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. 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.  Glider Design Figure 2-2.  Schempp-Hirth Ventus-2 glider with factory winglets  installed. Figure 2-1.  The Akaflieg Stuttgart FS-24 Phönix, made in Germany,  was first flown on November 27, 1957. 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.  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  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.  2-2