Multidisciplinary Design Optimization for Aeromechanics and Handling Qualities

A design optimization study is presented in which rotor dynamics and flight dynamics are simultaneously taken into account to maximize the damping of a rotor lag mode. The design variables include rotor, airframe, and flight control system parameters. The constraints address rotor stability and loads and handling qualities. Two design optimization cases are considered, one with only constraints computed from the linearized model of the helicopter and the other with additional constraints that require the integration of the nonlinear equations of motion. Both finite difference and semi-analytical gradients are used for some constraints. The optimization procedure increases the lag mode damping by up to 90%, while satisfying all of the constraints, primarily by reducing the blade torsion stiffness. The aeromechanic design problem is a multidisciplinary problem. The constraint active at the optimum is the level 1 handling qualities requirement in the pitch axis. Optimization provides a framework to manage multidisciplinary problems systematically and efficiently. Using semi-analytical gradients of the constraints computed from the linearized model yields the same results as with finite difference gradients, but more efficiently. The computational advantage increases with problem size. Further advances in computer hardware and in optimization algorithms, including efficient sensitivity analyses, will help make numerical optimization a practical design tool.