Helicopter Rotor Design Using a Time-Spectral and Adjoint-Based Method

Abstract

A time-spectral and adjoint-based optimization method was developed and applied to helicopter rotor design for unsteady level flight. The time-spectral method is a fast and accurate computational fluid dynamics algorithm for computing unsteady flows. It transforms the flow-governing equations into a periodic steady state by using a Fourier spectral derivative operator. An accompanying steady-state adjoint formulation was implemented in the time-spectral form of the governing equations to enable design optimization for unsteady flows. The time-spectral analysis was validated against conventional time-accurate computational fluid dynamics computation and flight test data of a UH-60A helicopter rotor during high-speed forward flight. A multidisciplinary analysis of blade structural dynamics was carried out through a comprehensive analysis coupling procedure that accounted for aeroelasticity and enforced vehicle trim. The adjoint-based design method was applied to optimize the blade shape of the UH-60A rotor. Power minimization was pursued with nonlinear constraints on the thrust and rotor drag force. The blade twist distribution, sectional airfoil shape, and outboard planform shape comprised over 100 design variables. Starting from the initial blade, the optimizer found a new design that showed improved performance. The validation results coupled with the multidisciplinary comprehensive analysis confirmed actual improvement: a 5% decrease in torque accompanied by a decrease in thrust of less than 1%.