AEROELASTIC OPTIMIZATION OF A HELICOPTER ROTOR

Structural optimization of a hingeless rotor is investigated to reduce oscillatory hub loads while maintaining aeroelastic stability in forward flight. Design variables include spanwise distribution of nonstructural mass, chordwise location of blade center of gravity and blade bending stiffnesses (flap, lag and torsion). A comprehensive aeroelastic analysis of rotors, based on a finite element method in space and time, is linked with optimization algorithms to perform optimization of rotor blades. The vehicle trim and blade steady response are calculated iteratively as one coupled solution using a modified Newton method. Eigenvalues corresponding to different blade modes are calculated from linearized perturbation equations using Floquet transition matrix theory. Sensitivity derivatives of blade response, hub loads and eigenvalues with respect to the design variables are derived using a direct analytical approach, and constitute an integral part of the basic blade response and stability analyses. This approach reduces the computation time substantially; a 80% reduction of CPU time to achieve an optimum solution as compared to the widely adopted finite difference approach. Through stiffness and nonstructural mass distributions, a 60-90% reduction in ail six 4/rev hub loads is achieved for a four-bladed soft-inplane rotor.