AIAA 2002-1717 Flutter Suppression for High Aspect Ratio Flexible Wings Using Microflaps

Miniature trailing edge effectors (MiTEs) are small flaps (typically 1% to 5% chord) actuated with deflection angles up to 90 degrees. Because of their small size, these devices provide the opportunity for high bandwidth control. The present study considers the use of many such control surfaces to increase the flutter speed of a high aspect ratio flexible wing. A finite element plate model is used to model the structural dynamics and an unsteady panel method provides the aerodynamic loads. Experimental flutter testing shows good agreement with the numerical stability analysis. The MiTE is modelled by a single panel element at the trailing edge with varying boundary conditions at its collocation point. In spite of the complex viscous aerodynamics of the MiTEs, the panel model proved to be adequate in simulating the steady and unsteady behavior. The use of these effectors for control is complicated by their nonlinear characteristics. Since the actuator is only effective at high deflection angles, it is only deflected in one of three positions: up, down, and neutral. The design of a nonlinear feedback controller has been performed using numerical optimization. Introduction The Gurney flap is a small (typically 1% ∼ 5% chord) flap used to increase the maximum lift of an airfoil section. It was developed and applied to racing cars by Robert Liebeck and Dan Gurney in 1960’s, although similar devices were employed in World War II aircraft such as the P-38 ∗Doctoral Candidate, Department of Aeronautics and Astronautics, AIAA Student Member †Professor, Department of Aeronautics and Astronautics, AIAA Fellow ‡Doctoral Candidate, Department of Aeronautics and Astronautics, AIAA Member Copyright c ©2002 by authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. and F8-F. Numerous wind-tunnel tests and numerical computations have been conducted on both single element and multi-element airfoils with Gurney flaps. These studies confirm that despite their small size, Gurney flaps with deflections near 90 degrees can increase maximum lift and the lift produced at a given angle of attack. Liebeck explained this effect, produced by a short region of separated flow directly upstream of the flap, with two counterrotating vortices downstream that effectively modify the trailing edge Kutta condition. This was verified to be correct for time averaged flow by flow visualizations and CFD simulations. In the present work, we consider the use of devices similar to Gurney flaps, not to increase maximum lift, but to provide high bandwidth, robust control. Miniature Trailing edge Effectors (MiTEs) are small movable control surfaces at or near the trailing edge, deflected to large angles to produce control forces and moments that may be used for flight control or structural mode control. The current study, begun in 1998, deals with the use of such actuators for aeroelastic control. MiTEs have distinct advantages over conventional control surfaces: High bandwidth actuation can be achieved due to their small size and inertia, enabling their use for flight control or for higher frequency structural mode control with significantly reduced power requirements. Spanwise variation and interdigitated deflections can produce rolling, pitching, and yawing moments, as well as the control of specific structural modes. Because the surfaces are deflected in a discrete manner (up, down, or neutral), no active servo-feedback is required, eliminating the expense of accurate, high-rate servo actuators and enabling a large number of these effectors to be fabricated at a low cost. The use of a large number of small, simple effectors also makes the system faulttolerant. The application of MiTEs for aeroelastic control is demonstrated here by designing an active control system that can suppress the flutter of a flexible wing. High aspect ratio flexible wings are of interest