ROTATING AERODYNAMIC- EXCITERS for in-flight flutter testing

Abstract

Telemetering, as used in in-flight testing, has several advantages including that of allowing what is known as real-time utilization; and thereby, in certain specific cases, the continuation of the flight programme in terms of the results obtained therein. This feature is especially attractive during the opening of the aircraft’s flutter envelope. It then becomes a matter of experimentally determining the aircraft’s aeroelastic stability throughout its flight envelope, and specifically at high speeds. In this connection, it’s common knowledge that in excess of a certain so-called critical speed, two or more vibratory modes of the structure can become coupled via the aerodynamic forces they respectively generate; and can lead to diverging oscillation liable to cause vibration failure. It’s easy to see that such a critical speed must be well within the permitted aircraft operation envelope and that approaching it during in-flight testing should only be considered with a certain amount of prudence and subject to strict monitoring of the structure’s behaviour. The most widely used monitoring system is to measure the transfer function relating an alternating force applied to the aircraft structure in flight to the displacements it causes at different points of that structure (figure 1). Progress in the flight envelope is made in speed steps, any variations in this transfer function being monitored between steps, and usually being reflected in terms of vibration frequencies and damping. Using telemetering, as in conducting these tests, is beneficial in several respects (figure 2). First it allows instant visual monitoring of the structure’s behaviour at its most significant points (rudders, bearing surface ends) by a team conveniently arranged on the ground. Then, further to a preliminary processing operation occurring in real-time, the test can be validated by merely observing the spectrums and the coherence functions existing between the forces applied and the structure’s response; a poor quality test, either due to a mismatched excitation or to the unexpected effect of an atmospheric turbulence, can be rerun without waiting for the aircraft to land. Finally, if adequate computing facilities are available, a comprehensive utilization of the values measured and their identification with a theoretical model lets the structure’s general behaviour be compared with the estimated figures, and thereby lets the aircraft resume the same test sequence at a higher speed or Mach number. The accuracy of the result and the speed at which it is obtained, so essential to the safe resumption of the flight, primarily depend on the extent and on the adequacy of the available information on the artificially applied forces. The design of “exciters” capable of creating controlled and measurable forces of an adequate level is thus the most vital constraint of the flutter testing facility.