Flexible Aircraft Dynamics with a Geometrically-Nonlinear Description of the Unsteady Aerodynamics

Abstract

The Unsteady Vortex-Lattice Method provides a medium-fidelity tool for the prediction of non-stationary aerodynamic loads in low-speed, but high-Reynolds-number, attached flow. Despite a proven track record in applications where free-wake modelling is critical, other models based on potential-flow theory, such as the Doublet Lattice and thin-aerofoil approximation, have been favoured in fixed-wing aircraft aeroelasticity and flight dynamics. This dissertation presents how the Unsteady Vortex-Lattice Method can be re-engineered as an enhanced alternative to those techniques for diverse situations that arise in flexible-aircraft dynamics. A historical review of the methodology is included, with latest developments and practical applications. Different formulations of the aerodynamic equations are outlined, and they are integrated with a nonlinear beam model for the full description of the dynamics of a free-flying flexible vehicle, which furnishes a geometrically-nonlinear description of both structure and aerodynamics. Nonlinear time-marching captures large wing excursions and wake roll-up, and the linearisation of the equations lends itself to a seamless, monolithic state-space assembly, particularly convenient for stability analysis. The aerodynamic model and the unified framework for the simulation of high-aspect-ratio planes are exhaustively verified by comparing them to lower- and higher-fidelity approaches. Numerical studies emphasising scenarios where the Unsteady Vortex-Lattice Method can provide an advantage over other state-of-the-art tools are presented. Examples of this comprise unsteady aerodynamics in vehicles with coupled aeroelasticity and flight dynamics, and in lifting surfaces undergoing complex kinematics, large deformations, or in-plane motions. Geometric nonlinearities are shown to play an instrumental, and often counter-intuitive, role in the aircraft dynamics. The Unsteady Vortex-Lattice Method is unveiled as a remarkable tool that can successfully incorporate them in the unsteady aerodynamics modelling.