This experimental study presented in this paper aims at understanding the complex aeroacoustics relationship between small-scale rotors in tandem configurations. The effect of the upstream rotor on the aeroacoustics of the multi-rotor system in a counter-rotating configuration was studied. Two rotors in tandem alignment were tested for varying separation distances, free-stream velocities, and tilt angles for the upstream rotor. The results are presented in terms of power spectral density of the acoustic pressures and the directivity of the overall sound pressure levels. The results show that the presence and position of an upstream rotor significantly affect the rotor system's noise signature. The tip-to-tip separation distance affects the magnitude of the radiated noise at the first blade passing frequency. Additionally, an increase in the free-stream velocity increases the radiated broadband noise. The OASPL directivity patterns show a minimal sensitivity to the free-stream velocity except for a few exceptional cases. Finally, the tilt angle of the upstream rotor has a significant effect on the OASPL directivity at higher tilt angles.

The aerodynamic noise characteristics of propellers in turbulent flow have been investigated experimentally. In this work, the effect of turbulence interactions on the noise signatures and aerodynamic loading of propellers were investigated using a turbulence-generating passive grid with a solidity ratio of 0.35 and a 3D printed NACA 0012 airfoil wing wake. These turbulence generating methods aim to simulate the fluctuating gusty wind in flight and airframe installation effect, respectively. The experiments were conducted at forwarding flight configuration with freestream inflow velocity ranging from 8 m/s to 24 m/s. Six-axis aerodynamic load measurements and far-field acoustic pressure measurements were performed simultaneously. The results show an increasing trend of thrust energy spectra for propeller operating with turbulence interactions in relative to clean laminar inflow. The acoustic spectrum of the propeller is rich in discrete tones and broadband noise. The results indicate that at a frequency band above 1000 Hz, an increase of noise spectra by about 2 dB/Hz for grid turbulence and around 5 dB/Hz to 15 dB/Hz for wing wake cases were recorded. The overall sound pressure level directivity appears to be less affected by the turbulence ingestion at a low rotational speed. However, at higher RPM, the results demonstrate enhanced radiation downstream.

The characteristics of aerodynamic noise and thrust of an isolated tilt-rotor system were experimentally investigated at hover and multiple tilting positions of the rotor disc. In this study, the effects of rotational speed, freestream flow velocity, and tilt angle were examined in an anechoic wind tunnel. Load measurements and far-field acoustic pressure measurements were performed in static condition and under freestream velocities ranging from 8 m/s to 24 m/s. Tests were conducted at a rotor diameter-based advance ratio between 0 and 0.3, and at rotor shaft tilt angle spanning from 0°to 30°. The aerodynamic coefficients, the sound power spectral density and the sound pressure level were studied and discussed. The results demonstrate dependency between the loading and noise signatures to the rotor tilting position. At a settings of rotation speed and freestream inflow velocity, the results show a significant noise attenuation at a higher tilting angle while maintaining a similar directivity of far-field noise radiation. However, at the same setting of rotational speed and inflow velocity, the aerodynamic thrust generating capability of the rotor is adversely affected by the rotor disc tilting angle.

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