e r t i c a l tail Lockheed- External vanes .5-25 Hz 60 Normal Exponential C-5A W i n g StAbi- Georgia l i z e r on top of s u r - 30 Dive only races n e a r t i p s Orurnman F-14 Wing f i n Aero-tab 5-50 Hz 15 Exponent la1 E x t e r n a l vane McDonnell- F-15 Normal 2-16 Hz 100-200 Linear Douglae- c o n t r o l 5-10 Hz 45 Frequency S t . LOUIS Ailerons Stab l l a t or W n e r a l F-111 Wing Aero-tab 35-2 Hz '(5 Exponential

D~nsmic 8 -

I (b) I n e r t i a l E x c i t a t i o n I

V e r t i c a l and SWEEP LAWS* is Frequency at S t a r t of Sweep Linear Frequency u0 h o t X is P r o p o r t i o n a l t o t h e ChWge I n Frequency p e r Cycle w ( t ) = u0e 2 Exponential wO Linear Period w ( t ) = u) "see r e f e r e n c e 7.

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Fig. 3 . C-% F l i g h t Flutter Tests - Vane and Pylop Assembly

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