The design of airfoils at low Reynolds numbers

This paper focuses on the design of airfoils at low Reynolds numbers (100.000~RN~500.000), specifically those applicable to radio-controlled model sailplanes. Two common types of airfoil lift and drag hysteresis are illustrated and explained in terms of the behavior of the upper-surface transitional separation bubble which is commonly present at these low Reynolds numbers. The theoretical section characteristics of several airfoils predicted by the Eppler computer program for the design and analysis of low-speed airfoils were compared with the recent experimental data of Althaus. Good correlation was found between the type of hysteresis and the general character of the pressure distribution. Also, the validity of the predicted section characteristics is discussed for this low Reynolds number regime. From the comparisons the desirable qualities of a low Reynolds number airfoil were determined. Based on this several airfoils for radio-controlled model sailplanes were subsequently designed and analyzed using the Eppler computer program.