Aerodynamic Interference and Reynolds Number Effects of Low-Speed Close-Coupled Biplanes

Modern aircraft design, given the trend to achieve smaller sizes and higher altitudes, urgently needs to introduce new configurations to achieve higher aerodynamic efficiency within a range of moderate Reynolds numbers (Re). The present study, which focuses on low-speed close-coupled biplanes, discovers a new mechanism that could increase the overall maximum lift-to-drag ratio of the configuration. This paper proposes a combination of geometrical parameters that achieves good aerodynamic performance through a set of constructive wing interferences of close-coupled NACA4412 biplanes. With this combination of parameters, the maximum lift-to-drag ratio of a biplane is improved by 3.69% relative to the maximum summative lift-to-drag ratio of two independent monoplanes at Re=3x106. Moreover, the maximum lift-to-drag ratio of a biplane is enhanced by 3.99% relative to that of a single monoplane and is 10.26% higher than the maximum summative lift-to-drag ratio of two independent monoplanes at Re=3x105. This paper studies three effects of the close-coupled wing interference. First, the upward wash effect on the upper wing induced by the lower wing is investigated. Second, the forward and upward pushing effects on the upper wing at the high pressure zone around the stagnation point of the lower wing is studied. Third, the pressure distribution melioration along the upper surfaces of both wings contributed by the positive pressure gradient zone near the narrow flow path between the upper and lower wings is examined. These effects collectively guarantee a high lift-to-drag ratio of a close-coupled biplane. The concept of the total pressure boundary layer is proposed to analyze the effects of the Reynolds number. The enhancement of the third effect is found to lead to a biplane's better performance over that of a monoplane when Re decreases.