Elliptical wing

Lore & Background

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 has disadvantages: the almost uniform lift distribution of a constant-aerofoil section elliptical wing can cause the entire span to stall simultaneously, potentially causing loss of control with little warning. To improve stalling characteristics, designers use a non-uniform aerofoil; for example, the Spitfire's wing was thinned towards the tips and twisted to give washout. 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 of a logarithmic term in the lift distribution near the wing tips. Elliptical wing planforms are more difficult to manufacture, as 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's leading and trailing edges are arcs of circles.

For a wing to have an elliptical area distribution, it is not necessary for both 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.

Reader's Guide

The elliptical wing's significance lies in its theoretical aerodynamic efficiency, but its practical legacy is shaped by compromises. The British aerodynamicist Frederick Lanchester researched the elliptical wing and wrote in detail about it during 1906, though it remained more a theoretical concept than a practical application due to overriding needs to compromise between an aircraft's aerodynamic and other design aspects. Ludwig Prandtl researched it in his lifting-line theory (1917–1918). The first successful project was the Bäumer Sausewind (1925), whose designers later joined Heinkel, leading to the He 70 and early He 111. The Supermarine Spitfire, designed by Reginald Mitchell, used a semi-elliptical wing to solve conflicting requirements: thin for high critical Mach number yet thick enough to house retractable undercarriage, armament, and ammunition. Mitchell has sometimes been accused of copying the He 70's wing shape, but aerodynamicist Beverley Shenstone noted the Spitfire's wing was much thinner and had a different section. Other aircraft with elliptical wings include the Republic P-47 Thunderbolt, Aichi D3A, Mitsubishi A5M, and Hawker Tempest II. Since 2009, Swift Aircraft have been developing the Swift, a two-seater with elliptical wings.

Did You Know?

• The first aircraft to use a purely elliptical wing was the Pigott Brothers' biplane, built in 1910 and probably tested in 1912.
• The Heinkel He 70 Blitz established eight world records for speed over distance, reaching 377 km/h (234 mph).
• The Spitfire's semi-elliptical wing was skewed so that the centre of pressure was close to the main spar, preventing wing twisting.
• Only early production models of the He 111 used an elliptical wing; a simpler design was adopted for economic reasons.