TIME SIMULATION OF FLUTTER WITH LARGE STIFFNESS CHANGES

Time simulation of flutter, involving large local structural changes, is formulated with a state-space model that is based on a relatively s mall number of constant generalized coordinates. Vibration modes are f irst calculated for a nominal finite element model with relatively lar ge fictitious masses located at the area of structural changes. A low- frequency subset of these modes is then transformed into a set of stru ctural modal coordinates with which the entire simulation is performed . These generalized coordinates and the associated oscillatory aerodyn amic force coefficient matrices are used to construct an efficient tim e-domain, state-space model for a basic aeroelastic case. The time sim ulation can then be performed by simply changing the mass, stiffness, and damping coupling terms when structural changes occur. It is shown that the size of the aeroelastic model required for time simulation wi th large structural changes at a few a priori known locations is simil ar to that required for direct analysis of a single structural case. T he method is applied to the simulation of an aeroelastic wind-tunnel m odel. The diverging oscillations are followed by the activation of a t ip-ballast decoupling mechanism that stabilizes the system, but may ca use significant transient overshoots.