The influence of propeller slipstream on lift and drag of deflected trailing edge flaps was previously described by this author in SAE paper 960C, as applied to the Breguet 941 STOL. The method calculating transition velocities proved to be very accurate. Recently the Ames Research Center conducted tests of a four propeller model, a close replica of the Breguet, applying the same method and obtaining very accurate results. This paper gives detailed analysis of how the method can determine CL and CD under transition conditions. Designers of STOL aircraft where thrust approaches large values compared with gross weight will find the procedure useful.