Lifting surface design using trailing edge devices, marginal aeroelastic stability and feedback control

Considerable promise is seen in the application of strain actuator technology as "Smart Materials" for fulfilling control functions of various kinds in aircraft. However, the strain and force capability of these actuators are known to impose limitations. To overcome these limitations, the aeromechanical design of fixed and rotary wings from control viewpoint have been investigated in this study. Three design analyses have been conducted using a simple model with a trailing-edge hinged flap or tab representing the strain actuators, including: (1) a fixed wing-flap-tab with constant forward speed, (2) a wing-flap with pulsating velocity superimposed on constant forward speed and (3) a helicopter rotor blade in hovering flight.; A generalized two-dimensional, time-domain, finite-state, Theodorsen-Greenberg unsteady aerodynamic model of the wing-flap in a pulsating velocity field has been extended and applied in these analyses to calculate the aerodynamic loads on the airfoils with a trailing-edge hinged control surface. State space representations of those lifting surface aerodynamics have been derived in time-varying format. Single- and multi-objective design optimizations have then been conducted to assess the capabilities of possible designs which maximize the amplification of the rotations and minimize force required of the control surfaces. As these optimal designs tend to approach aeroelastic instability, Linear Quadratic Gaussian (LQG) feedback controller design has been applied to assure the stability of resulting motions and improve the output performance. A discrete periodic time-varying LQG controller has also been developed to stabilize the systems in the presence of pulsating velocity effects. A dynamic inflow model has been linearized and included in the design analyses of the helicopter rotor blade to represent the three-dimensional transient behavior of the complete rotor.; The results of this research have shown the feasibility of applying properly designed trailing-edge control surfaces which make use of strain actuation concepts to both fixed and rotary wings. Further analyses and experiments are expected to be required to develop this concept of "Smart Structure" control surfaces to the point where they are ready for practical aircraft applications.