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FLIGHT FLUTTER T E S T I N G OF T H E P6M G. Kuchudowian, R. L. Goldman, D. M . Rohu - T h e Martin Co., Baltimore, Md.
Abstract On the P6M the shake behavior, i.e., the re- sponse to random excitation at subcritical speeds of lowly damped airplane modes, is as important a s the actual flutter speed, The approach is to first study the problem by means of analyses and wind-tunnel tests. With these predictions are compared flight test data obtained by spectral analysis of tape re- cordings of the airplane vibration responses to ran- dom aerodynamic turbulence.
A similar spectrum analysis approach has been used in high speed wind-tunnel tests. Furthermore, a resonance excitation technique has been developed for low speed wind-tunnel testing, and surprisingly well defined V-g curves have been obtained. The effect of various parameters on both shake and flutter of T-tails with and without dihedral have been studied.
Figure 1. Martin P6M "SEAMASTER" Preliminary flight tests yielded good correla- tion; they also yielded interesting information con- cerning a low frequency transonic snaking mode and degrees of freedom). Also, the swept back wings, concerning excitation by shed vortices. as might be expected, introduce flexibility and there- fore additional degrees of freedom to the dynamics behavior problem.
The approach to the problem of predicting the INTRODUCTION dynamic behavior of the P 6 M has been to use the standard flutter tools, Analysis, Model Test, and The P6M is a large four jet seaplane whose Flight Test. Since it would be dangerous not to know development provides a new weapons concept for the an aircraft's basic dynamic behavior prior to flight naval aviator. For the flutter engineer, however, test, we first studied the problem by analysis and in it introduces an aero-elastic problem that is fairly wind tunnel tests. The results predicted that the aircraft would be flutter free within the designed typical for most modern large scale aircraft. Looking flight envelope. During the current flight flutter at a picture of the P6M, Figure 1, we see, for in- stance, that the T-tail stabilizer sits on the tip of a test program we are, therefore, only concerned with tall, flexible, swept back fin which in turn is attached checking these predictions and determining whether to a long, slender, flexible hull (a system with many the aircraft behaves in any unusual manner.
In carrying out this approach we have developed V-SPEED - - B both a specific and a random excitation technique for obtaining experimental data. A specific resonance
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excitation technique was developed for use in low- speed wind tunnel tests, and a random excitation tech- nique was developed for flight flutter testing after earlier investigations in a high-speed wind tunnel.
This random excitation method, as will be shown later, is a technique involving spectral analysis of aircraft response to aerodynamic turbulence and has proven so far to be a reliable approach to our sub- critical investigations of the P6M.
MODE I
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Past experience on large flexible aircraft has shown that of equal importance as the prediction of Figure 2. Incompressible P6M T-Tail Analysis the flutter speed itself is the determination of the aircraft's sub-critical behavior. A large aircraft therefore not indicated on the plot. Mode 111, how- usually has vibration modes that are lowly damped ever, is one of the previously mentioned lowly at sub-critical speeds; this lowly damped sub-criti- damped modes. Mode III, as shown in Figure 2, cal response, although it is not immediately dangerous, stays close to the zero damping axis throughout the can limit both the life of the aircraft and its accepti- usable flight range. The illustration also shows bility.
that Mode I I I is characterized by a stabilizer yawing motion. Mode I (Figure 2) bears further considera- On the P6M it became apparent, through early tion, although this mode (hull lateral bending mode) is analysis and model tests and from early flight tests highly damped in this incompressible analysis it on a previous model, that the response of lowly suffers from the common transonic "snaking" inst- damped modes to random excitation at sub-critical bility and becomes lowly damped in the compressible speeds would be as important as the actual flutter analysis. (Reference 1).
speed. Changes were incorporated in the present P6M configuration to control the sub-critical behavior In order to provide quantitative correlation with and one purpose of the current flight flutter tests is the analysis, the usual series of flutter model tests, to check whether these changes are as effective as both low and high-speed, were undertaken. (Reference predicted.
1) The low-speed tests included tests of complete and empennage models while the high-speed testswere SUB-CRITICAL BEHAVIOR concerned only with empennage models.
Before proceeding, let me establish what we tests, a specific For the low-speed wind tunnel mean in the analytical sense by the phrase "lowly resonance excitation technique was developed for damped, sub-critical behavior . I t In typical V-g obtaining in-flight or more accurately tunnel flight plots, the lowly damped mode is characterized by damping information. (Reference 3) This technique a curve which runs relatively close to the zero incorporates the system of strings, springs, and damping axis. Such modes are susceptible to at- pulleys shown in Figure 3. By pulling on the control mospheric turbulence and if they are predominantly handle and varying the motor speed the model could tail modes they are continuously excited by turbulent its resonant modes at any one be excited in any of flow from the wing-engine area. When the excitation selected tunnel speed. The control handle was sharply band is broad enough and strong enough, all such modes are continuously excited and aircraft response can become large.
ANALYSIS ON THE P6M Turning now to the flutter analysis of the P6M, 2, we see a typical V-g plot based on an in Figure I incompressible analysis of the ship's T-tail. (Ref- TO erence 1) This analysis includes five degrees of P freedom, three of which are shown here, and takes into account the effects of sweep and the well-known detrimental effect of stabilizer dihedral for T-tails, Mode 1 1 , (Reference 2) the flutter mode, rises sud- denly from a highly damped condition to the flutter speed. This mode, a s shown here in the illustration is characterized by a stabilizer rolling or rocking Figure 3. Specific Resonance Excitation System motion. Modes Iv and V are all highly damped and M O D E 1 M O D E I I M O M --ZERO AIRSPEED released when the selected mode was at resonance.
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This removed the excitation force and permitted the motion to damp out. These damped motions were recorded and the logarithmic decriments of decay were then calculated for each mode. 10 RESPONSE By using this simple method, the damping in
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all the modes was measured at one tunnel speed and well defined experimental V-g curves of the type I shown in Figure 4 were quickly obtained. Figure 4 also indicates the analytical plot for comparison.
I Again we see the flutter mode 1 1 which is highly
FREQUENCY (CPS) -
damped but rises sharply to the critical speed. The Figure 5. Spectrum Analysis o f Flutter Model analysis to test comparison indicates some conserva- Response in High Speed Wind Tunnel tism in the analysis. Mode 1 1 1 as predicted in the analysis is lowly damped and in the peak area it w a s During the high-speed wind tunnel tests the exis- susceptible to tunnel turbulence.
tance of the lowly damped %naking" mode I was also experimentally verified. This low-frequency hull During the high-speed wind tunnel tests, mode lateral bending mode did not appear in the low-speed 1 1 was also found to be the flutter mode. In this case, tests but showed up a s expected inthe transonic range it was not possible to obtain sub-critical however, of the high-speed tests.
damping information through use of any of the usual specific excitation techniques. Instead strain gage RANDOM EXCITATION FLIGHT FLUTTER responses to tunnel turbulence were recorded on tape TESTING ON THE P6M for a few runs and analyzed. In Figure 5 we see a composite of the spectral analysis for these runs From the proceeding analysis and model tests, at several Mach numbers less than the flutter Mach the basic behavior of the aircraft was rather well number. The amplitude and width of the peaks known. Now we are only interested in checking these give some indication of the variation of damping in predictions, expecially, the "sub-critical behavior," the flutter mode a s the f1utte.r speed is approached.
by flight flutter testing. To do this a random exci- This type of analysis has a distinct advantage in that tation method was developed involving spectral anal- it makes use of a type of random excitation (tunnel it is to this to?ic that I ysis of aircraft response, and turbulence) that is always present in tunnel testing.
shall devote the remaining part of the paper.
This method is being further developed and it is planned to obtain quantitative evaluation of the tech- Normally, flight flutter testing employs methods nique in a forthcoming development program.
which require specific excitationtechniques using such devices a s control s u r f a c e s, explosive charges, shakers and so forth. (Reference 4) The limitations of these specific excitation methods center around SPEED rapid data evaluation and high costs for equipment, ever, is that flights have to be made specifically for the purpose of flight flutter testing.
The random excitation method a s applied to the P6M appears to us to overcome many of thesedif- ficulties. Random excitation as a vibration source is not a new concept but in fact has been suggested as an approach to this problem for some time. (Ref- erence 5) Today, with the use of efficient tape re- STRUCTURAL cording systems and corresponding spectrum ana-
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lyzers, this technique becomes fairly attractive. In fact, from our most recent results it appears to be a relatively inexpensive technique that gives results that are a s reliable a s those obtained using other more expensive and complicated methods. Of parti- cular importance is the fact that data canbe collected from every flight test run without resorting to special flights.
The technique takes advantage of atmospheric turbulence a s a source of random excitation. The "hash*' usually associated with normal flight response Figure 4. Experimental V-g Plot records comes primarily from this turbulence. In In practice this procedure is not so simple.
the past it was advantageous to get rid of this re- sponse by performing tests in turbulent free air. In The actual turbulence spectrum on the aircraft is modified by buffeting and flow separation, the tur- the random excitation method this "hash" is recorded bulence level can vary greatly in a normal flight and analyzed.
and in addition local resonances can confuse the meas- THEORETICAL RESPONSE O F AN AIRCRAFT urements.
TO ATMOSPHERIC TURBULENCE P6M FLIGHT TESTS Theoretically the procedure involves correla- tion of power spectrums of turbulence and power Because of these difficulties the random exci- spectrums of response at a given air speed and alti- tation method used on the P6M lacks the exactness tude. This process is illustrated in Figure 6. The required by the theoretical approach and instead con- input power spectrum of atmospheric turbulence, centrates on the spectral analysis of the response in-
$ ( w ) , a s derived by several authors (Reference 6
By analyzing the data from many formation alone.
and 7) using a statistical approach, is a function of a runs it is possible to minimize the effects of varia-
turbulence level L, airspeed U and frequency o . The
tions in turbulence and obtain a clear picture of the output power spectrums of aircraft response, $(a) , aircraft's behavior.
are easily obtained from analysis of accelerometer tape recordings. By taking the ratio of the output An early approach to this random excitation curve to the input curve, the square of the mechanical method of flight testing was used on a previous model admittance o r transfer function, fi21w) , is obtained.
P6M and involved a simplified harmonic analysis The admittance term is an indication of the energy of oscillograph records of aircraft flight response.
passing from the input to the output response and A particular investigation into transonic vrsnakingv9 on therefore is proportional to damping. The lower the this ship yielded significant results a s shown in At the response damping the larger the admittance.
Here the average response in a given fre- Figure 7.
peaks, the admittance terms are therefore a measure quency range 2.5 cps, is plotted against Mach number of the modal in-flight damping. By plotting the modal for different altitudes, and clearly shows a transonic admittance terms for several aircraft speeds it is "snakingv' boundary. On the basis of such information theoretically possible to obtain an indication of the it became possible to eliminate the vlsnaking"problem approach of flutter o r a measure of the sub-critical on the present model of the P6M.
behavior.
An improved random excitation technique has been developec! for the present P6M flight test pro- INPUT SPECTRUM
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gram for the evaluation of all sub-critical behavior.
The limited results obtained so far have yielded good correlation with Analysis and Model Tests.
The technique involves spectral analysis of air- craft acceleration responses and is illustrated in ATMOSPHERIC TURBULENCE Figure 8. Accelerometer responses are telemetered to the ground where they are visually monitored and
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w - recorded on tape. The frequencies are first increased I up to 16 times to provide longer sampling times and
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OUTPUT SPECTRUM
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I 0- Figure 6. Theoretical Aircraft Response to Figure 7 . Transonic Snaking Response Atmospheric Turbulence ACCELEROMETER 5 RECORDER TRANSMITTER
-0 FILTER
U LOG
TIME : 2S:OO - 26 :OO MIN
CONVERTER x-Y
2 R D CYCLE FLT. 5-1 '/29
D ANALYZER
RECORDER
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PLAY BACK
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IO
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SPECTRAL ANAlY SIS SYSTEM . o ,
O N THE P6M t , ?06
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Figure 8. Spectral Analysis System on the P6M easier handling by the available playback systems. The This gives us a large statistical source fromwhich to output is then placed on a continuous loop and detected obtain an average response.
on an Ampex record playback unit.
this store of information, a turbulence To unify factor, based on the rigid body response, is determined This signal i s then fed into a Technical Products for each run and the plots normalized to a unit rigid Wave Analyzer. This analyzer determines the ampli- body response level. By statistically integrating these tude and frequency of the complex wave input within normalized results, we obtain cross plots of responses the desired frequency range. These data are then i n each mode for various altitudes, speeds, and con- processed through a band pass filter and a log con- figurations. These plots yield the information neces- verter, and plotted on an X-Y recorder. Thus the sary to establish the dynamic behavior and the effec- tape loop is automatically scanned and a plot of the tiveness of changes designed to improve the in-flight mean RMS acceleration amplitude versus frequency damping.
is obtained as shown. In Figure 8 we see a completed analysis for one-minute of flight time at aconstant speed and altitude. The peaks coincide with the air- CONCLUSION craft vibration modes.
To date the amount of data analyzed has not been large enough to either prove o r disprove the As stated before only the response data are adequacy of this technique for flight flutter testing.
evaluated; no correlation of these plots with a specific We are presently analyzing several flights inorder turbulence input has been tried because of our in- to determine the repeatibility and clarity of the ability to represent the actual turbulence. T h i s does spectral plots. The method, although it still has some not lead to any difficulty, however, since information developmental problems, appears to offer the flutter I s obtained every minute while the aircraft i s flying.
engineer several attractive advantages, foremost of 2. Goldman, R. L. Flutter of T-tails with Dihedral- which is the fact that every minute of flight time Martin Engineering Report No. 8205, 1957.
yields dynamics information without the necessity for the conduct of special test flights for flutter.
3. Goldman, R. L. XPGM-1 T-tail Flutter Model Low
Speed Tests with Revised Fin Stiffness - Martin
The combination of analysis and model tests Engineering Report 9086 (Confidential), 1957.
has proven to be a good approach to sub-critical investigations on the P6M. In particular, a specific 4. Schwartz, M. D. Investigation of Flight Flutter resonance excitation method has been developed for Testing Techniques, -MIT, Aeroelastic and Struc- low-speed wind tunnel tests that yields damping in- tures Research Laboratory Report, 1951.
formation in all the modes at one tunnel speed. When this information is coupled with the random excitation
5. Bisplinghoff, R. L., Ashley, H., Halfman, R. L. -
technique of flight flutter testing, a means of correla- Aeroelasticity - Addison - Wesley Publishing Co., tion is provided that is more simple and apparently 1955.
a s accurate as any of the many more expensive and complicated techniques in use at the present time. 6. Liepmann, H. W. - On the Application of Statisti-
cal Concepts to the Buffeting Problem - Journal
of the Aeronautical Sciences, 1952.
References
7. Press, H. and Houbolt, J. C. - Some Applications
of Generalized Harmonic Analysis to Gust Loads 1. Tomassoni, J. and O'Hearne, C. Vibration and
-
Flutter Studies Models YP6M-1 and P6M-2 on Airplanes -Journal of the Aeronautical Sciences, Martin Engineering Repoi t No. 9204 Volume I-II7 1954.
(Confidential) 19 57.