Comparing Different Potential Flow Methods for Unsteady Aerodynamic Modelling of a Flutter Demonstrator Aircraft

Abstract

To further improve the performance of aircraft, the aspect ratios in recent designs grow larger. However, the resulting increased flexibility of the wings might have a detrimental effect on the flutter stability of the aerostructural system. Apart from structural changes that have a negative impact on the weight of the airframe, an Active Flutter Suppression (AFS) system can be employed to prevent such instabilities within the flight envelope. To develop these AFS control laws, accurate simulation and synthesis models of the flexible airframe are essential. Still today, the workhorse for modelling unsteady compressible aerodynamics for such models is the Doublet Lattice Method (DLM). However, there are some shortcomings regarding this method. One particular point is the absence of in-plane forces, which are essential for phenomena such as T-tail flutter. Also low frequency in-plane modes that might change the flutter mechanism for aircraft with high AR wings, can not be accurately modelled by the standard DLM. To address this issue, other unsteady aerodynamic methods based on potential flow are employed and compared to the standard DLM results. The DLM can be extended to include forces in x-direction. This enhanced formulation requires a modification of the boundary conditions at the box quarter chord points to recover complex directional lift forces as function of the reduced frequency. Alternatively, the unsteady 3D panel method USNEWPAN can be employed, which is based on the velocity potential and features thickness effects of the airfoil. The pressure integration over the contour then captures the in-plane forces. The solver USNEWPAN is similarly to the DLM, a compressible frequency domain method, which can also produce so called Aerodynamic Influence Coefficient (AIC) matrices that relate normal velocities at the collocation points to pressures. Complex valued Generalized Aerodynamic Forces (GAFs) are obtained using the normal modes of the aircraft structure and the corresponding differentiation matrices. These GAF matrices of the different aerodynamic methods are then used in subsequent flutter calculations.