a high location. Forward speed is then maintained by a gradual descent through the surrounding air, with the wings angled slightly down so that their lift also provides a small forward thrust to counter the drag of the wing. If the air is rising faster than the aircraft is descending through it, the glider will gain height and additional potential energy. Sources of such rising air include warm thermals and hill ridges. In the past, unpowered military gliders have been used for military applications. Today, the majority of use of all types of glider aircraft is recreational. Balloons Free-flying Balloons drift with the wind. The pilot controls the altitude either by heating the air more or by releasing ballast weight. The wind direction usually changes with altitude, so crude directional control can be obtained by changing altitude. A round tethered balloon is unstable in any significant wind. A kite balloon is streamlined to make it stable in strong winds. Today, the majority of manned balloon flights are recreational, whereas unmanned and usually free-flying balloons are widely used for meteorological measurement. Kites Kites are aircraft that are tethered to some other object (fixed or mobile) or other means that maintain tension in the kite line; and rely on virtual or real wind blowing over and under them to generate lift and drag. Kytoons are balloon kites that are shaped and tethered to obtain kiting deflections, and can be lighter-than-air, neutrally buoyant, or heavier-than air. Kites have been developed for commercial applications using kite control systems, including the airborne wind energy systems of high altitude wind power. See also Kite types Gliding Gliding competitions Gimli Glider, a passenger jet aircraft that ran out of fuel Gyroglider Colditz Cock, a glider built to escape from Colditz References External links Fédération Aéronautique Internationale This is the world organisation for air sports such as gliding, hang gliding and paragliding Aircraft