We present the development of a nonlinear reduced-order formulation for the simulation of geometrically-nonlinear responses of flexible aircraft and other aeroelastic systems. The theoretical foundation of the formulation will be presented first, based on a modal projection of the intrinsic description for beams, coupled with a 2-D unsteady aerodynamic description. We will then investigate the preservation of conservation laws in the proposed method and develop the numerical details in a practical implementation of the method in MATLAB. In this work we also developed a method of obtaining coeffcients of the nonlinear modal beam equations by means of a condensation process, based on the direct application of Guyan reduction of a high-fidelity 3D FE model. Structural and aeroelastic simulations will be presented to verify the implementation of the method against theory and published results, as well as demonstrating the numerical properties of the approach. Static trim, stability analysis and open-loop nonlinear flight simulations using the framework will be demonstrated on a highly-flexible flying wing and compared with published results, as well as carrying out control design and closed-loop nonlinear simulations to demonstrate the capabilities of the proposed reduced-order method.
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