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The type of operation for which an airplane is intended has a very important bearing on the selection of the shape and design of the wing for that airplane. Planform design and other issues are discussed in this section. Planform determines the stall characteristics of the wing. Angle of incidence of the wing improves flight visibility, enhances take-off and landing characteristics and reduces drag in level flight. Wing washout helps to control the aircraft near its stall angle, the angle at which the lift coefficient of the wing drops drastically.
Planform refers to the shape of the wing as seen from directly above. Wings may be rectangular or elliptical or delta shaped. Some wings taper from wing root to wing tip, with the taper along the leading edge or along the trailing edge or, in some cases, with a taper along both edges.
The aspect ratio of a wing is the relationship between the length or span of the wing and its width or chord. It is computed by dividing the span by the average chord.
A wing, for example, that has a span of 24 feet and a chord of 6 feet has an aspect ratio of 4. A wing with a span of 36 feet and a chord of 4 feet has an aspect ratio of 9. The actual size, in area, of both wings is identical (144 sq. ft.) but their flight performance is quite different because of their differing aspect ratios.
A wing with a high aspect ratio will generate more lift and less induced drag than a wing with a low aspect ratio.
For this reason, gliders have wings with high aspect ratios.
The angle of incidence is the angle at which the wing is permanently inclined to the airplanes longitudinal axis.
Choosing the right angle of incidence can improve flight visibility, enhance take-off and landing characteristics and reduce drag in level flight.
The angle of incidence that is usually chosen is the angle of attack at which the lift-drag ratio is optimum. In most modern airplanes, there is a small positive angle of incidence so that the wing has a slight angle of attack when the airplane is in level cruising flight.
To reduce the tendency of the wing to stall suddenly as the stalling angle is approached, designers incorporate in wing design a feature known as washout. The wing is twisted so that the angle of incidence at the wing tip is less than that at the root of the wing. As a result, the wing has better stall characteristics, in that the section towards the root will stall before the outer section of the wing. The ailerons, located towards the wing tips, are still effective even though part of the wing has stalled.
The same improved stall characteristics are
achieved by the device of changing the airfoil shape from the
root to the tip. The manufacturer incorporates a wing shape at
the tip, which has the characteristic of stalling at a slightly
higher angle of attack.
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Updated: May 02, 2008