Three basic forces act on gliders: lift, gravity and drag (airplanes have a fourth force: thrust):
- Lift is the all-important force, created by the wings and counteracting the weight, which allows an aircraft to stay aloft. In the case of a glider, the lift is enhanced through the use of highly efficient wings.
- Drag is the force that tends to slow a plane down. Drag reduction is critical on a glider, even more so than on a conventional airplane. In motorized aircraft, a pilot can simply increase the thrust (using the engines) to overcome drag. Since there is no engine on a glider, the drag must be minimized wherever possible or the plane won't remain in the air for long.
- Weight can be made to work for or against a glider. A lighter overall weight, for example, may allow the glider to stay aloft longer or travel further. A heavier weight, on the other hand, can be an advantage if greater speed is the objective. Many gliders contain ballast tanks that pilots can fill with water before takeoff. The added weight of the water allows greater speeds while in the air. If the pilot wished to reduce his weight, he can dump the tanks while in the air to lighten the plane.
Without an engine, a glider's first problem is getting off the ground and up to altitude. The most common launching method is an aero-tow. A conventional powered plane tows the glider up into the sky using a long rope. The glider pilot controls a quick-release mechanism located in the glider's nose and releases the rope at the desired altitude. Right after release, the glider and the tow plane turn in opposite directions and the glider begins its unpowered flight. The tow plane is then free to return to the airport and set up for another tow.
Another popular launching method is winch launching. An engine powers a large winch on the ground and a long cable connects the winch to another release mechanism located on the underside of the glider. When the winch is activated, the glider is pulled along the ground toward the winch and takes off, climbing rapidly. As the glider rises, the pilot can release the winch line as in an aero-tow and continue his flight.