A closed wing is a wing that effectively has two main planes which 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.[1]
Like many wingtip devices, the closed wing aims to reduce the wasteful effects associated with wingtip vortices which 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.