How to perform Flight Vibration Testing based on Operational Modal Analysis?

Abstract

Flutter is a dynamic instability caused by the interaction of the structural dynamics of the aircraft and unsteady aerodynamic forces. It occurs when one of the elastic modes of the airframe tends to negative damping above a critical speed. Prediction and flutter clearance are important problems in the design, testing and typecertification of sailplanes. From a practical point of view, Flight Vibration Testing (FVT) needs to be performed whenever a new sailplane is built or an existing type is modified. The application of Operational Modal Analysis (OMA) methods may provide improvements to identify modal parameters of multiple modes in one step without knowing the type of external excitation. If the identification is repeated for increasing flight velocities, it is possible to find the aeroelastic damping trends and to extrapolate to the stability boundary. The application of OMA needs a broadband excitation spectrum, which result from impulsive control kicks or continuing random excitation like gusts and turbulence in the atmosphere. To demonstrate the performance of the proposed method measured time histories are post-processed to identify all relevant mode shapes with natural frequencies and damping rates of the flying sailplane. Requirements for test equipment, software and the procedure to perform the FVT are summarized.