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## Glider Design 

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. 

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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 V$_{NE}$ for different heights. For instance, a popular two-seat composite glider shows 135 knots as the sea level V$_{NE}$, 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. 

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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 

