Glider

I

Introduction

Glider, heavier-than-air craft with no engine that stays aloft as a result of the aerodynamic forces acting upon it. In form, gliders resemble ordinary airplanes, but they are extremely light in weight, have a low wing load (the ratio of weight to wing area), and have a high aspect ratio (the ratio of the wingspan to the wing width). Glider wings are much longer and narrower than those of powered aircraft. A good modern glider of the sailplane type (see below), when flying level in still air, sinks at a rate less than 90 cm (36 in) per second, and therefore is able to climb in an air current that is rising at the rate of about 3 km/h (2 mph).

Experiments with gliders laid the foundations for the design of the first powered aircraft. Beginning in the 1870s, a number of pioneer aeronauts built gliders that made successful flights and provided information regarding the efficient design of wings and control systems. Among these pioneers were the German inventor Otto Lilienthal, the American inventors Octave Chanute, Orville and Wilbur Wright, and John Joseph Montgomery. The first powered airplane to fly successfully was designed by the Wrights as a direct result of their earlier work with gliders. See Airplane; Aviation.

The chief impetus to the modern development of gliders, and the art of flying them, came from the Germans. In the years following World War I (1914-1918), Germany, which was forbidden by the Treaty of Versailles to manufacture powered airplanes suitable for military use, turned to building gliders and to researching glider flight. German aeronautical engineers discovered the great efficiency of light craft with long, birdlike wings, and the meteorological conditions under which soaring flight could succeed.

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.

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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).

III

Glider Types

In general, gliders are of three types. Primary gliders, used entirely for instruction purposes, consist of little more than a girder framework to which the wings and the control and stabilizing surfaces are attached. The pilot sits on an open seat at the front of the framework. Secondary gliders, or sailplanes, are built like ordinary airplanes with a fuselage and an enclosed cockpit seating one or two people. They are designed for maximum aerodynamic efficiency. Gliders of the third type, cargo gliders, are used for military or peacetime purposes; these large aircraft are designed to carry heavy loads. They are built not to soar but to be towed in groups behind a powered plane to increase the payload of the plane. The chief advantages of the cargo glider are its carrying capacity and its low landing speed, which make possible the landing of a heavy payload in a space too restricted for conventional planes.

IV

Lifting Bodies

Experimental craft called lifting bodies were investigated in the late 1960s as potential space vehicles that would allow astronauts to glide in from outer space. The United States Air Force and the National Aeronautics and Space Administration (NASA) studied designs for a space vehicle that would acquire lift as it hurtled into the atmosphere of the earth and could then be maneuvered to a desired landing zone. Because conventional wings would snap off such a craft as it entered the atmosphere at high speed, the entire undersurface of the craft had to function as a lifting surface. This is the source of the term lifting bodies.

The lifting body program was started in the late 1950s. At that time NASA started experimenting with the M-2 and later with the HL-10 lifting bodies. These vehicles were dropped from B-52 bombers at high altitudes and they glided to earth without power but with aerodynamic control. In the late 1960s the air force tested the X-24 lifting body. Rocket engines on this craft pushed its speed up to about 2,200 km/h (about 1,350 mph). At that speed, the descent of the X-24 simulated a gliding reentry from space. Information gained in these tests was used to help design the space shuttle, which lands on the earth after a mission by gliding through the atmosphere (see Space Exploration).

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