An Approach for Nonlinear Aeroelastic and Flight Dynamic Analyses for Very Flexible Aircraft at Trim States of Large Deformations

Abstract

This paper presents a method for nonlinear aeroelastic and flight dynamic analyses of very flexible aircraft. Therefore, a nonlinear aeroelastic solver is used to obtain steady-state trim solutions, coupling a geometrically nonlinear vortex lattice method with the commercial finite element solver MSC Nastran in a geometrically nonlinear solution sequence. In addition, the updated modal properties, i.e. eigenfrequencies and mode shapes, are extracted at the preloaded trim state with large deformations. Subsequently, the structural model is linearized around the trim condition and fed into a nonlinear flight dynamic solver where the structural dynamics are described using a modal approach. The governing equations of motion of the free-flying aircraft employed are based on a derivation without the mean axes assumption and thus consider inertial coupling of rigid-body and elastic motions. With unsteady aerodynamic forces obtained from an unsteady vortex lattice implementation, the proposed method allows for nonlinear flight-dynamic time-domain simulations of a very flexible aircraft starting from trim states with large deformations. For the purpose of stability analysis, the unsteady vortex lattice method is linearized analytically and integrated with the linearized flight dynamic equations of motion into a linear monolithic state-space model. The aeroelastic and flight dynamic stability is then determined by means of the eigenvalues of the system's dynamics matrix. The proposed method is applied to a highly flexible generic UAV.