Lightweight glider thermal circle Heavy (ballasted) glider thermal cycle glider to climb reasonably well, the heavy glider’s advantage  during the cruising portion of flight outweighs the heavy  glider’s disadvantage during climbs. Water is often used as ballast to increase the weight of the  glider. However, the increased weight requires a higher  airspeed during the approach and a longer landing roll. Once  the cross-country phase is completed, the water ballast serves  no further purpose. The pilot should jettison the water ballast  prior to entering the traffic pattern. Reducing the weight of  the glider prior to landing allows the pilot to make a normal  approach and landing. The lighter landing weight also reduces  the loads that the landing gear of the glider must support. With the glide ratio data available to the pilot, provided by  the charts/graphs located in the GFM/POH for the glider,  the pilot can review or plot any specific combination of  airspeed and glide ratio/lift to drag ratio (L/D). The resulting  plot of L/D with airspeed (angle of attack) shows that glide  ratio increases to some maximum at the lowest airspeed.  Maximum lift-to-drag ratio (L/D$_{MAX}$)/glide ratio, occurs at  one specific airspeed (angle of attack and lift coefficient). If  the glider is operated in a steady flight condition, total drag is  at a minimum. This is solely based on airspeed. Any airspeed  (angle of attack lower or higher) than that for L/D$_{MAX}$/ glide ratio reduces the L/D$_{MAX}$/glide ratio and consequently  increases the total drag for a given glider’s lift. Note that a change in gross weight would require a change in  airspeed to support the new weight at the same lift coefficient  and angle of attack. This is why the glider GFM/POH has  different speeds for flying with or without ballast. The  configuration of a glider during flight has a great effect on  the L/D. One of the most important of which is the glider’s  best L/D/glide ratio. If the operating weight of a given glider is increased, the  stall airspeed, minimum controllable airspeed, minimum  sink airspeed, and the best L/D airspeed are increased by  a factor equal to the square root of the increase in weight.  [Figure 5-9]  Glide ratio is not affected by weight because,  while a heavier glider sinks faster, it does so at a greater  airspeed. The glider descends faster, but covers the same  horizontal distance (at a higher speed) as a lighter glider with  the same glide ratio and starting altitude. To help gliders fly faster, some gliders have tanks that can  hold up to 80 gallons of water. Higher speeds are desirable for  cross-country flying and racing. The disadvantages of these  ballasted gliders include reduced climb rates in thermals and  the possibility that suitable lift cannot be located after tow  release. To prevent this, the water ballast can be jettisoned at  any time through dump valves, allowing the pilot to reduce  the weight of the glider to aid in increased climb rates. The addition of ballast to increase weight allows the glider  to fly at increased airspeeds while maintaining its L/D ratio.  Figure 5-9  shows that adding 400 pounds of water ballast  increases the best L/D airspeed from 60 knots to 73 knots.  The heavy glider has more difficulty climbing in thermals  than the light glider, but if lift is strong enough for the heavy  Self-launching glider rate of climb is determined by design,  powerplant output, and glider weight. The rate of climb of  self-launching gliders may vary from as low as 200 fpm  to as much as 800 fpm or more in others. The pilot should  consult the GFM/POH to determine rate of climb under the  existing conditions. Rate of climb for the ground-launched glider primarily  depends on the strength of the ground-launching equipment.  When ground launching, rates of climb generally are quite  rapid, and can exceed 2,000 feet per minute (fpm) if the  winch or tow vehicle is very powerful. When aerotowing,  rate of climb is determined by the power of the towplane.  It is important when selecting a towplane to ensure that it  is capable of towing the glider, considering the existing  conditions and glider weight. Figure 5-9.  Effect of added weight on performance airspeeds. Rate of Climb 5-7 Operating Weight 800 pounds 1,200 pounds 1,600 pounds Stall Airspeed 36 knots 44 knots 50 knots Minimum Sink 48 knots 58 knots 68 knots Best L/D Airspeed 60 knots 73 knots 83 knots Figure 5-8.  Effect of added weight on thermaling turn radius.