Closed wing

A closed wing is a wing that effectively has two main planes that merge at their ends so that there are no conventional 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 associated with wingtip vortices that occur at the tips of conventional wings. Although the closed wing has no unique claim on such benefits, many closed wing designs do offer structural advantages over a conventional cantilever monoplane.

Characteristics

Wingtip vortices form a major component of wake turbulence and are associated with induced drag, which is a significant contributor to total drag in most regimes. 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 of fixed horizontal (spanwise) and vertical dimensions as viewed in the freestream flow direction, 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 with lifting surfaces fully occupying all four boundaries of the allowed rectangular area. However, the induced-drag performance of the ideal closed box wing can be approached very closely by open configurations such as the C-wing discussed below.

For any lifting system (or portion of a lifting system) that forms a closed loop as viewed in the freestream flow direction, the optimum lift (or circulation) distribution that yields the minimum induced drag for a given total vertical lift is not unique, but is defined only to within a constant on the closed-loop portion.