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A closed wing is a non-planar wing that uses a continuous surface, eliminating the wing tips. Closed wing designs include the annular wing (commonly known as the cylindrical or ring wing), the joined wing, the box wing and spiroid tip structures.〔 A closed wing can be thought of as the maximum expression of a wingtip device, which has the aim of eliminating the influence of the wingtip vortices which occur at the tips of conventional wings. These vortices form a major component of wake turbulence and are associated with induced drag, which negatively affects aerodynamic performance in most regimes. A closed wing surface has no wingtips whatsoever, and thus is capable of greatly reducing or eliminating wingtip drag, which has great implications for the improvement of fuel efficiency in the airline industry. ==Performance benefits== Closed wing surfaces exhibit a number of interesting structural and aerodynamic properties. The boxplane achieves the minimum possible induced drag for a given lift, wingspan, and vertical extent. Annular and joined wings can achieve span efficiencies greater than 1, and the annular wing exhibits half the vortex drag of a monoplane wing of the same span and lift.〔 〕 However, the concept of eliminating the influence of tip vortices through use of closed wings is an ill-conceived notion, according to Dr. Ilan Kroo, Professor of Aeronautics and Astronautics at Stanford University. There appears to be no particular advantage to a fully closed design; despite a decrease in local loading on any given point on the wing, the circulation is constant, thereby causing no change in the wake, and thereby the lift and interference drag associated with the surface. For this reason, closed wings remain mostly confined to the realms of studies and conceptual designs, as the engineering challenges of developing a strong, self-supporting closed wing for use in the large airliners which would benefit most from increases in efficiency have yet to be overcome. The C-wing benefits from many of the drag-reducing benefits of a closed wing design without the downsides of being a fully closed system. However, the design of a new airplane configuration is a multidisciplinary optimization and other factors should be considered such as aeroelasticity, stability, other types of drag etc.〔Aldo Frediani, V K I, lecture series: "''Innovative Configurations and Advanced Concepts for Future Civil Transport Aircraft''", June 06–10, 2005〕 Thus, it is conceptually incorrect to select/discard a configuration on the basis of pure induced drag considerations only. In fact, according to the literature (see section 20, page 35 of,〔Von Karman, T. and Burgers, J.M., "General Aerodynamic Theory-Perfect Fluids", Vol II of Aerodynamic Theory, 1935, pp 201-222, Editor in Chief Durand W.F〕 see also the analytical formulas reported in〔De Young, J., "''Induced Drag Ideal Efficiency Factor of Arbitrary Lateral-Vertical Wing Forms''", NASA Contractor Report 3357, 1980〕 and the discussion on the vertical aspect ratio〔Demasi Luciano, Dipace Antonio, Monegato Giovanni, and Cavallaro Rauno "An Invariant Formulation for the Minimum Induced Drag Conditions of Non-planar Wing Systems", AIAA Journal (2014), in press〕), increasing the so-called vertical aspect ratio of the wing dramatically reduces the minimum induced drag. However, as it can be easily realized, the weight and friction drag would also increase, showing that a design based on pure induced drag considerations may provide ''erroneous'' indications without appropriately considering other relevant factors. A theoretical discussion between C-wings and corresponding closed systems is provided in Demasi Luciano, Dipace Antonio, Monegato Giovanni, and Cavallaro Rauno "An Invariant Formulation for the Minimum Induced Drag Conditions of Non-planar Wing Systems", AIAA Journal (2014), in press, where the so-called "Quasi-Closed C-Wing Minimum Induced Drag Conjecture" (QCWMIDC) is discussed. In particular, it is emphasized in Figure 14 of that work that the quasi-closed C-wing presents practically the same optimum induced drag of the corresponding closed system. In addition to this conjecture, C-wings present another relevant induced drag property:〔 the so-called Quasi-closed C-wing zero gradient optimal circulation theorem which states that ''If the two tips of a C wing are brought indefinitely close to each other, then both the optimal circulation and its first derivative tend to zero at those points''. The Quasi-closed C-wing zero gradient optimal circulation theorem is also verified in the Figure, where several non-planar wing systems are analyzed. The parameter ε is the optimal aerodynamic efficiency ratio and represents the ratio between the aerodynamic efficiency of a given non-planar wing and the corresponding efficiency of a reference classical cantilevered wing with the same wing span and total lift. Both efficiencies are evaluated under their respective optimal conditions. For a classical cantilevered wing it is ε = 1. Back to closed systems, the closed wing concept is also used in the water medium, in surfboard fins also known as tunnel fins. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Closed wing」の詳細全文を読む スポンサード リンク
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