MODELING TURBULENCE SEEN BY MULTIBLADED ROTORS FOR PREDICTING ROTORCRAFT RESPONSE WITH 3-DIMENSIONAL WAKE

The state of the art of predicting rotorcraft response to turbulence i s based almost exclusively on modeling turbulence seen by one-bladed r otors, Accordingly, the difference between turbulence seen by one-blad ed and multibladed rotors, and related effects on response, are invest igated, This difference is fundamental in that it is due to cross-corr elation that always exists between turbulence excitations at two blade stations; the crass correlation is referred to as station-to-station cross-correlation when these two stations lie on the same blade and bl ade-to-blade cross-correlation when they lie on different blades. A tu rbulence-input matrix for multibladed rotors is developed in the time domain (correlation matrix) and in the frequency-time domain (instanta neous spectral density matrix), and earlier-studied turbulence models are recovered as special cases, The input matrix is then applied to is olated rotors with one and three blades executing napping motion; a li near airfoil theory with a three-dimensional finite-state wake model i s used, The second-order response statistics of flapping and hub shear are predicted, The statistics show that while the station-to-station cross-correlation effects on response are negligible, the effects of b lade-to-blade cross-correlation and dynamic wake are appreciable, More over, wake modeling with at least three harmonies is required for conv erged results.