Gliding is a recreational activity and competitive sport in which pilots fly un-powered aircraft known as gliders or sailplanes. Properly, the term gliding refers to descending flight of a heavier-than-air craft, whereas soaring is the correct term to use when the craft gains altitude or speed from rising air. When soaring conditions are good enough, experienced pilots can fly hundreds of kilometres before returning to their home airfields, and occasionally flights over 1,000 kilometres are made. However, if the weather deteriorates, they may need to land elsewhere, but motorglider pilots can avoid this by starting an engine.
While many glider pilots merely enjoy the sense of achievement, some competitive pilots fly in races around pre-defined courses. These competitions test the pilots' abilities to make best use of local weather conditions as well as their flying skills. Local and national competitions are organized in many countries and there are also biennial World Gliding Championships.
Powered aircraft and winches are the two most common means of launching gliders. These and other methods (apart from self-launching motor-gliders) require assistance from other participants. Gliding clubs have thus been established to share airfields and equipment, train new pilots and maintain high safety standards.
The first German gliding competition was held at the Wasserkuppe in 1920, organized by Oskar Ursinus. The best flight lasted two minutes and set a world distance record of 2 km. Within ten years, it had become an international event in which the achieved durations and distances had increased greatly. In 1931, Gunther Grönhoff flew 272 km (169 miles) from Munich to Czechoslovakia, further than had been thought possible.
In the 1930s, gliding spread to many other countries. In the 1936 Summer Olympics in Berlin gliding was a demonstration sport, and it was scheduled to be a full Olympic sport in the 1940 Games. A glider, the Olympia, was developed in Germany for the event, but World War II intervened. By 1939 the major gliding records were held by Russians, including a distance record of 748 km (465 miles).
During the war, civilian gliding in Europe was largely suspended. Although some military operations in WWII involved military gliders, they did not soar and so are unrelated to the sport of gliding. Nonetheless, several German fighter aces in the conflict, including Erich Hartmann, began their flight training in gliders.
Gliding did not return to the Olympics after the war, for two reasons: first, the shortage of gliders following the war; and second, the failure to agree on a single model of competition glider. (Some in the community feared doing so would hinder development of new designs.) The re-introduction of air sports such as gliding to the Olympics has been occasionally proposed by the world governing body, the FAI, but this has been rejected on the grounds of lack of public interest.
In many countries during the 1950s a large number of trained pilots wanted to continue flying. Many were also aeronautical engineers. They started both clubs and manufacturers, many of which still exist. This stimulated the development of both gliding and gliders; for example, the Soaring Society of America grew from 1,000 members then to its present total of 12,500. The increased numbers of pilots, greater knowledge and improving technology helped set new records, so that the pre-war altitude record was doubled by 1950, and the first 1,000 km (621 statute miles) flight was achieved in 1964. New materials such as glass fiber and carbon fiber, advances in wing shapes and airfoils, electronic instruments, GPS and improved weather forecasting have since allowed many pilots to make flights that were once extraordinary. Today over 500 pilots have made flights over 1,000 km.
Instead of Olympic competition there are the World Gliding Championships. The first event was held at the Wasserkuppe in 1937. Since WWII it has been held every two years. There are now six classes open to both sexes, plus three classes for women and two junior classes. Germany, the sport's birthplace, is still a center of the gliding world: it accounts for 30% of the world's glider pilots, and the three major glider manufacturers are still based there. However the sport has been taken up in many countries and there are now over 116,000 active glider pilots, plus an unknown number of military cadets. Each year many other people experience their first glider flight. It does not matter whether the countries are flat or mountainous, hot or temperate, because gliders can soar in most places.
Glider pilots can stay airborne for hours by flying through air that is ascending as fast or faster than the glider itself is descending, thus gaining potential energy. The most commonly used sources of rising air are
Ridge lift rarely allows pilots to climb much higher than about 600 m (2,000 ft) above the terrain; thermals, depending on the climate and terrain, can allow climbs in excess of 3,000 m (10,000 ft) in flat country and much higher above mountains; wave lift has allowed a glider to reach an altitude of 15,447 m (50,671 ft). In a few countries, gliders may continue to climb into the clouds in uncontrolled airspace, but in many countries the pilot must stop climbing before reaching the cloud base (see Visual Flight Rules).
When the air has little moisture or when an inversion stops the warm air from rising high enough for the moisture to condense, thermals do not create cumulus clouds. Without clouds or dust devils to mark the thermals, the pilot must use his skill and luck to find them using a sensitive vertical speed indicator called a variometer that quickly indicates climbs or descents. Typical locations to find thermals are over towns, freshly ploughed fields and asphalt roads, but thermals are often hard to associate with any feature on the ground. Occasionally thermals are caused by the exhaust gases from power stations or by fires.
As it requires rising heated air, thermalling is only effective in mid-latitudes from spring through into late summer. During winter the solar heat can only create weak thermals, but ridge and wave lift can still be used during this period.
Glider pilots have been able to use a technique called "dynamic soaring", where a glider can gain kinetic energy by repeatedly crossing the boundary between air masses of different horizontal velocity. However, such zones of high "wind gradient" are usually too close to the ground to be used safely by gliders.
Aerotows normally use single-engined light aircraft, although motor gliders have also been permitted to tow gliders. The tow-plane takes the glider to the desired height and place where the glider pilot releases the rope. A weak link is often fitted to the rope to ensure that any sudden loads do not damage the airframe of the tow-plane.
During the aerotow, the glider pilot keeps the glider in one of two positions behind the tow-plane. This position can either be the "low tow" position, just below the wake from the tow-plane, or the "high tow" position just above the wake. In Australia the convention is to fly in low tow, whereas in the United States and Europe the high tow prevails. One aerotow variation is to attach two gliders to one tow-plane, using a short rope for the high towed glider and the long rope for the low tow.
Gliders are often launched using a stationary ground-based winch mounted on a heavy vehicle. This method is widely used at many European clubs, often in addition to aerotowing. The engine is usually a large diesel, though hydraulic fluid engines and electrical motors are also used. The winch pulls in a 1,000 to 1,600 m (3,000 to 5,500-foot) cable, made of high-tensile steel wire or a synthetic fiber, attached to the glider. The cable is released at a height of about 400 to 700 m (1,300 to 2,200 feet) after a short and steep ride.
The main advantage of a winch launch is its lower cost, but the launch height is usually lower than an aerotow, so flights are shorter unless the pilot can quickly make contact with a source of lift within a few minutes of releasing the cable. Although there is a risk of the cable breaking during this type of launch, pilots are trained to deal with this.
Another launch method, the "autotow", is rarer nowadays. The direct towing method requires a hard surface, a powerful vehicle and a long steel cable. After gently taking up slack in the cable, the driver accelerates hard and the glider rises like a kite to as much as 400 m (1300 ft) if there is a good headwind and a runway of 1.5 km (1 mile) or more. This method has also been used on desert dry lakes.
A variation on this is the "reverse pulley" method in which the truck drives towards the glider that it is launching with the cable passing around a pulley at the far end of the airfield, with an effect similar to a winch launch.
The distance that a glider can fly for each meter it descends is expressed as its lift-to-drag ratio (L/D). Depending on the class, this can be between 44:1 and 70:1 in modern designs. This performance combined with regular sources of rising air enables gliders to fly long distances at high speeds. The record average speed for 1,000 km is 169.7 km/h (621 statute miles at 105 miles/h). Even in places with less favorable conditions (such as Northern Europe) most skilled pilots complete flights over 500 km (310 miles) every year.
Glider pilots are required to stay within gliding range of their home airfield for their early solo flights as student pilots. Cross-country flights are allowed when they have sufficient experience to find sources of lift away from their home airfield, to navigate and to land elsewhere if necessary. As the performance of gliders improved in the 1960s, the concept of flying as far away as possible became unpopular with the crews who had to retrieve the gliders. Pilots now usually plan to fly around a course (called a task) via turn-points, returning to the starting point.
In addition to just trying to fly further, glider pilots also race each other in competitions. The winner is the fastest, or, if the weather conditions are poor, the furthest round the course. Tasks of up to 1,000 km have been set and average speeds of 120 km/h are not unusual.
Initially, ground observers confirmed that pilots had rounded the turn-points. Later, the glider pilots photographed these places and submitted the film for verification. Today, gliders carry secure GNSS Flight Recorders that record the position every few seconds from GPS satellites. These recording devices now provide the proof that the turn-points have been reached.
National competitions generally last one week, with international championships running over two. The winner is the pilot who has amassed the greatest number of points over all the contest days. However, these competitions have as yet failed to draw much interest outside the gliding community for several reasons. Because it would be unsafe for many gliders to cross a start line at the same time, pilots can choose their own start time. Furthermore, gliders are not visible to the spectators for long periods during each day's contest and the scoring is complex, so gliding competitions have been difficult to televise.
In an attempt to widen the sport's appeal, a new format, the Grand Prix, has been introduced. Innovations introduced in the Grand Prix format include simultaneous starts for a small number of gliders, tasks consisting of multiple circuits, and simplified scoring. There is decentralized Internet based competition called the Online Contest where pilots upload their GPS data files and are automatically scored based on distance flown. 7,800 pilots worldwide participated in this contest in 2006.
On cross-country flights where strong lift is forecast, pilots fly with water ballast stored in tanks or bags in the wings and fin. The fin tank is used to reduce trim drag by optimizing the center of gravity, which typically would shift forward if water is stored only in the wings ahead of the spar. Ballast enables a sailplane to attain its best L/D at higher speeds but slows its climb rate in thermals, in part because a sailplane with a heavier wing loading cannot circle within a thermal as tightly as one with a lower, unballasted wing loading. But if lift is strong, typically either from thermals or wave, the disadvantage of slower climbs is outweighed by the higher cruising speeds between lift areas. Thus, the pilot can improve the speed over a course by several percent or achieve longer distances in a given time. If lift is weaker than expected, or if an off-field landing is imminent, the pilot can jettison the water ballast by opening the dump valves.
If lift is not found during a cross-country flight, for example because of deteriorating weather, the pilot must choose a field and 'land out'. Although inconvenient and often mistaken for "emergency landings", landing out (or "outlanding") is a routine event in cross-country gliding. The pilot has to choose a field where the glider can be landed safely, without damaging property such as crops or livestock.
The glider and the pilot(s) can be retrieved from the field using a purpose-built trailer. Alternatively, if the glider has landed in a suitable field, a tow-plane can be summoned to re-launch the aircraft (as long as the property owner gives permission). The glider pilot typically pays for the time the tow-plane is in the air, both to and from the field, so this alternative can become expensive.
All power units have to be started at a height that includes a margin that would still allow a safe landing-out to be made, if there were a failure to start. In a competition, using the engine ends the soaring flight. Unpowered gliders are lighter and, as they do not need a safety margin for starting the engine, they can safely thermal at lower altitudes in weaker conditions. Consequently, pilots in unpowered gliders may complete competition flights when some powered competitors cannot. Conversely, motor glider pilots can start the engine if conditions will no longer support soaring flight, while unpowered gliders will have to land out, away from the home airfield, requiring retrieval by road using the glider's trailer. Opinions differ whether difficult flights, where an engine was always available, are as satisfying as flights in pure gliders.
Aerobatic competitions are held regularly. In this type of competition, the pilots fly a program of maneuvers (such as inverted flight, loop, roll, and various combinations). Each maneuver has a rating called the "K-Factor". Maximum points are given for the maneuver if it is flown perfectly; otherwise, points are deducted. Efficient maneuvers also enable the whole program to be completed with the height available. The winner is the pilot with the most points.
Most clubs offer trial lessons to people interested in learning to glide. National gliding associations have contact details for their member clubs. Because most gliders are designed to the same specifications of safety, the upper weight limit for pilots, after allowing for a parachute, is usually 103 kg (228 pounds). People over 193 cm (6’ 4’’) will also have problems. The pupil flies with an instructor in a two-seat glider fitted with dual controls. The instructor performs the first launches and landings, typically from the back seat, but otherwise the pupil manages the controls. Some clubs offer courses over several days, with a mixture of winch and aerotow launches. It may take ab initios at least 50 training flights before they are deemed to have the skill and the airmanship necessary to fly solo.
If winches are used, the cost of learning to glide is much less than that of learning to fly powered aircraft. Training using aerotow costs more than using winches, though fewer launches (as few as 25) might be needed. Simulators are also beginning to be used in training, especially during poor weather.
After the first solo flight, further training with an instructor continues until the pupil is capable of taking a glider cross-country. In most countries pilots must sit examinations on the regulations, navigation, use of the radio, weather, principles of flight and human factors.