# Investigation and Design of the Transonic Laminar Flow Characteristics in a Laminar Aircraft
###### Abstract  
Reducing drag is critical to aircraft design. In recent years, laminar technology has become one of the most important feasible technologies for civil aircraft drag reduction design under many design constraints. However, various factors have a certain impact on the laminar flow characteristics in the state of transonic flight. Therefore, it is necessary to deeply understand the specific effects of various flight parameters on the characteristics of laminar flow. In this paper, a parameter sensitivity analysis for a central experimental wing in a special layout aircraft was carried out to investigate its transonic laminar characteristics. Then, the airfoil of the central experimental wing of the aircraft was designed for real flight. The RANS (Reynold-averaged Navier-Stokes) method combined with the \(\gamma-\text{Re}_{\theta}\) transition model based on local variables was used. The computational approach was validated by the wind tunnel tests and analyzed by the grid independence analysis. The sensitivity mainly focuses on the transition location and the length of the laminar flow zone of the central experiment under different boundary conditions. The transonic transition was affected by a variety of interacting factors that include _FSTI_ (free stream turbulence intensity), pressure gradient, _Re_ (Reynolds number), _Ma_ (Mach number) and \(a\) (angle of attack, degree). The essence of the transition is the disruption of flow stability caused by the increase in flow entropy. Among these factors, _FSTI_ directly affects global flow stability, and the pressure gradient affects local flow stability. _Ma_ and \(a\) can indirectly affect the flow stability by changing the pressure gradient. _Re_ can control the boundary layer properties to change the flow stability, whereas its effect is easily determined by the pressure gradient. Finally, the improved design of the airfoil with the central experimental wing was conducted. The design of weak shock wave and aerodynamic load on the rear part of the airfoil can improve the aerodynamic characteristics (\(C_{L}\), lift coefficient, increases by 0.28) of the airfoil, which can reduce the load burden on the outboard wing without affecting the laminar flow characteristics of the airfoil. In the next step, cross-flow instability will be considered. t 11 October  
\(C_{p}\) (pressure coefficient) curve can be created that favors flow stabilization and delays or suppresses transitions. Laminar drag reduction technology is one of the most important feasible technologies for civil aircraft drag reduction design under many design constraints [4]. It has become one of the major study objectives of aircraft designers. The surface finish of the wing was low due to the limited capacity of early aviation manufacturing in the past century. At the same time, the antifouling and deicing technologies on the wing surface are also not mature [5]. Therefore, the laminar flow characteristics of the aircraft were not obvious. At present, laminar flow design has gradually become possible with the advancement of design technology and the manufacturing process in the aviation industry. For civil aircraft, NLF (natural laminar flow) technology can maintain a laminar flow state with about 60% of the chord length on the airfoil at low speeds (around Mach Number 0.2) and with about 40% of the chord length on the airfoil at high speeds (around Mach Number 0.78). If NLF technology is adopted by the whole aircraft, the frictional drag can be reduced by about 30%, and the drag of the whole aircraft can be reduced by more than 15% [6; 7; 8]. This aerodynamic benefit is very obvious and more economical. Moreover, the reduction in fuel consumption greatly reduces carbon emissions, which is conducive to reducing air pollution. NASA (National Aeronautics and Space Administration), DLR (Deutsches Zentrum fur Luft- und Raumfahrt), and ONERA started research on NLF technology in the 1980s. They achieved a series of results that have been applied to Boeing/757 and A320 [9; 10; 11].