Gliders are un-powered heavier-than-air aircraft.
They can be divided into two broad categories, pure gliders and sailplanes.
Pure gliders are designed for descent only.
The first heavier-than-air aircraft to be flown, Sir George Cayley's Coachman Carrier (1853), was a pure glider. Otto Lilienthal is another significant contributor to the development of this type of aviation.
Other examples were the military gliders used during WWII to support paratroop operations. These craft were used for a single flight only. The troops landing by glider were referred to as airlanding as opposed to paratroops. A typical cargo plane could carry 8 to 10 soldiers, but that same plane could tow a glider with 20 men in it. Furthermore the glider could be released at some distance from the actual target, making it more difficult for the enemy to guess their intentions. Larger gliders were also used to land heavy equipment like anti-tank guns and jeeps, which was a major improvement in the power available to the otherwise lightly-armed paratroop forces.
The Orbiter vehicles or "space shuttles", which glide to earth at the end of each spaceflight, are also pure gliders.
A sailplane about to land
Sailplanes are specifically intended for the sport of gliding. Their design enables them to use energy from the atmosphere to "soar"; they can climb as well as descend. For more about soaring, please see the gliding, the hang gliding and paragliding articles.
To enable them to soar, sailplanes are designed to minimise drag. They have very smooth, narrow fuselages and very long, narrow wings with a high aspect ratio. New materials such as carbon fiber plastic and computer-aided design have increased performance. While early gliders have had glide ratios below 20 to 1, the latest open-class competition models can exceed ratios of 60 to 1 and maintain this efficiency over a wide range of air-speeds. (The ratio of 60:1 means that in smooth air the sailplane can horizontally travel 60 m while losing 1 m of altitude).
In modern cross-country gliding competitions, the winner is the pilot who is the fastest completing the task set for the day. To adapt the glider to the thermals and lift available at the time, the competition rules sometimes allow gliders to carry jettisonable water ballast. Heavier planes have a slight disadvantage climbing in rising air, but the extra wing load shifts the glider's performance curve into higher velocities, so that the plane can achieve the same glide ratio at a higher velocity. While 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 when the thermal conditions weaken in the evening.
Much more than in other types of aviation, glider pilots use an instrument called a variometer, which measures the climb or sink rate of the plane. Electronic variometers code their reading into an acoustic signal of variable amplitude and frequency, so that it can be used without drawing the pilot's attention away from watching the airspace and the weather. For more about this, please see the variometer article.
The most common method of launching gliders is behind a powered aircraft, although other methods are also used (see gliding).
A modern aerobatic glider.
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 variuos 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 after flying all programs of the competition.
A Scheibe SF25C - a typical old-style touring motorglider.
Some sailplanes ("self-launching motor gliders") are equipped with motors, usually retractable into the fuselage, powerful enough to allow the gliders to launch independently. Others ("self-sustaining motor gliders") are equipped with motors just powerful enough to allow the glider to climb slowly under its own power after an assisted takeoff. A third type, termed touring motorglider, has a conventional layout with a motor and propellor on the front of the aircraft. Some people argue that an engine makes the aircraft safer, because the pilot can avoid storms, and can go 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 invulnerability. In gliding and in single-engine flying in general, it is important never to be out of gliding range of a safe landing site.
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.
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. This does not apply in the United Kingdom, where glider pilots are regulated by the British Gliding Association using a "badge" system, and do not have to be licensed with the Civil Aviation Authority.
- In Conway's Game of Life a glider is a certain small structure that moves indefinitely in a direction.
- A comfortable swinging padded seat with a back, usable by more than one person, and typically used on a porch or veranda, is also called a glider.
- Sailplane Directory (http://www.sailplanedirectory.com/ndxtype.htm) - An enthusiast's web-site that lists manufacturers and models of sailplanes, past and present.
- USA organization (http://www.ssa.org) A good USA site for information on soaring and gliding with many links to clubs or operators and sources of equipment.
- Web-sites of the major modern sailplane manufacturers:
- Alexander Schleicher (AS) (http://www.alexander-schleicher.de/)
- DG Flugzeugbau (DG) (http://www.dg-flugzeugbau.de/index-e.html)
- Schempp-Hirth (http://www.schempp-hirth.com/en/)
- Stemme (http://www.stemme.de/daten/e/)