Design of Flight Control Laws for Aircraft with Flexible Wings Using Quantitative Feedback Theory

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Design of Flight Control Laws for Aircraft with Flexible Wings Using Quantitative Feedback Theory

C. L. Osmon

Aircraft composed of lightweight composite materials are extremely enticing since their structural weight is greatly reduced. However, the control of these aircraft is complicated by the resultant flexibility of the wings. Two avenues of approach are possible; stiffen the wings thus losing some of the weight reduction benefits, or design the lateral/directional flight control system cognizant of the wing's flexibility. In this thesis the second approach is taken. The design of three lateral/directional flight control systems for the sub-sonic flight envelope of the F-18 is presented. The Quantitative Feedback Theory (QFT) robust control design technique is used. These designs incorporate weighting matrices to distribute generalized aileron and rudder commands to the five control surfaces available on the F-18. The degree of freedom afforded by the use of ail control surfaces allows for the reduction of the load on the wings, while at the same time meeting military specifications for roll maneuvers. A baseline flight control system is designed and then improved upon; the final design incorporates load alleviation concepts to reduce the load on the wings thus avoiding wing twisting. All flight control systems designed in this thesis are shown to meet military specifications, as verified with nonlinear time simulations.