An elliptical wing is a wing planform whose leading and trailing edges each approximate two segments of an ellipse. It is not to be confused with annular wings, which may be elliptically shaped.
Relatively few aircraft have adopted the elliptical wing, an even-smaller number of which attained mass production; the majority of aircraft that did use this feature were introduced during the 1930s and 1940s. Perhaps the most famous aircraft to feature an elliptical wing is the Supermarine Spitfire, a Second World War-era British fighter aircraft. Another major application was the Heinkel He 70 "Blitz", a German fast mail plane and reconnaissance bomber; early versions of the He 111 bomber also used such a wing configuration before a simpler design was adopted for economic reasons.
Properties
Theoretically, the most efficient way to create lift is to generate it in an elliptical spanwise distribution across the wing. There is no inherent superiority to pure elliptical shapes, and wings with other planforms can be optimized to give elliptical spanwise lift distributions.
The basic elliptical wing shape also has disadvantages: The almost uniform lift distribution of a constant-aerofoil section elliptical wing can cause the entire span of the wing to stall simultaneously, potentially causing loss of control with little warning. To improve the stalling characteristics and give the pilot some warning, designers use a non-uniform aerofoil. For example, the wing of the Supermarine Spitfire was both thinned towards the tips and twisted to give washout, reducing the load on the tips so that the inner wing would stall first. Such compromises depart from the theoretical elliptical lift distribution, increasing induced drag. An elliptical spanwise lift distribution cannot be achieved by an untwisted wing with an elliptical planform because there is a logarithmic term in the lift distribution that becomes important near the wing tips.
Elliptical wing planforms are more difficult to manufacture. In it, either leading edge or trailing edge or both are curved, and the ribs change in a non uniform way along the wingspan. In practice, most elliptical wings are approximations, for example several sections of the Spitfire leading and trailing edges are arcs of circles.
The semi-elliptical wing
For a wing to have an elliptical area distribution, it is not necessary for both the leading and trailing edges to be curved. If one of these is straight, as in the semi-elliptical planform, the wing may still have an elliptical area distribution. Several aircraft of this type have been produced; one of the most successful being the American Seversky P-35.
During the postwar era, the semi-elliptical wing profile was extensively studied for its ground effect properties; it was postulated that it would be suitable for ground-effect vehicles (which operate close to the water, in ground effect, to avoid the higher induced drag that occurs out of ground effect). The low level of induced drag produced by a semi-elliptical wing would be beneficial for these vehicles.
History
The British theoretical aerodynamicist Frederick Lanchester was perhaps the first person to write in detail about the elliptical wing, having done so during 1907. Ludwig Prandtl independently rediscovered this in 1917–1918. Despite this early head-start, the elliptical wing was initially viewed as more a theoretical concept than one for practical application, in part due to the overriding needs to compromise between an aircraft aerodynamic properties and its other design aspects. It would be quite some time before practical use of the planform would be made.
The first aircraft to use the elliptical wing was the Bäumer Sausewind, a German light sports aircraft that performed its maiden flight on 26 May 1925. Its designers, the Günther brothers, would subsequently join the German aircraft manufacturer Heinkel and apply their designs, including the elliptical wing, to several different projects undertaken by the firm. During the early 1930s, Heinkel developed a fast mail plane and reconnaissance bomber, the Heinkel He 70 "Blitz", which featured the elliptical wing. It proved to have excellent performance for the era, establishing eight individual world records relating to speed over distance, having reportedly attained a maximum speed of 377 km/h (222 mph).
