High Solidity, Low Tip-Speed Rotors for Reduced eVTOL Tonal Noise

Abstract

This paper reports on a computational study conducted on an 8 ft diameter, fixed-pitch eVTOL rotor to examine the potential of using increased solidity and reduced tip speed to reduce the radiated acoustic signature. The study is conducted for the rotor operating in hover and in vertical climb, and at disk loadings between 6-12 lb/ft2. Relative to a "nominal" rotor of solidity σ=0.0646 (with N=2 blades and a root chord, c=15.82 cm), two 3σ rotors (the 3σ3 rotor with N=3 and root chord of 2c, and the 3σ5 rotor with N=5 and root chord of 1.2c) operating at reduced tip speed are considered, as is a single 5σ rotor (with N=5 and root chord of 2c) operating at a further reduced tip speed. The high solidity, low tip-speed rotors showed significant reductions in in-plane noise, both in hover as well as vertical climb, and over the range of disk loadings considered. The noise reductions observed with the 3σ5 rotor were significantly greater than those obtained by the 3σ3 rotor (operating at the same tip speed), and very similar to those of the 5σ rotor (operating at a lower tip speed). But the rotor torque and power penalty for the 3σ5 rotor was considerably lower than that for the 5σ rotor. Overall, a high solidity in the range of 0.2 for eVTOL rotors is quite advantageous, but further increase to around 0.3 appears acoustically unnecessary while being aerodynamically detrimental. At a solidity of 3σ, going from 3 wider chord blades to 5 narrower chord blades was hugely influential for in-plane noise reduction. Of the configurations studied, the best (the 3σ5 rotor) showed 16-24 dB reductions in in-plane noise in hover, reducing to 14.5-20 dB at 5/ms climb rate, and 12.5-16 dB at 10 m/s climb rate, with larger reductions seen at lower disk loadings. Relative to the solidity-σ rotor, the 3σ rotors had a torque penalty of 41-44%, and power penalties ranging from 1.5-5% in hover, increasing to 7.5-10% at 10 m/s climb rate.