Free-Flight Nonlinear Aeroelastic Simulations of the X-HALE UAV by an Extended Modal Approach

Kurzfassung

A recently proposed method that extends the classical modal approach captures geometrically nonlinear effects in large structural deflections. The extensions account for nonlinear force displacement relationship by generalized quadratic and cubic stiffness terms and a geometrically nonlinear displacement field by quadratic, cubic, and fourth-order mode components. These extensions make the method particularly suitable for aeroelastic applications involving highly flexible structures and nonlinearities due to nonconservative loads. In this work, the method is enhanced by rigid body degrees of freedom to simulate a maneuvering, very flexible aircraft. Special emphasis is put on the derivation of a set of coupled differential equations of motion in which as few assumptions as possible are made with respect to structural deformations. The use of the mean-axes constraints is explicitely avoided, all inertial and gyroscopic coupling terms between rigid body and elastic motion are included. The setup of the aeroelastic framework based on an unsteady vortex-lattice method in the time doamin is presented in detail. University of Michigan's X-HALE UAV is the test case of this work. The results of dynamic maneuvers including tail input scenarios and gust encounters are presented and compared with results from UM/NAST.