Abstract
This paper investigates the linearization, using perturbation methods, of the structural deformations in the nonlinear flight dynamic response of aircraft with slender, flexible wings. This has been achieved by first coupling a displacement-based, geometrically-nonlinear flexible-body dynamics formulation with the 3D Unsteady Vortex-Lattice Method, followed by a consistent linearization of the structural degrees of freedom, which are assumed to be small in a body-fixed reference frame. The translations and rotations of that reference frame can be arbitrarily large, however. The resulting system preserves all couplings between rigid and elastic motions and can be projected onto a few vibration modes of the unconstrained aircraft with arbitrarily-large, geometrically-nonlinear deformation at a trim condition. The dynamics of the system are then written in tensor form, with up to cubic terms due to the nonlinear rigid-body terms, and with a limited number of coefficients that can be pre-computed prior to the time-marching simulation. Numerical studies on a representative HALE UAV are presented to illustrate the approach and results are compared to the mean-axes solution.