A glider is a special kind of aircraft that has no engine. Paper airplanes are the most obvious example, but gliders come in a wide range of sizes. Toy gliders, made of balsa wood or styrofoam, are an excellent way for students to study the basics of aerodynamics. Hang-gliders are piloted aircraft that are launched by leaping off the side of a hill. The Wright brothers perfected the design of the first airplane and gained piloting experience through a series of glider flights from 1900 to 1903.More sophisticated 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 Space Shuttle flies as a glider during reentry and landing; the rocket engines are used only during liftoff.

Compared to a powered aircraft, a glider has only three main forces acting on it: lift, drag, and weight. Forces are vector quantities having both a magnitude and a direction. The weight acts through the center of gravity and is always directed towards the center of the earth. The magnitude of the weight is given by the weight equation and depends on the mass of the vehicle plus its payload. The lift and drag are aerodynamic forces and act through the center of pressure. The drag is directed opposite to the flight direction, and the lift is directed perpendicular to the flight direction. There are many factors that influence the magnitude of the lift and drag forces.

In order for a glider to fly, it must generate lift to oppose its weight. To generate lift, a glider must move through the air. But the motion of a glider through the air also generates drag. In a powered aircraft, the thrust from the engine opposes drag. But a glider has no engine to generate thrust. With the drag unopposed, a glider quickly slows down until it can no longer generate enough lift to oppose the weight.

So how does a glider generate the velocity needed for flight?

The simple answer is that a glider trades altitude for velocity. It trades the potential energy difference from a higher altitude to a lower altitude to produce kinetic energy, which means velocity. Gliders are always descending relative to the air in which they are flying.

How do gliders stay aloft for hours if they constantly descend?

The answer is that they are designed to be very efficient, to descend very slowly. If the pilot can locate a pocket of air that is rising faster than the glider is descending, the glider can actually gain altitude, increasing its potential energy. Pockets of rising air are called updrafts. Updrafts are found when a wind blowing at a hill or mountain has to rise to climb over it. Updrafts can also be found over dark land masses that absorb heat from the sun. The heat from the ground warms the surrounding air, which causes the air to rise. Rising pockets of hot air are called thermals. Large gliding birds, such as owls and hawks, are often seen circling inside a thermal to gain altitude without flapping their wings. Gliders do exactly the same thing.

• Forces on a Glider:
• Gliding Flight:

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