Closed wing
A wing without conventional tips, reducing induced drag.
A closed wing is a design where two main wing surfaces connect at their ends, eliminating conventional wingtips. Examples include the annular (or ring) wing, joined wing, box wing, and spiroid tip devices. Like other wingtip modifications, it aims to reduce the energy losses caused by wingtip vortices, which form at the tips of standard wings.
Wingtip vortices are a major source of wake turbulence and contribute to induced drag, a significant part of total drag in many flight conditions. By removing wingtips, a closed wing could potentially lower this drag. Beyond possible structural benefits over open cantilevered wings, closed wings have unique aerodynamic traits.
For a lifting system confined to a rectangular box of fixed span and height, the configuration that produces the least induced drag for a given total lift is a closed one—a box wing filling all four sides of that rectangle. However, open designs like the C-wing can come very close to matching this performance. For any closed-loop lifting system, the optimal lift distribution for minimum induced drag is not unique; a constant circulation can be added to the loop without changing total lift or induced drag. This explains why a C-wing can achieve nearly the same drag reduction as a fully closed system. Ultimately, while closed systems can greatly reduce induced drag compared to a conventional planar wing, there is no unique aerodynamic advantage to being closed rather than open.
Various closed wing configurations exist, including the box wing, rhomboidal wing, flat annular wing, and concentric wing and fuselage.
One early example was the Blériot III, built in 1906 by Louis Blériot and Gabriel Voisin, which had two annular wings mounted one behind the other. The later Blériot IV replaced the front annular wing with a biplane and added a canard foreplane, managing short hops before being destroyed. Based on G.J.A. Kitchen’s work, Cedric Lee and G. Tilghman Richards built and flew several annular-wing aircraft, starting with a biplane and followed by monoplanes, the last of which remained in use until 1914.
In 1944, German designer Ernst Heinkel began work on the Lerche, an annular-wing VTOL fighter, but the project was soon dropped. During the 1950s, the French company SNECMA developed the Coléoptère, a single-person VTOL annular-wing aircraft that proved dangerously unstable and was abandoned.
- First known aircraft
- Blériot III (1906)
- Designers of early annular wing
- Cedric Lee and G. Tilghman Richards
- Notable postwar program
- SNECMA Coléoptère (1950s)
- Fuel reduction claimed by spiroid wingle
- over 10%
- First flight of flynano nano
- 11 June 2012
- Only crewed annular closed wing aircraft
- OW-1 (2007)
Lore & Background
Wingtip vortices form a major component of wake turbulence and are associated with induced drag. A closed wing avoids the need for wingtips and thus might be expected to reduce wingtip drag effects. In addition to potential structural advantages over open cantilevered wings, closed wing surfaces have some unique aerodynamic properties: for a lifting system constrained to fit within a rectangular box, the configuration that provides the absolute minimum induced drag for a given total vertical lift is a closed system, i.e. a rectangular box wing. However, the induced-drag performance of the ideal closed box wing can be approached very closely by open configurations such as the C-wing.
An early example of the closed wing was on the Blériot III aircraft, built in 1906 by Louis Blériot and Gabriel Voisin. The later Blériot IV replaced the forward annular wing with a biplane and added a canard foreplane. Based on the work of G.J.A. Kitchen, Cedric Lee and G. Tilghman Richards built and flew several annular-wing aeroplanes. In 1944, the German designer Ernst Heinkel began working on an annular-wing VTOL multirole single-seater called the Lerche, but the project was soon abandoned. During the 1950s, the French company SNECMA developed the Coléoptère, a single-person VTOL annular wing aircraft, which proved dangerously unstable. The Belarusian built OW-1 experimental aircraft has since its maiden flight in 2007, been the only crewed annular closed wing aircraft to have successfully maintained stable horizontal flight.
Reader's Guide
The closed wing remains 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 that would benefit most from increases in efficiency have yet to be overcome. Although closed systems can produce large induced-drag reductions relative to a conventional planar wing, there is no significant aerodynamic advantage that uniquely accrues to their being closed rather than open. The C-wing, a theoretical configuration in which much of the upper centre section of a box wing is removed, can achieve very nearly the same induced-drag performance as a corresponding box wing. The Spiroid winglet, a design currently under development by Aviation Partners, is a closed wing surface mounted at the end of a conventional wing, and the company announced that the winglets fitted to a Gulfstream II reduced fuel consumption in the cruise phase by over 10%. The Prandtl box wing, suggested by Ludwig Prandtl in 1924, is claimed to be theoretically efficient for wide-body jet airliners, potentially allowing even larger aircraft to use current infrastructure. The closed wing is also used in water, for surfboard fins of the type also known as the tunnel fin.
Did You Know?
- The Blériot III, built in 1906, was an early example of a closed wing aircraft with two annular wings mounted in tandem.
- The Spiroid winglet, a closed wing surface, reduced fuel consumption on a Gulfstream II by over 10%.
- The OW-1 experimental aircraft, first flown in 2007, is the only crewed annular closed wing aircraft to have achieved stable horizontal flight.
- A closed box wing can provide the absolute minimum induced drag for a given total vertical lift when constrained to a rectangular box.
More in Aircraft Components, Part 3 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced