II.

Glider Flight

Updrafts in the atmosphere, on which the glider pilot depends for motive power, are principally of two kinds: ridge currents and thermal currents. Ridge currents are formed when a steady wind blows against the side of a ridge or a range of hills. Such currents can be quite strong but are limited to an area relatively close to the windward edge of the ridge. Thermal currents are formed by heat rising from the ground. Such currents occur over a bare field on a hot day, for example. Thermal currents are always present under cumulus clouds; extremely strong, dangerous currents are under the towering, anvil-shaped clouds of thunderstorms.

In gliding flight, the craft must be launched from the ground, usually by catapulting it by using long elastic cords or by towing it aloft with a winch, an automobile, or a powered airplane. When a glider is launched by a tow, the glider pilot cuts loose the towline after reaching the desired altitude. Once in the air, the pilot directs the glider in search of upcurrents. If he or she simply wishes to remain in the air, the pilot may fly back and forth along a ridge where a suitable current exists. If making a cross-country flight, however, the pilot flies by “cloud chasing” or “thermal sniffing”—searching for thermal currents that will give the glider lift. When such a current is found, the pilot will spiral the craft to remain within the current while gaining altitude. After reaching the maximum altitude to which the current will lift the glider, the pilot glides away to find another current. Glider flights are generally restricted to daylight hours because of meteorological conditions as well as the need for visibility.

Modern gliders can reach high speeds and can stay in the air for long periods of time, even without the presence of strong updrafts. High-performance gliders can have a glide ratio of 40 or more. The glide ratio is the relation between horizontal and vertical distance the glider travels; a glide ratio of 40, for example, means that for every kilometer the glider loses in altitude, the craft travels a horizontal distance of 40 kilometers. Gliders used in competition are capable of maintaining air speeds of over 160 km/h (100 mph) on a course that is 300 km (186.41 mi) long. The world record for the highest altitude reached by a glider is 14,938 m (49,009 ft); the straight-line distance record is 1,460.8 km (907.7 mi).