On the uniqueness of the minimum-state representation

A nonlinear indicial approach in conjunction with computational fluid dynamics (CFD) was developed to analyze the transonic aeroelastic response and to determine the flutter boundary of general two-degree-of-freedom airfoil systems. The unsteady aerodynamic lift and moment were obtained from linear and nonlinear indicial response functions used in conjunction with the generalized Duhamel superposition principle. The use of the nonlinear indicial function was shown to be very successful in adressing issues in the transonic flow regime. For a two-degree-of-freedom typical section model, the predictions based on the present nonlinear indicial function showed very good agreements with results by the direct CFD method.