Closed wing

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.

Closed wing surfaces feature several structural and aerodynamic properties. The boxplane may achieve minimal 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.

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