Closed wing

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A closed wing is a non-planar wing surface concept. The term "closed wing" encompasses the annular wing (better known as the ring wing), the boxplane, and the joined wing. Spiroid winglets are a closed wing surface attached to the end of a conventional wing. Closed wing designs are in a sense the maximum expression of wingtip devices, which aim to eliminate the influence of the wingtip vortices which occur at the tips of conventional wings. These vortices, which are a major component of wake turbulence, are associated with induced drag, which negatively affects aerodynamic performance in most regimes. The elimination of the aircraft's wingtips, and thus the great reduction or total elimination of wingtip drag has great implications for the improvement of fuel economy in the airline industry.

A closed wing surface achieves the minimum possible induced drag for a given lift, span, and vertical extent, and in some cases a span efficiency greater than 1. However, there is no particular advantage to the 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.

[edit] History

The Blériot III, with its two closed-surface wings
The Blériot III, with its two closed-surface wings

The closed wing surface is a concept that has been explored many times in the past. The oldest known implementation was the Blériot III aircraft, built in 1906 by Louis Blériot and Gabriel Voisin. The aircraft's lifting structure consisted of two annular wings mounted in tandem, with two tractor propellors powered by an engine mounted inside the diameter of the forward wing. The Blériot IV, a variation on this design which replaced the forward annular wing with a canard biplane setup similar to the 1903 Wright Flyer, was able to leave the ground in a series of small hops before being damaged beyond repair.

During the 1950s, the French company SNECMA developed an aircraft called the Coléoptère, a single-person VTOL design equipped with an annular wing. Despite the development and testing of several prototypes, the French were unsuccessful in their attempts to produce a safe, stable aircraft, and the design was abandoned.

The Convair Model 49 AAFSS, and an early '80s Lockheed concept, the "Flying Bog Seat" (http://i9.photobucket.com/albums/a94/WtMiller/scan0003-1.jpg) were later attempts to create a working design. However, the closed wing surface principle is misunderstood, and never gained popularity. Recently, Annular wings have been tested as surfboard fins also known as tunnel fins

[edit] Variations

Two sorts of annular wing: the plane of the ring faces the direction of travel, or the edge of the ring faces the direction of travel, with the aircraft suspended beneath, in helicopter fashion.

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