Performance and noise emission analysis of coaxial co-rotating propellers

The trade-off between performance and noise emissions has progressively established itself as a key factor in the emergence of Urban Air Mobility. This work addresses the relationship between the propulsive efficiency and noise emissions of drone-sized coaxial, co-rotating propellers operating in hover, for varying phase offset ϕ and axial separation Δz between the rotors. Experimentally, an antagonistic behavior emerged between power loading and broadband noise, while a modulation of the tonal noise with varying phase offset was observed. The physical mechanism modulating tonal noise is explained using an analytical approach accounting for the acoustic interferences that occur for unevenly spaced rotors, based on Goldstein's formulation for the tonal noise generated by rotating dipole sources. Furthermore, a combination of a commercial panel method and a rotating dipole source model is proposed as a fast-turnaround approach to predict aerodynamic loads and tonal noise. The acoustic predictions obtained for the second shaft harmonic are consistent with the experimental trends, given the inherent limitations of the solver employed, and assess the potential of the proposed method for a multidisciplinary early-stage design phase.