A glider , sometimes called a sailplane , is a rigid, fixed-wing aircraft designed for soaring flight that is commonly used for the sport of gliding. Other types of unpowered aircraft include foot-launched aircraft (such as hang gliders and paragliders) or rotary winged gliders (gyrogliders). Other fixed winged gliders have been built that not were intended to soar, such as military gliders. These were then developed by a number of countries, particularly during World War II, for landing troops. A glider, the 'Colditz Cock' was even built secretly by POWs as a potential escape method in 1944. Some experimental aircraft such as the North American X-15 and spacecraft such as the Space Shuttle make unpowered landings.

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Use of engines

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 before retracting their propellers and are known as 'self-launching' gliders. There are also 'touring motor gliders', which can also launch themselves and 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

Although many unpowered aircraft had flown before the 1920s, the sporting use of gliders then began at Wasserkuppe in Germany, evolving rapidly in the 1930s to an Olympic demonstration sport. Sport and recreation are now the main application of gliders. 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. International competitions began in 1937.

Launch, flight and landing

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.

Once launched gliders generally try to gain height using thermals, ridge lift or lee waves and can remain airborne for hours. This is known as 'soaring'. Experienced pilots fly cross-country, often on pre-declared tasks of hundreds of kilometers and sometimes further. They often fly in competition with each other.

Many gliders have a retractable undercarriage that is raised to reduce drag in flight. This is lowered shortly before landing. Pilots usually land back the airfield from which they took off, but a landing is possible in any flat field at least 250 metres long. This is possible because 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.

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 ^[1] though it was first described by Wolfgang Späte in 1938. ^[2] 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.

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.

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. ^[3]

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.

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.

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.

See also

References

1. MacCready Theory
2. Letters
3. NASA's web site for Space Shuttle Glider at www.nasaexplores.com