Factors Affecting Glider Performance (Part Three) – Weight

Weight

Glider lift, drag, and glide ratio characteristics are governed solely by its design and construction, and are predetermined at takeoff. The only characteristic the pilot controls is the weight of the glider. In some cases, pilots may control glider configurations, as some high-performance gliders may have a wing extension option not available on other models. Increased weight decreases takeoff and climb performance, but increases high-speed cruise performance. During launch, a heavy glider takes longer to accelerate to flying speed. The heavy glider has more inertia, making it more difficult to accelerate the mass of the glider to flying speed. After takeoff, the heavier glider takes longer to climb out because the heavier glider has more mass to lift to altitude than does the lighter glider (whether ground launch, aerotow launch, or self-launch). [Figure 5-7]

Figure 5-7. Effect of weight on takeoff distance and climb-out rate and angle.

Figure 5-7. Effect of weight on takeoff distance and climb-out rate and angle.

The heavy glider has a higher stall speed and a higher minimum controllable airspeed than an otherwise identical, but lighter, glider. The stall speed of a glider increases with the square root of the increase in weight. If the weight of the glider is doubled (multiplied by 2.0), then the stall speed increases by more than 40 percent (1.41 is the approximate square root of 2; 1.41 times the old stall speed results in the new stall speed at the heavier weight). For example, a 540-pound glider has a stalling speed of 40 knots. The pilot adds 300 pounds of water ballast making the new weight 840 pounds. The new stalling speed is approximately 57 knots (square root of √ 300 + 40 = 57).

When circling in thermals to climb, the heavy glider is at a disadvantage relative to the light glider. The increased weight of the heavy glider means stall airspeed and minimum sink airspeed are greater than they would be if the glider were operating at a light weight. At any given bank angle, the heavy glider’s higher airspeeds mean the pilot must fly larger diameter thermalling circles than the pilot of the light glider. Since the best lift in thermals is often found in a narrow cylinder near the core of the thermal, larger diameter circles generally mean the heavy glider is unable to exploit the strong lift of the thermal core, as well as the slower, lightweight glider. This results in the heavy glider’s inability to climb as fast in a thermal as the light glider. [Figure 5-8]

Figure 5-8. Effect of added weight on thermaling turn radius.

Figure 5-8. Effect of added weight on thermaling turn radius.

The heavy glider can fly faster than the light glider while maintaining the same glide ratio as the light glider. The advantage of the heavier weight becomes apparent during cruising flight. The heavy glider can fly faster than the light glider and still retain the same lift-to-drag (L/D) 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.

Figure 5-9. Effect of added weight on performance airspeeds.

Figure 5-9. Effect of added weight on performance airspeeds.

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/DMAX)/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/DMAX/ glide ratio reduces the L/DMAX/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.