Coupled rotor/wing optimization procedure for high speed tilt-rotor aircraft

A multidisciplinary optimization procedure is developed to investigate the design trade-offs associated with coupled rotor/wing performance in high speed tilt-rotor aircraft. The aerodynamic efficiency of the rotor in both hover and high speed cruise are coupled with the aerodynamic and aeroelastic performance of the wing while maintaining structural integrity of the wing/rotor configuration. The objectives are to maximize the hover figure of merit and the high speed cruise propulsive efficiency of the rotor and to minimize the wing weight. Constraints on the rotor include the first natural frequency in hover, the autorotational inertia and the blade weight. To avoid whirl flutter instabilities, constraints are imposed on the real part of the stability roots in the windmill flight condition. Constraints are also imposed on wing root stresses in both hover and cruise. An isotropic box beam model is used to represent the structural properties of the wing-box section. Design variables include rotor and wing planform variables and individual wall thicknesses in the wing. The Kreisselmeier-Steinhauser function approach is used to formulate the multiobjective optimization problem. A nonlinear programming technique based on the Broyden-Fletcher-Goldfarb-Shanno method is used as the optimization algorithm. The two-point exponential expansion approximation technique and a variable move limit scheme are used to reduce the computational effort. The optimum design is compared with an existing advanced tilt-rotor performance which is used as the baseline design. The results show significant improvements in both aerodynamic and structural performance without degradation of the aeroelastic stability.