A NUMERICAL PROCEDURE AND AN EXPERIMENTAL ANALYSIS FOR STUDYING FLUTTER CONDITIONS OF A NON-LINEAR WING MODEL

摘要

The problem of concentrated non-linear stiffness on pylon joints has been treated by many authors, due to the usual closeness of the bending and torsional mode of the wing with respect to the pitch mode of the store. Some very simple approaches consider linearized stiffnesses to compute boundary worse behaviours with linear FEM codes. In our approach, structural dynamic modification method has been used together with the Describing Function linearization, since the concentrated non-linearity was a-priori known as well as it location. The numerical results obtained trough this way have demonstrated to be in closeness with the classical linear approach, whilst the computing time was far less with respect to the other method. The big fault in using the linear method is obviously referred to the fact that each calculation considers the constant stiffness without taking into account the different effect of bilinear joint for smaller and larger amplitudes. A validation of the iterative calculation has been performed by setting a variable stiffness non-linear joint for the model scale wing. Experimentally, static bilinear stiffness has been verified, as well as the Hilbert Transform identification method has been used, to detect stiffening effects belonging to the pitch mode of the store. For higher sine excitation a jump phenomena is detectable and the frequency shift becomes considerable up to the coupling with the first bending mode of the wing. Unstable conditions are shown by different graphs particularly referred to the initial disturbance displacement.