History of Glider

An unpowered flying device that attempts to copy the flight of soaring birds as accurately as possible.

Early development.
Otto Lilienthal made hundreds of flights with several designs of hang gliders before his fatal crash in 1896. His gliders were of rigid construction, generally without movable control surfaces, and were controlled by shifting the pilot's weight. These gliders had no landing gear other than the pilot's legs.

The next major advance in glider design was made by Wilbur and Orville Wright, who were inspired by Lilienthal. The Wrights' 1902 glider in its final version made hundreds of perfectly controlled glides and set a distance record of 622 ft (189 m) and a duration record of 26 s on the dunes of Kitty Hawk, North Carolina. This glider did not depend on weight shift for control, but had aerodynamic controls consisting of movable elevator, rudder, and wing tips. This system, in principle, has been used to the present time, even on very large and fast gliders and powered aircraft. The success with this glider led the Wrights to construct a slightly larger aircraft, with engine and propellers, that enabled them to take off and fly from level ground for the first time on December 17,1903. The first successful powered vehicle, now called an airplane, was not a practical or useful aircraft, but with engineering development did reach that goal in a few years.

Methods of flight.

In October 1911, Orville Wright made a gliding flight of nearly 10 min duration, and demonstrated that gliders could stay up for long periods in the rising air caused by the wind blowing against a sand dune or hill. This condition of flight, called slope soaring, was the basic method of soaring flight until about 1930. Thermal soaring, the next step, was accomplished by flying in areas of rising convection currents, which are almost always present to some degree in the atmosphere. The development of thermal soaring and its practice is now principally due to the use of the variometer, calibrated as a sensitive rate-of-climb instrument, which enables the pilot to find the thermal and make the best use of it. By the use of thermal flight, the modern glider can fly almost anywhere in the world for extended time and distances over 500 mi (800 km) in one flight. Other methods of soaring make use of clouds and standing-wave phenomena in the atmosphere. These techniques have enabled gliders to achieve altitudes of 46,000 ft (14,000 m) under the proper conditions. High-performance gliders (sailplanes;)may be launched by towing behind powered aircraft to a release height of about 2000 ft (700 m), by winch-launching to about 800 ft (250 m), or by car towing, which is used to a lesser extent. Some gliders have been fitted with a small motor and propeller, which enables them to take off and climb to an altitude where rising air permits them to soar unpow-ered. Francis M. Rogallo

Sailplanes.
Sailplane construction traditionally has been of wood and plywood, although the use of aluminum alloy has become common. The greater strength and stiffness of modern aluminum alloy permits higher aspect ratios and improved performance. The use of fiber glass as primary structure has also come into prominence, since it is possible to produce the external shapes in accurate molds with greater precision, resulting in improved performance.
Sailplanes are equipped with dive brakes on the wings for emergency descent and for landing in small areas. Properly designed brakes hold the aircraft to its "never-exceed" dive speed in a vertical dive. The laminar-flow airfoil achieves its lowest drag in a certain range of lift coefficient and corresponding speed range. By the use of properly designed flaps, this range can be shifted to higher or lower speeds at the pilot's control, which provides improved performance at a wider range of speeds.
As sailplane performance increases, each new drag item becomes important, and there has been a tendency to use reclining, and almost full reclining, positions for the pilots. This permits fuselages with overall heights as low as 30 in. (75 cm) but introduces control system and visibility problems.

Flexible-wing gliders.

In the late 1950s the National Aeronautics and Space Administration (NASA) investigated various methods of returning crewed spacecraft to Earth. Two kinds of glider were investigated: aircraft with rigid delta or lifting body shapes and very high landing speeds, which later evolved into the space shuttle; and flexible-wing craft which could be packed like parachutes and deployed for slow, controlled landings in almost any open field, a concept that had been proposed 10 years earlier. After NASA demonstrated flexible-wing capability, many segments of the Department of Defense and industry also became interested in flexible-wing gliders for a variety of applications. Crewed and radio-controlled flights were made with flexible-wing gliders with or without power or towed by cars or aircraft. Some gliders were completely flexible, and some were stiffened with springy battens, aluminum tubes, or fabric tubes either pressurized or ram-inflated. Although extensive military and space applications are still undeveloped, flexible wings have had an effect on sport flying devices such as kites, hang gliders, and deployable gliders or gliding canopies used by sky divers. The completely flexible, deploy-able gliders have maximum glide ratios of only 3 to 4, which are very low compared to those of sailplanes, but a substantial change from the zero glide of the traditional parachute or the glide ratio of less than unity obtained from modified parachutes. Flexible, deployable gliders are used by sky divers who have demonstrated great ability to maneuver, penetrate the wind, and land on a chosen spot.

Modern foot-launched hang gliders with aluminum tube frames are the result of many improvements by private individuals and small manufacturers. Hang gliders have flown over 100 mi (160 km) in straight-line distance, have reached about 20,000 ft (6000 m) altitude, and have remained aloft more than 15 h. But it is not so much their performance that makes hang gliders popular, as their low cost, their convenience of folding into a small package for transport or storage, and the fact that no license is required for glider or pilot. Hundreds of thousands of people have learned to fly hang gliders.

Propellers and motors of about 10 hp (7.5 kW) have been attached to some hang gliders, enabling them to take off from level ground and climb in still air if necessary, while still able to soar in updrafts with the engine off. Some hang gliders have wheels that can be used optionally. In the summer of 1979 five powered hang gliders took off from California and, after many stops for rest and refueling, landed on the east coast.
Reference : McGraw - Hill Encyclopedia of Science and Technology