Uniform Downwash With Rotors Having A Finite Number Of Blades

This paper deals with factors that influence the hovering performance of helicopter rotors. First, a discussion is presented classical on the shortcomings of the blade element-momentum theory when it is used to define the optimum rotor with a finite number of blades. A discussion of the blade loadings shows that, when real blades are considered, the inverse twist distribution indicated by classical theory is not an optimum distribution. Fixed-wing aerodynamic theory is used to demonstrate that the loading on a single blade is better represented by treating the rotor blade as a wing in a linearly varying spanwise velocity field. A comparison between experimentally obtained loading on a single blade and the loading calculated from fixed-wing theory is presented. An approximation for the velocity induced by a rolled-up vortex is suggested and a comparison of this approximation with a measured velocity distribution near the core region of a tip vortex is shown to be very good.