Hans,

Dr. Selig's airfoils are typically geared towards low speed, low reynolds number flight such as that found in RC aircraft. If you aren't familiar with reynolds number, it's effectively the vicosity of air, and with the size wings we typically run and the speed at which they fly, the wings are prone to stall earlier than normal due to something called laminar seperation bubbles. NACA series airfoils were designed for larger chord wings at higher speeds in which these seperation bubble do not occur. Hope this helps a bit.

Wallis