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Computer drawing of the Wright 1902 glider in descending flight.
 The tangent of the glide angle between the flight path and the ground equals
 the change in height divided by the distance traveled.

A glider is a special kind of aircraft that has no engine. Paper airplanes are the simplest aircraft to build and fly, and students can learn the basics of aircraft motion by flying paper airplanes. Toy gliders, made of balsa wood or styrofoam, are an inexpensive way for students to study the basics of aerodynamics, while having fun building and flying the aircraft. Hang-gliders are piloted aircraft that are launched by leaping off the side of a hill or by being towed aloft. Piloted gliders are launched by ground based catapults, or are towed aloft by a powered aircraft then cut free to glide for hours over many miles. The Wright Brothers perfected the design of the first airplane and gained piloting experience through a series of glider flights from 1900 to 1903. The Space Shuttle flies as a glider during reentry and landing; the rocket engines are used only during liftoff.

If a glider is in a steady (constant velocity and no acceleration) descent, it loses altitude as it travels. The glider's flight path is a simple straight line, shown as the inclined red line in the figure. The flight path intersects the ground at an angle a called the glide angle. If we know the distance flown d and the altitude change h, we can calculate the glide angle using trigonometry:

tan(a) = h / d

where tan is the trigonometric tangent function. The ratio of the change in altitude h to the change in distance d is often called the glide ratio.

If the glider is flown at a specified glide angle, the trigonometric equation can be solved to determine how far the glider can fly for a given change in altitude.

d = h / tan(a)

Notice that if the glide angle is small, the tan(a) is a small number, and the aircraft can fly a long distance for a small change in altitude. Conversely, if the glide is large, the tan(a) is a large number, and the aircraft can travel only a short distance for a given change in altitude. We can think of the glide angle as a measure of the flying efficiency of the glider. On another page, we will show that the glide angle is inversely related to the lift to drag ratio. The higher the lift to drag ratio, the smaller the glide angle, and the farther an aircraft can fly.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Jun 12 2014

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