Gliding

Gliding (or soaring) is a recreational activity and competitive sport where pilots fly un-powered aeroplanes 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.

The words gliding and soaring are also used to describe the ways birds capable of flight remain aloft without flapping their wings; the mechanics of this process are explained in the article on bird flight, while this article focuses on aircraft.

Recreation versus sport

While recreational glider enthusiasts enjoy the freedom, scenic views and enjoyment of controlling the aircraft, others compete (up to World Championship level), or practise competing, by flying as quickly as possible around a circuit defined by "turning-points". These competitions test the pilots' (and, in two-seat gliders, the co-pilots') ability to recognise and make use of local weather conditions, their flying skills and navigational abilities. There are also glider aerobatics competitions.

All methods of launching gliders (apart from self-launching motor-gliders) require assistance from other participants and so sailplane pilots band together within clubs to share an airfield and launch equipment, and to maintain high safety standards. Since assistance is also needed to rig and retrieve gliders as well as to train new pilots, there is an important social aspect to the sport.

History

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 first German gliding competition was held at the Wasserkuppe in 1920, organised by Oskar Ursinus, and ten years later had become an international event. The sport has since been taken up in many countries. It does not matter whether the countries are flat or mountainous, hot or temperate, because gliders can soar in most places. Germany, however, remains the world centre of gliding, and all the major glider manufacturers are still based there. In Olympic Games in 1936 in Berlin gliding was a demonstration sport and was scheduled to be a full Olympic sport in the 1940 games.

Soaring

Soaring is achieved by flying through a mass of air that is ascending as fast or faster than the sailplane is descending, thus gaining potential energy. The most commonly used rising masses of air are thermals (updrafts of warm air), 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) above the terrain; thermals, depending on the climate and terrain, can exceed 10,000 ft (3,000 m) in flat country and much higher in the mountains; wave lift has allowed gliders to achieve altitudes approaching 50,000 ft (15,000 m).

Thermals are formed on the ground through the warming of the surface by sunlight. In thermal flight, the glider pilot finds streams of air that are moving upwards as a result of being heated by contact with sun-lit earth. If the air contains enough moisture, the water will condense from the rising air and form cumulus clouds. (In hot countries, thermals can be marked by dust devils.) Well-formed cumulus clouds (the fluffy, cotton-wool type of cloud) with sharply defined flat bases often form at the tops of strong thermals. Once a thermal is encountered, the pilot banks to keep the plane turning in a circle within the thermal and so can gain altitude. Climb rates depend on conditions, but several metres per second is common. When thermals are produced in an approximate line often because of the local winds or terrain, cloud streets may become visible and allow the pilot to fly straight while climbing in continuous lift.

Sometimes thermals do not create cumulus clouds. This can happen when the air has little moisture or when an inversion stops the thermal from rising high enough for the moisture to condense. Without clouds or dust devils to mark the thermals, the pilot must use his skill and luck to find them, and a sensitive instrument in the aircraft called a variometer or vertical speed indicator that indicates rates of climb or descent. 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 the thermal is caused by the exhaust gases from power stations or, less commonly, by the burning of the stubble of cereal crops.

As it requires rising heated air, thermalling is typically only effective in mid-latitudes from spring through into late summer. Other latitudes often have a lower inversion, a layer of warm air which stops the air in the thermals from rising to high altitudes. During winter the solar heat can only create weak thermals.

A pilot who is ridge soaring looks for air that is being lifted as it flows up the sides of hills. In places where a steady wind blows, a ridge may allow virtually unlimited time aloft. Ridge lift is present whenever the wind blows in any weather but sometimes it is augmented by thermals when the slopes also face the sun.

Mountain waves give long stretches of rising air and allow gliders to climb high, long before the sun has started heating the ground. Most sailplane altitude records have therefore been set by using mountain waves from long mountain ranges all over the world. The current World Distance Record of 3008 km by Klaus Ohlmann (on 21 January 2003) was also flown in the mountain wave in South America. Long, stationary lenticular (lens-shaped) clouds, perpendicular to the wind direction, frequently mark the crests of atmospheric waves. Waves can produce powerful lift and sink.

In a few countries gliders can continue to climb into the clouds in uncontrolled airspace but in many countries the pilot must stop climbing at cloud-base (see Visual Flight Rules).

Supplementary oxygen is required in gliders for flights above 12,500 feet above sea level in countries where such altitudes are possible, to avoid the life-threatening risk of hypoxia, also known as anoxia.

A good visual and interactive explanation of how a glider functions and the main types of lift can be found at http://www.yorksoaring.com/whatissoaring.html .

Other sources of lift

The boundary where two air masses meet is known as a convergence zone. Glider pilots can gain altitude by flying along this intersection as if it were a ridge of land. This can occur in sea breezes or in desert regions. A rare convergence phenomenon known as Morning Glory has also been used by sailplane pilots in Australia. Convergence may occur over considerable distances and so may permit virtually straight flight while climbing.

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 different horizontal velocity. However, such zones of high "wind gradient" are usually much too low to be used safely by aircraft, so dynamic soaring is a technique only really useful to radio control model aircraft and to birds, notably to the albatrosses who during long flights can be seen repeatedly pulling up, turning, and diving back down through the wind gradient close to the surface of the ocean.

Badges

Achievements in gliding have been marked by the awarding of badges since the 1920s. For the lower badges national glider federations set their own criteria. For example, in the United States an "A" badge is issued for the first solo, while "B" and "C" require longer flights and more training. A bronze badge shows preparation for cross-country work, including spot landings and a pair of two hour flights.

The higher badges follow the standards set down by the Federation Aeronautique Internationale. Earning the Silver Badge shows that a glider pilot has achieved an altitude gain of at least 1000 m, made a five-hour duration flight, and has flown cross-country for a straight-line distance of at least 50 km: usually, but not invariably, in separate flights. The FAI Sporting Code defines the rules for observers and recording devices to validate the claims for badges. In the United States alone, over 6000 Silver Badges have been issued.

The Gold and Diamond Badges require pilots to fly higher and farther. A pilot with the three "Diamonds" has flown 300 km to a pre-defined goal, has flown 500 km in one flight (but not necessarily to a pre-defined goal) and gained 5000 m in height. The FAI also issues diplomas for 1000 km and thereafter in increments of 250 km. The ultimate challenge is to add a 2000 km diploma for a single flight exceeding that distance. Only a few people have ever achieved it.

National federations also issue other badges. For example, The Soaring Society of America also issues badges for going above 25,000 feet (7,620 m) and for enough cross-country flying to circle the world. The British Gliding Association issues a 750 km diploma, because only two flights over 1000 km have ever been possible in the UK's climate.

Launch methods

Gliders are launched into the air by one of several methods, the most common are "aerotowing" and "winching".

Aerotows normally use single engined light aircraft, but lately, powerful self-launching motor gliders and microlight planes have also been permitted to tow gliders. The tow aircraft takes the glider to the desired height and place and the pilot releases the rope. Aerotow ropes are typically made of polypropylene rope and are between 50 and 60 metres in length. At the tow plane end, a weak link is fitted to the rope to ensure that any sudden loads imposed by the glider getting out of station do not damage the airframe of the tow plane.

During the aerotow, the glider pilot keeps the glider "in station" behind the tow plane. This can either be the "low tow" position, just below the slipstream of the tow plane propellor, or the "high tow" position just above the slipstream. Over the years there has been great debate about which of these two positions is the safest, and there has been no universal agreement. In Australia the convention is to fly in low tow, whereas in the United States the high tow prevails.

One interesting aerotow variation is to perform a "dual tow" in which two gliders are attached to the one tow plane, using ropes of different lengths, short rope on high tow, long rope on low tow. This certainly looks spectacular, but requires skill and precise flying by all concerned.

Gliders are often launched using a stationary ground-based winch, sometimes mounted on a heavy vehicle. This method is widely used in many European countries, often in addition to aerotowing. The engine is usually from a large car or a diesel truck (sometimes using LPG), though hydraulic fluid engines and electrical motors are sometimes used. The winch pulls in a 1000 to 1600 m long cable made of steel wire or a synthetic fibre which is attached to the glider. The glider releases the cable at a height of about 400 to 500 m after a very short and steep ride. The main advantage of the winch launch is that it costs much less than an aero-tow. One disadvantage of winch launching is that the launch height is variable with the wind strength and cable run length, which could mean that the duration of flights is slightly shorter unless the pilot is fortunate enough to make contact with a thermal or other source of lift within a few minutes of releasing the cable.

Rarely gliders can also be launched from the top of a hill into a stiff breeze using a rubber band, or "bungee". For this launch method, the glider's main wheel rests in a small concrete trough. The hook that is normally used for winch-launching is used instead to attach the middle of the bungee. Each end of the bungee is then pulled by 3 or 4 people. One group runs slightly to the left, the other to the right of the glider. Once the tension in the bungee is high enough, the pilot releases the wheel brake and the glider's wheel pops out of the trough. The glider gains just enough energy to leave the ground and fly away from the hill.

Another launch method, now rarely used, is the "autotow". This needs a long runway, a large pick-up truck and a length of 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 metres if there is a good headwind and a 1.5 km runway. In some places a low autotow is used just to get the glider airborne but thereafter ridge lift from a cliff is used. A variation on this is the "reverse pulley" method in which the car drives towards the glider that it is launching; the cable connecting the car and glider passes around a pulley at the far end of the airfield. The reverse has the advantage that, at the end of the launch run, the cable is already in the correct position for the next launch.

Glider licenses in the U.S. differentiate between aerotows and ground launch methods, due to the widely different techniques. Glider pilots who want to use both launch methods need to show proficiency in both when getting tested for their license.

Cross-country

Gliders can stay airborne for hours if the conditions are good. This enables gliders to fly long distances at surprisingly high speeds. Although Klaus Ohlmann's world record is obviously not a typical flight, even in less favourable places in Europe good pilots usually have flights over 500 kilometres every year at average speeds of 80 km/h or faster.

In addition to just trying to fly further, gliders also race each other. 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 win contests by being the fastest around a pre-defined course (called a task) back to the starting point, or, if the weather is not as good as expected, the furthest round the course. Originally proof of getting to the turning points was by observing the gliders from the ground. Later the pilots took photographs of the turn-points but nowadays gliders carry secure devices that record the position every few seconds from GPS satellites. National competitions generally last one week but international championships are normally over two weeks. The winner is the pilot who has amassed the greatest number of points over all the contest days. Because it would be unsafe for many gliders to cross a start line at the same time, pilots can choose their own start time. Gliders are not visible to spectators for long periods of each day's contest and scoring is complex, so gliding has been a difficult sport to televise. This means that soaring is a sport in which most contestants are still amateurs. However, a new format contest has been introduced see Sailplane Grand Prix. Also gaining popularity in recent years is an informal online contest called the OLC where pilots upload their GPS data files and are automatically scored based on distance flown. Nearly 9,000 pilots worldwide participate.

Soaring pioneer Paul MacCready developed a mathematical theory for optimizing cross country soaring speeds. His theory allows the optimal cruising speed between thermals to be computed, accounting for thermal strength, sailplane performance and other variables. The theory accounts for the fact that if a pilot flies faster between thermals, the next thermal is reached sooner. However the glider also sinks faster, requiring the pilot to spend more time circling to regain the altitude. The MacCready speed represents the optimal tradeoff between cruising and circling. Most competition pilots make use of MacCready theory to optimize their flight speeds, and have the necessary calculations programmed in their flight computers.

On cross country flights, pilots often fly with water ballast, which is stored in integral water tanks in the wings and sometimes also in the tail fin. The ballast makes the glider fly faster, but it also makes the glider climb more slowly in thermals. However, if the thermals are strong, the disadvantage of slower climbs become small compared to the advatage of higher cruising speeds between thermals. Thus, the pilot can signficantly improve the speed over the course by several percent. In competitions, pilots may add more than 150 liters of water, giving a total wing loading of 50 kg/sqm or more. However, to prevent damage to the glider while landing, the pilot should dump the water before landing.

Outlandings

Sometimes a pilot on a cross-country flight finds that the weather is not as good as expected. In these circumstances, the pilot must choose a field and 'land-out'. Landing out is a routine event in cross-country gliding, though they are often mistaken for 'emergency landings'. They are entirely normal, although they are an inconvenience. The pilot has to choose from the air a field that is safe to land in and which does not cause damage to the property.

The glider and pilot can be retrieved by pilot's ground crew using a purpose-built trailer which can easily be towed by a car. Alternatively, if the glider has landed in a suitable field, a tow plane can be summoned to re-launch the aircraft (with the permission of the field's owner).

To avoid the inconvenience of landing out, some gliders have a small engine and a retractable propeller. Some of these engines are not powerful enough to launch the glider, but they can provide enough power to allow gliders to stay airborne and so to return to their home airfields. However, an engine has to be started at a height that includes a margin that would still allow a safe outlanding to be made, if the engine were to fail to start. Consequently gliders without an engine will sometimes be able to thermal safely below that height, find lift and continue on their task. An engine also adds to the weight and expense of a glider.

Hazards

Although considered a relatively safe form of aviation, there are hazards in gliding. Gliders surround the pilot with a strong structure and most accidents occur at a low speed causing no injuries. Training and safe procedures are central to the ethos of the sport. Nevertheless a small number of fatal accidents occur every year, almost all caused by pilot error. Causes include:

mid-air collisions: gliders pilots fly to small areas of lift, especially thermals and ridges, and so there is a risk of collision. Pilots must comply with the rules of the air and keep a good look-out. They also usually wear parachutes. In Switzerland, Austria and Germany, the Flarm warning system is used to avaid mid-air collisions between gliders
incorrect procedure during launch, especially when using a winch. A wing catching the ground, unlocked airbrakes, too rapid rotation into the full climb and too slow a speed during the launch can cause problems
incorrect landing procedures: a poorly trained pilot can undershoot or overshoot the landing area and so may collide with the field boundary
low speeds while turning before landing, especially if excessive rudder is used, can cause a spin. The training of glider pilots emphasises spin avoidance and spin recovery
in-flight structural failures: these occur rarely because large safety margins are used when building gliders. Very high loads are needed to cause structural failures but they can arise during aerobatics or when incorrectly recovering from a loss of control. Annual inspections of gliders and training reduce the already minimal chances of structural problems
unconnected controls and other fittings: gliders are designed to be rigged each day. If the glider is not assembled properly, a pilot will not be able to control the glider. The controls of most of the gliders built in the last twenty years connect automatically
outlandings: there is some risk of striking power lines or other unseen objects by cross-country pilots during an outlanding
contact with terrain: pilots who fly in mountainous terrain are especially wary of the risk of flying too close, especially in turbulent conditions
hypoxia: pilots who fly high should be aware of the need to use supplementary oxygen, incidents are therefore rare
failure to anticipate severe weather: storms may cause rapid and unpredictable changes in the wind when landing.

Learning to glide

Most clubs offer trial lessons to people interested in learning to glide and will accept bookings by phone. The links to national organisations below give the contact details for the nearest clubs. The pupil flies with an instructor in a two-seat glider fitted with dual controls. The instructor does the first launches and landings but otherwise the pupil uses the controls. People with the skill to drive a car can usually learn to fly a glider. Some clubs offer courses over several days, though, with a mixture of winch and aerotow launches, it often takes ab initios at least 50 training flights before they are allowed to fly solo.

If winches are used, the cost of learning to glide is much less than that of learning to fly powered aircraft. However the cost is much greater if aerotowing is the only available method of launching, even though fewer launches might be needed, perhaps as few as 30. Further training continues after the first solo until the pupil is judged capable of taking a glider cross-country. Some studying is required on topics such as the regulations, use of the radio, weather and navigation.

Notable people who were also glider pilots

Neil Armstrong - astronaut
Kalpana Chawla - astronaut
John Denver - singer/songwriter
Richard C. du Pont - director of military glider program
Steve Fossett - entrepreneur and record breaker
Barron Hilton - hotel magnate
Paul MacCready - aviation inventor
Steve McQueen - actor
Mike Melvill - Spaceship One test pilot
Robert Pearson - airline pilot who glided a Boeing 767
Derek Piggott - movie stunt pilot
Christopher Reeve - actor
Hanna Reitsch - test pilot
Cliff Robertson - actor
Peter Scott - naturalist (founder of World Wildlife Fund)
Peter Twiss - test pilot and former holder of the World Air Speed Record

Some national gliding associations

Related sports

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. Another variation of the sport is radio-controlled gliding, where the operator of the aircraft flies the model sailplane from the ground via radio transmission.

External links

Aeronautics | Gliding | Past Olympic sports

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