Document
FLIGHT FLUTTER T E S T I N G OF MULTI-JET AIRCRAFT
J . Bartley - Boeing Airplane Co., Seattle, Washington
Abstract Structural characteristics and internal wing fuel distribution of the jet transports a r e generally similar Extensive flight flutter tests have been con- to the B-52, although the structural frequencies are somewhat higher.
ducted by BAC on B-52 and KC-135 prototype air- planes. The paper will discuss the need for and importance of these flight flutter programs to Boeing Initial appraisal of the B-52 flutter problems airplane design. Basic concepts of flight flutter testing indicated that a comprehensive theoretical analysis of multi-jet aircraft and analysis of the test data will would require approximately 20 degrees of freedom, be presented. Exciter equipment and instrumentation a prohibitive number for the computing machinery will be discussed. available at that time. The alternative which was employed in these tests decided upon was to build dynamically scaled flutter models for wind tunnel flutter testing. Results of INTRODUCTION a the wind tunnel flutter investigations indicated During the past 6 years the Boeing Airplane marked sensitivity of flutter speeds to moderate Company has accumulated an extensive experience changes in wing and nacelle strut stiffness and weight with the flight flutter testing of multi-jet aircraft, distribution. Also, flutter occurred in approximately including the B-52, 707-80 commercial prototype and 5 different modes all of which involved strong coupling the KC-135 tanker. This has been occasioned by the of the wing and fuselage.
complex flutter characteristics associated with the FLIGHT TEST EQUIPMENT AND PROCEDURE general design of these airplanes involving a high aspect ratio wing carrying flexibly-mounted nacelle pods and a long slender fuselage. The resulting as- Although the wind tunnel flutter investigations sembly presents a large number of possible flutter indicated adequate flutter speed margins for the nom- modes of the basic structure. Figure 1 shows the inal B-52 configuration, it was decided to embark on number of anti-symmetrical modes and frequencies a flight flutter program which would provide maximum of interest from a flutter standpoint for one distri- safeguards against the occurrence of unanticipated bution of fuel on the B-52. These data were obtained flutter on this airplane. This decision was based on from a ground vibration test of a B-52 flutter model, the feeling that the overall complexity of the B-52 and the frequencies shown are model values which are structure made it necessary t o provide an additional 4.5 times actual airplane frequencies. measure of safety over and above that provided by the wind tu.mel test results. A systematic monitoring Furthermore, added complicktion comes from of the flutter behavior of the airplane as test speeds a r e increased in increments up to the design speed the fact that fuel is carried internally throughout the limit was established as the basic flight flutter test wing and fuselage, and in the case of the B-52, the external tanks are mounted on the outboard wing, plan. Telemetering of response data to a ground sta- presenting a wide variation in fuel configurations tion permitting a crew of flutter personnel to analyze to be cleared for flutter. Figure 2 illustrates the the behavior of the airplane carefully during flight distribution of fuel tanks in the B-52 wing and fuse- flutter tests was considered an essential part of the plan to provide maximum overall flight safety.
lage.
BODY FREQ MODE CPM AA 1st BODY SIDE BDG.
BB 1st WING BENDING CC R H. NAC. STRUT BDG.
L H. OUTBD. NAC. PITCH DD L H NAC. STRUT BDG.
R H OUTBD. NAC. PITCH EE WING BDG 8 TORS.[SOME CHDWISE) 654 BODY ROLL RR BODY TORSION 715 FF BODY TORSION 7 2 0 OO2nd WING BDG.
GG 2nd BODY SIDE BDG.
HH VERT. TAIL BDG., HORIZ. TAIL 1218 RESPONSE JJ LH.OUTBD NAG. ROLL 8 YAW R H INBD. NAC. ROLL 8 YAW WING RESPONSE, VERT. TAIL BDG.
HORIZ. TAIL RESPONSE K K EXT TANK PITCH. WING BDG. 8 TORS, H O R I Z . ~ VERT. TAIL RESPONSE M PP HORIZ. TAIL YAW 8 BDG. 17J7 LL VERT TAIL BDG., HORIZ. TAIL 2730 RESPONSE -N MM VERT. TAIL ROT. 8 BDG , HORIZ. 2880 H TAIL RESPONSE L VERT. TAIL ROTATION FOO 3740 -R NN HIGHER EMPENNAGE MODE Figure 1. Antisymmetrical Modes and Frequencies - High Gross Weight Excitation of the airplane structural modes is
A
provided by two methods: through control impulse and by an oscillating airfoil shaker located at one wing tip. In the simpler of the two methods, the in- put pulse from abrupt displacement of the control surfaces is used to excite response in those modes of vibration most easily excited by each control sur- face, generally the lower frequency modes. Tests are conducted at successive speeds, in increments of 5 to 20 knots, up to limit test speed based on pre- dicted placard or design speed limit as shown in Figure 3. Trend in the rate of decay of the response (damping) with increasing airspeed is used as an in- dication of approach to flutter in each mode which can be excited by the dontrol pulse.
A telemetered record of response to an elevator impulse is shown inFigure 4. Note that the pulse excites two super- Figure 2. B-52 Fuel Distribution imposed modes at nearly the same frequency (this is most noticeable on the trace of wing chordwise re- sponse). The mode of lower frequency damps out The general philosophy of flight flutter testing rapidly leaving the higher frequency mode to decay at Being is to employ it as a check o r confirmation by itself. Figure 5 shows the samping curve vs air- of margins of safety predicted by wind tunnel testing speed obtained for one mode using control impulse o r analysis, and not as an investigative technique.
testing techniques.
That is, flight test plans call for configurations to be flown only at speeds which have been cleared pre- Generally, a great deal of judgment on the part viously with adequate margins by wind tunnel tests of the ground crew is involved in analysis of the decay or by analysis.
1 04
OVERALL PING (9) VERSUS
SPEED§ - FLIGHT ~LUTT€R
AIRSPEED - CONTROL IMPULSE
YB -52
TESTING 8-52 AIRPLANE MACH NO.
VSRllCAL DlSPLACfMINl 8 5 111.5 110 CPM 0, ALTIWDI 9 01 1000 fill u I 2 0 s a 0.
3 01
w 0 01 , I D ,so , . o .IO ,eo . I O .IO . P O 100 110 I 2 0 $10 1 . 0 110 I60 170 110 $90 600 IRUf AlRSPffD ~ KNOTS AUfOll VIBRATOR AND NACA 5TANDARD ATMOIPHfRI TRUE AIR SPIfD. KNOTS
* CONIROI IMPULSI
@ CONTROL IMPULSI ONLY Figure 5. Overall Damping (g) Versus Airspeed - Figure 3. Test Speeds - Flight Flutter YB-52 Control Impulse Testing B-52 Airplane data. Repeatability is only fair, although it tends the disadvantage of being limited in the number of to improve a s damping decreases. modes which can be excited, generally 2 or 3, and mode separation is not altogether satisfactory.
Responses from 29 locations on the airplane, and force input from the wing tip vibrator, are re- An alternate method of flight testing employs corded on a Miller Model J oscillograph installed an electric motor-driven airfoil installed at the right i n the airplane. Figure 6 shows the location of pick- wing tip of the test airplane. The unit which was ups on fixed structure and the airfoil force vector.
designed and constructed in the Structural Test Unit The double headed vectors indicate the measurement at Boeing, has a programmed frequency sweep which of angular motion about the axis of the vector. In covers the range of critical frequencies of the air- addition, there a r e 7 control surface and tab deflec- plane. The sweep from the lower to the upper limit tion indicators. The 5 starred locations in Figure 6, of frequency is accomplished in about 7 minutes. The plus the vibrator force, are telemetered to the ground slow rate of sweep is required in order to allow each station using a Bendix FM TXV-13 transmitter and structural resonance sufficient time to build up and TGRS receiving station. Flight test time required decay as the vibrator continues through its sweep.
for each test condition, using this technique, averages A section of Brush record showing typical response about 3 minutes including analysis. However, it has to the vibrator sweep is given in Figure 7.
Figure 4. Telemetered Record of Typical Airplane Response to Wing Tip Vibrator Excitation Initial efforts at providing controlled mechani-
LOCATl ON OF PICKUPS AND
cal vibratory excitation on a B-52 airplane in flight were aimed toward the use of a rotating unbalance
MEASUREMENT
DIRECTION OF
vibrator. Such a unit, hydraulically driven, was designed, fabricated and installed in the tail of the YB-52 airplane. Required to provide a reasonably uniform rotating force Vector over the frequency range, with good speed control and powerful braking in the event of control failure, the tail vibrator emerg- ed a very complex system which taxed the limit of auxiliary power available on the airplane. Although it provided adequate excitation of wing and body mMes, the tail vibrator, because of its overall complexity, failed to perfom as reliably as is necessary for flight test work. It was replaced bythe more reliable airfoil vibrator unit upon completion of the early phases of B-52 flight flutter testing.
FORCE Theairfoil vibrator is comprised on an unswept
* * - TELEMtTERED TO GROUND
tapered airfoil driven by a 1/2 horsepower DC elec- tric motor. The airfoil has an area of 2 square feet, with a 2-foot span, 16-inch root chord, 8-inch tip Figure 6. Location of Pickups and Direction chord, and a thickness ratio of 6 percent. The axis of Measurement of rotation is along the quarter chord, and the airfoil is mass bdlanced uniformly along the span to main- tain the center of gravity slightly forward of the ro- Flight test time required for each test condition, tational axis. This provides a safequard against which employs both control impulse and vibrator flutter involving the airfoil in the event of a free sweep, averages about 15 minutes including analysis.
Figure 7. Telemetered Record of Airplane Response to Elevator Impulse airfoil resulting from failure of the driving system.
The oscillatory angle of the airfoil canbe varied from 0 to a maximum of rt4 degrees. The oscillatory fre- quency can be varied between 85 and 600 cycles per minute. Both the angle of attack and frequency of oscillation can be controlled by the pilot during flight. In addition, the programmed automatic sweep of @e frequency range is provided by electronic con- trol of the amplidyne power supply for the electric drive motor. Frequency control during the program- med sweep is within 1/2 percent of the prescribed frequency.
An emergency stop is provided which will halt oscillatory motion of the shaker in less than 1 cycle.
Figure 8. Wing Tip Vibrator - YB-52 This may be used to collect damping data from decay of the shaker-induced structural oscillation.
The weight of the entire unit at the wing tip is approximately 150 pounds. The vibrator weight is When the vibrator is used, force to produceunit counterbalanced by an equivalent weight at the oppo- response is plotted against airspeed since this ratio site wing tip to maintain symmetry of weight distri- tends toward zero as damping of a mode decreases.
bution of the outboard wing of the test airplane. of vibration are excited through use of More modes the airfoil vibrator than with the pulse technique Figure 8 shows the airfoil installed at the wing (roughly 8 or 9 compared with 2 or 3) and frequency tip of the B-52 airplane.
separation is highly superior. Figure 9 shows plots of force/displacement amplitude versus speed for 6 The entire drive unit (motor, gear box, support, of the modes which were excited by the vibrator etc.) is housed in the wing tip fairing. during testing of one B-52 configuration.
RESPONSE DATA USING WING TIP
VIBRATOR 5 4 2 AIRPLANE
R.H. W.S. 1377 VIRTICAL R.H. W.S. 1377 ANGULAR 12OCPM U S ) s.0 s.0 4.0 3.0 g 2.0 2.0
3 1 . 0
1.0 YI a 420 460 500 5 4 0 420 460 5 0 0 340 420 460 5 0 0 540 1RUE AIR S N E O - KNOlS a.n. STABILIZER 362 VERTICAL R H. SlABlLlZER R.H. W.S. 1377 V€RlICAL 315 CPM I AS1 190 CPM (51 5 0 10.0 4.0 8.0 3.0 6.0 2.0 4.0 I .o 2.0 '420 460 500 540 420 460 SO0 140 420 460 5 0 0 540
TRUE AIR SPfEO - KNOTS
(I) Spm.tricml (Ais) Anti-symetricd Figure 9. Response Data Using Wing Tip Vibrator B-52 Airplane During level flight test conditions, both methods what smaller number of configurations were tested of excitation are employed at each test speed, and the on the B-52 with 1000 gallon external tanks andon plots of damping and response to vibrator input are B-52 airplanes and jet transports without external made concurrently. Flight flutter tests in level flight tanks.
are conducted up to level flight maximum speed (400 knots EAS, M = .89 at 19,500 f t for the B-52). Be- The external tanks carried on B-52 production yond this speed, up to 400 knots EAS, M = .93, the airplanes contain baffles which prevent significant tests require diving the airplane and the interval of shift of fore-and-aft center of gravity during transient time available at test conditions is necessarily brief. response conditions. Holes in the baffles allow fuel Therefore, control impulse testing only is employed to flow through slowly thereby permitting a substan- at these speeds. By the time the level flight high tial shift in fore-and-aft center of gravity for sustained speed is reached, the modes of concern have been climb or dive attitudes. The flight speeds associated identified from the combined shaker and impulse with sustained climb are limited by power considera- testing, so it is relatively. safe at that point to con- tions and do not present a critical flutter problem.
tinue on up in speed employing control impulse only. However, sustained dive attitudes at high speeds are possible, and configurations with external tank fuel Because the amplitude of airplane response to distributed forward in the tank a r e studied inthe wind pulse and airfoil excitation i s quite small (one-half tunnel and checked in the flight test program. The to three-fourths of an inch double amplitude at the external tanks of the test airplane are divided into 3 compartments, and each compartment is loaded with wing tip) it is essential that the tests be flown in (in smooth air. Although flutter tests have been dis- the proper amount of ballast mixture to represent continued because of turbulence, it has been a rare a level flight condition of the flutter test airplane) occurrence and not a major problem. High speed the weight and cg of external tank fuel in the uncom- partmented tank on an airplane i n a 25" dive attitude.
buffet becomes significant only at the maximum test Figure 11 illustrates this simulation. The ballast is Mach number, M = .93, where strong buffet is made up of a mixture of water and glycerin (anti- encountered.
freeze).
Results of wind tunnel flutter tests have indi- cated that variation of outboard internal and/or ex- ternal wing fuel is more effective i n altering flutter
3000 GALLON COMPARTMENTED
characteristics than variation of inboard wing and
TEST TANK
body fuel. Accordingly, the configurations tested in the flight flutter program involve a more detailed breakdown of fuel in these tanks than in the main wing and body fuel tanks. An illustration of the 1ANK 514.
S A . SIA number of flight flutter test configurations involving 14.
combinations of outboard wing internal and external T I T
tank loadings is shown in Figure 10.
Twenty-eight configurations were tested on B- Asome- 52's carrying 3000 gallon external tanks.
FLIGHT FLUTTER TEST CONFIGUR-
ATIONS 6-52 WITH 3000-GAL.
EXTERNAL TANKS
Figure 11. 3000 Gallon Compartmented Test Tank €Xt.."rnl i d RESULTS in I . r . 1 llipht oltilud.
Before discussing flight test results and com- paring with wind tunnel data, some description of the nature o f our wind tunnel testing should be presented.
The wind tunnel program has been conducted using dynamically scaled models of the complete B-52, 707 and KC-135 basic structure. A flutter model of the B-52 airplane is shown in Figure 12.
Figure 10. Flight Flutter Test Configurations B-52 Structural stiffnesses of the wing, fuselage, nacelle strut and empennage structure are repre- with 3000-Gal. External Tanks ation of damping in this mode was experienced at maximum true airspeed during testing of configura- tions carrying empty external tanks with a capacity of 3000 gallons. Wind tunnel tests had indicated adequate flutter margins for these configurations.
A detailed reanalysis was made of structural representation of the airplane on the part of the elastic model. A carefully controlled stiffness test of the airplane nacelle strut and local wing attach- ment structure revealed that the flutter model was considerably out of scale in this parameter. Cor- rection of this deficiency resulted in good correlation between model and airplane data where airplane con- figurations had been flown near enough to flutter to permit a reliable extrapolated prediction of the critical speed, Figure 13.
Figure 12. Flutter Model of the B-52 Airplane COMPARISON OF WIND TUNNPL AND FLlGHT FLU TER RESULTS sented by single dural spars which are covered by slotted balsa sections forming the geometric external contour of the model. The flutter model tests have been conducted in low-speed wind tunnels, with maxi- mum test speeds being in the neighborhood of 200 is flown in the wind tunnel miles per hour. The model on the rod-trunnion arrangement shown in Figure 12, gradually increasing tunnel velocity until flutter occurs 1 I I Measurements of damping ma in the most critical mode.
of the various modes present in the model below the I critical flutter speeds are not obtained. Wind tunnel
turbulence provides generous excitation of the model, o&-u 0 15 100
so that flutter occurs once the critical speed is O U l b O A R D WING FUtL - yo FULL reached.
12,000 FI A L l l l U D f - H I N D TUNNEL M O D f L DATA
W I N D TUNNfL MODEL D A l A f M P l V 3000 GALLON f X I E R N A 1 1ANKS - -
Yb-SZ AIRPLANE FllGHl I f 5 1 DATA - Because the procedure used up to the present tests at Boeingdiffers in conducting wind tunnel flutter Figure 13. Comparison of Wing Tunnel and Flight from that employed in flight flutter tests, it is not Flutter Results possible to obtain a direct comparison of wind tunnel model and airplane flutter characteristics in the The figure shown is for configurations flown stable area below the critical flight speed. A s stated with various amounts of fuel in the outboard wing previously, the policy at Boeing has been to avoid flying into a region of known or suspected flutter. and with empty 3000 gallon external tanks. Similar A s a consequence, our experience has been pri- correlation exists for B-52 configurations carrying marily one of negative agreement; that is, the wind empty 1000 gallon external tanks.
tunnel results predict freedom from flutter up to a specified limit, and the flight flutter tests provide It is noteworthy, in considering the application of these flight test techniques to the B-52, 707 and confirmation.
KC-135 flight flutter programs, that wind tunnel tests Actually, during the early B-52 flight flutter had shown that potential flutter modes are of the testing, correlation with previous wind tunnel test %on-explosive” type. That is, evidence of a flutter results could be classified as no better than fair. condition (reduced damping trend) appears on the Although no flutter incidents occurred, the mode of model at speeds appreciably below the critical speed.
Furthermore, because of the low frequencies associ- the airplane which exhibited lowest damping during the ated with the basic structure of these airplanes and flight test program had not fluttered nor indicate low rate of divergence of damping during the wind tunnel testing of comparable the large masses involved, the configurations. The mode involved was a symmetrical the flutter oscillations against time is low.
higher order mode of the wing coupled with body vertical bending. There was an appreciable chordwise In summary, flight flutter tests have been con- component of wing motion. The frequency was ap- ducted on B-52, 707 and KC-135, airplanes totalling proximately 160 cpm. Finally, indication of deterior- approximately 250 hours of flight time. The airfoil vibrator has been used successfully on about 25 flights of the KC-135 airplane and 85 flights of B-52air- planes. Almost 450 sweeps have been conducted during the flutter testing of these airplanes using the airfoil vibrator.
The flight flutter techniques employed provide adequate safeguard against catastrophic flutter of the