Planform

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This article is about the term as used in aviation. For its general use, see Plan view.

In aviation, a planform is the shape and layout of a fixed-wing aircraft's wing. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military aircraft and airliners.

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Straight wing
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Swept wing
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Variable sweep wing
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Delta wing (tailless)

Low-speed[edit]

A Spitfire built in 1945 shows off its elliptical planform

Some of the primary concerns in optimizing a wing for efficient flight are area, span, and taper ratio. Span and area are often expressed together as aspect ratio, which is, roughly, the ratio of the wingspan, measured from tip to tip, to the average chord, measured from leading edge to trailing edge. The drag of a wing consists of two components, the induced drag, which is related to the production of lift and diminishes as wingspan increases, and the parasite drag, largely due to skin friction, which is proportional to the area. It is therefore desirable for a wing to have the least area compatible with the desired landing speed, combined with the greatest span compatible with requirements of maneuverability, structural stiffness, ground handling and so on. A high aspect ratio, which implies a small area combined with a large span, is therefore a rough indicator of high aerodynamic efficiency. Small, low-altitude general aviation airplanes typically have aspect ratios of six or seven; airliners of 12 or more; and high-performance sailplanes of 30 or more.

In theory, induced drag is at its minimum when the spanwise distribution of lift is elliptical. A number of factors influence induced drag, however, and as a practical matter a wing of elliptical planform, like that of the Supermarine Spitfire fighter of World War II, is not necessarily the most efficient. The planforms of the wings of jet airliners, which are highly optimized for efficiency, are far from elliptical.

The ratio of tip chord to root chord is called the taper ratio. Taper has the desirable effect of reducing the root bending stress by shifting the lift inboard, but it has been argued by some noted designers, including John Thorp and Karl Bergey, that an untapered rectangular planform is best for airplanes of less than 6,000 pounds gross weight.

High-speed[edit]

The delta-winged F-106 Delta Dart

At higher speeds nearing the speed of sound, a new form of drag appears: wave drag. Wave drag is considerably more powerful than induced drag, and must be avoided at all costs to improve performance. Doing so demands a wing that is as thin as possible, with a slowly changing profile over a wide chord. Of course, this is basically the opposite goal to low speed wings, which presents a problem.

Just as on the lower speed designs, making an ideal high speed planform is difficult for practical reasons. In this case, a very thin wing makes it difficult to use the internal room to store fuel and landing gear, makes the wing far less stiff torsionally, and causes increased induced drag when flying slower.

Solutions to this problem come in many forms, notably the use of the swept wing and delta wing, both of which avoid the shock wave at the leading edge. Other designs make the wing as thin as possible, leading to designs like the F-104 Starfighter.

Gallery[edit]

See also[edit]