Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the leisure activity and sport of gliding. This unpowered aircraft can use naturally occurring currents of rising air in the atmosphere to gain altitude. Sailplanes are aerodynamically streamlined, so they can fly a significant distance forward for a small decrease in altitude.
In North America, the term 'sailplane' is also used to describe this type of aircraft. In other parts of the English-speaking world, the word 'glider' is more common.
Types
Gliders benefit from producing very low drag for any given amount of lift. This factor is best achieved with long, thin wings, a slender fuselage, and smooth surfaces with an absence of protuberances. Aircraft with these features are able to soar, meaning they may climb efficiently in rising air produced by thermals or hills. In still air, sailplanes can glide long distances at high speed with a minimum loss of height in between.Sailplanes have rigid wings and either skids or undercarriage. In contrast, hang gliders and paragliders use the pilot's feet for the start of the launch and for the landing. These latter types are described in separate articles, though their differences from sailplanes are covered below. Sailplanes are usually launched by winch or aerotow, though other methods, such as auto tow and bungee, are occasionally used.
These days, almost all gliders are sailplanes, but in the past, many gliders were not. These types did not soar. Instead, they were simply engineless aircraft towed by another aircraft to a desired destination and then cast off for landing. The prime example of a non-soaring glider was the military glider. They were often used just once, then usually abandoned after landing, having served their purpose.
Motor gliders are gliders with engines that can be used for extending a flight and even, in some cases, for take-off. Some high-performance motor gliders may have an engine-driven retractable propeller, which can be used to sustain flight. Other motor gliders have enough thrust to launch themselves before the propeller is retracted and are known as "self-launching" gliders. Another type is the self-launching "touring motor glider", where the pilot can switch the engine on and off in flight without retracting the propeller.
History
's gliders achieved brief wing-borne hops from around 1849. In the 1890s, Otto Lilienthal built gliders using weight shift for control. In the early 1900s, the Wright Brothers built gliders using movable surfaces for control. In 1903, they successfully added an engine.After World War I, gliders were built for sporting purposes in Germany. Germany's strong links to gliding were in large part due to post-World War I regulations, where Germany was not permitted to construct or fly motorized planes. Accordingly, the country's aircraft enthusiasts often turned to gliders. Many of these enthusiasts were actively encouraged by the German government, particularly at flying sites suited to gliding flight like the Wasserkuppe.
The sporting use of gliders rapidly evolved in the 1930s; it is now their main application. As their performance improved, gliders began to be used for cross-country flying. They now regularly fly hundreds or even thousands of kilometres in a day when the weather is suitable.
Design
Glider airframes include a fuselage, wings, and empennage section. Self-launching gliders also include built-in engines.Early gliders had no cockpit; 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.
The early gliders were made mainly of wood with metal fastenings, stays, and control cables. Later fuselages made of fabric-covered steel tubes were married to wood and fabric wings for lightness and strength. New materials such as carbon fiber, fiberglass, 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 to the trailing edges of the wings on some gliders to optimise lift and drag at a wide range of 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 losing only 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 36:1, and modern 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 Gimli Glider, a Boeing 767 that ran out of fuel mid-flight and was found to have a glide ratio of 12:1, or to the Space Shuttle with a glide ratio of 4.5:1.
High aerodynamic efficiency is essential to achieve a good gliding performance, so gliders often have aerodynamic features seldom found in other aircraft. The wings of a modern racing glider are designed by computers to create a low-drag laminar flow airfoil. After the wings' surfaces have been shaped by a mould to great accuracy, they are then highly polished. Vertical winglets at the ends of the wings decrease drag, improving wing efficiency. 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. The extra weight provided by the water ballast is advantageous if the lift is likely to be strong; it 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 downforce from the horizontal stabilizer and the resultant drag from that downforce. 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 a small amount of 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 are designed to EASA Certification Specification CS-22. These define minimum standards for safety in a wide range of characteristics, such as controllability and strength. For example, gliders 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
The two most common methods of launching sailplanes are by aerotow and by winch. When aerotowed, the sailplane is towed behind a powered aircraft using a rope about long. The sailplane pilot releases the rope after reaching the desired altitude. However, the rope can be released by the towplane also in case of emergency. Winch launching uses a powerful stationary engine located on the ground at the far end of the launch area. The sailplane is attached to one end of of cable, and the winch rapidly winds it in. The sailplane can gain about of height with a winch launch, depending on the headwind. Less often, automobiles are used to pull sailplanes into the air, either by pulling them directly or through the use of a reverse pulley in a similar manner to the winch launch. Elastic ropes are occasionally used at some sites to launch gliders from slopes if there is sufficient wind blowing up the hill. Bungee launching was the predominant method of launching early gliders. Some modern gliders can self-launch by using retractable engines or just retractable propellers. These engines can use internal combustion or battery power.Once launched, gliders try to gain height using thermals, ridge lift, lee waves or convergence zones and can remain airborne for hours. This is known as "soaring". By finding lift sufficiently often, experienced pilots fly cross-country, often on pre-declared tasks of hundreds of kilometers, usually back to the original launch site. Cross-country flying and aerobatics are the two forms of competitive gliding. For information about the forces in gliding flight, see lift-to-drag ratio.
Glide slope control
Pilots need some form of control over the glide slope to land the glider. In powered aircraft, this is done by reducing engine thrust. In gliders the drag of the glider must be increased. Parasitic drag can be increased by use of airbrakes. On the other hand, lift-induced drag can be increased by use of spoilers. These reduce lift generated near the spoilers; they also cause the pilot to increase the angle of attack so the rest of the wing generates more lift to compensate. 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 of the glider, called "L-over-D". 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 glide angle and make it difficult to bring the glider to Earth in a short distance.;Sideslipping: A slip is performed by crossing the controls so that the glider is no longer flying aligned with the airflow. This will present one side of the fuselage to the airflow significantly increasing 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-chord or near the spar. They are raised into the airflow to eliminate the lift from the wing area behind the spoiler, disrupting the spanwise distribution of lift and increasing lift-induced drag. Spoilers significantly increase drag.
;Air brakes: Air brakes, also known as dive brakes, are devices whose primary purpose is to increase drag. On gliders, the spoilers act as air brakes. They are positioned on top of and below the wing. When slightly opened, the upper brakes will spoil the lift, but when fully opened, they will present a large surface, such that they can provide significant drag. Some gliders have terminal velocity dive brakes, which provide enough drag to keep the speed below the maximum permitted speed, even if the glider were pointing straight down. This capability is considered a safer way to descend without instruments through clouds than the only alternative, which is an intentional spin.
;Flaps: Flaps are movable surfaces on the trailing edge of the wing, inboard of the ailerons. The primary purpose of flaps is to increase the camber of the wing, so they increase the maximum lift coefficient while reducing the stall speed. Another feature that some flapped gliders possess is negative flaps, which can deflect the trailing edge upward a small amount. This feature is included on some competition gliders to reduce the pitching moment acting on the wing, reducing the downward force that must be provided by the horizontal stabiliser; this reduces the induced drag acting on the stabilizer. On some types the flaps and ailerons are linked, known as 'flaperons'. Simultaneous movement of these allows a greater rate of roll.
;Parachute: Some high-performance gliders from the 1960s and 1970s were designed to carry a small drogue parachute because their air brakes were not particularly effective. This was stored in the tail cone of the glider during flight. When deployed, a parachute causes a large increase in drag; however, it 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.