Experimental aeroacoustic investigation of tandem co-rotating rotors in non-axial inflow

This study experimentally investigates the aeroacoustic characteristics of tandem, co-rotating rotors operating under non-axial inflow conditions. Measurements were conducted in an anechoic wind tunnel over a range of advance ratios and tilt angles to provide a sub-scale representation of the noise generated under moderate non-axial inflows. Far-field noise was characterised using multiple in-plane and out-of-plane microphone arrays to resolve directivity, spectral content, blade passing harmonics, and broadband components. The results show that the noise from the tandem, co-rotating rotors is strongly influenced by both advance ratio and tilt angle. Operation at low tilt angles leads to significant amplification of higher blade passing harmonics and broadband noise, particularly at out-of-plane observer locations, indicating increased unsteady loading. Increasing the tilt angle substantially reduces higher harmonic and broadband noise whilst the blade passing frequency component remains comparatively insensitive. These findings highlight the sensitivity of tandem-rotor noise to operating conditions and provide insight into noise mitigation opportunities during transition flight in urban air mobility applications.