Glider

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

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, see the gliding, the hang gliding and paragliding articles.

Related Topics:
Gliding - Hang gliding - Paragliding

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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 sailplane's pilot releases the rope after reaching the required 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).

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Staying aloft without an engine

Flights of five hours (and much longer) by sailplanes are common. These flights are possible because glider pilots seek out rising air masses (lift) that has been created by one or more of several naturally occurring weather phenomena. 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, 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 presense of lift and sink. Having located a thermal, a glider pilot will circle within the area of rising air to gain height. Another form of lift is formed when the wind meets a mountain, cliff or hill. The air mass is deflected up the windward face of the mountain forming lift and sailplanes can climb in this rising air by flying along these features. This is commonly 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. The third main type of lift used by glider pilots occurs downwind of the mountains because the airflow can generate standing waves with alternating areas of lift and sink. 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 sailplane pilots in Australia. Another form of lift results from the convergence of air masses, as with a sea-breeze front.

Related Topics:
Thermals - Cumulus - Variometer - Windward - Appalachians - Andes - South America - Standing waves

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Sailplane 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 sailplanes to soar more effectively than primary gliders, the designs minimised drag. Sailplanes now have very smooth, narrow fuselages and very long, narrow wings with a high aspect ratio.

Related Topics:
Primary glider - Drag - Fuselages - Aspect ratio

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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 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.

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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 17:1 means that in smooth air a sailplane can travel forward 17 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 15 meter gliders such as the ASG29 have a glide ratio of over 50:1. The latest open-class sailplanes with spans of 26 meters can exceed ratios of 60:1 and maintain this efficiency over a wide range of air-speeds.

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Due to the critical role that aerodynamic efficiency plays in the performance of a sailplane, sailplanes often have state of the art aerodynamic features seldom found in other aircraft. A modern racing sailplane will have a specially designed low-drag laminar flow airfoil and a wing produced in a mold with a wing surface that is smooth to within a few thousandths of an inch. 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 may disturb the smooth flow of air over the wing.

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Modern gliders are also designed to carry jettisonable water ballast. 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 climbing in rising air, the same glide angle is achieved 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 because of weaker thermal conditions. To avoid undue stress on the airframe, gliders must jettison the water ballast before landing.

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