Flow field and acoustic assessment of twin rotors in hover conditions

Abstract The paper presents part of the experimental activities in the GARTEUR Action Group RC/AG-26 framework to study the acoustic and aerodynamic characteristics of small rotors, including the influence of the rotor-rotor interactions. Two rotors with three-bladed propellers with a diameter D=393.7 mm were tested at a constant rotational speed of 5200 RPM, for different rotating verses and geometry configurations. Two rotor configurations were assessed in hover, i.e., either a baseline comprised of one isolated rotor, or two rotors arranged side-by-side. The aerodynamic loads, flow field velocity and acoustics emissions, were investigated using a six-component load cell, Particle Image Velocimetry and microphone array measurements, respectively. The aerodynamic characterisation of the isolated rotor was performed for a variety of rotating speeds. The interference between the slipstreams due to the side-by-side rotors was studied for co-rotating and counter-rotating verses at distances between the rotor axes of d=1.02D, d=1.1D, and 1.2D. The results showed that the slipstreams of the side-by-side rotors deflect and vary with the inter-axial distance. The rotor-rotor interaction, which is found to be related to the rotor distance, affects also the acoustic emissions. A remarkable loss of thrust is observed for the closest distance d=1.02D. Regarding acoustic emissions, the overall sound pressure level increases as the rotors run in counter-rotating verses compared to the co-rotating.