Ducted Co-Rotating Rotors As An Alternative For UAV Propulsion: An Experimental Aeroacoustic Study

Abstract

The coaxial, co-rotating configuration of rotors, also referred to as stacked rotors, is a promising alternative to counter-rotating rotors, simpler and lighter mechanically, while still providing improved disk loading relative to an isolated rotor. Ducting, on the other hand, is commonly used on rotorcraft for improved efficiency and safety, as well as some reduction of noise emissions. In this work, the two solutions are combined. Aeroacoustic performance of ducted stacked rotors is evaluated experimentally in an anechoic chamber, and compared to similar cases of an isolated rotor, a single ducted rotor, and unducted stacked rotors. Although changes to blade geometry necessitated by the presence of a duct prevent testing of truly analogous cases, an attempt is made at a fair comparison between the different configurations. The results show similar efficiency of the ducted stacked configuration relative to a single ducted rotor, but with an apparent detriment to the acoustic performance. Compared to unducted stacked rotors, the ducted configuration provides improved efficiency and similar noise levels. Furthermore, a range of blade pitch angles, with varying increments between the upstream and downstream rotors, is tested for the ducted stacked rotors. The impact of dissimilar pitch between the rotors on the aeroacoustic performance of the system is small but distinct, with certain pitch configurations generating less noise than others. This suggests that there is significant potential in optimising rotor blades specifically for coaxial operation.