Acoustic Characteristics of Urban Air Mobility-Scale Rotors of Increasing Size

This study examines the acoustics of Urban Air Mobility-scale (UAM), variable-RPM electric Vertical Takeoff and Landing (eVTOL) rotors in hover. The high-solidity, 5-bladed rotors operate at low tip speeds and a nominal 6psf disk loading. Rotor diameter varies from 4 to 14 ft, and variations in disk loading and solidity are also considered. Far-field in-plane tonal and broadband noise increases of 11 dB and 7.4 dB are observed, going from 4 to 14 ft diameter, with the 14 ft rotor carrying 12.25 times the thrust. But with A-weighting, tonal noise from the 14 ft rotor is 9.1 dBA lower than the 4 ft rotor, due to its lower blade passage frequency. Over the range of rotor sizes, broadband noise was observed to be 3–6.6 dB higher than tonal, unweighted, while completely dominating when A-weighting is used. Increasing from 6 psf to 12 psf disk loadings results in increased tip Mach numbers, and increases of Formula: see text in tonal noise and 6.8–8 dB in broadband noise. At high disk loading, A-weighted broadband noise continues to dominate, but tonal noise comes to within 5.6 dBA of broadband for the 4 ft rotor. Going to a lower solidity 2-bladed rotor operating at higher tip Mach number shows similar characteristics of increase in unweighted tonal noise, relative to broadband, but continued dominance of broadband with A-weighting. Moving from an in-plane observer to elevation angles below the rotor, the tonal noise is seen to drop away sharply, while the broadband noise shows a gentler increase.