Safe Flutter Tests Using Parametric Flutter Margins

Abstract

The recently developed numerical parametric flutter margin method is applied to perform safe flutter tests in which the flutter-onset conditions of a nominal configuration are positively identified, while the system is stabilized by an added concentrated mass. A known excitation force is applied to the added mass, and the collocated normal acceleration response is measured. Fourier transforms of the two signals are used for calculating flutter margins that change sign at flutter-onset conditions of the nominal system. The stability of the modified model at these conditions facilitates safe, flutter-free, testing conditions with a robustly measured response. The method is applied in wind-tunnel flutter tests of a clamped wing with a tip store, to which a stabilizing mass is added at a forward location. The added mass is connected through a thin cord and a pulley to a weight outside the tunnel. The excitation is performed by cutting the cord, and the mass acceleration is extrapolated from other accelerometers on the wing. The process is repeated at several airspeeds to compute frequency response functions and pinpoint the flutter-onset velocity of the nominal system. The resulting flutter boundaries are within a few percent of those measured by direct-flutter tests.