During the Princeton airfoil tests, several attempts were made to design an F3B airfoil (hot thermal/duration airfoil) that out-performed the RG15, which was then among the best. During the design process at Princeton, the objective was to develop an airfoil with lower drag everywhere. In hindsight that objective is not physically possible. To improve on the RG-15 performance requires making some compromises. The question is then where should the emphasis on improving performance be placed and, in turn, where is performance less critical.
Ask F3B pilots and you will hear that they want all of it: better speed, better distance and better thermal performance. There is nothing to give up! No tradeoffs? Actually, there is. Between the distance speed and thermal speed there is a wide lift range (speed range) that is less critical and rarely used. This little used region of performance can be traded in exchange for better performance elsewhere. According to predictions and UIUC wind tunnel tests, the S7012 gives up performance in this range (between thermal and distance speeds) for improved performance where it is most needed. Also, the speed range for thermalling has been reduced. These tradeoffs lead to the following benefits. In exchange for the narrower thermal speed range, the lower thermal drag should increase duration times by 4% - a difference that can be detected by a pilot. Relative to the RG15, the S7012 has lower drag in the speed lift range of the polar. Thus, speed should be improved. Also, for distance, the new airfoil will match the performance of the RG15 in weak course conditions. Then when conditions are strong, the new airfoil might surpass the RG15 by 1-2 laps.