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Glider
Single-seat high performance fiberglass Glaser-Dirks DG-808 over the Lac de Serre Ponçon in the French Alps
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A glider or sailplane is a type of glider aircraft used in the sport of gliding.[1][2] Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight. Foot-launched aircraft (such as hang gliders and paragliders) are described in separate articles. Gliders have also been used for purposes other than recreation, for example for military purposes and for research.

Sports gliders benefit from creating the least drag for any given amount of lift, and this is best achieved with long, thin wings and a fully faired narrow cockpit. Aircraft with these features are able to climb efficiently in rising air and can glide long distances at high speed with a minimum loss of height in between.

Contents

Use of engines

Although many gliders do not have engines, there are some that use engines occasionally (see Motor glider). The manufacturers of high-performance gliders now often list an optional engine and a retractable propeller that can be used to sustain flight if required; these are known as 'self-sustaining' gliders. Some can even launch themselves before retracting their propellers and are known as 'self-launching' gliders. There are also 'touring motor gliders', which can also launch themselves and can switch off their engines in flight, though without retracting their propellers. [3]

History

Otto Lilienthal in flight

The first heavier-than-air (i.e. non-balloon) man-carrying aircrafts were Sir George Cayley's series of gliders which achieved brief wing-borne hops from around 1849.[4] Santos Dumont, Otto Lilienthal, Percy Pilcher, John J. Montgomery, and the Wright Brothers are other pioneers who built gliders to develop aviation. After World War I gliders were built for sporting purposes in Germany (see link to Rhön-Rossitten Gesellschaft) and in the United States (Schweizer brothers). The sporting use of gliders rapidly evolved in the 1930s and is now the main application. As their performance improved, gliders began to be used to fly cross-country and now regularly fly hundreds or even thousands of kilometers in a day[5][6] if the weather is suitable.

Glider design

Early gliders had no cockpit and the pilot sat on a small seat located just ahead of the wing. These were known as "primary gliders" and 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 than primary gliders, the designs minimized drag. Gliders now have very smooth, narrow fuselages and very long, narrow wings with a high aspect ratio and winglets.

Cockpit of a typical modern glider (Glaser-Dirks DG-101G ELAN).
Click on the image for an explanation of the instrumentation.
A glider releasing its water ballast

The early gliders were made mainly of wood with metal fastenings, stays and control cables. Later fuselages made of fabric-covered steel tube were married to wood and fabric wings for lightness and strength. New materials such as carbon-fiber, fiber glass and Kevlar have since been used with computer-aided design to increase performance. The first glider to use glass-fiber extensively was the Akaflieg Stuttgart FS-24 Phönix which first flew in 1957. This material is still used because of its high strength to weight ratio and its ability to give a smooth exterior finish to reduce drag. Drag has also been minimized by more aerodynamic shapes and retractable undercarriages. Flaps are fitted on some gliders so that the optimal lift of the wing is available at all speeds.

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. Comparing some typical gliders that might be found in the fleet of a gliding club - the Grunau Baby from the 1930s had a glide ratio of just 17:1, the glass-fiber Libelle of the 1960s increased that to 39:1, and nowadays flapped 18 meter gliders such as the ASG29 have a glide ratio of over 50:1. The largest open-class glider, the eta, has a span of 30.9 meters and has a glide ratio over 70:1. Compare this to the infamous Gimli Glider, a Boeing 767 which ran out of fuel mid-flight and was found to have a glide ratio of only 12:1, or to the Space Shuttle with a glide ratio of 4.5:1.[7]

Due to the critical role that aerodynamic efficiency plays in the performance of a glider, gliders often have aerodynamic features seldom found in other aircraft. The wings of a modern racing glider have a specially designed low-drag laminar flow 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 or multiple blow holes positioned in a span-wise line along the wing are used to trip 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 carry jettisonable water ballast (in the wings and sometimes in the vertical stabilizer). 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. Moving the center of mass toward the rear by carrying water in the vertical stabilizer reduces the required down-force from the horizontal stabilizer and the resultant drag from that down-force. Although heavier gliders have a slight disadvantage when climbing in rising air, they achieve a higher speed at any given glide angle. This is an advantage in strong conditions when the gliders spend only little time climbing in thermals. The pilot can jettison the water ballast before it becomes a disadvantage in weaker thermal conditions. Another use of water ballast is to dampen air turbulence such as might be encountered during ridge soaring. To avoid undue stress on the airframe, gliders must jettison any water ballast before landing.

Most gliders are built in Europe and in general these are designed to EASA Certification Specification CS-22 (previously Joint Aviation Requirements-22). These define minimum standards for safety in a wide range of characteristics such as controllability and strength. For example it must have design features to minimize the possibility of incorrect assembly. Automatic connection of the controls during rigging is the common method of achieving this.

Launch and flight

Double aerotow

The two most common methods of launching sailplanes are by aerotow and by winch[8]. When aerotowed, the glider is towed behind a powered aircraft using a rope about 60 meters (about 200 ft) long. The glider's pilot releases the rope after reaching the desired altitude, but the rope can also be released by the towplane in an emergency. Winch launching uses a powerful stationary engine located on the ground at the far end of the launch area. The glider is attached to one end of 800-1200 metres (about 2,500-4,000 ft) of wire cable and the winch then rapidly winds it in. More rarely, automobiles are used to pull gliders into the air, by pulling them directly or through the use of a pulley in a similar manner to the winch launch. Elastic ropes (known as bungees) are occasionally used at some sites to launch gliders off slopes, if there is sufficient wind blowing up the hill. Bungee launching was the predominant method of launching early gliders. Many modern sailplanes also known as motor gliders also provide self-launching capability by the use of retractable engines and/or propellers.

Once launched sailplanes generally try to gain height using thermals, ridge lift or lee waves and can remain airborne for hours. This is known as 'soaring'. Experienced pilots fly cross-country, often on pre-declared tasks of hundreds of kilometers and sometimes further. They often fly in competition with each other. For information about the forces in gliding flight, see lift-to-drag ratio.

Glide slope control

Red spoilers on PW-6 glider near touchdown

Pilots needs some form of control over the glide slope to land the glider. In powered aircraft, this is done by reducing engine thrust. In gliders, other methods are used to either reduce the lift generated by the wing, increase the drag of the entire glider, or both. Glide slope is the distance traveled for each unit of height lost. In a steady wings-level glide with no wind, glide slope is the same as the lift/drag ratio (L/D) of the glider, called "L-over-D". Reducing lift from the wings and/or increasing drag will reduce the L/D allowing the glider to descend at a steeper angle with no increase in airspeed. Simply pointing the nose downwards only converts altitude into a higher airspeed with a minimal initial reduction in total energy. Gliders, because of their long low wings, create a high ground effect which can significantly increase the L/D and make it difficult bring the glider to earth in a short distance.

  • Slipping - A slip is performed by crossing the controls (rudder to right with ailerons to left, for example) so that the glider is no longer flying aligned with the air flow. This will present one side of the fuselage to the air-flow significantly increased drag. Early gliders primarily used slipping for glide slope control.
  • Spoilers - Spoilers are movable control surfaces in the top of the wing, usually located mid-cord or near the spar which are raised into the air-flow to eliminate (spoil) the lift from the wing area behind the spoiler. Spoilers have minimal impact on drag and their primary purpose is lift reduction.
  • Air brakes - Air brakes, also known as dive brakes, are devices whose primary purpose is to increase drag. On gliders, air brakes also act as spoilers. They may be positioned on top of the wing and on some types below the wing also. When slightly opened the upper brakes will spoil the lift, but when fully opened will present a large surface and so can provide significant drag. Some older gliders have terminal velocity dive brakes which provide enough drag to keep its speed below maximum permitted speed, even if the glider were pointing straight down.
  • Flaps - Flaps are movable surfaces on the trailing edge of the wing. The primary purpose of flaps is to change the camber of the wing and so change the L/D ratio of the wing. This reduces the stall speed and so allows reduced landing speeds. It was possible to lower the flaps on some older gliders by up to 90 degrees to increase drag significantly as well as increasing lift when landing.
  • Parachute - Some high performance gliders from the 1960s and 1970s were designed to carry a small drogue parachute because their air brakes are not particularly effective. This is stored in the tail-cone of the glider during flight. When deployed, a parachute causes a large increase in drag, but has a significant disadvantage over the other methods of controlling the glide slope. This is because a parachute does not allow the pilot to finely adjust the glide slope. Consequently a pilot may have to jettison the parachute entirely, if the glider is not going to reach the desired landing area.

Landing

A typical training glider, Schleicher ASK 21 just before landing

Early glider designs used skids for landing, but modern types generally land on wheels. Some of the earliest gliders used a dolly with wheels for taking off and the dolly was jettisoned as the glider left the ground, leaving just the skid for landing. A glider may be designed so the center of gravity (CG) is behind the main wheel so the glider sits nose high on the ground. Other designs may have the CG forward of the main wheel so the nose rests on a nose-wheel or skid when stopped. Skids are now mainly used only on training gliders such as the Schweizer SGS 2-33. Skids are around 100mm (3 inches) wide by 900mm (3 feet) long and run from the nose to the main wheel. Skids help with braking after landing by allowing the pilot to put forward pressure on the control stick, thus creating friction between the skid and the ground. The wing tips also have small skids or wheels to protect the wing tips from ground contact.

In most high performance gliders the undercarriage can be raised to reduce drag in flight and lowered shortly before landing. Wheel brakes are provided to allow stopping once on the ground. These may be engaged by fully extending the spoilers/air-brakes or by using a separate control. Although there is only a single main wheel, the glider's wing can be kept level by using the flight controls until it is almost stationary.

Pilots usually land back at the airfield from which they took off, but a landing is possible in any flat field about 250 metres long. Ideally, should circumstances permit, a glider would fly a standard pattern, or circuit, in preparation for landing, typically starting at a height of 300 metres (1,000 feet). Glide slope control devices are then used to adjust the height to assure landing at the desired point. The ideal landing pattern positions the glider on final approach so that a deployment of 30-60% of the spoilers/dive brakes/flaps brings it to the desired touchdown point. In this way the pilot has the option of opening or closing the spoilers/air-brakes to extend or steepen the descent to reach the touchdown point. This gives the pilot wide safety margins should unexpected events occur.

Instrumentation and other technical aids

Schempp-Hirth Janus-C in flight, showing instrument panel equipped for "cloud flying," configured in the basic-T, with airspeed, turn and bank and altitude displays across the top row; below a GPS-driven computer, with wind and glide information, drives two electronic variometer displays to the right. The yaw string and compass are above the glare shield

In addition to an altimeter, compass, and an airspeed indicator, gliders are often equipped with a variometer, turn and bank indicator and an airband radio (transceiver), each of which may be required in some countries. An Emergency Position-Indicating Radio Beacon (ELT) may also be fitted into the glider to reduce search and rescue time in case of an accident.

Much more than in other types of aviation, glider pilots depend on the variometer, which is a very sensitive vertical speed indicator, to measure the climb or sink rate of the plane. This enables the pilot to detect minute changes caused when the glider enters rising or sinking air masses. Both mechanical and electronic 'varios' are usually fitted to a glider. The electronic variometers produce a modulated sound of varying amplitude and frequency depending on the strength of the lift or sink, so that the pilot can concentrate on centering a thermal, watching for other traffic, on navigation, and weather conditions. Rising air is announced to the pilot as a rising tone, with increasing pitch as the lift increases. Conversely, descending air is announced with a lowering tone, which advises the pilot to escape the sink area as soon as possible. (Refer to the variometer article for more information).

Gliders' variometers are sometimes fitted with mechanical devices such as a "MacCready Ring" to indicate the optimal speed to fly for given conditions. These devices are based on the mathematical theory attributed to Paul MacCready[9] though it was first described by Wolfgang Späte in 1938.[10] MacCready theory solves the problem of how fast a pilot should cruise between thermals, given both the average lift the pilot expects in the next thermal climb, as well as the amount of lift or sink he encounters in cruise mode. Electronic variometers make the same calculations automatically, after allowing for factors such as the glider's theoretical performance, water ballast, headwinds/tailwinds and insects on the leading edges of the wings.

Soaring flight computers, often used in combination with PDAs running specialized soaring software, have been designed for use in gliders. Using GPS technology these tools can:

  • Provide the glider's position in 3 dimensions by a moving map display
  • Alert the pilot to nearby airspace restrictions
  • Indicate position along track and remaining distance and course direction
  • Show airports within theoretical gliding distance
  • Determine wind direction and speed at current altitude
  • Show historical lift information
  • Create a GPS log of the flight to provide proof for contests and gliding badges
  • Provide "final" glide information (ie showing if the glider can reach the finish without additional lift).
  • Indicate the best speed to fly under current conditions

After the flight the GPS data may be replayed on computer software for analysis and to follow the trace of one or more gliders against a backdrop of a map, an aerial photograph or the airspace.

Because collision with other gliders is a risk, the anti-collision device, FLARM is becoming increasingly common in Europe and Australia. In the longer term, gliders may eventually be required in some European countries to fit transponders once devices with low power requirements become available.

Swift S-1 of the UK Swift Aerobatic Display Team at Kemble 2009

Markings

To distinguish gliders in flight, very large numbers/letters are sometimes displayed on the fin and wings. Registrations on narrow fuselages are difficult to read. These numbers were first added for use by ground-based observers in competitions, and are therefore known as "competition numbers" or "contest ID's"[11]. They are unrelated to the glider's registration number, and are assigned by national gliding associations. They are useful in radio communications between gliders, so glider pilots often use their competition number as their call-signs.

Fibreglass gliders are white in color after manufacture. Since fibreglass resin softens at high temperatures, white is used almost universally to reduce temperature rise due to solar heating. Color is not used except for a few small bright patches on the wing tips; these patches (typically bright red) improve gliders' visibility to other aircraft while in flight. Non-fibreglass gliders (those made of aluminum and wood) are not subject to the temperature-weakening problem of fibreglass, and can be painted any color at the owner's choosing; they are often quite brightly painted.

Comparison of gliders with hang gliders and paragliders

There is sometimes confusion about gliders, hang gliders and paragliders. In particular paragliders and hang gliders are both foot-launched. The main differences between the types are:

Paragliders Hang gliders Gliders/Sailplanes
Undercarriage: Pilot's legs used for take-off and landing Pilot's legs used for take-off and landing Aircraft takes off and lands using a wheeled undercarriage or skids
Wing structure: entirely flexible, with shape maintained purely by the pressure of air flowing into the wing in flight and the tension of the lines. prone to collapse in turbulence. generally flexible but supported on a rigid frame which determines its shape and thus does not collapse in turbulence, but note that rigid wing hang gliders also exist rigid surface to wings that totally encases structure
Pilot position: sitting ‘supine’ in a seated harness. usually lying ‘prone’ in a cocoon-like harness suspended from the wing. Seated, and 'supine' are also possible. sitting in a seat with a harness surrounded by a crash-resistant structure.
Speed range (stall speed – max speed): slower – hence easier to launch and fly in light winds, can get into trouble when winds pick up, poor wind penetration and no pitch control, cannot dive for speed, although some pitch variation can be achieved with speed bar. faster – much faster, up to 145 km/h (90+ mph), hence easier to launch and fly in stronger conditions with better wind penetration, and can outrun bad weather, full pitch control even faster - maximum speed up to about 280 km/h (170 mph); stall speed typically 65 km/h (40mph). Able to fly in windier turbulent conditions and can outrun bad weather. Exceptional penetration into the wind. Semi- or fully aerobatic.
Maximum glide ratio: about 12, relatively poor glide performance makes long-distances more difficult about 17 for flexible wings, though up to 20 for rigid wings. Glide performance enables longer-distance flying, 700km (430+ mile) record about 70[12], high glide performance enabling long distances, 3000km (1800+ mile record)
Turn radius: tighter turn radius, allowing circling in the rapidly rising center of thermals somewhat larger turn radius, not allowing such a high rate of climb in thermals even greater turn radius but still able to circle tightly in thermals
Landing-out: smaller space needed to land, offering more landing options from cross-country flights. Also easier to carry back to the nearest road longer approach & landing area required, but can reach more landing areas due superior glide range can land in less than 200 metres and can often reach another airfield. Specialised trailer needed to retrieve by road
Learning: quicker to get ‘into the air’ with most skills learned in the air; flying tandem with an instructor is rarely necessary during instruction basic control skills are learned in ground school, and in flights close to the ground prior to high flights; teaching is done in a two seat glider with dual controls
Convenience: pack smaller (easier to transport and store); lighter (easier to carry); quicker to rig & de-rig; transported in the trunk of a car more awkward to transport & store; longer to rig and de-rig; transported on the roof of a car trailers are typically 10 m (30 ft) long. Rigging & de-rigging takes about 20 minutes
Cost: cheaper but less durable more expensive but more durable long lasting (several decades), so active second hand market in all price ranges, but cost of new gliders very high. Often syndicated.

Competition classes of glider

DG Flugzeugbau DG-1000 of the Two Seater Class

Eight competition classes of glider have been defined by the FAI[13]. They are:

  • Standard Class (No flaps, 15 m wing-span, water ballast allowed)
  • 15 metre Class (Flaps allowed, 15 m wing-span, water ballast allowed)
  • 18 metre Class (Flaps allowed, 18 m wing-span, water ballast allowed)
  • Open Class (No restrictions except a limit of 850 kg for the maximum all-up weight)
  • Two Seater Class (maximum wing-span of 20 m), also known by the German name "Doppelsitzer"
  • Club Class (This class allows a wide range of older small gliders with different performance and so the scores have to be adjusted by handicapping. Water ballast is not allowed).
  • World Class (The FAI Gliding Commission which is part of the FAI and an associated body called Organisation Scientifique et Technique du Vol à Voile (OSTIV) announced a competition in 1989 for a low-cost glider, which had moderate performance, was easy to assemble and to handle, and was safe for low hours pilots to fly. The winning design was announced in 1993 as the Warsaw Polytechnic PW-5. This allows competitions to be run with only one type of glider.
  • Ultralight Class, for gliders with a maximum mass less than 220 kg.

Major manufacturers of gliders

The full list of gliders and manufacturers, past and present, shows that a large proportion have been and are still made in Germany, the birthplace of the sport. The principal German manufacturers are:

though there are other specialist manufacturers in Germany, Poland and in other eastern European countries.[14]

See also

Gliding as a sport

Other unpowered aircraft:

Unpowered flying toys and models:

Other uses of the word "glider"

References

  1. ^ FAA Glider handbook
  2. ^ Definition of gliders used for sporting purposes in FAI Sporting Code
  3. ^ British CAA definition in LASORS
  4. ^ [http://www.flightglobal.com/pdfarchive/view/1954/1954%20-%202909.html Flight magazine 1954
  5. ^ [http://records.fai.org/gliding/#current List of FAI ratified world records
  6. ^ On-line contest web page
  7. ^ Space Shuttle Technical Conference pg 258
  8. ^ Piggott, Derek (2002). Gliding: A handbook on soaring flight. A & C Black. ISBN 0-7136-6148-8.  
  9. ^ "MacCready Theory". http://home.att.net/~jdburch/polar.htm. Retrieved 2006-08-24.  
  10. ^ Pettersson, Åke (Oct-Nov 2006). "Letters". Sailplane & Gliding (British Gliding Association) 57 (5): 6.  
  11. ^ Reference to competition numbers on FAI web site
  12. ^ Handicap list 2008 of the Deutscher Aero Club - accessed 2008-08-07
  13. ^ Competition classes as defined by FAI
  14. ^ Simons, Martin (2002). Sailplanes 1965-2000. Eqip. ISBN 3-9808-8381-7.  

External links

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Simple English

Gliders are aircraft which do not have a motor. They are sometimes called 'sailplanes'. Gliders are controlled by their pilots by using control-sticks. Some gliders can only carry one person, but some gliders can carry two people. In gliders with two seats, each pilot has a control-stick. Gliders always have seats for the pilots.

Gliders have long wings so that they will only lose height slowly. In some places the air goes up faster than the glider is going down. The pilot of a glider can make it climb by flying to these places. Good pilots can travel long distances by always finding rising air. Some pilots race each other over hundreds of kilometres each day. Other pilots just fly for fun.

Gliders cannot get into the air by themselves. They are pulled into the air by an aircraft with a motor or they are pulled up by motor on the ground.

There are two other types of gliders. Hang-gliders have frames to give the wings their shape, but do not have seats for their pilots and do not have control-sticks. The wings of paragliders are like parachutes and so do not have frames to give them their shape. These other two types of glider are often launched from the tops of hills or cliffs.



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