Hover Performance of Corotating and Counterrotating Coaxial Rotors

Abstract

Hover testing was performed on a 2-m-diameter coaxial corotating (stacked) rotor system to investigate the effect of azimuthal spacing, or index angle, on performance. Individual rotor thrust and power measurements were performed as a function of the index angles. It was found that the contributions of individual rotors to the total stacked rotor performance were remarkably dependent on the index angle. For an index angle of 10°, that is, when the lower rotor trails the upper rotor by 10°, a nearly 3.6% increase in total system power loading was observed compared to that for an index angle of –10°. Numerical calculations using the U.S. Army's Rotorcraft Comprehensive Analysis System (RCAS) were used to better understand this behavior in terms of self-induced and interference power constituents. For the coaxial, counterrotating (CCR) rotor, the upper rotor performance was close to that of an isolated two-bladed rotor. The lower rotor operating in the wake of the upper rotor showed a relatively poor performance, requiring 70% more induced power than the upper rotor. The improvement in performance for a CCR rotor results from a larger thrust share and better performance of the upper rotor as compared to the lower rotor.