A glider or sailplane is a type of glider aircraft used in the leisure activity and sport of gliding (also called soaring). This unpowered aircraft can use naturally occurring currents of rising air in the atmosphere to gain altitude. Sailplanes are aerodynamically streamlined, so they can fly a significant distance forward for a small decrease in altitude. In North America, the term 'sailplane' is also used to describe this type of aircraft. In other parts of the English-speaking world, the word 'glider' is more common.
Types
Gliders benefit from producing very low drag for any given amount of lift. This factor is best achieved with long, thin wings, a slender fuselage, and smooth surfaces with an absence of protuberances. Aircraft with these features are able to soar, meaning they may climb efficiently in rising air produced by thermals or hills. In still air, sailplanes can glide long distances at high speed with a minimum loss of height in between. Sailplanes have rigid wings and either skids or undercarriage. In contrast, hang gliders and paragliders use the pilot's feet for the start of the launch and for the landing. These latter types are described in separate articles, though their differences from sailplanes are covered below. Sailplanes are usually launched by winch or aerotow, though other methods, such as auto tow and bungee, are occasionally used. These days, almost all gliders are sailplanes, but in the past, many gliders were not. These types did not soar. Instead, they were simply engineless aircraft towed by another aircraft to a desired destination and then cast off for landing. The prime example of a non-soaring glider was the military glider (such as those used in the Second World War). They were often used just once, then usually abandoned after landing, having served their purpose. Motor gliders are gliders with engines that can be used for extending a flight and even, in some cases, for take-off. Some high-performance motor gliders (known as "self-sustaining" gliders) may have an engine-driven retractable propeller, which can be used to sustain flight. Other motor gliders have enough thrust to launch themselves before the propeller is retracted and are known as "self-launching" gliders. Another type is the self-launching "touring motor glider", where the pilot can switch the engine on and off in flight without retracting the propeller.
History
Sir George Cayley's gliders achieved brief wing-borne hops from around 1849. In the 1890s, Otto Lilienthal built gliders using weight shift for control. In the early 1900s, the Wright Brothers built gliders using movable surfaces for control. In 1903, they successfully added an engine. After World War I, gliders were built for sporting purposes in Germany. Germany's strong links to gliding were in large part due to post-World War I regulations, where Germany was not permitted to construct or fly motorized planes. Accordingly, the country's aircraft enthusiasts often turned to gliders. Many of these enthusiasts were actively encouraged by the German government, particularly at flying sites suited to gliding flight like the Wasserkuppe. The sporting use of gliders rapidly evolved in the 1930s; it is now their main application. As their performance improved, gliders began to be used for cross-country flying. They now regularly fly hundreds or even thousands of kilometres in a day when the weather is suitable.
Design
Glider airframes include a fuselage, wings, and empennage section. Self-launching gliders also include built-in engines. Early gliders had no cockpit; 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 minimized drag. Gliders now have very smooth, narrow fuselages and very long, narrow wings with a high aspect ratio and winglets.
The early gliders were made mainly of wood with metal fastenings, stays, and control cables. Later fuselages made of fabric-covered steel tubes were married to wood and fabric wings for lightness and strength. New materials such as carbon fiber, fiberglass, 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 minimized by more aerodynamic shapes and retractable undercarriages. Flaps are fitted to the trailing edges of the wings on some gliders to optimise lift and drag at a wide range of speeds. 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 losing only 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 36:1, and modern 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. Compare this to the Gimli Glider, a Boeing 767 that ran out of fuel mid-flight and was found to have a glide ratio of 12:1, or to the Space Shuttle with a glide ratio of 4.5:1.
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