multirotor aerodynamics, multirotor acoustics, hover, URANS, VPM, FWH, aeroacoustics, DEP, distributed electric propulsion, electric aircraft


Multirotor configurations introduce complicated aerodynamic and aeroacoustic interactions that must be considered during aircraft design. In this paper we explore two numerical methods to model the acoustic noise caused by aerodynamic rotor-on-rotor interactions of rotors in hover. The first method uses a conventional mesh-based unsteady Reynolds-average Navier-Stokes (URANS) solver, while the second consists of a meshless Lagrangian solver based on the viscous vortex particle method (VPM). Both methods are coupled with an aeroacoustics solver for tonal and broadband noise predictions. Noise predictions are validated for single and multi-rotor configurations, obtaining with the VPM a similar accuracy than URANS while being two orders of magnitude faster. We characterize the interactions of two side-by-side rotors in hover as the tip-to-tip distance and downstream spacing are varied. At an observer located six diameters away, multirotor noise is the strongest above and below the rotors, increasing by about 10 dBA directly underneath as the rotors are brought closer together. The interactions show no sensitivity to blade loading distribution, indicating that multirotor interactions are not alleviated with a lighter tip loading. We found that noise can be mitigated by spacing the rotors in the downstream direction—with the optimal spacing being about half a diameter—achieving a noise decrease of about 4 dBA without any aerodynamic penalties.

Original Publication Citation

Alvarez, E. J., Schenk, A., Critchfield, T., and Ning, A., “Rotor-on-Rotor Aeroacoustic Interactions of Multirotor in Hover,” Journal of the American Helicopter Society (in review).

Document Type

Peer-Reviewed Article

Publication Date


Permanent URL






Ira A. Fulton College of Engineering and Technology


Mechanical Engineering

University Standing at Time of Publication

Associate Professor