glider

[glahy-der]

glider, type of aircraft resembling an airplane but having at most a small auxiliary propulsion plant and usually no means of propulsion at all. The typical modern glider has very slender wings and a streamlined body. The unpowered variety is launched by an elastic shock cord, a rope, or a cable, attached to the front of the glider and pulled by a launching crew, a winch, a tow car, or a tow plane. Gliders can be towed behind airplanes over great distances. The powered variety can take off and climb on its own. 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 hang glider, with nylon or Kevlar stretched over an aluminum frame, can reach an altitude of 20,000 ft (6,100 m) and stay aloft up to 15 hours; in 1979 five hang glider pilots flew their machines (fitted with auxiliary motors) across the United States. A paraglider is an parachutelike airfoil made of nylon and Mylar from which the pilot is suspended by a series of ropes. Paraglider pilots must "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).

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

glider

Nonpowered heavier-than-air craft capable of sustained flight. Early experimenters in glider flight included George Cayley, who built the first man-carrying glider in 1853, and Otto Lilienthal (1848–1896), who introduced tail stabilizers on his first practical man-carrying craft in 1891. Improvements by Octave Chanute (1832–1910) in 1896 and by Wilbur and Orville Wright in 1902 perfected the control needed for developing the Wrights' powered airplane in 1903. The slender-winged glider was launched by being towed behind an airplane or a car. Gliders were used in World War II to carry troops. Today they are mainly used for recreation; the sailplane type is built for soaring on the lift from thermals. Seealso hang gliding.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Glider

Gliders or Sailplanes are heavier-than-air aircraft primarily intended for unpowered flight. See also gliding and motor gliders for more details.

Terminology

A "glider" is an unpowered aircraft. The most common types of glider are today used for sporting purposes. The design of these types enables them to climb using rising air and then to glide for long distances before finding the next source of lift. This has created the sport of gliding, or soaring. The term "sailplane" is sometimes used for these types, implying a glider with a high soaring performance. In addition to high-performance sailplanes, the term 'glider' also encompasses hang gliders and paragliders. Like sailplanes these can use upwardly moving air to soar but differ in not having a fuselage, control surfaces or a control column. Descriptions of these variants are in separate articles and so the rest of this article is only about conventional gliders and sailplanes.

Although many gliders do not have engines, there are some that use engines occasionally (see Motor glider). The manufacturers of high-performance gliders now often list an optional engine and a retractable propeller that can be used to sustain flight if required; these are known as 'self-sustaining' gliders. Some can even launch themselves and are known as 'self-launching' gliders. There are also 'touring motor gliders', which can switch off their engines in flight though without retracting their propellers. The term "pure glider" (or equivalently, but less commonly "pure sailplane") may be used to distinguish a totally unpowered glider from a motorized glider, without implying any differential in gliding or soaring performance.

History

In China, kites rather than gliders were used for military reconnaissance. However the Extensive Records of the Taiping Era (978) suggests that a true glider was designed in the 5th century BC by Lu Ban, a contemporary of Confucius. There is also a report from the History of Northern Dynasties (659) and Zizhi Tongjian (1084) that Yuan Huangtou in Ye made a successful glide, taking off from a tower in 559.

Abbas Ibn Firnas invented the first weight shift aircraft (hang glider) and is also claimed as the inventor of the first manned glider in 875 by fixing feathers to a wooden frame fitted to his arms or back. Written accounts at the time suggest that he made a ten minute flight. Abbas was seriously injured in the resulting crash.

The first heavier-than-air (i.e. non-balloon) aircraft to be flown in Europe was Sir George Cayley's series of gliders which achieved brief wing-borne hops from around 1804. Santos Dumont, Otto Lilienthal, Percy Pilcher, John J. Montgomery, and the Wright Brothers are other pioneers who built gliders to develop aviation. After the First World War gliders were built for sporting purposes in Germany (See link to Rhön-Rossitten Gesellschaft) and in the United States (Schweizer brothers). The sporting use of gliders rapidly evolved in the 1930s and is now the main application. As their performance improved gliders began to be used to fly cross-country and now regularly fly hundreds or even thousands of kilometers in a day, if the weather is suitable.

Military gliders were then developed by a number of countries, particularly during World War II, for landing troops. A glider was even built secretly by POWs as a potential escape method at Oflag IV-C near the end of the war in 1944. The space shuttle orbiters do not use their engines after re-entry at the end of each spaceflight, and so land as gliders.

Launch methods

The two most common methods of launching gliders are by aerotow and by winch. When aerotowed, the glider is towed behind a powered aircraft using a rope about 60 meters (about 200 ft) long. The glider's pilot releases the rope after reaching the desired altitude, but the rope can also be released by the towplane in an emergency. Winch launching uses a powerful stationary engine located on the ground at the far end of the launch area. The glider is attached to one end of 800-1200 metres (about 2,500-4,000 ft) of wire cable and the winch then rapidly winds it in. More rarely, powerful automobiles are used to pull gliders into the air, by pulling them directly or through the use of a pulley in a similar manner to the winch launch. Elastic ropes can also be used to launch gliders off slopes if there is sufficient wind blowing up the hill. The glider will then gain height using ridge lift.

Staying aloft without an engine

Glider pilots can stay airborne for hours by flying through air that is ascending as fast or faster than the glider itself is descending, thus gaining potential energy. The most commonly used sources of rising air are

thermals (updrafts of warm air);
ridge lift (found where the wind blows against the face of a hill and is forced to rise); and
wave lift (standing waves in the atmosphere, analogous to the ripples on the surface of a stream).

Ridge lift rarely allows pilots to climb much higher than about 600 m (2,000 ft) above the terrain; thermals, depending on the climate and terrain, can allow climbs in excess of 3,000 m (10,000 ft) in flat country and much higher above mountains; wave lift has allowed a glider to reach an altitude of 15,447 m (50,671 ft). In a few countries, gliders may continue to climb into the clouds in uncontrolled airspace, but in many countries the pilot must stop climbing before reaching the cloud base (see Visual Flight Rules).

Moving forward

After climbing in lift, gliders move on to find the next source of lift, or to land. As the glider descends, the air moving over the wings generates lift. The lift force acts slightly forward of vertical because it is created at right angles to the airflow which comes from slightly below as the glider descends, see Angle of attack. This horizontal component of lift is enough to overcome drag and allows the glider to accelerate forward. The ratio of lift to drag is the same as the height lost for each metre of forward travel, Glide ratio.

Landing

When glider pilots wish to land at an airfield or for an outlanding, they typically descend to about 250-300 m (800-1000 ft) AGL to enter a standard traffic pattern. The positioning must be continuously monitored and may be adjusted throughout this stage because gliders do have not the option to go around for a second try. Modern gliders are equipped with spoilers to control the rate of descent. These give the pilot wide safety margins so that they can land exactly the chosen place, or, if necessary, land much shorter or longer should unexpected events occur.

Glider design

Early gliders had no cockpit and the pilot sat on a small seat located just ahead of the wing. These were known as "primary gliders" and they were usually launched from the tops of hills, though they are also capable of short hops across the ground while being towed behind a vehicle. To enable gliders to soar more effectively than primary gliders, the designs minimized drag. Gliders now have very smooth, narrow fuselages and very long, narrow wings with a high aspect ratio and winglets.

The early gliders were made mainly of wood with metal fastenings, stays and control cables. Later fuselages made of fabric-covered steel tube were married to wood and fabric wings for lightness and strength. New materials such as carbon-fiber, glass-fiber and Kevlar have since been used with computer-aided design to increase performance. The first glider to use glass-fiber extensively was the Akaflieg Stuttgart FS-24 Phönix which first flew in 1957. This material is still used because of its high strength to weight ratio and its ability to give a smooth exterior finish to reduce drag. Drag has also been minimized by more aerodynamic shapes and retractable undercarriages. Flaps are fitted on some gliders so that the optimal lift of the wing is available at all speeds.

With each generation of materials and with the improvements in aerodynamics, the performance of gliders has increased. One measure of performance is the glide ratio. A ratio of 30:1 means that in smooth air a glider can travel forward 30 meters while only losing 1 meter of altitude. Comparing some typical gliders that might be found in the fleet of a gliding club - the Grunau Baby from the 1930s had a glide ratio of just 17:1, the glass-fiber Libelle of the 1960s increased that to 39:1, and nowadays flapped 18 meter gliders such as the ASG29 have a glide ratio of over 50:1. The largest open-class glider, the eta, has a span of 30.9 meters and has a glide ratio over 70:1. Compare this to the infamous Gimli Glider, a Boeing 767 which ran out of fuel mid-flight and was found to have a glide ratio of only 12:1, or to the Space Shuttle with a glide ratio of 3:1.

Due to the critical role that aerodynamic efficiency plays in the performance of a glider, gliders often have state of the art aerodynamic features seldom found in other aircraft. The wings of a modern racing glider have a specially designed low-drag laminar flow airfoil. After the wings' surfaces have been shaped by a mold to great accuracy, they are then highly polished. Vertical winglets at the ends of the wings are computer-designed to decrease drag and improve handling performance. Special aerodynamic seals are used at the ailerons, rudder and elevator to prevent the flow of air through control surface gaps. Turbulator devices in the form of a zig-zag tape or multiple blow holes positioned in a span-wise line along the wing are used to trip laminar flow air into turbulent flow at a desired location on the wing. This flow control prevents the formation of laminar flow bubbles and ensures the absolute minimum drag. Bug-wipers may be installed to wipe the wings while in flight and remove insects that are disturbing the smooth flow of air over the wing.

Modern competition gliders are also designed to carry jettisonable water ballast (in the wings and sometimes in the vertical stabiliser). The extra weight provided by the water ballast is advantageous if the lift is likely to be strong, and may also be used to adjust the glider's center of mass. Although heavier gliders have a slight disadvantage when climbing in rising air, they achieve a higher speed at any given glide angle. This is an advantage in strong conditions when the gliders spend only little time climbing in thermals. The pilot can jettison the water ballast before it becomes a disadvantage in weaker thermal conditions. Another use of water ballast is to dampen air turbulence such as might be encountered during ridge soaring. To avoid undue stress on the airframe, gliders must jettison any water ballast before landing.

Pilots can land accurately by controlling their rate of descent using spoilers, also known as air brakes. These are metal devices which extend from either the upper-wing surface or from both upper and lower surfaces, thereby destroying some lift and creating additional drag. A wheel-brake also enables a glider to be stopped after touchdown, which is particularly important in a short field.

Classes of glider

Eight classes of glider have been defined by the FAI. They are:

Standard Class (No flaps, 15 m wing-span, water ballast allowed)
15 metre Class (Flaps allowed, 15 m wing-span, water ballast allowed)
18 metre Class (Flaps allowed, 18 m wing-span, water ballast allowed)
Open Class (No restrictions except a limit of 850 kg for the maximum all-up weight)
Two Seater Class (maximum wing-span of 20 m), also known by the German name "Doppelsitzer"
Club Class (This class allows a wide range of older small gliders with different performance and so the scores have to be adjusted by handicapping. Water ballast is not allowed).
World Class (The FAI Gliding Commission which is part of the FAI and an associated body called Organisation Scientifique et Technique du Vol à Voile (OSTIV) announced a competition in 1989 for a low-cost glider, which had moderate performance, was easy to assemble and to handle, and was safe for low hours pilots to fly. The winning design was announced in 1993 as the Warsaw Polytechnic PW-5. This allows competitions be run with only one type of glider.
Ultralight Class, for gliders with a maximum mass less than 220 kg.

Major manufacturers of gliders

DG Flugzeugbau GmbH
Schempp-Hirth GmbH
Alexander Schleicher GmbH & Co
Rolladen-Schneider Flugzeugbau GmbH (taken over by DG Flugzeugbau)

See also the full gliders and manufacturers list, past and present.

Instrumentation and other technical aids

Gliders must be equipped with an altimeter, compass, and an airspeed indicator in most countries, and are often equipped with a variometer, turn and bank indicator and an airband radio (transceiver), each of which may be required in some countries. An Emergency Position-Indicating Radio Beacon (ELT) may also be fitted into the glider to reduce search and rescue time in case of an accident.

Much more than in other types of aviation, glider pilots depend on the variometer, which is a very sensitive vertical speed indicator, to measure the climb or sink rate of the plane. This enables the pilot to detect minute changes caused when the glider enters rising or sinking air masses. Both mechanical and electronic 'varios' are usually fitted to a glider. The electronic variometers produce a modulated sound of varying amplitude and frequency depending on the strength of the lift or sink, so that the pilot can concentrate on centering a thermal, watching for other traffic, on navigation, and weather conditions. Rising air is announced to the pilot as a rising tone, with increasing pitch as the lift increases. Conversely, descending air is announced with a lowering tone, which advises the pilot to escape the sink area as soon as possible. (Refer to the variometer article for more information).

Gliders' variometers are sometimes fitted with mechanical devices such as a "MacCready Ring" to indicate the optimal speed to fly for given conditions. These devices are based on the mathematical theory attributed to Paul MacCready though it was first described by Wolfgang Späte in 1938. MacCready theory solves the problem of how fast a pilot should cruise between thermals, given both the average lift the pilot expects in the next thermal climb, as well as the amount of lift or sink he encounters in cruise mode. Electronic variometers make the same calculations automatically, after allowing for factors such as the glider's theoretical performance, water ballast, headwinds/tailwinds and insects on the leading edges of the wings.

Soaring flight computers, often used in combination with PDAs running specialized soaring software, have been specifically designed for use in gliders. Using GPS technology these tools can:

Provide the glider's position in 3 dimensions by a moving map display
Alert the pilot to nearby airspace restrictions
Indicate position along track and remaining distance and course direction
Show airports within theoretical gliding distance
Determine wind direction and speed at current altitude
Show historical lift information
Create a secure GPS log of the flight to provide proof for contests and gliding badges
Provide "final" glide information (ie showing if the glider can reach the finish without additional lift).
Indicate the best speed to fly under current conditions

After the flight the GPS data may be replayed on specialized computer software for analysis and to follow the trace of one or more gliders against a backdrop of a map, an aerial photograph or the airspace. A 3-D view is shown here with a topographical background.

Because collision with other gliders is an ever-present risk, the anti-collision device, FLARM is becoming increasingly common in Europe and Australia. In the longer term, gliders may eventually be required in some European countries to fit transponders once devices with low power requirements become available.

Glider markings

Like all other aircraft, gliders are required to be painted with a national aircraft registration number, known as a "tail number" or in the U.S. as an "N-number". The required size of these numbers varies from country to country. The size range is from 1 cm to 30 cm, sometimes depending on the age of the aircraft.

To distinguish gliders in flight, very large numbers/letters are sometimes displayed on the fin and wings. These numbers were added for use by ground-based observers in competitions, and are therefore known as "competition numbers" or "contest ID's". They are unrelated to the glider's registration number, and are assigned by national gliding associations. They are useful in radio communications between gliders, so glider pilots often use their competition number as their call-signs.

Fibreglass gliders are white in color after manufacture. Since fibreglass resin softens at high temperatures, white is used almost universally to reduce temperature rise due to solar heating. Color is not used except for a few small bright patches on the wing tips; these patches (typically bright red) improve gliders' visibility to other aircraft while in flight. Non-fibreglass gliders (those made of aluminum and wood) are not subject to the temperature-weakening problem of fibreglass, and can be painted any color at the owner's choosing; they are often quite brightly painted.

Aerobatic gliders

Another - less widespread - form of gliding is aerobatics. Gliders have been developed specifically for this type of competition, though most gliders can perform simpler aerobatic maneuvers such as loops and chandelles. Aerobatic gliders usually have stronger and shorter wings than the gliders that are used in cross-country racing to withstand the high g-forces that are experienced in some maneuvers.