A full-span free-wake model using circular-arc vortex elements and incorporating rotor trim analysis

Abstract

An analytical method incorporating a free-wake analysis with a rotor trim algorithm is developed for predicting rotor wake geometry and blade loading distribution for a helicopter in hover and forward flight. The wake model of this method is based on a multi-filament full-span free wake, using sophisticated circular-arc vortex elements. The blade model is based on the second-order lifting-line theory to improve the calculation of three-dimensional tip effects. A rotor trim method is given, based on the finite-difference approximation of the blade flapping motion and the trim Jacobian matrix. This is subsequently coupled into the free-wake model. In order to demonstrate the capability of the coupled method, comparisons between the calculated results and the available experimental data of the rotor blade flapping motion for a model rotor are presented covering conditions from hover through forward flight. The free-wake geometry and the blade loading distribution at a low advance ratio are calculated for the H-34 helicopter rotor, and the blade loading distribution is compared with both the measured data and calculated results on the basis of the undistorted wake.