Stability and Open-Loop Dynamics of Very Flexible Aircraft Including Free-Wake Effects

High Altitude Long Endurance (HALE) aircraft, with all of their intentions, are usually sizable and flexible, and hence very likely subject to large deflections under flight loads. The large deflections will in turn significantly affect the dynamic/static loads of the aircraft. Therefore, a new methodology, coupling a nonlinear structural model that can accurately simulate the large deformations with a nonlinear aerodynamics model, is required to perform a consistent nonlinear aeroelastic analysis. This paper presents the formulation and results of such a nonlinear aeroelastic method with applications to a Goland wing and a modeled HALE type wing. The developed nonlinear aeroelastic model includes a nonlinear finite element implementation of a geometrically exact nonlinear beam model that accounts for the nonlinear dynamics of curved and twisted anisotropic beams, and a general unsteady nonlinear vortex-lattice method (UVLM) for unsteady aerodynamic prediction. UVLM gives the wake position as part of the solution and allows the aerodynamic grid (lattice) to be deformed with the structures. The governing equations of the coupled dynamic system are integrated simultaneously and interactively to yield the wing response and the consistent flowfield in the time domain. Prior to performing the nonlinear aeroelastic analysis, careful validation of the nonlinear aerodynamic model and the nonlinear structural model were carried out by comparing their solutions with counterpart linear solutions