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A "glider" is an unpowered aircraft. The most common types of glider are today used for sporting purposes. The design of these types enables them to climb using rising air. This has created the sport of gliding. Although many gliders do not have engines, there are some that use engines occasionally.

Early gliders had no cockpit and the pilot sat on a small seat located just ahead of the wing. 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, the designers minimized 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, struts and control cables. New materials such as carbon-fiber, glass-fiber and Kevlar have since been used with computer-aided design to increase performance. Drag has also been minimized 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.

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 glider have a specially designed low-drag 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 are used to direct 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 competition gliders are also designed to carry jettisonable water ballast (in the wings and sometimes in the vertical stabiliser). The extra weight provided by the water ballast is advantageous if the lift is likely to be strong, and may also be used to adjust the glider's center of mass. To avoid undue stress on the airframe, gliders must jettison any water ballast before landing.

Pilots can land accurately by controlling their rate of descent using spoilers, also known as air brakes. These are metal devices which extend from either the upper-wing surface or from both upper and lower surfaces, thereby destroying some lift and creating additional drag. A wheel-brake also enables a glider to be stopped after touchdown, which is particularly important in a short runway.

UNIT 3

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