Assessment of synchrophasing for a pair of rotors in close proximity

This paper investigates the aerodynamics and aeroacoustics of a pair of rotors within the "Radiation and Propagation for Multirotor Systems" project from the Group for Aeronautical Research and Technology in Europe (GARTEUR) Action Group 26. The study employs validated high-fidelity computational fluid dynamics (CFD), the in-house Helicopter Multi-Block Solver 3 (HMB3), to investigate a pair of dual-bladed 13x7 rotors in hover and edgewise flights. The study aims to reduce rotor noise by introducing a phase offset between the starting positions of the rotors in tandem configurations. The rotors maintained a constant RPM, and a tip Mach number of 0.4, close to typical values for full-scale urban air mobility (UAM) rotorcraft. Results indicated a performance loss from tandem rotors compared to isolated configurations, especially for the downstream rotor. However, the performance of the downstream rotor is slightly recovered in the tandem offset configuration. A comprehensive synchrophasing study based on cumulative effect of the rarefaction and compression in the acoustic wave is also presented, showing different levels of noise reduction without compromising performance. More than 20 dB of noise reduction was observed in several one-third octaves bands, and up to 7 dB of overall noise reduction was achieved at optimal de-phased cases for the region between the two rotors. In the meantime, the most beneficial phases can be adjusted to achieve maximum noise reduction at different locations, including upstream, interrotor, and downstream regions. Furthermore, transforming from a single isolated rotor to the optimal de-phased tandem offset configuration while producing the same amount of lift achieved almost 10 dB noise reduction, suggesting a perceived noise that is 50% lower in reality.