The original group of Selig airfoils23 was designed with the aid of both the Eppler and Somers Computer Code for the Design and Analysis of Low-Speed Airfoils20 and the experimental results of Althaus9. The approach taken was to compare the theoretical and experimental results of several airfoils, then to determine from these comparisons what factors (in the velocity distribution and boundary layer development) were needed to produce good, low-iZn airfoils. These factors were subsequently incorporated into the design of several new airfoils for RC sailplanes.

A common feature of most of the Selig airfoils is a long, gradual pressure recovery region on the upper surface called a bubble ramp. As a result of this ramp, the upper-surface transition point moves forward slowly and continuously with increasing angle of attack. This feature usually gives the polars an appear­ance more closely resembling those measured at high Rn. These are general design principles. Surprisingly, even today the details of how “gradual” and how “slowly”, and how best to achieve this are topics of research.

The performance of the Selig airfoils is often markedly different from one to the next. This is because the airfoils were designed to have a variety of char­acteristics in the hope that trends would emerge, leading ultimately to a better understanding of low-Rn airfoils. Implicit in this approach is the recognition that low-Rn airfoil design is still in the early stages of development.

The S-designation is also used by Somers; however, Selig uses four digits in the airfoil name, while Somers uses only three.