What is s-series airfoil?

The "S-series" airfoil designations are a family of airfoil shapes developed by Thomas A. Aeronautical Engineering professor for high-speed applications. These airfoils are optimized for low Reynolds number conditions, which are common in unintentional stall conditions. The airfoils have a maximum thickness at the 30% chord and are well-suited for gliders, sailplanes, and other high-performance aircraft.