and landed vertically, but never achieved transition to horizontal flight. Examples with the wing mounted on top of the fuselage have been proposed but never built. Annular (planar): the wing is shaped like a disc with a hole in it. A number of Lee-Richards annular monoplanes flew shortly before the First World War. Joined wing: a tandem-wing layout in which the front low wing sweeps back and/or the rear high wing sweeps forwards such that they join at or near the tips to form a continuous surface in a hollow diamond or triangle shape. The Ligeti Stratos is a rare example. Rhomboidal wing: a joined wing consisting of four surfaces in a diamond arrangement. The Edwards Rhomboidal biplane of 1911 had both wings in the same plane but failed to fly. Wings can also be characterised as: Rigid: stiff enough to maintain the aerofoil profile in varying conditions of airflow. A rigid wing may have external bracing and/or a fabric covering. Flexible: The surface may be flexible, typically a thin membrane. Requires external bracing and/or wind pressure to maintain the aerofoil shape. Common types include the Rogallo wing, parafoil and most kites. An otherwise rigid structure may be designed to flex, either because it is inherently aeroelastic as in the aeroisoclinic wing, or because shape changes are actively introduced. Wing planform The wing planform is the silhouette of the wing when viewed from above or below. See also variable geometry types which vary the wing planform during flight. Aspect ratio The aspect ratio is the span divided by the mean or average chord. It is a measure of how long and slender the wing appears when seen from above or below. Low aspect ratio: short and stubby wing. Structurally efficient, high instantaneous roll rate, low supersonic drag. They tend to be used on fighter aircraft, such as the Lockheed F-104 Starfighter, and on very high-speed aircraft including the North American X-15. Moderate aspect ratio: general-purpose wing, very widely used, for example on the Douglas DC-3 transport. High aspect ratio: long and slender wing. More efficient aerodynamically, having less induced drag at subsonic speeds. They tend to be used by high-altitude subsonic aircraft such as the Lockheed U-2 spy plane and by high-performance sailplanes such as the Glaser-Dirks DG-500. Most variable geometry configurations vary the aspect ratio in some way, either deliberately or as a side effect. Chord variation along span