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Gliders are un-powered heavier-than-air aircraft.

They can be divided into two broad categories, true gliders and sailplanes.

1. True gliders

True gliders are designed for descent only.

The first heavier-than-air aircraft to be built and flown, Sir George Cayley's Coachman Carrier (1853) was a true glider.

Other examples were the military gliders[?] used in the second world war 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 - due largely to having two sets of wings. Furthermore the glider could be released at some distance from the actual target, making it somewhat more difficult for the enemy to guess their intentions. Larger gliders were also used to land heavier equipment like anti-tank guns and jeeps, which was a major improvement in the power of the otherwise lightly-armed paratroop forces.

The Orbiter vehicles or "space shuttles", which glide to earth at the end of each spaceflight, also fall into this category.

2. Sailplanes

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 article.

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 of worse than 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 60km while losing 1000m of altitude]. In modern gliding competitions, the pilot who is the fastest completing the task set for the day is the winner. 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 gliders 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 and rely on an instrument called a variometer, which measures the climb/sink rate of the plane. Because of their continuous use in gliders, electronic variometers are common, which 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/scanning airspace and weather. Only in gliders, a special variometer system called 'total energy compensation' is used. In this setting, the variomter does not measure the pure climb/sink rate, which changes dramatically when the plane's speed changes because of kinetic/potential energy conversion. Ideally, it only measures true gain/loss of glider energy due to vertical airflows. Total energy compensation in variometers can be achieved all mechanically without microprocessor electronics and has been used in gliders from the early days on.

The commonest method of launching gliders is to be towed behind a powered aircraft, although a variety of other methods are also widely used (see gliding).

Scheibe motor glider.
Larger version

Some sailplanes ("self-launching motor gliders") are actually 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. 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 go 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.

See also: gliding

External links:

  • General:
    • Sailplane Directory (http://www.sailplanedirectory.com/ndxtype.htm) - An enthusiast's web-site that lists manufacturers and models of sailplanes, past and present.

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