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Aspect ratio

Aspect ratio is the ratio of width to height. It is used in a number of fields, but is most commonly used when talking about aircraft or images.

Aspect ratio for images

In imaging terms, aspect ratio is the width of the image divided by its height (usually expressed as "x:y"). For instance, the aspect ratio of a non-HDTV television screen is 4:3, or 1.33:1. The aspect ratio of a standard 35mm film frame is around 1.35:1, although cameramen sometimes use only the part of the frame which will be visible on a television screen. Letterboxed videos are frequently in 16:9 aspect ratio.

Within the motion picture industry, the convention is to assign a value of 1 to the image height, so that, for example, a Cinemascope frame is described as 2.35:1 or just "2.35". This way of speaking comes about because the width of a film image is restricted by the presence of sprocket holes and, usually, an optical sound track[?] on the projection print[?]. Development of various camera systems therefore centers on the placement of the frame in relation to these lateral constraints; the height of image can be adjusted freely, so the ingenuity goes into getting different widths. One clever widescreen process, VistaVision[?], used standard 35mm film running sideways through camera gate, so that the sprocket holes were above and below frame and the width was not restricted. The most common projection ratios in American theaters are 1.85 and 2.35.

The term is also used in the context of computer graphics to describe the shape of an individual pixel in a digitized image. Most digital imaging systems use square pixels--that is, they sample an image at the same resolution horizontally and vertically. But there are some devices that do not, so a digital image scanned at twice the horizontal resolution to its vertical resolution might be described as being sampled at a 2:1 aspect ratio, regardless of the size or shape of the image as a whole.

See also: motion picture terminology, widescreen, pan and scan

Aspect ratio for aircraft

In aircraft terms, the aspect ratio is the span of the airplane's wing divided by its standard mean chord (SMC). It can be calculated more easily, however as span squared divided by wing area:

AR = b2/S

Aspect ratio is a powerful indicator of the general performance of a wing. Much of the drag created by a wing, notably at low speeds, comes from an effect called induced drag[?] which occurs at the wing-tips. High pressure air on the bottom has a "short cut" to the low-pressure air on top by circling around the tip of the wing. This causes a vortex to form, which robs the plane of energy. By reducing the amount of wing tip area, making it skinny or pointed for instance, you reduce the amount of energy lost to this process, and thus reduce effective drag. This is why high performance gliders have very long, skinny wings; they cannot compensate for induced drag by increasing engine power.

Why don't all aircraft have high aspect-ratio wings? There are two main reasons:

  1. Structural: the deflection along a high aspect-ratio wing tends to be much higher than for one of low aspect ratio, thus the stresses and consequent risk of fatigue failures are higher - particularly with swept-wing designs.
  2. Maneuverability: a high aspect-ratio wing will haver a lower roll rate[?] than one of low aspect ratio, due to higher drag and greater moment of inertia, thus rendering them unsuitable for fighter aircraft.



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