What is a glider?
A glider is a special kind of aircraft that has no engine. There are many different types of gliders. Paper airplanes are the simplest gliders to build and fly. Balsa wood or Styrofoam toy gliders are an inexpensive vehicle for students to have fun while learning the basics of aerodynamics. Hang-gliders are piloted aircraft having cloth wings and minimal structure. Some hang-gliders look like piloted kites, while others resemble maneuverable parachutes. Sailplanes are piloted gliders that have standard aircraft parts, construction, and flight control systems, but no engine. The Space Shuttle returns to earth as a glider; the rocket engines are used only during liftoff. Even the Wright Brothers gained piloting experience through a series of glider flights from 1900 to 1903.
In flight, a glider has three forces acting on it as compared to the four forces that act on a powered aircraft. Both types of aircraft are subjected to the forces of lift, drag, and weight. The powered aircraft has an engine that generates thrust, while the glider has no thrust.
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. 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, and it then falls to earth.
So how does a glider generate the velocity needed for flight?
For paper airplanes and balsa gliders, the aircraft is given an initial velocity by throwing the aircraft. Some larger balsa gliders employ a catapult made from rubber bands and a tow line to provide velocity and some initial altitude. Hang-glider pilots often run and jump off the side of a hill or cliff to get going. Some hang-gliders and most sailplanes are towed aloft by a powered aircraft and then cut loose to begin the glide.
The powered aircraft that pulls the glider aloft gives the glider a certain amount of potential energy. The glider can trade 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.