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FLIGHT FLUTTER TESTING THE B-58 AIRPLANE P. T. Maha_ey _ Convair, Ft. Worth, Texas ||m|mmmmmm Abstract To be frank about it, when we started planning this program back in 1952, we weren't sure what The flight flutter tests on the B-58 airplane will portion of the flight envelope of the airplane would be described, and the philosophy of flight flutter be critical for flutter. By the time we were ready testing at Convair, Forth Worth, discussed. A de- to flight test the airplane, we were pretty sure that the critical region was transonic speed at low altitude.
scription of the instrumentation used in the airplane However, there were still enough unknowns to cause and in the telemetering receiving station on the us to proceed rather cautiously.
ground will be given. The methods used for excit- Lug the airplane and the flight test procedure will INSTRUMENTATION AND TELEMETERING be covered. Also described will be the type of data obtained and its reduction. An evaluation of the TECHNIQUES procedure and instrumentation will be given with a With this as a background, I would like now to discussion of desirable improvements for future proceed with a description of the instrumentation which testing.
we used on the B-58. We approached this problem with the thought of pushing the state of the art to a INTRODUCTION certain extent, but at the same time staying with items which we felt pretty sure would work. We wanted To lay the ground work for what we have done to get both frequency response data and damping in the program, I would first like to describe the records. Basically our thought was to determine the problem with which we were faced and our philosophy principal response frequencies in flight and to take of approach to it. To begin with, we had to consider damping records corresponding to these as a function a low load factor airplane designed to fly into the high of speed. We also wished to telemeter this informa- speed flight regime which had hitherto been breached tion. By telemetering we could accomplish several only by research airplanes and a few fighters. To things.
make matters worse from the flutter prediction stand- point, we had four pylon mounted nacelles on a delta First, we wished to be able to proceed with wing planform. This was the first time anyone had more than one speed increment per flight. This produced a configuration like this. So we had very automatically ruled out recording the data in the air- little background information on which to draw.
plane and reducing it later on the ground.
The basic approach to the flutter problem onthe Second, this procedure would relieve the flight B-58 on which we decided was as follows. We would crew of the responsibility of monitoring the records put the basic emphasis on flutter models. Analysis in flight in addition to their other duties.
would be used to predict the character of flutter to be expected and the flutter trends arising from the variation of parameters. Finally, flight flutter Third, we would be able to display a number of channels of information on the ground. Also we could testing would be employed to demonstrate that the employ certain kinds of bulky, special equipment on the airplane was flutter-free.
elevons and at about their midspan to excite a high ground such as band pass filters and automatic sweep frequency vibration mode which flutter model tests plotters.
had indicated might produce elevon flutter.
Fourth, the flutter information could be monitor- We used the same frequency control unit for both ed by flutter specialists.
types of excitation. This was operated by the flight To cover the desired frequency range from 1 to test engineer from his post in the third crew station in the airplane. The heart of the unit was a variable 40 cycles per second, we had to provide two types of excitation. For the range of 1 to7 cps, we introduced frequency electrical oscillator. The flight test a sinusoidal electrical signal into the airplane auto- engineer was able to set desiredfrequencies manually, pilot servos. This produced a sinusoidal oscillation or to activate an automatic frequency sweep mechan- ism. Selector switches enabled him to use either the of the control surfaces about the trim flight position.
high or the low frequency range, and to direct the The amplitude was proportional to the input voltage excitation to the appropriate autopilot servos or and could then be adjusted in flight by turning a knob.
vibrators.
We had used this system on the B-36 and YB-60 airplanes and knew it would work. However, the We used two types of pickups to detect response.
characteristics of the autopilot and power control For linear motion we employed strain-gauge type system limited its useful frequency range.
Statham accelerometers. These had ranges varying For the range from 5 to 40 cps, we decided to from ±2g to ±15g, depending on the location. They were fluid damped and had built-in electric heaters use vibrators of the type developed by our San Diego Division. These are inertia shakers, hydraulically to maintain a constant 165°F operating temperature.
powered, and electrically controlled. The ones we used had overall dimensions of 4.5 x 4.5 x 8.5 inches For detecting angular motion of the rudder and and weighed 25 lbs. The force output increased lin- elevons, we used Eclipse-Pioneer AY503-8 autosyns.
With our instrumentation, these were capable of mea- early w_.n frequency from 40 lbs. at 7.5 cps to 150 lbs.
at 40 cps. We installed one vibrator in the tip of the suring surface deflections down to 1/20 of a degree.
vertical tail, and one in the trailing edge of each wing. Figure 1 shows the location of these pickups. The output from the 9 encircled pickups was telemetered The wing vibrators were placed just ahead of the Figure 1. Pickups and Vibrator Locations For B-58 Flight Flutter Tests
along with the excitation signal. The signals from all
of the pickups were simultaneously recorded on tape
in the airplane.
On the ground the telemetered signals were dis-
played on two Sanborn direct writing oscillographs
as shown in Figure 2. Before going into the recorder,
however, each signal w a s passed through a variable
band pass filter. The filters were used as required
to remove any unwanted hash from the traces.
Figure 3. Frequency Sweep Recorder
corder. This is shown in Figure 3. The pen was
driven across the paper in proportion to the excitation
frequency by a circuit similar to that of a frequency
2. Sanborn Recorders and Filters
Figure
meter. The paper was moved up and down in pro-
portion to the amplitude of the signal from the pickup
We recorded frequency sweeps directly with a 4 shows a typical sweep
being monitored. Figure
special unit made by adapting a two axis Brown re- record from this equipment.
4. Fin Frequency Sweep Taken in Flight
Figure
FLIGHT TEST PROCEDURES First, the instrumentation and techniques that we used proved to be practical and worked pretty Next I would like to describe out test procedure much the way we expected. This is not to say that in flight. I wanted to say "typical" test procedure, they always worked perfectly, but they proved to be but conditions varied so much from flight to flight servicable.
that there wasn't a set pattern. Basically, however, We went through the following procedure. The flight Secondly, we are pretty well convinced that test engineer informed the ground station when he was frequency sweeping yields more information than any ready to start. He then activated the automatic other one thing that we can do. Damping records excitation sweep on the tail, the response to which essentially confirm what we expect from the sweep was recorded on the ground. Next, automatic sweeps data. In a since damping records give one dimen- were taken for symmetric and antisymmetric ex- sional information while sweeps give two dimen- sional data.
citation of the wing. While the wing sweeps were being taken, the tail sweep record was reviewed to determine the major response frequencies. These were then For another thing, we have found that the ampli- transmitted by radio to the flight test engineer with tude of excitation is important. We don't know how a request for damping records. He then proceeded to specify the minimum acceptable level, but we know to set the requested frequencies manually and to give from our experience that low excitation amplitudes short bursts of excitation to the tail for damping tend to give erratic damping values. These values records. During this period the wing sweeps were also tend to indicate lower damping than actually reviewed for major response frequencies. These exists. On the B-58 fin which has an exposed span frequency values were then passed on to the flight of about fifteen feet excitation double amplitudes of test engineer as soon as he finished with the tail about one inch gave much better results than am- damping records. The procedure of excitation and plitudes of one quarter of an inch. On the wing, recording of damping records was then repeated for amplitudes of one inch also gave better results than the wing.
amplitudes of one quarter of an inch. I am not able to define all the pertinent parameters, but I am sure In the ground station we had a group of about that the ratio of the excitation amplitude to the ran- six flutter engineers. These men monitored the dom steady state amplitude is important. We try to information as it was received. They determined obtain excitation amplitudes of at least three to four damping and response frequency on apreliminary basis times the normal random amplitude. I suspect that within a few seconds and added these new points to the boundry layer thickness may also have a bearing the plots of data previously taken. All during this on this problem. At any rate, the amount of ex- time, the new data points were being monitored and citation amplitude required to give good flutter data considered by a senior member of the flutter crew.
is a subject on which research is needed.
If everything appeared in order at the conclusion of the planned testing at the speed point, the senior flutter engineer would give his O.K. for the airplane IMPROVEMENTS IN FUTURE FLUTTER crew to increase speed to the next scheduled point.
TESTING Normally this increment was one tenth of a Mach number.
I might pass along the following comments on The procedure described above takes about ten what we consider to be needed improvements in the minutes to accomplish three sweeps and six damping field of flutter testing. One very important practical runs. In practice, however, we found that we never problem is the amount of time required to obtaiv quite followed this procedure for one reason or another.
data. This definitely needs to be shortened. But One thing which effected the plan was the time avail- directly opposed to this requirement is the need tc able. We were limited in telemetering range to obtain more complete and better data. I think the about ninety miles radius, and it doesn't take long to best solution of this dilemma lies in automatic data fly by at high speed. Also, we often found it necessary reduction equipment. Our sweep plotter is a stele to make repeat runs to get good data. As a result in this direction.
of this, other items in the flight test plans, and the inevitable descrepancies which always show up from Another thing which would be a definite im- time to time in experimental airplanes and instru- provement in our system would be to record infor- mentation, we usually were in the position of trying mation on how much response is being obtained for to finish one point and start another.
given input. Our current B-58 instrumentation doe., not give this. However, I think that this could bc achieved if the necessary development work were done TEST RESULTS on the instrumentation. I believe it is entirel 3 feasible to obtain an automatic sweep plot in term., I have some comments and observations that I of response amplitude per pound of excitation or pel might pass on as a result of our experience on the degree of control surface rotation.
B-58.
A very basic need has become clearly apparent CONCLUSION during this program. I think this is a need which applied to all of us who are engaged in flight flutter To us at this time, it appears that the best testing. This is to be able to predict in advance approach to the problem lies through frequency re- what our test results should be. To do a real engin- sponse data. It is technically feasible to obtain in- eering job on flutter, we need to make our pre- formation of this type which would be directly com- dictions in terms that we can measure directly on an parable to the airplane data by both calculation and airplane in flight. Then we could spot check enough model test. This would be costly, but I believe it points to prove that our engineering predictions were would save money in the long run if we could do a correct and greatly reduce the costly task of proving good job in this respect. Certainly it would enable that an airplane is free from flutter. us to do a better, safer, and shorter job of flight flutter testing.