Experimental Investigation of Coaxial Co-Rotating and Counter-Rotating Rotor Acoustics in Hover

An experimental investigation was conducted on a 1.108 m radius coaxial co-rotating (or stacked) rotor and a coaxial counter-rotating (CCR) rotor of identical geometry to compare the acoustics and loads of both rotor configurations in hover. The rotors were operated at a tip Mach number of 0.40, tip Reynolds number of 765,000 and an axial spacing of 1.55 chord lengths, and the index angle between the upper and lower blades of the stacked rotor was varied. The overall sound pressure level (OASPL) was significantly larger for the CCR rotor. For example, total rotor noise at -45◦ angle of elevation was 6 dB greater for the CCR rotor than the stacked rotor at 8◦ collective. These increases in OASPL were driven by large increases in tonal noise for the CCR rotor, of up to 10 dB higher than the stacked rotor at some angles of elevation. This was attributed to additional tonal noise occurring at harmonics of 2Nb/rev, due to vibratory loads from 2Nb/rev blade crossings. The results of the experimental study suggest that, in terms of reduced noise and increased hover efficiency, the stacked rotor is preferable to the CCR rotor for eVTOL vehicle configurations that implement compact or closely spaced designs.