Glider

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Gliders

Gliders are heavier-than-air aircraft primarily intended for unpowered flight.

Terminology

A "glider" is an unpowered aircraft. However, the term is also used to refer to motorgliders, which are aircraft that switch off their engines in flight. The design of gliders enables them to climb using rising air instead of merely descending. This has created the sport of gliding, or soaring. The term "sailplane" is also used, implying a high soaring performance. Minimalist variations of the sport also occur in hang gliding and paragliding.

The term "pure glider" (or equivalently, but less commonly "pure sailplane") may be used to distinguish an unpowered glider from a motorglider, without implying any differential in gliding or soaring performance.

History

In ancient China, manned kites were used for military reconnaissance. The first glider seems to have been designed in 500 BC by Lu Ban, a contemporary of Confucius, although this was more of a toy than a genuine aircraft. Some records mention manned gliders in China by AD 500.

Abbas Ibn Firnas, invented (apparently independently of the Chinese) an early form of parachute in 875. It has also been claimed that before he died he developed a glider, and made a ten minute flight. Lacking a 'tail' his aircraft could neither steer nor land properly, and he was injured in the resulting crash.

The first heavier-than-air (i.e. non-balloon) aircraft to be flown in Europe, Sir George Cayley's Coachman Carrier (1853), was a pure glider. 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 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 Colditz Castle 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.

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 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 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. For more about these and other methods (see gliding).

Staying aloft without an engine

Glider pilots can stay airborne for hours. This is possible because they seek out rising air masses (lift) from the following sources:.

Thermals

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.

Ridge lift

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.

Mountain wave

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.

Convergence

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 vortexes 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 [http://www.dropbears.com/brough/index.html.

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 minimised 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 minimised by more aerodynamic shapes and retractable undercarriages.

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.

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 spanwise 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 gliders are also designed to carry jettisonable water ballast in the wings and often in the fin. This is advantageous if the lift is likely to be strong, and may also be used to adjust the glider's centre of gravity. Although heavier gliders have a slight disadvantage when climbing in rising air, the same glide angle is achieved at a higher speed. 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 the water ballast before landing.

Pilots can land accurately by controlling their rate of descent using spoilers. These are either on the upper-wing surface or on both the upper and lower surfaces. A wheel-brake also enables a glider to be stopped after touchdown, which is particularly important in a short field.

Classes of glider

For competitions several 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)
Two Seater Class (maximum wing-span of 20 metres)
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 International 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.

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 list of glider manufacturers, past and present, and the list of gliders.

Instrumentation and other technical aids

Gliders are equipped with an altimeter, compass and an air-speed indicator, and often with a radio, though in some countries the radio is not compulsory. In European countries with crowded airspace there are also proposals that all gliders should fly with anti-collision devices such as transponders but this is still uncertain.

Much more than in other types of aviation, glider pilots depend on an instrument known as a variometer (a very sensitive vertical climb indicator), which measures the climb or sink rate of the plane. This enables the pilot to detect rising and sinking air. Both mechanical and electronic 'varios' are usually fitted to a glider. The electronic variometers produce a beeping noise of variable amplitude and frequency depending on the strength of the lift, so that the pilot can concentrate on watching for other traffic, on navigating and on the weather. (Refer to the variometer article for more information). Rising air is announced to the pilot as a rising tone which the pilot may choose to react to by turning the glider to circle in the lift. Alternately descents are announced with different tone and the pilot will typically accelerate to escape the sink as soon as possible. The same instrument will often also suggest an ideal speed when flying straight after allowing for factors such as water ballast, headwinds/tailwinds and insects on the leading edges of the wings.

Gliders' variometers are often fitted with devices such as the "MacCready Ring" to indicate the optimal speed to fly for given conditions. These devices are based on the mathematical theory developed by Paul MacCready. MacCready theory solves the problem of how fast a pilot should cruise in 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.

Soaring flight computers, in combination with PDAs and specialized soaring software, have been specifically designed for glider use. Using GPS technology these tools are able to:

Provide the glider's position in 3 dimensions by a moving map display
Alert the pilot to nearby airspace restrictions
Indicate contest task position along with managing required course direction and distance
Show airports within gliding distance (ignoring sink/lift)
Calculate and display information to help in remaining aloft
Determine wind direction and speed at current altitude
Show historical lift information
Create a secure GPS log of the flight, required for contest flying

...and a host of other soaring related data.

The flight computer's GPS log may be replayed on specialized computer software to analyse past flights, including watching one or more gliders fly together in a two or three dimension 3-D view. The 3-D representation is shown here with a typical topographical background showing map details such as roads, cities and airports. The glider ("CD") has just executed a series of tight thermalling turns in the Austrian Alps. Other backgrounds might be a satellite image or an FAA sectional map.

Glider markings

To distinguish gliders in flight, large numbers/letters are sometimes put on the fin and wings. Because these numbers were once needed by ground-based observers in competitions, these are known as "competition numbers". They are in addition to the glider's registration and are assigned by national gliding associations. They are useful in radio communications between gliders and so they are often the glider's call-sign.

Fiberglass gliders are white in color after manufacture. Fibreglass resin softens at high temperatures, white reduces temperature rise due to the sun. Color is not used except for a few small bright patches on the wing tips, where high strength is not required, to improve gliders' visibility to other aircraft. The wings of older aluminum and wooden gliders are very often quite brightly painted, not having a temperature-weakening problem.

Aerobatic gliders

Another - less widespread - form of gliding is aerobatics. In this type of competition, the pilot fly a program of maneuvers (such as inverted flight, loop, roll, and various combinations). Each maneuver has a rating called the "K-Factor." This number of points is given if the maneuver is flown perfectly, otherwise a number of points is subtracted. The winner is the pilot with the highest sum of points in each skill based category.

Motor gliders

Some gliders ("self-launching motor gliders") are equipped with propellers that retract into the fuselage. The motor is powerful enough to allow these gliders to launch independently. Recently electric self-launchers such as the Antares have been developed. Others ("self-sustaining motor gliders," also referred to as "turbo" or "sustainer" gliders) are equipped with motors just powerful enough to allow the glider to climb slowly but they must be launched like unpowered gliders. A third type, termed touring motorglider, has a conventional layout with a motor and propellor on the front of the aircraft.

The most important point in favor of powered gliders (retractable engine high-performance types) is that it helps pilots to avoid outlandings. Outlandings, while they are not necessarily dangerous, can be an expensive and time-consuming nuisance for competitive pilots who need to be back home at a set time. Another consideration is that a retrieve crew is needed on stand-by. However the sense of achievement in completing a difficult cross-country is lessened if an engine has been available.

Some people argue that an engine makes the aircraft safer, because the pilot can avoid storms, and can go on to an airstrip to land. An opposing view is that motor gliders are against the spirit of the sport, and, more importantly, that they sometimes give pilots a false sense of security. Even in a motor glider, it is important never to be out of gliding range of a 'landable' area.

More recently, pilot licensing terms have changed in Europe. Powered gliders are now categorized into gliders with retractable propellers/engines, which can be flown with an ordinary glider pilot license (GPL), and touring motor gliders (TMG), which require a specific license extension to the standard GPL. In the United Kingdom, where gliding is regulated by the British Gliding Association, pilots of self-sustaining gliders, like those of pure gliders, do not have to be licensed with the United Kingdom Civil Aviation Authority.

In the United States, a private glider pilot certificate allows the pilot to fly unpowered gliders, self-launching motor gliders (including touring motor gliders and gliders with retractable engines or propellors) and sustainer motor gliders. An instructor must provide instruction and sign the logbook of the pilot to authorize the launch method, which may be by airplane towing, ground launch (winches, bungee, auto tow, etc.) or, in the case of a suitable motor glider, by self-launching.

External links

For more information on gliders and learning to glide, see article on gliding and contact the national gliding federation
- Links to all national gliding federations

Information about all types of glider:
- Sailplane Directory - An enthusiast's web-site that lists manufacturers and models of gliders, past and present.

FAI webpage
- FAI records- sporting aviation page with international world soaring records in distances, speeds, routes, and altitude

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