Effects of Leading-Edge, Suction-Surface Roughness on Propeller Performance for Unmanned Aircraft Applications

Abstract This paper presents a study of roughness on the leading-edge, suction-surface of a propeller intended for unmanned aircraft applications, evaluating the effect on thrust coefficient and propeller efficiency. The motivation for this study is to understand the potential impact of ice accretion on propeller performance relevant to unmanned aircraft flight conditions, occurring at low Reynolds numbers. Under such conditions, the propeller can be vulnerable to boundary layer separation at local spanwise locations. The propeller test article used in this study has a 16-in (406mm) diameter and 10-in (254mm) pitch. Surface roughness was simulated with protruding domes, having height and spacing based on blade element chord length. The area covered by the field of domes varied by spanwise and chordwise extents to determine correlations between propeller performance and surface roughness area. The spanwise extent begins at 30% of the propeller span. Experiments were conducted in a low-subsonic wind tunnel with a 3-ft (1m) by 3-ft (1m) test section and propeller dynamometer instrumented for torque, thrust and shaft speed. Initial experiments were conducted with a stock, unmodified propeller to compare baseline experimental and analytical results. Propeller test articles were fabricated using a resin 3D printer such that the structural stiffness and surface finish differed slightly from the stock propeller. A baseline, fabricated test article without roughness was tested for comparison with the stock propeller. Subsequent fabricated test articles with roughness were compared to the baseline fabricated test article to account for material and manufacturing influences. The experimental procedure varied airspeed from 20-ft/s (6-m/s) to 50-ft/s (15-m/s) and rotational speeds from 3000-RPM to 6000-RPM to span a Reynolds number operating range. Results show propeller thrust coefficient and efficiency relative to advance ratio. In some cases, surface roughness had a positive influence on propeller performance. A numerical study was conducted to visualize surface roughness flow field interactions to explain experimental results. Observations from this study provide insight into effects of surface roughness, such as ice, on propeller performance. Conclusions from this evaluation inform unmanned aircraft operators of potential effects on low flight speed performance in adverse weather.