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# Glide Ratio

## Description

Gliding flight is heavier-than-air flight without the use of thrust; the term volplaning also refers to this mode of flight in animals. It is employed by gliding animals and by aircraft such as gliders. This mode of flight involves flying a greater distance horizontally than vertically and therefore can be distinguished from a simple descent like a parachute.

Although the human application of gliding flight usually refers to aircraft designed for this purpose, most powered aircraft are capable of gliding without engine power. As with sustained flight, gliding generally requires the application of an airfoil, such as the wings on aircraft or birds, or the gliding membrane on gliding possum. However, gliding can be achieved with a flat (uncambered) wing, as with a simple paper plane, or even with card-throwing. However some aircraft with lifting bodies and animals such as the flying snake can achieve gliding flight without any wings by creating a flattened surface underneath.

Glide ratio

When flown at a constant speed in still air a glider moves forwards a certain distance for a certain distance downwards. The ratio of the distance forwards to downwards is called the glide ratio. The glide ratio (E) is numerically equal to the lift-to-drag ratio under these conditions; but is not necessarily equal during other manoeuvres, especially if speed is not constant. A glider’s glide ratio varies with airspeed, but there is a maximum value which is frequently quoted. Glide ratio usually varies little with vehicle loading however, a heavier vehicle glides faster, but maintains its glide ratio.

Glide ratio (or “finesse”) is the cotangent of the downward angle, the glide angle (γ). Alternatively it is also the forward speed divided by sink speed (unpowered aircraft).

Glide number (ε) is the reciprocal of glide ratio but sometime it’s confused.

Related formulas

## Variables

 E glide ratio (dimensionless) ΔS distance of forward travel (m) ΔH altitude lost in that distance (m)