Like any aerodynamic object, airplanes have a Lift-to-Drag ratio, to define the ability of the aircraft to glide in flight, i.e. without no engine assistance, being the main characteristic of gliders.The Lift-to-Drag ratio of an airplane depends not only on its aerodynamic forms, its wings and its air resistance, but also its weight and speed.
In fact, the Lift-to-Drag ratio is a mathematical ratio, calculated by dividing the horizontal distance that the airplane glides by the height it descends within the same time. In other words, the bigger the Lift-to-Drag ratio of an aircraft is, the greater will be its ability to glide.
To give an order of magnitude, airliners have a Lift-to-Drag ratio of between 15 and 22. That’s to say, in the event of total engine failure, they can glide 9.3 to 13.5 miles ( 15 to 22 kilometres ) for a drop in altitude of only 3,300 feet ( 1,000 metres ). Gliders have a Lift-to-Drag ratio of about 45, which can increase up to 70 for the highest performance glider, the ETA.
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