Efficient Low-Reynolds-Number Airfoils

Abstract

A low speed wind tunnel investigation is presented characterizing the effect of leading and trailing edge flaps on thin flat plate airfoils at low Reynolds numbers. Flap size and setting angle were varied. Tests were conducted at Reynolds numbers of 40,000, 60,000, and 80,000, consistent with the operational regime of micro aerial vehicles. Performance was contrasted against a circular arc profile and three conventional airfoils. Testing encompassed force balance and surface pressure measurement. The numeric data were complemented by on and off surface flow visualization as well as the development of an analytic prediction model. The results indicate that the deployment of leading and trailing edge flaps can significantly improve the airfoil’s efficiency—not just compared with a flat plate but also a 5% camber circular arc profile. For the tested geometries, a leading and trailing edge flap angle of 15 deg was optimal. Conventional airfoil performance degraded significantly at low Reynolds numbers, yielding efficiencies far below any of the flat plate airfoils for and 60,000. Increasing the Reynolds number to 80,000 showed a dramatic improvement in the performance of the conventional airfoils.