Within the frame of the DLR internal Project TuLam a short and medium range transport aircraft with a natural laminar flow forward swept wing (NLF-FSW) was designed, [1]. Top level aircraft requirements were taken from the reference aircraft Airbus A320, in particular the design Mach number of MD = 0.78 was kept. The basic idea then was to exploit the beneficial effect of a combination of forward sweep and taper which leads to a wing with low leading-edge sweep (-17° compared to +27 for an A320), while in the recompression zone the sweep is the same as for the backward swept one. Due to the low leading-edge sweep, attachment line transition and cross-flow instability growth can be controlled solely by natural means, i.e. contour shaping. Hence, it was possible to design a wing with the transition line on the upper surface located approximately at the shock position of 65% chord without additional wave drag penalties. In order to verify the NLF-FSW concept, a wind tunnel model was derived from the final wing-body configuration and subsequently tested in the European Transonic Windtunnel ETW, see Fig. 1 (left). Polars were measured at design as well as off design conditions, with the Mach number ranging from low-speed at M = 0.2 to high-speed at M=0.82. All polars were run with transition free on the upper surface as well as transition fixed at approximately 5% chord. Transition detection was performed at selected polar points using the temperature step technique (typical result see Fig 1). However, Reynolds number for all polars was with 16. Mio only 2/3 of the free flight value in order to prevent turbulent wedges induced by leading edge contamination from small particles in the tunnel as far as possible. Therefore, measured drag polars have to be corrected for free flight conditions; the respective methodology will also be presented here.