Annular wings

A closed wing is a non-planar wing that closes back on itself, so that there are no 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 devices.

Like many wingtip devices, the closed wing aims to reduce the wasteful effects of wingtip vortices which occur at the tips of conventional wings. However such benefits are difficult to realize. Many closed wing designs do offer structural advantages over a conventional cantilever monoplane.

Wingtip vortices form a major component of wake turbulence and are associated with induced drag, which reduces aerodynamic performance in most regimes. A closed wing avoids the need for wingtips and thus might be expected to reduce wingtip drag effects.

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 any claimed aerodynamic advantages are illusory; 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. 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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