Natural Laminar Flow at Cruise Mach Number 0.78: First Results of ETW Concept Verification Tests

Abstract

From 2014 to 2017 the German Aerospace Center DLR conducted an internal project called TuLam (Toughen up Laminar Technology). Within the frame of this project 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. Of course, due to the low leading-edge sweep, attachment line transition and cross-flow instability growth could successfully be controlled 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. Because the wing features a slotted Krueger as leading-edge high-lift device, the lower surface is considered full chord turbulent. In order to overcome isobar de-sweeping in the wing-body junction, a customized belly fairing was developed [3]. In order to verify the NLF-FSW concept, a clean wing wind tunnel model was derived from the final wing-body configuration and subsequently tested in the European Transonic Windtunnel. 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 procedure of ETW. Reynolds number was 16.0 Mio for all polars with transition line detection by employing the temperature sensitive paint (TSP) technique. In order to minimize the occurrence of turbulent wedges that are induced by small particles in the tunnel, Reynolds number was chosen to be only two third of the design Reynolds number of ReD = 24.0 Mio. Therefore, measured drag polars have to be corrected for free flight conditions.