High-Fidelity Computational Analysis on the Noise of a Side-by-Side Hybrid VTOL Aircraft

Abstract

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. First, the influence of the finest wake-grid spacing (Δfinest) on the side-by-side rotor acoustics is studied. No significant difference in overall sound pressure level (OASPL) is found between the two wakegrid spacing cases: Δfinest = 5% Ctip and 10% Ctip. Physical noise sources, such as impulsive self blade–vortex interactions, rotor-to-rotor blade–vortex interactions, and separated flow and blade interactions, are investigated in details based on high-fidelity CFD simulation results and noise-integrand analyses. The effect of rotor overlap on rotor acoustics is also assessed. It is found that the maximum OASPL difference among the overlap cases is about 1 dB, but the radiation patterns are different for each overlap case. Furthermore, the noise of the side-by-side rotor with 0% and 25% overlaps is compared against the noise of a conventional helicopter and various background noises. The side-by-side rotor noise is comparable with the helicopter noise around 1000 Hz. The results also show that the side-by-side rotor noise could not be fully masked by the freeway background noise even at an altitude of 1500 ftand exceed Uber's noise guideline of 62 dBA at 500 ft. Thus, noise reduction technology should be pursued to lower UAM aircraft noise, which is one of major technical barriers for UAM public acceptance.