Numerical Simulations of the Aeroelastic Response of an Actively Controlled Flexible Wing
Hall, Benjamin D.
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A numerical simulation for evaluating methods of predicting and controlling the response of an elastic wing in an airstream is discussed. The technique employed interactively and simultaneously solves for the response in the time domain by considering the air, wing, and controller as elements of a single dynamical system. The method is very modular, allowing independent modifications to the aerodynamic, structural, or control subsystems and it is not restricted to periodic motions or simple geometries. To illustrate the technique, we use a High Altitude, Long Endurance aircraft wing. The wing is modeled structurally as a linear Euler-Bernoulli beam that includes dynamic coupling between the bending and torsional oscillations. The governing equations of motion are derived and extended to allow for rigid-body motions of the wing. The exact solution to the unforced linear problem is discussed as well as a Galerkin and finite-element approximations. The finite-element discretization is developed and used for the simulations. A general, nonlinear, unsteady vortex-lattice method, which is capable of simulating arbitrary subsonic maneuvers of the wing and accounts for the history of the motion, is employed to model the flow around the wing and provide the aerodynamic loads. Two methods of incorporating gusts in the aerodynamic model are also discussed. Control of the wing is effected via a distributed torque actuator embedded in the wing and two strategies for actuating the wing are described: a classical linear proportional integral strategy and a novel nonlinear feedback strategy based on the phenomenon of saturation that may exist in nonlinear systems with two-to-one internal resonances. Both control strategies can suppress the flutter oscillations of the wing, but the nonlinear controller must be actively tuned to be effective; gust control proved to be more difficult.
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