Maneuver Trim Optimization Techniques for Active Aeroelastic Wings

123 A new method for performing trim optimization of Active Aeroelastic Wing (AAW) technology is presented. The method is based on posing the trim problem as a linear program and solving it with the simplex method. Trim optimization is then integrated with the structural design process in a sequential manner, such that new optimal deflections of the control surfaces are computed for every structural design iteration. The use of the simplex method for trim optimization allowed the elimination of nonphysical constraints that had to be imposed when a gradient-based method was used. This resulted in significantly better objectives for the trim optimization. The sequential AAW design process was demonstrated on a lightweight fighter type aircraft performing symmetric and antisymmetric maneuvers at subsonic and supersonic speeds. The concurrent trim and structural optimization resulted in a significantly lighter structure compared to a structure designed with conventional control technology and to a structure employing AAW technology with fixed control surface deflections. Introduction An emerging and promising technology for addressing the problem of adverse aeroelastic deformation, such as control surface reversal, is Active Aeroelastic Wing (AAW) technology. It has recently been a key area of study for both the government and industry and is defined by Pendleton et. al., as "a multidisciplinary, synergistic technology that integrates air vehicle aerodynamics, active controls, and structures together to maximize air vehicle performance". AAW technology exploits the use of leading and trailing edge control surfaces to aeroelastically shape the wing, with the resulting aerodynamic forces from the flexible wing becoming the primary means for generating control power. With AAW, the control surfaces then act mainly as tabs and not as the primary sources of control power as they do with a conventional control * Graduate Research Assistant, Student Member AIAA † Assistant Professor, Senior Member AIAA ‡ Postdoctoral Fellow, Member AIAA Copyright © 2000 by P.S. Zink, D.N. Mavris, and D.E. Raveh. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. philosophy. As a result, wing flexibility is seen as an advantage rather than a detriment, since the aircraft can be operated beyond reversal speeds and still generate the required control power for maneuvers. Hence, there is potential for significant reductions in structural weight and actuator power. Figure 1 illustrates conceptually the differences between AAW technology and a conventional control approach. The hypothetical example shows the cross section of two wings deforming due to aeroelastic effects. The wing on the left, employing AAW technology, is twisting in a positive way with the use of both leading and trailing edge surfaces, while the conventionally controlled wing on the right, which uses only the trailing edge surface, is twisting in a negative way. This adverse twist due to the deflection of the trailing edge surface is associated with reduced control surface effectiveness and control surface reversal in which the increase in camber due to the deflection of the surface is offset by the negative twist of the wing. LE up TE up Positive Twist TE LE Aeroelastic Twisting Moment TE down Adverse Twist TE AAW Approach Conventional Approach V∞ Figure 1 AAW Technology vs. Conventional Control Since AAW technology is multidisciplinary in nature, structural design using the technology necessarily requires detailed information about the vehicle structures, aerodynamics, and controls, and in particular, is heavily dependent on control law design, which in turn depends on the flexible structure. As a result, there is a need for an AAW design process in which the structure and control laws are optimized concurrently. In consideration of AAW technology’s use of redundant control surfaces, important constituents of the technology are control surface gear ratios which dictate how one control surface deflects with respect to a single basis surface. Two gear ratio scenarios are illustrated in Figure 2, in which the deflections of the leading edge inboard (LEI), leading edge outboard (LEO), and trailing edge inboard (TEI) surfaces are