Aeroelastic optimization of advanced geometry and composite helicopter rotors

Using an analytical approach, sensitivity analyses and aeroelastic optimization procedures are developed for advanced geometry and composite rotors. Aeroelastic and sensitivity analyses of the rotor based on a finite element method in space and time are linked with automated optimization algorithms to perform optimization of a four-bladed, soft-inplane hingeless rotor. For the advanced geometry rotor, design variables include nonstructural mass and its placement, blade bending stiffness (flap, lag and torsion) and blade geometry (sweep, droop and planform taper) at five spanwise stations. The objective function constitutes minimization of all six vibratory hub loads subject to constraints on frequency placement, autorotational inertia and aeroelastic stability of the blade in forward flight. Optimization results show reduction in the 4/rev hub loads of 25-60 percent. In another part of this study, aeroelastic optimization is performed for a composite rotor with the blade span modeled as a single-cell as well as a two-cell box-beam. The design variables are the ply angles of the cross-section of the blade spar. Optimization is performed for several layups and configurations and it is concluded that elastic stiffnesses reduce the objective function by about 20-40 percent; composite couplings yield a further reduction in the objective of about 10-15 percent. Using vibratory hub loads alone in the objective function can cause an increase in the blade root bending moments leading to higher dynamic stresses. This issue is addressed by performing a multi-objective optimization on the composite rotor by using a combination of vibratory hub loads and vibratory bending moments in the objective function. Optimum designs for the multi-objective optimization show a reduction in the objective function of about 30 percent from the starting design; 20 percent of this reduction is due to elastic stiffness and 10 percent due to composite coupling (flap bending-torsion coupling). As compared to the starting design, the optimum solution results in a 15-60 percent reduction of the 4/rev hub loads as well as a reduction in the peak-to-peak flap and lag bending moments of 11 and 14 percent, respectively, compared to the starting design. Starting from an initially infeasible design with a 3 percent requirement on lag mode damping, the optimum solution with composite chordwise bending-torsion coupling results in an increase in lag mode damping of 130-200 percent, compared to the starting design.