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In aerodynamics, the lift-to-drag ratio (or L/D ratio) is the lift generated by an aerodynamic body such as an aerofoil or aircraft, divided by the aerodynamic drag caused by moving through air. It describes the aerodynamic efficiency under given flight conditions. The L/D ratio for any given body will vary according to these flight conditions.
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 ...
Glide ratio is dependent on an aircraft's class, and can typically range from 44:1 (for modern designs in the Standard Class) up to 70:1 (for the largest aircraft). A good gliding performance combined with regular sources of rising air enables modern gliders to fly long distances at high speeds.
Glide slope is the distance traveled for each unit of height lost. In a steady wings-level glide with no wind, glide slope is the same as the lift/drag ratio (L/D) of the glider, called "L-over-D". Reducing lift from the wings and/or increasing drag will reduce the L/D allowing the glider to descend at a steeper angle with no increase in airspeed.
Mammals tend to rely on lower glide ratios to increase the amount of time foraging for lower energy food. [8] An equilibrium glide, achieving a constant airspeed and glide angle, is harder to obtain as animal size increases. Larger animals need to glide from much higher heights and longer distances to make it energetically beneficial. [9]
One measure of performance is the glide ratio. For example, a ratio of 12:1 means that in smooth air a glider can travel forward 12 metres while only losing 1 metre of altitude. Some performance figures as of 2006: Topless gliders (no kingpost): glide ratio ~17:1, speed range ~30–145 km/h (19–90 mph), best glide at 45–60 km/h (28–37 mph)
Good trackers can cover nearly as much ground as the distance they fall, approaching a glide ratio of 1:1. The fall rate of a skydiver in an efficient track is significantly lower than that of one falling in a traditional face-to-earth position; the former reaching speeds as low as 40 metres per second (90 mph), the latter averaging around the 54 m/s (120 mph) mark.
The speed to fly is the optimum speed through sinking or rising air mass to achieve either the furthest glide, or fastest average cross-country speed. [1] Most speed to fly setups use units of either airspeed in kilometers per hour (km/h) and climb rate in meters per second (m/s), or airspeed in knots (kn) and climb rate in feet per minute (ft ...