Experiments of Propeller-Induced Flow Effects on a Low-Reynolds-Number Wing

Abstract

Novel findings are discussed in this paper that will be especially beneficial to designers and modelers of small-scale unmanned air vehicles and high-altitude long-endurance vehicles that both operate at low Reynolds numbers (). Propeller-induced flow effects in both tractor and pusher configurations on a rectangular wing using the Wortmann FX 63-137 airfoil (a common low-Reynolds-number high-lift airfoil) are presented in this paper. Significant performance benefits can be found for a wing in the tractor configuration. Experiments, including trip tests and upper-surface oil flow visualization, show and verify that the propeller slipstream induces early transition to turbulent flow in the regions within the slipstream and the premature formation of a separation bubble in the regions outside the slipstream. The result is a reduction of pressure drag and an increase in lift of the wing where lift-to-drag ratios are as high as 10–12 (a maximum of 70% increase in lift-to-drag ratio from a clean wing configuration) and are measured at both low and high angles of attack up to stall (0–16 deg). Similar performance benefits are not observed in pusher configuration results where only increased local flow velocity and varying inflow angle effects are apparent. Thus, contrary to the design rules for optimal performance of wings at high Reynolds numbers, at low Reynolds numbers, a propeller in the tractor configuration exhibits significant performance improvements, especially in cruise configurations (low angles of attack), as compared with a propeller in the pusher configuration or even a clean wing.