Gliding is a
recreational activity and competitive
sport where individuals fly un-powered
aeroplanes usually called
gliders or
sailplanes. Properly, however, "gliding" is the term used for a descending flight of any heavier-than-air craft, when its own weight is its sole motive force. When the craft gains altitude or speed from the atmosphere during the flight, the correct term is "soaring".
While recreational glider enthusiasts enjoy the freedom, scenic views, and sheer enjoyment of controlling the planes, others concentrate on building their own craft, while still others fly in competitions, where the goal is to complete circuit around designated "turning-points", as quickly as possible. These competitions test the pilot's (and the co-pilot, in two-seater gliders) ability to recognise and make use of local weather conditions as well as their flying skills and navigational abilities. There are also glider
aerobatics competitions.
All developments in heavier-than-air flight between
1853 (
Sir George Cayley's coachman), and
1903 (
Wright brothers) involved gliders (See
History of Aviation). However, the
sport of gliding only emerged after the
First World War, and the reason for its development can be traced to the
Treaty of Versailles. The peace settlement imposed severe restrictions on the manufacture and use of single-seater powered aeroplanes in
Germany. Thus, in the 1920s and 1930s, while aviators and aircraft makers in the rest of the world were working to improve the performance of powered aeroplanes, the Germans were designing, developing and flying ever more efficient gliders, and discovering ways of using the natural forces in the atmosphere to make them fly further and faster . The sport has since taken hold in many countries, not only those offering large areas of relatively flat land but also those where the terrain provides more challenging flying. Germany, however, remains the world centre of gliding, as evinced by the fact that all the major glider manufacturers are based in that country.
Soaring is usually achieved by flying through a mass of air that is ascending as fast or faster than the sailplane is descending, and thus gaining
potential energy. The most commonly exploited rising masses of air are
thermals[?] (updrafts of hot ground layer air caused by local differences in air temperature), ridge lift (found where the wind blows against the face of a hill and is forced to rise), and wave lift (
standing waves in the atmosphere, analogous to the ripples on the surface of a stream). Ridge lift rarely allows pilots to climb much higher than about 2,000 ft (600 m); thermals, depending on the climate and terrain, can go as high as 10,000 ft (3 000 m) in flat country and even higher in the mountains; wave lift has allowed gliders to achieve altitudes in excess of 30,000 ft (10 000 m).
On rare occasions, glider pilots have been able to use a technique called "dynamic soaring", where a sailplane can be made to gain kinetic energy by repeatedly crossing the boundary between air masses of differing horizontal velocity. However, such zones of high "wind gradient[?]" are usually much too low to be used safely by aircraft, and dynamic soaring is a technique only really useful to birds, notably to the albatrosses who during long migrations can be seen repeatedly pulling up, turning, and diving back down through the wind gradient close to the surface of the ocean.
In thermalling, the pilot attempts to find bubbles of air that are moving straight up as a result of being heated by contact with sun-lit earth. Typical spots to find thermals are over freshly ploughed fields and asphalt roads, but most of the time thermals are hard to assiciate with features on even ground. As it requires rising heated air, thermalling is typically only effective in mid-latitudes from spring through into late summer, other latitudes tend to have vertical air temperatures-gradients suppressing thermal convection, and during winter there is too little solar heat to start thermals. Once a thermal is encountered, the pilot banks sharply to keep the plane turning in a small circle within the thermal. This way they can ride it upward until it cools off and stops lifting.
Ridge running instead looks for air that is being mechanically lifted as it flows up the sides of hills or other vertical changes in the landscape (including buildings in some cases). Ridge running works in any clime[?] or weather, but can only be used in certain locations. Often a combination of ridge and thermal gliding is used. Ridge lift can kick off strong thermals.
Mountain wave flying is a variation of ridge-running allowing the glider to climb much higher. Most sailplane altitude records were set by flying large scale mountain waves in the top flow of long mountain ranges all over the world.
Glider pilots learn to spot the characteristic cloud formations that usually accompany zones of rising air or "lift". Well-formed cumulus clouds (the fluffy, cotton-wool type of cloud) with sharply defined flat bases often form at the tops of strong thermals, and long, stationary lenticular (lens-shaped) clouds, perpendicular to the wind direction, frequently mark the crests of atmospheric waves.
Gliders are initially launched into the air by one of several methods, most commonly "aerotowing" (being towed behind a powered aircraft by means of a detachable cable) or "winching" (using a stationary ground-based winch possibly mounted on a heavy vehicle). Other less common methods are "auto-towing" (towing behind a car or truck), "reverse auto-towing" (like auto-towing except the launch cable is threaded through a fixed pulley at one end of the airfield, so the truck is driving towards the aircraft) and "bungy-launching" (propulsion from a steep hillside using an elastic rope).
Sailplanes are normally launched by aero-tow with a single engine tow-plane. Lately, strong self-launching motor gliders and microlight planes have also been permitted to tow gliders.
A very economical method for launching gliders is the use of a truck-mounted diesel-engine purpose-built winch. (Launch costs of about EUR 3 are an order of magnitude less compared with an aero-tow.) The winch pulls in a 1000-1200m long steel rope attached to the sailplane, which releases the rope in about 400-500m altitude after an amazingly short and steep ride comparable to a rocket lauch. This method is prevalent in gliding clubs all over Germany.
Outlandings are common in cross country gliding. These are often mistaken for 'emergency landings', but they are entirely normal. The pilot's crew retrieves the plane in a purpose-built trailer which can easily be towed by a car. Modern expensive gliders often have a retractable propeller/two-stroke engine, allowing them to return to their home airfield and avoid the hassle of outlandings. Some of these are powerful enough for independent take-off.
Two minimalistic variations of the sport are
hang gliding, where instead of a fully-fledged plane with full control surfaces and an enclosed cockpit the craft used is basically a fabric flying wing, and
paragliding, where a sophisticated kind of parachute is flown.
See also: glider