Aeroacoustic Analysis of a Side-by-Side Hybrid VTOL Aircraft

This paper investigates the acoustics of a side-by-side Urban Air Mobility (UAM) aircraft with 0%, 5%, 15%, and 25% rotor overlaps in forward flight based on high-fidelity Computational Fluid Dynamics (CFD) simulations. The CFD and acoustics simulations are carried out using the HPCMP CREATETM-AV rotorcraft simulation and analysis tool Helios and the acoustic prediction tool PSU-WOPWOP. The influence of the finest wake-grid spacing size on acoustic prediction of the side-by-side rotor with 0% overlap is studied based on two wake-grid spacing cases: 5% Ctip and 10% Ctip. No significant difference in overall sound pressure level (OASPL) is found between the two cases. The effect of rotor overlap on rotor acoustics is also assessed, and it is shown that the 25% overlap case yields higher OASPL than the other overlap cases particularly due to stronger rotor-to-rotor blade-vortex interactions (BVIs). Furthermore, the noise of the side-by-side rotor with 0% and 25% overlaps is compared against similar-sized helicopter noise and various background noise levels. It is shown that the side-by-side rotor noise is comparable with the noise of a similar-sized four-bladed helicopter in cruise. The OASPL difference of the overhead case is less than 10 dB, which does not meet the guideline that the UAM noise should be 15 dB lower than similar-sized helicopter noise. The results also show that the side-by-side rotor noise could not be fully masked by the freeway background noise at an altitude of 1,500 ft. Thus, noise reduction technology should be developed to overcome the barrier of public acceptance of UAM noise.