The glider uses gravity and updrafts of air to keep it flying slope soaring relies on wind rising off dunes or hillsides, while thermal soaring exploits convection currents in the air. In soaring the glider is repeatedly maneuvered through updrafts to reach altitudes as high as 46,000 ft (14,000 m). It can then glide down through air that is not rising. In a powered glider the engine can be turned on to keep the glider aloft when there are no updrafts. A sailplane, a glider which is built especially for soaring and sustained flight, can travel as much as 500 mi (800 km) in this manner. The usual flight controls in a glider consist of a pedal to operate the rudders and a control stick to operate the elevators and ailerons.
Otto and Gustav Lilienthal of Germany made the first successful piloted glider flight in 1891. The Lilienthals demonstrated the superiority of curved over flat surfaces in flight and encouraged others to make glider experiments, at least until Otto's death in a glider crash in 1896. At the beginning of the 20th cent. the Wright brothers constructed and flew many gliders. They introduced land skids, wing warping, and other improvements that characterize present-day gliders. In World War II troop-transport gliders were used for aerial invasions. The gliders were launched and towed by cargo aircraft to the invasion area, where they were released.
Early gliders were launched from hills or by running forward the machine maintained stability while in flight by the pilot's shifting body weight. These techniques have been resurrected in modern hang gliding, a development based on NASA experiments with flexible-wing gliders in the 1950s. The
kite —raise the airfoil into the air by running and using the wind—before launching themselves from a cliff or the like.
See T. L. Knauff, Glider Basics from First Flight to Solo (1982) D. Piggott, Gliding (5th ed. 1987).
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