This paper discusses the design optimization of a thin composite wing box structure for a commuter tilt-rotor aircraft. The strength, stiffness, and aeroelastic characteristics of the cantilevered wing govern its structural design, and pose multiple constraints on its optimization. In this investigation the wing deflection, internal load distribution, and the natural frequencies are calculated using the finite element method. In lieu of a formal aeroelastic analysis of the preliminary wing design for whirl flutter, limits are imposed on the magnitudes of its primary vibration frequencies in bending and torsion and their placements relative to each other and to the critical rotor frequencies in helicopter and airplane flight modes. In addition, multiple linear regression equations are developed and used to capture the skin and shear web panel buckling responses in the optimization process. Two skin ply patterns based on different blend ratios of 0, +/-45, and 90 degree plies are considered for the laminated wing skin. The optimum design is obtained based on three maneuver loading conditions associated with the airplane mode. The results of this investigation indicate that the first horizontal bending and first torsion frequency constraints are more critical than that associated with the vertical bending mode. Both design cases result in the same optimal weight although the individual structural members are different in size reflecting alternate load paths.
