Force appropriation for flight flutter testing

In this paper, the multivariate mode indicator function (MMIF) method has been applied to simulated flutter test data in order to evaluate the potential application of normal mode force appropriation to flight flutter testing where there are many close modes and noise levels are severe. It is seen that the approach is capable of estimating accurate modal frequencies and force patterns from corrupted test data, for use in the appropriation of particular modes. Damping ratios are accurately estimated from a decay response following a resonance dwell. The complex mode indicator function (CMIF) has also been considered. The practical limitations of in-flight force appropriation are discussed, and the effects of the limited excitation possibilities are demonstrated by simulation. The number and position of exciters used are arguably the most important factors in determining the success of this technique. The application of force appropriation to flight flutter testing may well depend on the availability of small detachable vanes which allow direct excitation of all areas of the aircraft structure.