Glide ratio
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Glide ratio is an aviation term that refers to the distance an aircraft will move forward for any given amount of lost altitude (the cotangent of the downward angle).
<math>{L \over D}={{\Delta h} \over {\Delta s}}={v_{h}\over v_{hor}}<math>
For instance, general aviation aircraft like the Cessna 150 will move forward 7 feet for every foot of altitude lost, resulting in a glide ratio of 7:1. A modern sailplane may have a glide ratio as high as 60:1, with fabric hang gliders averaging about 10:1 and airliners about 25:1.
The glide ratio of an aircraft is often strongly correlated to the efficiency of the wing, known as the lift-to-drag ratio. Overall drag is also important, for instance a cargo plane will typically have a lower glide ratio than a passenger plane, but in generally these differences are a smaller effect than the wing itself.
Importance of the glide ratio in gliding
Although the best glide ratio is important when measuring the performance of a glider, its ability to achieve a good glide ratio at high speed determines its success when racing (see article on gliding).
The loss of height can be measured at several speeds and plotted on a "polar curve" to calculate the best speed to fly in various conditions, such as when flying into wind or when in sinking air. Other polar curves can be measured by loading the glider with water ballast. When ballast is carried, the best glide ratio is achieved at higher speeds.