Time-Spectral Method for CFD Prediction of Helicopter Rotor Vibratory Loads

Abstract

A Fourier collocation based time-spectral method is used to simulate helicopter rotor ows under high-speed transonic and highly-loaded dynamic stall conditions. Computations are performed using fully coupled CFD/Comprehensive Analysis, where the Comprehensive Analysis supplies nite element CSD and aircraft trim capabilities. The focus is on the prediction of vibratory airloads of an isolated rotor. The accuracy of spectral method is compared with a time-accurate analysis, both trimmed consistently to the same ight condition. Two UH-60A ights are simulated { Counters 8534 and 9017 from the U. S. Army/NASA Airloads Program { a high vibration ight and a deep dynamic stall ight. For the high vibration ight, the fastest and minimal implementation of the method, with 11 time instances for a 4-bladed rotor, predicts the vibratory lift and pitching moments within 5{10% accuracy of time-accurate simulations in about one-fth the computational time. The largest errors occur in vibratory chord forces (10 to 20%). The minimal implementation is not adequate under dynamic stall conditions, as expected, where signicant higher-harmonic content (6/rev and higher) produce aliasing errors in the harmonics of interest.