Nonlinearities in Flutter Evaluation

Flutter and other aeroelastic instability phenomena have had a significant influence on airplane development and the airworthiness criteria governing the design of civil airplanes. Flutter analyses are usually carried out with the hypothesis of structural linearity, but experience shows that a satisfactory description of the flutter phenomena cannot be made apart from nonlinearities. Nonlinearities can be lumped, typical of control surface mechanisms or distributed, such as those associated with riveted/bolted connections. Because of the huge amount of riveted/bolted joints almost everywhere in the structure, their contribution to nonlinearities can be considered as “distributed” and consequently their effect on normal modes can be neglected. On the contrary experience has taught that lumped nonlinearities strongly affect the aeroelastic stability. The present paper is focused on one of the approaches that can be followed to investigate the flutter behaviour in frequency domain when in presence of nonlinearities in the control circuit mechanism. The I-23 aircraft (Institute of Aviation, Poland) has been used as a test-case of the methodology. In particular the Harmonic Balance technique has been exploited to perform “pseudo-linear” flutter analyses with varying control circuit stiffness. Thanks to a Dynamic Substructuring approach the pseudo-linear flutter analyses have been conducted with a considerable computational time saving because all nonlinearities have been considered at the boundaries between substructures. This way of modelling allows not to recalculate the Generalized Aerodynamic Forces at every changing of control circuit parameters. Finally a set of nonlinear time analyses, by means of a nonlinear State Space representation, have been performed in order to “validate” the Harmonic Balance technique.