Planform
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In aviation, a planform is the shape and layout of an airplane'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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Low-speed planforms
The primary concern in low speed flight is the aspect ratio, the comparison of the length of the wing measured out from the fuselage, span, compared to the length from front to back, chord. Wings with higher aspect ratios, that is, wings that are longer and skinnier, have lower drag for any given amount of lift than a wing of the same area that is shorter and fatter. This is due to an effect known as induced drag, caused by airflow over the tip of the wing. As the size of the tip decreases compared to the wing's overall size, the magnitude of the induced drag is reduced.
This is why gliders, which want as low a drag as possible, use very long and skinny wings. In a general sense all planes would want to use wings like those on a glider, but practical problems intrude. The primary issue is strength; it is simply harder to build a longer wing than a short one.
There are other ways to reduce induced drag as well, mostly by changing the shape of the wing to reduce the size of the tip. The elliptical wing found on the Supermarine Spitfire and Republic P-47 is by demonstration, the most efficient, but difficult to build. A practical and simple compromise is to taper the wing towards the tip, a feature that can be found on almost all modern aircraft (including gliders).
High-speed planforms
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 in order 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 the "perfect" high speed planform is difficult for practical reasons. In this case a very thin wing makes it difficult to use the internal room for storage of fuel and landing gear, as well as making the wing considerably less stiff torsionally as well as leading to 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 "fool" the air into thinking it is flowing over a thinner wing with more chord. Other designs simply made as thin a wing as possible, leading to designs like the F-104 Starfighter.