Safe flutter tests using parametric flutter margins

A recently developed numerical Parametric Flutter Margin method is applied to perform safe flutter tests where 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 co-located normal acceleration response is measured. Fourier transforms of the two signals are used for calculating flutter margin, which become zero, when the flutter onset conditions of the nominal system are met. The stability of the actual test model at these conditions facilitates safe, flutter-free, testing conditions with robustly measured response. The ability to exceed the nominal-system flutter onset provides a convenient identification of the flutter mode. The method is applied in wind-tunnel flutter tests of a clamped wing with a tip store, to which a stabilizing mass is added. The added mass is connected through a thin chord and a pulley to a weight outside the tunnel. The excitation is performed by cutting the chord, which provides a ramped indicial force excitation, and the acceleration response of the stabilizing mass is extrapolated from other accelerometers on the wing. The process is repeated at several airspeeds to compute response functions and pinpoint the flutter speed of the nominal system. The response functions are somewhat scattered mainly due to unknown aerodynamic noise. The results, however are quite accurate in comparison with direct tests that risked the model by actually driving it into flutter. The test success opens the door to more elaborate wind-tunnel and flight tests based on the structurally safe Parametric Flutter Margin method.