Evaluation of Propeller Self-Aspirated Flow Controls to Suppress Boundary Layer Separation at Low Reynolds Number

Abstract This paper presents experimental results for propellers with self-aspirated, jet-blowing flow controls to suppress boundary layer separation at low Reynolds number operating conditions associated with low-speed, unmanned aircraft. The motivation is to improve aircraft range and endurance by reducing propeller wake losses and thereby increasing propeller efficiency. Test articles were produced using computer aided design drawings based on commercially available propellers and fabricated with a resin 3D printer. Each propeller was integrated with novel flow controls consisting of either an array of circular jets or a singular rectangular jet acting as self-aspirated flow controls, channeling flow through the blade from the pressure side to the suction side. A preliminary study was conducted with a scaled airfoil based on the mid-span of a 10-in diameter and 5-in pitch propeller, modified with circular self-aspirated vortex generator jets. The airfoil was evaluated in a closed-loop, 11.8-in by 11.8-in by 39.8-in water tunnel test section with chord-based Reynolds number ranging from 15K to 90K at 0-deg to 30-deg angles of attack. Each experimental configuration was characterized by the velocity field evaluated using particle image velocimetry. Subsequent wind tunnel tests with 16-in by 10-in propellers, one with a rectangular jet fabricated at 20-deg and the other at 40-deg relative to the propeller suction surface. Experimental data was collected from a propeller dynamometer in a 3-ft by 3-ft wind tunnel test section at Reynolds numbers ranging from 30K to 60K based on propeller chord at 75% span, corresponding to 20-, 30-, 40- and 50-ft/s freestream velocities. Thrust coefficient and propeller efficiency at varying advance ratios are compared to baseline unmodified propeller to assess propeller performance. This study revealed that self-aspirated flow controls at or just upstream of the expected separation chord location on a propeller are effective suppressing boundary layer separation under low Reynolds number conditions.