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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. 

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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 

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. 

## Rate of Climb 

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. 

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. 

