The paper investigates the coupled nonlinear aeroelasticity and flight mechanics of very flexible lightweight aircraft. A geometrically-exact composite beam formulation is used to model the nonlinear flexible-body dynamics, including rigid-body motions. The aerodynamics are modeled by a general 3-D unsteady vortex-lattice method, which can capture the instantaneous shape of the lifting surfaces and the free wake, including large displacements and interference effects. The coupled governing equations are solved in a variety of ways, allowing linear and nonlinear time-domain simulations of the full vehicle and frequency-domain linear stability analysis around trimmed configurations. The resulting framework for the Simulation of High-Aspect Ratio Planes (SHARP) provides a medium-fidelity representation of flexible aircraft dynamics, based on an intuitive and easily linearizable structural representation using displacements and the Cartesian rotation vector, time-domain aerodynamics, and at relatively low computational costs. Previous verification studies on the structural dynamics and aerodynamics modules are complemented here with studies on the flexible-body implementation and on the integrated simulation methodology. A numerical investigation is finally presented on a representative high-altitude long-endurance model aircraft, investigating its stability properties and its open-loop dynamic response. Copyright © 2011 by Joseba Murua, Henrik Hesse, RafaelPalacios and Michael Graham.