A case of flutter developed at a speed lower than had been flown previously. This incident precipitated the routine procedure of pulsing control surfaces as well as the firing of explosive charges during speed build-ups. In the interest of rapid evaluation of results, simple methods of data reduction were used. A case history is presented where in the pulse technique predicted flutter by extrapolating decay rates obtained at subcritical speeds; in addition, a case is presented where no valid extrapolation could be made

Lenk, E. J.

Stringham, R. H., Jr.

English

NASA Technical Reports Server

FLIGHT FLUTTER T E S T I N G  USING PULSE TECHNIQUES 
R, H .  Stringhum, Jr., E. J .  Lenk - D o u g h  Aircruft Co., 
EL Segzmdo, Culiforniu 
Abstract 
A case of flutter developed at a speed lower than 
had been flown previously. This incident precipitated 
the routine procedure of pulsing control surfaces a s  
well a s  the firing of explosive charges during speed 
build-ups. In the interest of rapid evaluation of re- 
sults, simple methods of data reduction were used. A 
case history is presented where in  the pulse technique 
predictedflutter by extrapolating decay rates obtained 
at subcritical speeds; i n  addition, a case is presented 
where no valid extrapolation could be made. 
INTRODUCTION 
The need for systematically evaluating the 
structural stability of aircraft by flight testing has 
arisen out of the need for confirming the results of 
the flutter analysis as well asfor  searchingout modes 
unforeseen by the analysis. It ts the purpose of this 
paper to describe how the pulse technique has been 
used to fulfill this need during the flight testing of 
airplanes designed by the El Segundo Division of the 
Douglas Aircraft Company. 
Methods used for the generation of pulses are 
described and the results of their application shown. 
The pulse technique has been used at Douglas because 
of its simplicity as compared to other methods such 
as the frequency response technique. Also, a mini- 
mum of auxiliary equipment is required, and data can 
be obtained without prolonged speed stabilization which 
is an advantage when exploring the speed envelope 
beyond the airplanes level flight capabilities. 
GATHERING AND DATA INTERPRETATION 
In conducting pulse tests the structural response 
has been measured primarily with accelerometers and 
occasionally with strain gages. Outputs have been 
recorded by oscillographs installed in the test a i r -  
plane. With accelerometers, low pass filters usually 
have been used for suppressing the high frequency 
disturbances excited by buffeting, turbulence, and 
noise. 
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 
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- 
torted to establish the decay envelope. Ideally the 
damping speed plot thus obtained will  form a smooth 
curve enabling an extrapolation to the flutter speed. 
GENERATION OF PULSES 
In view of the means of data reduction the 
primary requirement of pulsing is that the airplane 
structure be excited in the proper mode o r  modes 
at an amplitude substantially above the noise level. 
In an attempt to fulfill this requirement, pulses have 
been generated primarily by two methods: (1) man- 
ual control pulses, and (2) the firing of explosive 
charges. 
For piloted aircraft, the advantages of manual 
control surface pulses are obvious in that no special 
https://ntrs.nasa.gov/search.jsp?R=19760003016 2020-03-22T19:28:43+00:00Z
equipment is required and the number of pulses per 
flight is practically unrestricted. However the shape 
and magnitude of the force-time curve a r e  important. 
Thus limitations are imposed upon the manual pulse 
by the response characteristics of the control system, 
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. 
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 
modes have presented a greater problem. Attempts 
to excite symmetrical wing modes with elevator con- 
trol  pulses have been ineffective; however, there has 
been some success in exciting the first bending sym- 
metrical stabilizer mode with the elevator. 
The second pulse method which has been ex- 
tensively employed is that of firing explosive charges. 
With this method, control over the force-time curve 
is possible, allowing a broader frequency spectrum 
to be examined a s  compared to the manual pulse 
method. Also, the pulse shapes formed by explosive 
charges are likely to be more consistent. Of course, 
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 
Group. This device has been called an "impulse 
generator", with a length of 3-3/4 inches and a cross 
section of 1-1/2 x 1-1/2 inches. The breech of the 
impulse generator has been designed to accept a 
standard Mark 1 bomb ejector cartridge. These 
units have been installed on wing tips, stabilizer 
tips, and fin tips. 
A wing tip installation is shown by Figure 1 
consisting of four units. Here the nozzles can be 
seen firing upwards. Thermostatically controlled 
heating blankets are wrapped around each unit to 
insure that the ignition delay time and burning rate 
remain unchanged with ambient temperature. Uni- 
formity of ignition delay and burning rates are always 
desirable, but a r e  especially important when synchro- 
I 
t 
I 
Figure 1. Impulse Generator Installation 
nization between two pulses is required, e.g., when 
exciting symmetrical modes. 
Figure 2 shows an oscillograph record illus- 
trating the satisfactory excitation of the first sym- 
metrical wing mode during low-speed flight with an 
external store configuration. It can be seen that the 
wing tips are in phase following the firing,with a 
well-defined decay envelope. At high-speed, although 
the "hash" level was considerably higher than for the 
low speed case, it was still  possible to sketch a rea- 
sonable decay envelope for computing the damping. 
Antisymmetric modes were excited by aileron and 
rudder pulses. 
DEVELOPING PULSE SHAPES 
Some work has been done at  Douglas, E l  
Segundo in shaping the pulse of the explosive charges 
in order to emphasize the response of a given struc- 
tural vibration mode. The impulse generator, when 
used with a standard ejector cartridge, generates a 
force curve similar to that shown at the top of Figure 
3. The pulse rises sharply, reaching a peak value of 
about 1000 pounds in 7 milliseconds. With this pulse, 
one would expect the higher frequencies to be excited 
at the expense of the lower. The lower curve of Fig- 
ure 3 shows an approximate half-sine pulse as gen- 
erated by a specially developed reload. This half- 
sine reaches a peak value of about 500 to 700 pounds in 
approximately 17.5 milliseconds; longer rise times, 
it was found, could not be developed by reloading the 
--I,--- - RIGHT WING TIP NORMAL- ApN--- 
RIGHT STORE NORMAL 
-CUI- 
~~ 
-__y_- - 
LEFT STORE NORMAL- 
._I____-_rr_l_yc 
LEFT WING TIP NORMAL - - tv- 
Figure 2. Symmetrical Wing Mode Excited by Dual Impulse Generators (Low Speed) 
70 
PULSE GENERATED BY 
EJECTOR CARTRIDGE 
N 
4 I-=.oo~sEc. TIME 
Figure 3. Pulse Generated by Ejector Cartridge 
ejector cartridge without an unacceptable reduction in 
peak force and a deterioration of reliability. 
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 
computer damping. Aerodynamic forces were not 
simulated. The computer study showed that best re- 
sults, as measured in terms of maximum displace- 
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 
standard cartridge load, a rise time of 1/4 the period 
gave the maximum amplitude response per peak force 
input. 
The analog results indicated that standard ejec- 
tor cartridge loads were satisfactory for frequencies 
of about 40 cps. However, our critical flutter modes 
have been from 5 to 30 cps and, therefore, special 
reloads have been used. These special loads have 
operated effectively down to 12 cps. For lower fre- 
quencies, reliance has been placed upon control sur- 
face pulses. 
APPLYING THE PULSE METHODS 
The success, a s  well as lack of success, in 
using the pulse methods described can best be shown 
by citing three cases wherein- the pulse method was 
used. 
Case 1: 
A small attack airplane experienced a fin- 
rudder flutter at a speed lower than the airplane had 
been flown previously. This flutter had not been pre- 
dicted by analysis. No systematic pulsing ip conjunc- 
tion with the speed build-up program had been done 
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. 
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- 
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 
e 
0.7 0.8 0.9 1.0 
VELOCITY 
Figure 4. Single-Engine Airplane Fin Tip Response 
This damping plot indicates a definite trend to 
neutral stability. Since tests were carried out at an 
altitude higher than where flutter had originally oc- 
curred, the speed where the actual flutter occurred 
is not plotted. Figure 4 also shows a plot of the 
damping data obtained after making the fix. All 
flights were made with a rudder damper installed 
and adjusted with one degree of free-play. This 
damper arrangement was used to limit the rudder 
amplitude, thereby preventing destructive oscillations 
in case the flutter speed was exceeded. 
Case 2: 
The next example concerns aflight test program 
wherein flights were conducted in the speed region 
where fin stability was predicted to be marginal. 
Manual rudder pulses failed to excite the instability 
or definitely indicate approaching instability. The 
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 
by an impulse generator at a speed 5 h o t s  lower than 
where a rudder pulse had been made. Figure 5 shows 
the oscillograph record of the oscillating surface with 
the amplitude limited by the free-play rudder damper. 
71 
FIN-RUDDER 
EXCITED BY IMPULSE GIENERATOR 
C.G. LATERAL ,-- 
WING TIP NORMAL- 
-,. ".... 
IL CONE LATERAL 
Figure 5. Fin-Rudder Flutter Excited by Impulse Generator 
This case does not speak well for the pulse 
technique in that a flutter case was actually allowed 
to develop; however, it does illustrate the importance 
of proper excitation. We  believe that the investigation 
of this case was complicated by static friction, as 
are most investigations of flutter involving control 
surfaces. 
Case 3: 
A twin-engine airplane had been given control 
surface pulses during the initial speed build-ups but, 
as attention had been directed to modes which were 
thought to be critical but in fact were not, a mode 
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 
the cockpit area for the pilot to be aware of its exis- 
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 
instability. After stiffening the structure, pulse tests 
were again made. These results are also shown in 
Figure 6. Although the damping appears to be good, 
the data is scattered and a definite trend is not indi- 
cated; consequently, the flutter speed for the fixcould 
not be predicted by extrapolating the damping plot. 
CONCLUSIONS 
Based on experience gained from flutter flight 
testing in general and from using the pulse techniques 
in particular, the following conclusions have been 
reached: 
(1) It is  possible to fly beyond the critical speed 
without exciting flutter. 
TWIN-ENGIN€ AIRPLANE 
HORIZONTAL STABILIZER RESPONSE 
DAMPING 
%C/CC 
8, @ 
10 1 13, 
13 
t I 
0.7 0.0 0.9 I .o 
VELOCITY 
Twin-Engine Airplane Horizontal Stabilizer 
Response 
Systematic pulsing is necessary to minimize 
the possibility of flying into a dangerous 
speed range. 
Where flutter is known to exist, proper 
pulsing has yielded damping data which 
could be extrapolated to the flutter speed. 
A conscientious effort must be made to 
instrument and watch for unpredictedflutter 
modes; we must not be distracted by watch- 
ing only those which have been predicted to 
be critical. 
Although it does not always establish the 
flutter speed, the pulse technique is useful 
in showing the margin of damping within 
the speed range of the airplane. 
72 


Flight flutter testing using pulse techniques

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