A closed wing is a non-planar wing planform concept. The term closed wing encompasses a number of designs, including the annular wing (commonly known as the cylindrical or ring wing), the joined wing, and the box wing. 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.
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.
The use of closed wings in aircraft has been explored many times in the past. The oldest known implementation of the surface 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 propellers powered by an engine mounted inside the diameter of the forward wing. The Blériot IV was a variation on this design, which replaced the forward annular wing with a canard biplane setup similar to the 1903 Wright Flyer. This aircraft was able to leave the ground in a series of small hops before being damaged beyond repair. An aircraft known as the "Kitchen Doughnut" flew in Chicago in 1911; it had two ring wings, one mounted atop the other.
George Tilghman Richards and Cedric Lee designed an annular wing monoplane. Flown by Gordon England in 1913; it crashed but a second was built. After that was wrecked, a third was completed and flown.
During the 1950s, the French company SNECMA developed the Coléoptère, a single-person VTOL annular wing aircraft. The aircraft proved dangerously unstable despite the development and testing of several prototypes, and the design was abandoned. Later proposals for closed-wing designs included the Convair Model 49 Advanced Aerial Fire Support System (AAFSS) and the 1980s Lockheed "Flying Bog Seat" concept.
Dr. Julian Wolkovitch continued to develop the idea in the 1980s, claiming it was an efficient structural arrangement in which the horizontal tail provided structural support for the wing as well as acting as a stabilizing surface.
The Spiroid winglet, a design currently under development by Aviation Partners, is a closed wing surface mounted at the end of a conventional wing. The company announced that the winglets fitted to a Gulfstream II reduced fuel consumption in the cruise phase by over 10%.
During 2011, NASA's Aeronautics Research Mission Directorate's Environmentally Responsible Aviation Project asked teams from Boeing, Lockheed Martin, and Northrop Grumman to study technologies that could help aircraft meet NASA's goal to reduce future aircraft fuel consumption by 50% compared to the aircraft of 1998. The Lockheed Martin team proposed using a box wing design along with other advanced technologies.
Several other types have been proposed but have not been built.
Proposals with concentric wing and fuselage
Proposals with the wing mounted on top of the fuselage
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