Frequency-domain identification of time-varying systems for analysis and prediction of aeroelastic flutter

In this paper a different approach to wind tunnel flutter testing is presented. This procedure can now be performed as one continuous test, resulting in a major time saving. Both analysis of the current behaviour of the structure, and prediction towards higher velocities, are important for flight flutter testing, and are dealt with in this paper. The recently developed time-varying weighted non-linear least-squares estimator (TV-WNLS) (Lataire and Pintelon, 2011 [1]) is applied to the aeroelastic flutter problem. Smooth variation of the transfer function coefficients is forced through the TV-WNLS estimator, and the obtained polynomials are used as basis for predicting the damping ratio towards higher velocities. Selection of the model order is based on linear variation of the airspeed and the evaluation of Theodorsen's unsteady aerodynamics for the frozen time-varying aeroelastic system at a certain constant velocity. Therefore, providing a physical justification for the extrapolation of the damping ratio towards higher velocities. The method is applied to wind-tunnel measurements on a cantilevered wing. It is shown that the proposed method outperforms flutter speed prediction by classic damping ratio extrapolation and a non-parametric analysis of the time-varying signal.