Appendix 3
6. FLIGHT TEST PROCEDURES. Due to the potential dangers involved in conducting flight flutter tests, there is always the desire of the crew to get the tests over with and a general tendency to want to skip speed points and short cut the test. For this reason, a flight plan should be established prior to beginning the test and followed as closely as possible.
A chase plane should be used for all tests.
Testing should begin at a low airspeed point to establish a data base; i.e., frequency correlation with zero velocity modes. Follow-on increments should be based on the flight history of the airplane. If the airplane has previously flown to some limit speed, then enough test points should be checked to that pre-achieved speed to develop a data trend. For unexplored speeds, the increments should be smaller and no more than two speed data points checked at any one altitude during a flight unless data trends show continuously increasing, or very high, damping with no indications of ensuing shifts in this trend. More data points may be checked per flight if telemetry and real time data analysis systems are available. Between VC and VD, the number of airspeed increments should be increased.
Atmospheric turbulence should be avoided as much as possible during these tests to eliminate superposition of unwanted random signals on the data records and to preclude possible structural overload at speeds near VD.
If inertia shakers, aero vanes, or other sinusoidal exciters are used, frequency sweeps should be conducted at each speed point. It is also recommended that shaker dwells be performed at selected airspeeds as a check on the damping characteristics established from sweeps. These are conducted by tuning the shaker(s) to each peak frequency, allowing time for the airplane to stabilize, then cutting the shaker power and recording the response of each transducer. If pilot induced control impulses are used, each axis should be pulsed at least twice in each direction wherein the airplane is allowed to stabilize, with bands off controls, prior to the next impulse or speed point.
7. DATA REDUCTION AND INTERPRETATION. The methods used for analyzing flight flutter data will depend on (1) the type of excitation used, (2) the availability of electronic analysis equipment, (3) the degree of accuracy required, and (4) the time allowed for data reduction. Generally, absolute damping values are not necessary to achieve the objective of the flight tests wherein monitoring of damping versus airspeed trends is the primary concern. The approximation methods addressed briefly in this section will allow development of reliable trends provided consistent procedures are followed in reducing the response traces throughout the test.
Appendix 3 when impulse excitation is used, the damping can be obtained by measuring the decay rate directly from the response traces. For cases where several frequencies are being excited by the impulse, it may be necessary to reverse the transient and play it into a tuned filter. The frequency of the tuned filter can then be varied to yield discrete modal responses (reference 7).
when continuously forced oscillation techniques are used to excite the structure, either the amplitude response method or the vectorial analysis method as developed by Kennedy and Pancu (reference 6) can be used to reduce the data. The amplitude response method is advantageous for general aviation applications since it provides a good approximate damping level, requires a minimum of electronic equipment, and the data can be quickly reduced. For this method it is assumed that the relative damping ratio is approximately inversely proportional to the maximum resonant amplitude of the respective modes. Therefore, if the amplitude for a given mode is increasing as the airspeeds increase, a reduction in stability will be indicated. The approximation method is briefly outlined as follows:
a. The product of response amplitude times damping will be constant for a given exciter force; i.e., (Ag) (gg) = c
b. If the exciter force relative to frequency remains consistent, then at a given airspeed, the net structural damping will be (Ay) (8y) = C = (Ag) (8g) then:
Ev = (Ag) (8g) / Av where:
Ag = peak amplitude on ground Ay = peak amplitude in air at test velocity gg = damping measured on ground at zero velocity gv = net structural damping at test velocity The net damping (g) can be defined as the actual structural damping measured on the ground (8g) minus the analytica.
aerodynamic damping (g) determined from the flutter analysis with zero structural damping; i.e., 8v = 8g - g, where g is negative
d. To permit use of this method, the shaker force and sweep rate must remain consistent from run to run.
A gradual increase in peak response (Av) by a factor of 3 or a rapid increase in A by a factor of 2 will normally require stopping the test to inspect the airplane, instrumentation, etc. It is also possible to experience a very sharp rise in damping followed by a sharp decrease leading to violent flutter, thus making it difficult to predict trends without the aid of reliable analyses.
Appendix 3 It is advisable to check the effects of variation in sweep frequency on the peak response damping using techniques included in reference 2. It is also advantageous to supplement continuously forced damping measurements with damping obtained from decay records. Flutter margin predictions per the techniques of Zimmerman and Weissenburger (Journal of Aircraft, 1964, Volume 1, Number 4) may in turn be a beneficial data presentation approach to supplement the characteristic velocity versus damping plots.
7 (and 8)
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Appendix 4 APPENDIX 4. ACKNOWLEDGMENTS, REFERENCES AND BIBLIOGRAPHY
ACKNOWLEDGMENTS
1. Acknowledgment is given to Mr. George S. Rasmussen, Structures DER, for his contribution in the development of the data presented in appendix 2 for whirl mode flutter analysis.
REFERENCES:
1. Airframe and Equipment Engineering Report No. 45, 99Simplified Flutter Prevention Criteria for Personal Type Aircraft." FAA, Engineering and Manufacturing Division, Flight Standards Service, Washington, D.C.
2. Introduction to the Study of Aircraft Vibration and Flutter, Scanlan and Rosenbaum, McMillan Company, New York, 1951.
3. Aeroelasticity, Bisplinghoff, Ashley, and Halfman, Addison-Wesley Publishing Co., Reading, Mass., 1955,
4. Application of Three-Dimensional Flutter Theory to Aircraft Structures, Air Force Technical Report (A.F.T.R.) 4798, 1942.
5. Tab Flutter Theory and Application, Air Force Technical Report, A.F.T.R., 5153, 1944.
6. The Use of Vectors in Vibration Measurement and Analysis, Kennedy and Pancu, Journal of Aeronautical Sciences, J.A.S., Vol. 14, 1947.
7. A Survey of Flight Flutter Testing Techniques, AGARD Flight Test Manual, Vol. 11, Chapter 14.
8. Mechanical Vibrations, Thomson, Prentice-Hall, New York, 1953.
9. An Analytical Treatment of Aircraft Propeller Precession Instability, Reed and Bland, NASA TN D-659.
10. Propeller Nacelle Whirl Flutter, Houbolt and Reed, Journal of Aeronautical Sciences (J.A.S.) March, 1962.
11. Review of Propeller-Rotor Whirl Flutter, NASA TR-264, July, 1967.
12. Cantilevered Power Package Whirl Mode Flutter Program, G.S. Rasmussen & Associates, Report No. 650, June 1, 1971.
13. Wind-Tunnel Measurement of Propeller Whirl-Flutter Speeds and Static Stability Derivatives and Comparison with Theory, NASA TN D-1807, August 1963.
14. Laguerre's Method Applied to the Matrix Eigenvalue Problem Beresford Parlett, A Paper Prepared Under Contract AT (30-l)-1480 with the U.S.A.E.C.
Appendix 4 BIBLIOGRAPHY:
1. Manual on Aeroelasticity, North Atlantic Teaty Organization Advisory Group for Aeronautical Research and Development (AGARD).
2. Analytical Methods in Vibration. Mierovetch, McMillan Company, New York, 1967.
3. An Introduction to the Theory of Aeroelasticity, Fung, John Wiley and Co., New York, 1955.
4. Dynamics of Structures, Hurty and Rubinstein, Prentice-Hall, New York, 1964.
5. Sane Aspects of Ground and Flight Vibration Tests, Mazet, ONERA TN No.
34, 1956.
6. A Survey of Aircraft Subcritical Flight Flutter Testing Methods, Rosenbaum, A.R.A.P. Report No. 218, 1974.
7. Subcritical Flutter Testing and System Identification, Houbolt, A.R.A.P.
Report No. 219, 1974.
8. Proceedings of the Flight Flutter Testing Symposium, May 1958, Washington, D.C., O.S.R.-9-0269.