Calibration-related pseudo-Reynolds number trends in transonic wind tunnels

Until the mid-seventies there were many variable density transonic wind tunnels which had been calibrated at only one total pressure. Today this still holds for some tunnels. If Reynolds number, i.e., total pressure, is varied in such tunnels during a test, small systematic errors in freestream Mach number M0 do then result. In M0-sensitive measurements this causes exaggerated (pseudo) Reynolds number trends. For the examples of afterbody pressure drag, transonic shock location, and transonic maximum lift, it is shown that up to 100% of these trends can be attributed to the lack of a full Reynolds number calibration. The remaining few percent must be due to true Reynolds number effects and to additional, yet unknown, systematic errors.