Glider aircraft are heavier-than-air craft that are supported in flight by the dynamic reaction of the air against their lifting surfaces, and whose free flight does not depend on an engine. Mostly these types of aircraft are intended for routine operation without engines, though engine failure can force other types of aircraft to glide.
There are a wide variety of types differing in the construction of their wings, aerodynamic efficiency, location of the pilot and controls. Some may have power-plants to take off and/or extend flight. Some are designed simply to descend, but the most common varieties exploit meteorological phenomena to maintain or even gain height. These types are principally used for the air sports of gliding, hang gliding and paragliding. Perhaps the most familiar type is the paper aeroplane.
History of gliders
Early accounts of flight are often sketchy and it is unclear whether each craft was a kite, parachute or a truly controllable aircraft. In China, kites rather than gliders were used for military reconnaissance. However the Extensive Records of the Taiping Era (978) suggests that something similar to a 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 and piloted the first manned ornithopter in 875. Abbas was seriously injured in the resulting crash. In circa 1630-1632 Ottoman Turkey, Hezârfen Ahmed Çelebi reputedly flew from the Galata Tower by glider, across the Bosporus, landed in Doğancılar square in Üsküdar -and was rewarded by Murad Khan with him a sack of golden coins and exile to Algeria.
The first heavier-than-air (i.e. non-balloon) man-carrying aircraft that were based on published scientific principles were Sir George Cayley's series of gliders which achieved brief wing-borne hops from around 1849. Thereafter gliders were built by pioneers such as Jean Marie Le Bris, John J. Montgomery, Otto Lilienthal, Percy Pilcher, Octave Chanute and Augustus Moore Herring to develop aviation. Of these Lilienthal was the first to make repeated successful flights, achieving over 2,000 and was the first to use rising air to prolong the flight.
The Wright Brothers developed a series of three manned gliders after preliminary tests with a kite as they worked towards achieving powered flight. They returned to glider testing in 1911 by removing the motor from one of their later designs.
Development of gliders
After World War I recreational gliders were built 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 developed during World War II by a number of countries for landing troops. A glider - the Colditz Cock - was even built secretly by POWs as a potential escape method at Oflag IV-C near the end of the war in 1944.
Development of flexible-wing hang gliders
Foot-launched aircraft had been flown by Lilienthal and at the meetings at Wasserkuppe in the 1920s. However the innovation that led to modern hang gliders was in 1951 when Francis Rogallo and Gertrude Rogallo applied for a patent for a fully flexible wing with a stiffening structure. The American space agency NASA began testing in various flexible and semi-rigid configurations of this Rogallo wing in 1957 in order to use it as a recovery system for the Gemini space capsules. Charles Richards and Paul Bikle developed the concept producing a wing that was simple to build which was capable of slow flight and as gentle landing. Between 1960-1962 Barry Hill Palmer used this concept to make foot-launched hang gliders, followed in 1963 by Mike Burns who built a kite-hang glider called Skiplane. In 1963, John W. Dickenson began commercial production.
Development of paragliders
January 10, 1963 American Domina Jalbert filed a patent US Patent 3131894 on the Parafoil which had sectioned cells in an aerofoil shape; an open leading edge and a closed trailing edge, inflated by passage through the air – the ram-air design. The 'Sail Wing' was developed further for recovery of NASA space capsules by David Barish. Testing was done by using ridge lift. After tests on Hunter Mountain, New York in September 1965, he went on to promote ‘slope soaring’ as a summer activity for ski resorts (apparently without great success). NASA originated the term ‘paraglider’ in the early 1960’s, and ‘paragliding’ was first used in the early 1970’s to describe foot-launching of gliding parachutes.
The main application today of glider aircraft is sport and recreation.
Gliders were developed from the 1920s for recreational purposes. As pilots began to understand how to use rising air, gliders were developed with a high lift-to-drag ratio. These allowed longer glides to the next source of 'lift', and so increase their chances of flying long distances. This gave rise to the popular sport known as gliding although the term can also be used to refer to merely descending flight.
Gliders were mainly built of wood and metal but the majority now have composite materials using glass, carbon fibre and aramid fibres. To minimise drag, these types have a fuselage and long narrow wings, ie a high aspect ratio. Both single-seat and two-seat gliders are available.
Initially training was done by short 'hops' in primary gliders which are very basic aircraft with no cockpit and minimal instruments. Since shortly after World War 2 training has always been done in two-seat dual control gliders, but high performance two-seaters are also used to share the workload and the enjoyment of long flights. Originally skids were used for landing, but the majority now land on wheels, often retractable. Some gliders, known as motor gliders, are designed for unpowered flight, but can deploy piston, rotary, jet or electric engines.
Gliders are classified by in the FAI for competitions into glider competition classes mainly on the basis of span and flaps. However a class of ultralight sailplanes, including some known as microlift gliders, has been defined by the FAI based on a maximum weight.
Unlike a glider (sailplane), a hang glider is capable of being carried, foot launched and landed solely by the use of the pilot’s legs.
In a hang glider the shape of the wing is determined by a structure, and it is this that distinguishes them from the other main type of foot-launched aircraft, paragliders, technically Class 3. Some hang gliders have engines, and are known as powered hang gliders. Due to their commonality of parts, construction and design, they are usually considered by aviation authorities to be hang gliders, even though they may use the engine for the entire flight. Some flexible wing powered aircraft, Ultralight trikes, have a wheeled undercarriage, and so are not hang gliders.
A paraglider is a free-flying, foot-launched aircraft. The pilot sits in a harness suspended below a fabric wing. Unlike a hang glider whose wings have frames, shape of a paraglider's wing is formed by its suspension lines and the pressure of air entering vents in the front of the wing. The concept of ram-air inflated wings has been refined so that the best of these have a glide ratio of 10 at 45 km/h. Like sailplanes and hang gliders, paragliders use rising air to gain height and this process is the basis for most recreational flights and competitions, though aerobatics and 'spot landing competitions' also occur. Launching is often by stepping from a slope, but winch launches are also used. Paramotors are attached to some types which are known as powered paragliders. These in turn have spawned paraplanes, which are wheeled and motorized but which still use ram-air wings.
Military gliders were used mainly during the Second World War for carrying troops and heavy equipment (see Glider infantry) to a combat zone. These aircraft were towed into the air and most of the way to their target by military transport planes, eg C-47 Dakota, or by bombers that had been relegated to secondary activities, eg Short Stirling. Once released from the tow near the target, they landed on as close to target as possible. The advantage over paratroopers were that heavy equipment could be landed and that the troops were quickly assembled rather than being dispersed over a drop zone. The gliders were treated as disposable leading to construction from common and inexpensive materials such as wood, though a few were retrieved and re-used. By the time of the Korean War, transport aircraft had also become larger so that that even light tanks could be dropped by parachute, and so gliders were not used.
Even after the development of powered aircraft, gliders have been built for research. The NASA Parasev Rogallo flexible wing is an example that was built to investigate alternative methods of recovering spacecraft. Although this application was abandoned, publicity inspired hobbyists to adapt the flexible wing airfoil for modern hang gliders.
In contrast to the flying wings, lifting bodies have been also developed using unpowered prototypes. Although the idea can be dated to Vincent Justus Burnelli in 1921, interest increased as an alternative for returning spacecraft. Traditional space capsules have very little control over where they land, whereas a steerable craft using wings would have more options. The lifting bodies use the fuselage itself to generate lift without employing the usual thin and flat wing. The objective of the lifting body is to minimize the drag and structure of a wing for very high supersonic or hypersonic flight as might be experienced during the re-entry of a spacecraft. This can be compared with a flying wing that seeks to minimise drag at subsonic speeds by eliminating non-lifting surfaces. Examples of type are the Northrop HL-10 and Martin-Marietta X-24.
Rocket powered aircraft consume their fuel quickly and so most must land unpowered, unless there is another type of engine. The first was the Lippisch Ente. Later examples include the Messerschmitt Me 163 rocket-powered interceptor and the Messerschmitt Me 323 military glider which was tested with rocket engines to assist take-off. The American series of research aircraft starting with the Bell X-1 in 1946 up to the North American X-15 spent more time flying unpowered than under power. The Space Shuttle orbiters and the Russian Buran are the culmination of these types and are by far the fastest type of gliders to date. The latest examples of rocket glider are the privately-funded SpaceshipOne which is intended for sub-orbital flight and the XCOR EZ-Rocket which is being used to test engines.
Most unpowered rotary-wing aircraft are kites rather than gliders, ie they are usually towed behind a car or boat rather than being capable of free flight. These are known as rotor kites. However rotary-winged gliders, 'gyrogliders', were investigated that could descend like an autogyro or helicopter, using the lift from rotors to reduce the vertical speed. These were evaluated as a method of dropping people or equipment from other aircraft.
A 'radio-controlled glider' is a type of radio-controlled airplane that normally does not have any form of propulsion. Like piloted gliders they can remain airborne for extended periods by using the lift produced by slopes and thermals. They are controlled remotely from the ground with a transmitter. A paper aeroplane, also known as a 'paper plane' or 'paper dart', is another example of a model glider.
Glide bombs are bombs with aerodynamic surfaces to allow a gliding flightpath rather than a ballistic one. This increases the protection of the carrying aircraft that is attacking a heavily defended target. Remote control systems allow the carrying aircraft to direct the bomb to the target. These were developed in Germany from as early as 1915. In World War Two they were most successful as anti-shipping weapons. Some air forces today are equipped with gliding devices that can remotely attack airbases with a cluster bomb warhead.
Published - July 2009
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