Flutter Clearance of a Non-linear aircraft

Flight flutter testing is always carried out under the assumption that aircraft are
linear. Recently, this assumption has started to come under question, especially as far as
military aircraft are concerned. This paper deals with possible methodologies for flight flutter
testing of aircraft that are no longer assumed linear. Simulated flight testing is performed for a
simple non-linear aeroelastic system with cubic stiffness. The flutter speeds predicted using
some of the classical linear flutter prediction methods as well as a non-linear method are
compared. It is shown that, for non-linear system undergoing Hopf Bifurcations, classical
linear flutter prediction can predict the flutter envelope with reasonable accuracy. However,
fully non-linear system identification and stability analysis can not only predict the flutter
point but also determine whether it is a linear or non-linear flutter point (i.e. whether
divergent or Limit Cycle Oscillations will ensue). Additionally, the non-linear method can
predict the amplitudes of LCOs that will occur post-critically. The application of the nonlinear
method was successful for noise free data, but the problem of noise corruption still
needs further investigation.