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Gliding

From Academic Kids

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A modern glider crossing the finish line at speed

Gliding (or soaring) is a recreational activity and competitive sport where individuals fly un-powered aeroplanes known as gliders or sailplanes. Properly, the term gliding refers to descending flight of a heavier-than-air craft when gravity (its own weight) is its sole motive force; soaring is the correct term to use when the craft gains altitude or speed from movements of the atmosphere during the flight.

The word soaring is also used to describe the way 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.

Contents

Recreation vs. sport

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 a circuit around designated "turning-points" as quickly as possible. These competitions test the pilot's (and, in two-seat gliders, the co-pilot's) ability to recognise and make use of local weather conditions, their flying skills and navigational abilities. There are also glider aerobatics competitions.

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, as evinced by the fact that all the major glider manufacturers are still based there.

Good gliding weather. Well formed  humilis, with white bases, suggests active  and light winds.
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Good gliding weather. Well formed cumulus humilis, with white bases, suggests active thermals and light winds.

Soaring

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 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).

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, and dynamic soaring is a technique only really useful 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. Dynamic soaring is now often used by radio control model aircraft pilots to achieve speeds in excess of 200 mph (320 km/h) while slope soaring.

In thermal flight, the glider pilot attempts to find 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. 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 sharply to keep the plane turning in a small circle within the thermal and so can ride upward. Rates of climb depend on conditions, but several metres per second is common.

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 layer of warm air Capping inversion which stops the air in the thermals from rising higher. During winter the solar heat can only create weak thermals.

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) Sometimes a Capping inversion stops the thermal from rising high and so the moisture does not condense into clouds. Without clouds to mark the thermals, the pilot must use his skill to find them. Typical spots to find thermals are over towns, freshly ploughed fields and asphalt roads, however most of the time thermals are hard to associate with any feature on the ground.

A pilot who is ridge running 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 climate or weather, but can only be used in certain locations. Often a combination of ridge and thermal gliding is used. The air that is already rising from the ridge can be augmented by strong thermals.

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 in mountain waves from long mountain ranges all over the world. The current World Distance Record (http://records.fai.org/gliding/#current) of 3008 km by Klaus Ohlmann (on 21 Jan 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.

A rare phenomenon known as Morning Glory has also been used by sailplane pilots in Australia.

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 C shows that a glider pilot has achieved an altitude gain of at least 1000m, made a five hour flight, and has flown cross-country for at least 50km, usually in separate flights. The FAI Sporting Code (http://www.fai.org/sporting_code/sc3.asp) defines the rules for observers and recording devices to validate the claims for badges. In the United States alone, over 6000 Silver Cs have been issued.

The gold and diamond badges require higher and longer flights. A pilot with the three "Diamonds" has flown 300km to a pre-defined goal, has flown 500km in a flight (but not necessarily to a pre-defined goal) and gained 5000m in height. The FAI also issues diplomas for 1000km and thereafter in increments of 250km. 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 750km diploma because only two flights over 1000km have ever been possible in the UK's climate.


Launch methods

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A Piper Pawnee, a common aerotow tug aircraft.
A Pawnee aerotowing a glider.
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A Pawnee aerotowing a glider.
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A Ventus 2a being winch-launched.
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A bungee launch

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

Aerotows normally use single engined 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 USA 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. 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 also used in places. The winch pulls in a 1000 to 1600 m long cable made of steel wire or a synthetic fiber which is attached to the glider. The glider releases the cable at a height of about 400 to 500 m after an amazingly short and steep ride. A winch launch costs between EUR 3 and EUR 10, which is much less than an aero-tow. Because many launches are needed to train new pilots, winching is a useful way of keeping the cost down. The disadvantage of winch launching is that the duration of flights tends to be short unless the pilot is fortunate enough to make contact with a thermal or other source of lift shortly after releasing the tow rope.

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. This method is still regularly used by a club in the UK.

Another rare method 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. A variation on this is the "reverse pulley" method in which the car drives towards the glider that it is launching. The cable that joins the car and glider passes around a pulley at the far end of the airfield.

Cross-country

Gliders can stay airborne for hours if there are places where the air is going up faster than the glider is going down. This enables gliders to fly long distances at surprisingly high speeds. Although the 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, the concept of flying as far away as possible became unpopular in the 1960s with the crews who had to retrieve the gliders. Pilots now win contests by being the fastest around a pre-defined course 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 so gliding has been a difficult sport to televise. This means that gliding is a sport in which everyone is still an amateur. However a new format has been announced see Sailplane Grand Prix (http://www.cnvv.net/wsgp/en/accueil-en.htm)

Outlandings

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Pilot and crew about to de-rig a glider

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 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 of course).

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.

Learning to glide

A Blanik L-23. A common training glider
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A Blanik L-23. A common training glider

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 will fly with an instructor in a two-seat glider fitted with dual controls. The instructor will do the first launches and landings but otherwise the pupil uses the controls. People with the skill to drive a car can 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. 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.

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.

See also: glider

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