Concurrently Optimized Aeroelastic Couplings and Rotor Stiffness for Alleviation of Helicopter Aeromechanical Instability

Abstract

The effectiveness of optimized aeroelastic couplings and rotor stiffness properties is examined for improving the aeromechanical stability characteristics of a helicopter with a soft-in-plane rotor, over a wide range of conditions, to enable the elimination of auxiliary lag dampers. A refined optimization procedure is developed that is robust and numerically efficient. When this procedure is used, results indicate that it is possible to reduce significantly the peak instability levels, while enforcing constraints on design variables and the rotating flap and lag frequencies. Concurrent optimization of the aeroelastic couplings and rotor stiffness parameters, rather than a sequential optimization strategy, yielded a design that provided maximum improvement in aeromechanical stability characteristics. The optimized design for the ground contact condition also resulted in improved lag damping in hover and forward flight. By the appropriate selection of additional design parameters such as landing gear stiffness and damping, it is possible to altogether alleviate instabilities in the optimized design.