The earlier gliders were made mainly of wood with metal fastenings, stays, and control cables. Subsequent designs led to a fuselage made of fabric-covered steel tubing glued to wood and fabric wings for lightness and strength. New materials, such as carbon fiber, fiberglass, glass reinforced plastic (GRP), and Kevlar® are now being used to developed stronger and lighter gliders. Modern gliders are usually designed by computer-aided software to increase performance. The first glider to use fiberglass extensively was the Akaflieg Stuttgart FS-24 Phönix, which first flew in 1957. [Figure 2-1] Fiberglass 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 were installed when technology improved and are fitted to the trailing edges of the wings on some gliders to minimize the drag and to allow lower landing speeds.
Most high-performance gliders are built of composites, instead of metal or wood, with a gel-coat finish. The gel coat is susceptible to damage from exposure to ultraviolet (UV) radiation from the sun, as well as prolonged exposure to moisture. At some soaring sites, pilots can keep the glider assembled in a hangar, but the composite glider is more frequently rigged before flying and derigged after flying. The transition to high-performance gliders necessitates development of checklists and discipline during glider assembly and disassembly. Other considerations for gel-coat care include extreme cold soaking. There is evidence that flying a composite glider with a gel-coat finish to very high and cold altitudes followed by a quick descent to warmer levels can seriously reduce the life of the gel coat. Composite gliders appear to be more susceptible to flutter than metal gliders. Flutter is a function of true airspeed. The GFM/POH of composite gliders sometimes presents a table of the indicated VNE for different heights. For instance, a popular two-seat composite glider shows 135 knots as the sea level VNE, 128 knots at 10,000 feet MSL, 121 knots at 13,000 feet MSL, etc. Read the GFM/ POH carefully and obey the limitations set forth in the manual.
With each generation of new materials and development and improvements in aerodynamics, the performance of gliders has increased. One measure of performance is glide ratio. A glide ratio of 30:1 means that in smooth air a glider can travel forward 30 feet while only losing 1 foot of altitude. Glide ratio is discussed further in Chapter 5, Glider Performance.
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 wing surfaces have been shaped by a mold with great accuracy, they are highly polished and painted with a gel coat (light fiberglass spray/sealer). Some high performance gliders have winglets installed at the ends of the wings. These winglets are computer designed to decrease drag and improve handling performance. [Figure 2-2] To continually ensure the best in aerodynamics, manufacturers use specially designed seals in the vicinity of the flight controls (i.e., ailerons, rudder, and elevator) to prevent the flow of air in the opposite direction through the control surface gaps, which causes turbulence over the area.
Additional high-technology designs include such items as bug wipers. These are very similar to a car windshield wiper. They may be installed to wipe the wings while in flight and remove insects that are disturbing the smooth flow of air over the wing by sliding back and forth along the leading edge of the wing. [Figure 2-3] Bug wipers can be operated by small electrical motors or by aerodynamics.
Modern competition gliders carry water ballast that can be jettisoned. This water acts as 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 gravity (CG) during flight. Moving the CG toward the rear by carrying water in the vertical tail section reduces some of the required down force from the horizontal stabilizer aerodynamics 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 that may be encountered during ridge soaring. To avoid undue stress on the airframe, gliders may jettison any water ballast before landing. [Figure 2-4] This is discussed further in Chapter 5, Glider Performance.
Most gliders are built in Europe and are designed to meet the requirements of the European Aviation Safety Agency (EASA), similar to the United States Federal Aviation Administration (FAA). The EASA Certification Specification CS-22 (previously Joint Aviation Requirements (JAR)-22), defines 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 (gliders are often stowed in disassembled configuration with at least the wings being detached). Automatic connection of the controls during rigging is the common method of achieving this.
Throughout the years, flying gliders has not only been a recreational past time but are built and used for sport as well. Many glider pilots take part in gliding competitions that usually involve racing. Modern gliding competitions now comprise closed tasks; everyone races on an aerial route around specified turnpoints, plus start and finish points that bring everybody back to base. The weather forecast and the performance of the gliders, as well as the experience level of the pilots, dictate the length of the task. Today, most of the points are speed points, and the rule is to set the task so all pilots have a fair chance of completing it.
With the advent of global positioning systems (GPS), new types of tasks were introduced, such as speed or distance tasks within assigned areas and speed or distance tasks with pilot-selected turn points. Despite the use of pilot-selected turn points made possible by GPS, tasks over a fixed course are still used frequently. The Fédération Aéronautique Internationale (FAI), the world’s air sports federation, is a nongovernmental and nonprofit international organization with the basic aim of furthering aeronautical and astronautical activities worldwide. The FAI Gliding Commission is the sporting body overseeing air sports at the international level so that essentially the same classes and class definitions are followed in all countries.
The following is an overview of the seven classes of gliders that are currently recognized by the FAI and are eligible for European and World Championships:
- Standard class—no flaps, 15 meter (49.2 feet) wingspan, water ballast allowed.
- 15 meter class—flaps allowed, 15 meter (49.2 feet) wingspan, water ballast allowed.
- 18 meter class-—flaps allowed, 18 meter (59 feet) wingspan, water ballast allowed.
- Open class—no restrictions on wingspan, except a limit of 850 kg (1,874 pounds for the maximum all-up weight). Open classes may have wingspans in excess of 85 feet or more. [Figure 2-5]
- Two-seat class—maximum wingspan of 20 meters (65.6 feet), also known by the German name of Doppelsitzer. [Figure 2-6]
- Club class—this class allows a wide range of older, small gliders with different performance. The scores must 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 Organization Scientifique et Technique du Vol à Voile (OSTIV), announced a competition in 1989 for a lowcost glider that had moderate performance, was easy to assemble and 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.
Glider airframes are designed with a fuselage, wings, and empennage or tail section. Self-launching gliders are equipped with an engine that enables them to launch without assistance and return to an airport under engine power if soaring conditions deteriorate.