Validation of Natural Laminar Fin at Flight Reynolds Number

A natural laminar flow design was applied to the vertical tail of a high-subsonic, narrow-body aircraft to reduce carbon dioxide emissions. Its effectiveness was experimentally validated under flight Reynolds number conditions in the European Transonic Windtunnel. A total of 116 test cases were conducted to measure five-component aerodynamic forces, structural deformations, surface pressure distributions, and boundary-layer transition locations. Structural deformations were found to be negligible, and the measured aerodynamic forces and pressure distributions showed good agreement with numerical simulations. Transition locations, determined using temperature-sensitive paint, varied consistently with Reynolds number. At the design point---Mach number 0.78 and Reynolds number based on the mean aerodynamic chord of 23.6 million---approximately 30 % of the vertical tail surface of the wind tunnel model was confirmed to be laminar. Linear stability analysis revealed that Tollmien–Schlichting instability dominated at higher Reynolds numbers and inboard regions, while crossflow instability prevailed at lower Reynolds numbers and outboard regions. Notably, crossflow instability was completely suppressed near the leading edge, as intended by the natural laminar flow design.