Gliders or Sailplanes are heavier-than-air aircraft primarily intended for unpowered flight. They have been used not only for sport but also for observations. Coming in various sizes and classes, they have been and are still flown all over the world. See also gliding and motor gliders for more details.
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 instead of merely descending. 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.
Although many gliders do not have engines, there are some that use engines occasionally. 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 motorgliders, 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.
Minimalist variations of gliders also occur in hang gliding and paragliding. The soaring performance is lower than a sailplane but they too can climb without using an engine.
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 fifth century B.C.E. by Lu Ban, a contemporary of Confucius. There is also a report that Yuan Huangtou, Ye made a successful glider flight taking off from a tower in 559.
Abbas Ibn Firnas is also claimed as the inventor 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  although this is almost certainly an exaggeration of the flight's duration. Because his aircraft lacked a 'tail', he could neither steer nor land properly, and he 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 in Germany for sporting purposes (See link to Rhön-Rossitten Gesellschaft). 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 (km) 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 Orbiter vehicles or "space shuttles" do not use their engines after re-entry at the end of each spaceflight, and so land as gliders.
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 (m) (about 200 feet (ft)) long. The glider's pilot releases the rope after reaching 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, automobiles are used to pull gliders into the air or gliders are launched from sloping ground or cliffs.
Glider pilots can stay airborne for hours. This is possible because they seek out rising air masses (lift) from the following sources:
The most commonly used source of lift is created by the sun's energy heating the ground which in turn heats the air above it. This warm air rises in columns known as thermals. Soaring pilots quickly become aware of visual indications of thermals such as: cumulus clouds, cloud streets, dust devils and haze domes. Also, nearly every glider contains an instrument known as a variometer (a very sensitive vertical speed indicator) which shows visually (and often audibly) the presence of lift and sink. Having located a thermal, a glider pilot will circle within the area of rising air to gain height. In the case of a cloud street thermals can line up with the wind creating rows of thermals and sinking air. A pilot can use a cloud street to fly long straightline distances by remaining in the row of rising air.
Another form of lift occurs when the wind meets a mountain, cliff or hill. The air is deflected up the windward face of the mountain forming lift. Gliders can climb in this rising air by flying along the feature. This is referred to as "ridge running" and has been used to set record distance flights along the Appalachians in the USA and the Andes Mountains in South America. Another name for flying with ridge lift is slope soaring.
The third main type of lift used by glider pilots are the lee waves that occur near mountains. The obstruction to the airflow can generate standing waves with alternating areas of lift and sink. The top of each wave peak is often marked by lenticular cloud formations.
Another form of lift results from the convergence of air masses, as with a sea-breeze front.
More exotic forms of lift are the polar vortices which the Perlan Project hopes to use to soar to great altitudes . A rare phenomenon known as Morning Glory has also been used by glider pilots in Australia.
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 balance drag and allows the glider to move forward.
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.
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 so-called 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 1:1.
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. 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.
For competitions several classes of glider have been defined by the Fédération Aéronautique Internationale (FAI). They are:
See also the full gliders and manufacturers, past and present.
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. Maximizing time in the best lift areas in an otherwise invisible air mass can therefore be realized. 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:
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.
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. Some countries allow registration numbers as small as 1cm in height; other countries specify a minimum height of two inches, three inches, or 12 inches, 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." 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.
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.
Some gliders are motor gliders, equipped with propellers that are either fixed, or retract into the fuselage. The motor can be powerful enough to allow these gliders to launch independently, or just powerful enough to allow the glider to climb slowly, requiring launching like unpowered gliders. A third type, termed touring motor glider, has a conventional small airplane layout with an engine and propeller on the front of the aircraft.
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