Acoustic optimization of a contra-rotating propeller rig

Abstract

With the advance of unmanned aerial vehicles as a viable alternative for urban transportation, it is imperative that these vehicles have a low impact on community noise. Contra-rotating propeller technologies offer interesting advantages over traditional single-propeller equivalents, such as improved aerodynamic performance and redundancy in case of failure, but can be exceedingly noisy when not carefully designed. This work presents a parametric study for the acoustic optimization of a custom-made contra-rotating propeller rig. The rig consists of two commercially available motors mounted on a rotating stand built in-house and mounted inside an anechoic chamber. A far-field microphone arc is used to collect acoustic pressure data, and a load cell is used to measure total thrust. We investigate the effect of variations in the number of blades (between 2 and 6 on both rotors) and axial spacing (between 0.1 and 1 rotor diameter) on the radiated noise, while adjusting the rpm to maintain constant thrust. Overall sound pressure levels, far-field spectra, and directivities (overall SPL, tones, and broadband) are assessed to determine trade-offs and optimal choices in designing a contra-rotating propeller rig.