AEROELASTIC OPTIMIZATION OF A HELICOPTER ROTOR WITH COMPOSITE TAILORING

A sensitivity analysis and aeroelastic optimization for a four-bladed, soft-inplane, uniform, composite hingeless rotor with a generic cross-section is developed using an analytical approach. Shear degrees of freedom are eliminated by using static condensation. The composite blade spar is modeled as a single-cell box-beam as well as a two-cell box-beam. The design variables used in this study are the ply angles of the laminated walls of the composite box-beam. Aeroelastic and sensitivity analyses of the rotor based on a finite element in space and time are linked to an automated algorithm to perform optimization studies. The objective function minimizes the 4/rev hub loads, with constraints on blade frequency and aeroelastic stability in forward flight. A sensitivity study of the 4/rev loads, blade stability and frequency show that torsion stiffness is a dominant section property for the optimization problem. Optimum design solutions for a two-cell box-beam configuration with no elastic coupling show a reduction of more than 10 percent in the objective function from the starting design. The influence of elastic coupling shows an additional reduction in the objective function of over 10 percent. Starting from an initially infeasible design, the optimum design solution for a two-cell box-beam configuration with negative pitch-lag and positive extension-torsion coupling results in an increase in lag mode damping of over 50 percent compared to the baseline layup.