Coupled flight dynamics and aeroelasticity of very flexible aircraft based on commercial finite element solvers

An integrated formulation of flight dynamics and aeroelasticity that allows large body frame motions coupled with small elastic displacements was developed to analyze free-flying aircraft described by detailed finite element models. In this paper the formulation is applied to an existing experimental vehicle: the University of Michigan's X-HALE. Designed to be aeroelastically representative of a very flexible aircraft, the X-HALE shows significant couplings between rigid-body and elastic degrees of freedom due to very low-frequency natural vibration modes. At the same time, the lightly instrumented X-HALE Risk Reduction Vehicle experiences relatively moderate deflections at typical operating trim conditions, so providing an appropriate test case for the present formulation. Numerical results for flight dynamic/aeroelastic stability around steady rectilinear flight are presented to discuss the relevance of different couplingsand they are compared with the outputs from a commercially available linear flutter solver. The integrated formulation is next extended by allowing large trim deflections,and analyses conducted using the statically linear and statically nonlinear models are compared with each other to point out the influence of geometric nonlinearities on the stability of the examined vehicle.