Quantification of Swirl Recovery in a Coaxial Rotor System

Experiments were performed in hover on a 2 m diameter, isolated single rotor system, a coaxial counter-rotating rotor (CCR) system, and coaxial co-rotating (stacked) rotor system to quantitatively extract the swirl recovery of a CCR system. The performance comparison was based on induced power factors obtained from curve fits to momentum theory. The three systems were tested up to a CT =σ of 0.1 and tip Mach number was maintained around 0.4 for all experiments. The azimuth spacing between the upper and lower blades, defined as the index angle, was varied in the stacked rotor system. Measurements revealed that the stacked rotor performance was highly dependent on the index angle. When the lower rotor trails the upper rotor by index angle of 10°, there was a 4% performance gain on the stacked rotor, compared to the CCR rotor. Examining and isolating the effect of index angle on the performance of the stacked rotor, the swirl recovery in the CCR rotor was measured to be nearly 4%. Vibratory loads induced by the aerodynamic interaction between two rotors were also investigated.