Aerodynamic and Acoustic Characterization of a Ducted Propeller in a Small Hover Anechoic Chamber

This study investigates the aerodynamic performance and acoustic characteristics of a sub-scale ducted propeller relative to an open propeller of common geometry for static hover and low-speed axial flow conditions. The propeller test article is a four-bladed modified optimum hovering rotor, while the duct consists of a blended geometry between a referenced leading edge design and a NACA 0018 trailing edge. Low-fidelity modeling tools are leveraged to provide a predictive comparison for both isolated and ducted propeller systems. The ANOPP-PAS code was found to model the fundamental acoustic blade passage frequency directivity of the isolated propeller quite well, with adjustments made to the simulation case to account for
additional torque loading believed to be due to “over-tripping” of the blades. Both momentum theory and the Ducted Fan Design Code yielded good agreement with the measured thrust levels of the ducted propeller system in hover, with the latter prediction method showing good agreement for low-speed axial flow conditions as well. These good agreements, however, were limited to the higher tested propeller
rotation rates. Overall, the ducted propeller exhibited slightly improved propulsive efficiency in hover, and poorer efficiency in low-speed axial flow relative to the isolated propeller. Acoustically, the ducted configuration was seen to exhibit acoustic shielding at the propeller blade passage frequency, however an increase in higher blade passage frequency harmonics was also observed. There was also an increase
in measured broadband noise associated with the incorporation of the duct, which was more prevalent for the hover operating conditions.