Aeroacoustic Analysis of Asymmetric Lift-Offset Helicopter in Forward Flight

Abstract

In recent years, the University of Maryland has worked on an asymmetric lift-offset compound helicopter. The configuration consists of a single main rotor helicopter with the addition of two key ways to increase the forward speed: a stubbed wing on the retreating fuselage side, and a slowed down rotor. Experiments and simulations have shown that the novel concept provides improved thrust potential and lift-to-drag ratios in high-speed forward flight. This study aims to determine whether the asymmetric lift-offset configuration also provides aeroacoustic benefits in forward flight in addition to its aerodynamic advantages. The aerodynamic results from previous computational and experimental studies are recreated using the Mercury framework, an in-house CFD solver based on Reynolds-Averaged NavierStokes (RANS). The acoustic analysis is performed using an acoustic code based on the Ffowcs William-Hawkings equation to solve for the noise propagating from the surfaces of the aircraft. It was found that for an advance ratio of 0.5 the wing-lift offset configuration can produce 56.8% more thrust at the same collective angle without any penalties in total noise. When the configurations produce equal thrust it was found that the wing-lift offset case has a 4 dB reduction in maximum overall sound pressure level. At an advance ratio of 0.3 with trim for equivalent thrust between configurations, a 3 dB maximum OASPL reduction was obtained with the inclusion of the wing.