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

A time-spectral and adjoint-based optimization procedure that is particularly efficient for the analysis and shape design of helicopter rotors is developed. The time-spectral method is a fast and accurate algorithm to simulate periodic, unsteady flows by transforming them to a steady-state analysis using a Fourier spectral derivative temporal operator. An accompanying steady-state adjoint formulation for periodic unsteady problems is then possible and enables an unsteady flow design optimization procedure. The time-spectral CFD analysis is validated against conventional time-accurate CFD and flight test data for a UH-60A Black Hawk helicopter rotor in high speed forward flight. A multidisciplinary structural dynamics and comprehensive analysis coupling is employed for validation study to include blade structural dynamics and to enforce vehicle trim. Application of the adjointbased design method is carried out to optimize blade shape for a UH-60A. An uncoupled aerodynamics only calculation is performed for design application, holding the blade deformations and trim angles fixed to the previously calculated values. Minimization of power is pursued with non-linear constraints on thrust and drag force, resulting in a simplified form of the trim condition. The blade twist distribution, airfoil section shapes, and outboard planform shape comprise over 100 design variables. Starting from the initial configuration, the optimizer found a new design that shows good performance improvement, amounting to a 2% decrease in torque and 7% increase in thrust compared to the baseline UH-60A. The results demonstrate the potential of the time-spectral and adjoint-based design method for helicopter rotors.