Helicopter rotor flapping angles from hover to low speed forward flight are calculated and compared with the measured data in this paper. The analytical method is based on a second order lifting line/full span free wake model as well as a fully coupled rotor trim model. It is shown that, in order to accurately predict the lateral flapping angle at low advance ratio, it is necessary to use free wake analysis to account for the highly non uniform inflow induced by the distorted wake geometry at rotor disc plane.
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