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FLIGHT FLUTTER T E S T I N G U S I N G PULSE TECHNIQUES R, H . Stringhum, Jr., E. J . Lenk - D o u g h Aircruft Co., EL Segzmdo, Culiforniu GATHERING AND DATA INTERPRETATION Abstract A case of flutter developed at a speed lower than In conducting pulse tests the structural response had been flown previously. This incident precipitated has been measured primarily with accelerometers and the routine procedure of pulsing control surfaces a s occasionally with strain gages. Outputs have been well a s the firing of explosive charges during speed recorded by oscillographs installed in the test air- build-ups. In the interest of rapid evaluation of re- plane. With accelerometers, low pass filters usually sults, simple methods of data reduction were used. A have been used for suppressing the high frequency case history is presented where i n the pulse technique disturbances excited by buffeting, turbulence, and predictedflutter by extrapolating decay rates obtained noise.
at subcritical speeds; i n addition, a case is presented where no valid extrapolation could be made.
This filtering has been necessary in view of the method by which data has been reduced. Data reduction has consisted simply of measuring the decay envelope directly from the oscillograph rec- ord, computing the percent of critical damping, and INTRODUCTION plotting this damping as a function of speed. In this way, the damping is plotted for each frequency ap- pearing on the record in a form sufficiently undis- The need for systematically evaluating the torted to establish the decay envelope. Ideally the structural stability of aircraft by flight testing has damping speed plot thus obtained will form a smooth arisen out of the need for confirming the results of curve enabling an extrapolation to the flutter speed.
the flutter analysis as well asfor searchingout modes unforeseen by the analysis. It t s the purpose of this paper to describe how the pulse technique has been GENERATION O F PULSES used to fulfill this need during the flight testing of airplanes designed by the El Segundo Division of the Douglas Aircraft Company. In view of the means of data reduction the primary requirement of pulsing is that the airplane are structure be excited in the proper mode o r modes Methods used for the generation of pulses at an amplitude substantially above the noise level.
described and the results of their application shown.
In an attempt to fulfill this requirement, pulses have The pulse technique has been used at Douglas because been generated primarily by two methods: (1) man- of its simplicity as compared to other methods such as the frequency response technique. Also, a mini- ual control pulses, and (2) the firing of explosive charges.
mum of auxiliary equipment is required, and data can be obtained without prolonged speed stabilization which is an advantage when exploring the speed envelope For piloted aircraft, the advantages of manual beyond the airplanes level flight capabilities. control surface pulses are obvious in that no special equipment is required and the number of pulses per flight is practically unrestricted. However the shape I and magnitude of the force-time curve a r e important.
Thus limitations are imposed upon the manual pulse t by the response characteristics of the control system, I together with the rapidity by which the pilot can move the control. Based upon experience, it has been found that pilot technique is an important part of ob- taining a satisfactory pulse. Usually, sharply applied control inputs of iow amplitude have resulted in better excitation than those of large amplitude. Large amplitude inputs have invariably resulted in pulses of prolonged duration which fail to disturb the struc- tural modes.
Figure 1. Impulse Generator Installation For single engine type airplanes with fairly rigid control systems, manual control surface pulses have been effective in exciting antisymmetric modes with frequencies as high as 20 cps. Symmetric nization between two pulses is required, e.g., when a greater problem. Attempts exciting symmetrical modes.
modes have presented to excite symmetrical wing modes with elevator con- trol pulses have been ineffective; however, there has Figure 2 shows an oscillograph record illus- been some success in exciting the first bending sym- trating the satisfactory excitation of the first sym- metrical stabilizer mode with the elevator. metrical wing mode during low-speed flight with an external store configuration. It can be seen that the The second pulse method which has been ex- wing tips are in phase following the firing,with a tensively employed is that of firing explosive charges. well-defined decay envelope. At high-speed, although With this method, control over the force-time curve the "hash" level was considerably higher than for the is possible, allowing a broader frequency spectrum low speed case, it was still possible to sketch a rea- to be examined a s compared to the manual pulse sonable decay envelope for computing the damping.
method. Also, the pulse shapes formed by explosive Antisymmetric modes were excited by aileron and charges are likely to be more consistent. Of course, rudder pulses.
a means for containing and firing the charge is re- quired and, for this purpose, a breech-nozz' le assem- bly has been developed by the Douglas Armament DEVELOPING PULSE SHAPES Group. This device has been called an "impulse Some work has been done at Douglas, E l generator", with a length of 3-3/4 inches and a cross Segundo in shaping the pulse of the explosive charges section of 1-1/2 x 1-1/2 inches. The breech of the in order to emphasize the response of a given struc- impulse generator has been designed to accept a tural vibration mode. The impulse generator, when standard Mark 1 bomb ejector cartridge. These used with a standard ejector cartridge, generates a units have been installed on wing tips, stabilizer force curve similar to that shown at the top of Figure tips, and fin tips.
3. The pulse rises sharply, reaching a peak value of A wing tip installation is shown by Figure 1 about 1000 pounds in 7 milliseconds. With this pulse, one would expect the higher frequencies to be excited consisting of four units. Here the nozzles can be at the expense of the lower. The lower curve of Fig- seen firing upwards. Thermostatically controlled ure 3 shows an approximate half-sine pulse as gen- heating blankets are wrapped around each unit to erated by a specially developed reload. This half- insure that the ignition delay time and burning rate sine reaches a peak value of about 500 to 700 pounds in remain unchanged with ambient temperature. Uni- approximately 17.5 milliseconds; longer rise times, formity of ignition delay and burning rates are always desirable, but a r e especially important when synchro- it was found, could not be developed by reloading the
- - I , - - - -
RIGHT WING TIP NORMAL- ApN---
RIGHT STORE NORMAL
-CUI- ~~
LEFT STORE NORMAL-
-__y_- - ._I____-_rr_l_yc -
- t v -
LEFT WING TIP NORMAL
Figure 2. Symmetrical Wing Mode Excited by Dual Impulse Generators (Low Speed) dicted by analysis. No systematic pulsing ip conjunc-
PULSE GENERATED BY
tion with the speed build-up program had been done
EJECTOR CARTRIDGE prior to the flutter incident. This incident led to a
flight program with an unstable configuration using manual rudder pulses and pulses by impulse genera- tors while cautiously approaching the flutter speed.
N N It was believed necessary to obtain a damping plot of the known unstable configuration in order to demon- strate the value of the pulse technique in predicting the approach to instability, thereby establishing a method whereby a”fix” could be demonstrated. There-
4 I-=.oo~sEc. TIME
fore, a speed build-up program was conducted where- in the decay rates, measured from the fin response, were plotted vs speed, a s shown by Figure 4.
SINGLE-ENGINE AIRPLANE FIN TIP RESPONSE Figure 3. Pulse Generated by Ejector Cartridge ejector cartridge without an unacceptable reduction in peak force and a deterioration of reliability.
e An analog computer study was made to deter- mine the response of a cantilever wing to the pulse shapes shown by Figure 3. Mass and elastic proper- ties of the wing were included together with normal 0.7 0.8 0.9 1 . 0 computer damping. Aerodynamic forces were not VELOCITY simulated. The computer study showed that best re- sults, as measured in terms of maximum displace- Figure 4. Single-Engine Airplane Fin Tip Response ment response of the first bending mode per peak force input, could be expected when the rise time of a half- sine pulse equaled about 1/3 the period of the first bending frequency. For pulse shapes generated by a This damping plot indicates a definite trend to standard cartridge load, a rise time of 1/4 the period neutral stability. Since tests were carried out at an gave the maximum amplitude response per peak force altitude higher than where flutter had originally oc- input.
curred, the speed where the actual flutter occurred is not plotted. Figure 4 also shows a plot of the The analog results indicated that standard ejec- damping data obtained after making the fix. All tor cartridge loads were satisfactory for frequencies flights were made with a rudder damper installed of about 40 cps. However, our critical flutter modes and adjusted with one degree of free-play. This have been from 5 to 30 cps and, therefore, special damper arrangement was used to limit the rudder reloads have been used. These special loads have amplitude, thereby preventing destructive oscillations For lower fre- operated effectively down to 12 cps.
in case the flutter speed was exceeded.
quencies, reliance has been placed upon control sur- face pulses.
Case 2: The next example concerns aflight test program APPLYING THE PULSE METHODS wherein flights were conducted in the speed region The success, a s well as lack of success, in where fin stability was predicted to be marginal.
using the pulse methods described can best be shown Manual rudder pulses failed to excite the instability by citing three cases wherein- the pulse method was or definitely indicate approaching instability. The used. technique was for speeds to be advanced with control surface pulses, followed by an impulse generator firing at a slightly lower speed. Flutter was excited Case 1 : at a speed 5 h o t s lower than by an impulse generator A small attack airplane experienced a fin- where a rudder pulse had been made. Figure 5 shows rudder flutter at a speed lower than the airplane had the oscillograph record of the oscillating surface with been flown previously. This flutter had not been pre- the amplitude limited by the free-play rudder damper.
FIN-RUDDER
EXCITED BY IMPULSE GIENERATOR
C . G . LATERAL , - -
WING TIP N O R M A L -
-,. "....
IL CONE LATERAL
Figure 5. Fin-Rudder Flutter Excited by Impulse Generator This case does not speak well for the pulse TWIN-ENGIN€ AIRPLANE technique in that a flutter case was actually allowed HORIZONTAL STABILIZER RESPONSE to develop; however, it does illustrate the importance of proper excitation. W e believe that the investigation of this case was complicated by static friction, as are most investigations of flutter involving control surfaces.
DAMPING %C/CC Case 3: 8, @ A twin-engine airplane had been given control 1 0 1 1 3 , surface pulses during the initial speed build-ups but, t I as attention had been directed to modes which were 0.7 0.0 0.9 I .o thought to be critical but in fact were not, a mode VELOCITY involving horizontal stabilize'r yawing was not detected as becoming unstable. The flutter frequency was rel- atively low and did not involve sufficient response in Twin-Engine Airplane Horizontal Stabilizer the cockpit area for the pilot to be aware of its exis- Response tence. After the flutter incident, speed build-ups were again made with proper attention given to the stabilizer yawing mode. The damping measured from the tests is presented by Figure.6 and shows the approach to tests instability. After stiffening the structure, pulse Systematic pulsing is necessary to minimize were again made. These results are also shown in the possibility of flying into a dangerous Figure 6. Although the damping appears to be good, speed range.
the data is scattered and a definite trend is not indi- cated; consequently, the flutter speed for the fixcould Where flutter is known to exist, proper not be predicted by extrapolating the damping plot. pulsing has yielded damping data which be extrapolated to the flutter speed.
could CONCLUSIONS A conscientious effort must be made to instrument and watch for unpredictedflutter modes; we must not be distracted by watch- Based on experience gained from flutter flight ing only those which have been predicted to testing in general and from using the pulse techniques be critical.
in particular, the following conclusions have been reached: Although it does not always establish the flutter speed, the pulse technique is useful (1) It is possible to fly beyond the critical speed in showing the margin of damping within without exciting flutter. the speed range of the airplane.