Reynolds Number Effects on the Performance of Small-Scale Propellers

Abstract

The growing use of small UAVs and MAVs has increased the interest in determining the performance of small-scale propellers that power them. These small-scale propellers operate at much lower Reynolds numbers than propellers for larger aircraft; typical Reynolds numbers for small-scale propellers are less than 100,000 based on the velocity and chord at the 75% span station. The low Reynolds numbers of the propellers make predicting the performance of these propellers difficult as the performance of the propellers generally change with Reynolds numbers. In order to understand the Reynolds number effects on small-scale propellers, the performance data of twenty-seven off-the-shelf propellers and four 3D-printed propellers were measured in both static and advancing-flow conditions. The diameters of these propellers ranged from 2.25 in to 9 in. From these tests, the performance of the propellers was shown to improve as the Reynolds number increased. This improvement was seen as either an increase in the thrust coefficient, a decrease in the power coefficient, or both. For propellers in an advancing flow, performance improvement was also seen as an increase in the propeller efficiency. Results from some of the propellers tested showed an efficiency improvement of around 10%. Nevertheless, the maximum efficiency achieved by the small-scale propellers is still low when compared to typical values of larger propellers.