Robust Design for Aeroelastically Tailored / Active Aeroelastic Wing

A study of multidisciplinary design concerning the incorporation of aeroelastic tailoring, control surface blending, and active aeroelastic wing concepts is presented. The design process incorporates response surfaces, fast probability integration and modal-basis multidisciplinary design optimization to characterize the design space. The wing box skins of a representative fighter configuration with multiple wing control surfaces are sized to minimum weight. A design of experiments approach is developed for the gear ratios in control surface blending. Design optimization is conducted for each set of gearing functions. The control surface gear ratios are then treated as “noise” in the structural design process, and a robust structural design is sought to account for the change in control laws that historically occur during the aircraft design process. The motivation for this methodology investigation is derived from the common occurrence of control law changes throughout the lifetime of an aircraft. Introduction Aeroelastic tailoring and active aeroelastic wing technologies are envisioned for application in the structural design of future advanced fighter aircraft with the goals of reducing weight and increasing maneuverability. Aeroelastic tailoring is the concept of using the directional stiffness properties of composites to design an aircraft structural component to deform under load in such a way as to benefit the performance of the aircraft. For example, Bohlmann, Eckstrom, and Weisshaar proposed aeroelastic tailoring for an oblique wing concept. In this case structural wash-out of the forward swept part of the wing was used to counteract the 'natural', untailored tendency of that part of the wing to wash-in, while structural wash-in of the aft swept part * Graduate Research Assistant, Student Member AIAA † Assistant Professor, Aerospace Engineering, Senior Member AIAA ‡ Senior Engineering Specialist, Senior Member AIAA § Associate Professor, Aerospace Engineering, Member AIAA Copyright © 1998 by Lockheed Martin Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. of the wing was used to counteract the 'natural', untailored tendency of that part of the wing to wash-out thus providing a lateral load balance. They discovered that some of the benefits of aeroelastic tailoring included reduced aileron deflection required for trim and reduced hinge moments thus reducing required actuator weight and power. Active aeroelastic wing (AAW) technology, which has recently been a key area of study for both the government and industry, as defined by Pendleton et. al., is "a multidisciplinary, synergistic technology that integrates air vehicle aerodynamics, active controls, and structures together to maximize air vehicle performance". AAW technology uses leading and trailing edge control surfaces to twist the wing which then becomes the primary surface for generating control power. 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. Since the AAW is more flexible than a comparable traditional wing, it has a lower weight. Because aeroelastic tailoring and AAW technologies drive wing deformation to some desirable shape, they are complementary technologies that in combination should produce significant weight savings over untailored, traditionally controlled aircraft. Since both technologies are "multidisciplinary and synergistic" in nature it is important to consider their impact from the outset of the design process. This in and of itself presents a challenge since application of the technologies, particularly AAW technology, require detailed information of the structure, aerodynamics, and controls of the aircraft. In the beginning of the design process this kind of detailed knowledge is limited. Much effort is being expended within the aerospace research community in establishing design knowledge early in the design process while at the same time keeping design freedom open. As examples, References 3 and 4 used finite element method and equivalent laminated plate analysis, respectively, in conjunction with a Design of Experiments/Response Surface Methodology (DOE/RSM) to generate wing weight response surface equations as a function of wing geometry. These equations were then incorporated into