Ballast is nonstructural weight that is added to a glider. In soaring, ballast weight is used for two purposes. Trim ballast is used to adjust the location of the CG of the glider so handling characteristics remain within acceptable limits. Performance ballast is loaded into the glider to improve high-speed cruise performance.
Removable trim ballast weights are usually made of metal and are bolted into a ballast receptacle incorporated in the glider structure. The manufacturer generally provides an attachment point well forward in the glider cabin for trim ballast weights. These weights are designed to compensate for a front seat pilot who weighs less than the minimum permissible front seat pilot weight. The ballast weight mounted well forward in the glider cabin helps place the CG within permissible limits, which allows the maximum shift in CG with the minimum addition of weight.
Some trim ballast weights are in the form of seat cushions, with sand or lead shot sewn into the unit to provide additional weight. This type of ballast, which is installed under the pilot’s seat cushion, is inferior to bolted-in ballast because seat cushions tend to shift position. Seat cushion ballast should never be used during acrobatic or inverted flight.
Sometimes trim ballast is water placed in a tail tank in the vertical fin of the fuselage. The purpose of the fin trim ballast tank is to adjust CG location after water is added to, or drained from, the main wing ballast tanks. Unless the main wing ballast tanks are precisely centered on the CG of the loaded aircraft glider, CG location shifts when water is added to the main ballast tanks. CG location shifts again when water is dumped from the main ballast tanks. Adjusting the amount of water in the fin tank compensates for CG shifts resulting from changes in the amount of water ballast carried in the main wing ballast tanks. Water weighs 8.35 pounds per gallon. Because the tail tank is located far aft, it does not take much water to have a considerable effect on CG location. For this reason, tail tanks do not need to contain a large volume of water. Tail tank maximum water capacity is generally less than two gallons of water.
Although some older gliders employed bags of sand or bolted-in lead weights as performance ballast, water is used most commonly to enhance high-speed performance in modern sailplanes. Increasing the operating weight of the glider increases the optimum speed to fly during wings-level cruising flight. The resulting higher groundspeed provides a very desirable advantage in cross-country soaring and in sailplane racing.
Water ballast tanks are located in the main wing panels. Clean water is added through fill ports in the top of each wing. In most gliders, the water tanks or bags can be partially or completely filled, depending on the pilot’s choice of operating weight. After water is added, the filler caps are replaced to prevent water from sloshing out of the filler holes.
Drain valves are fitted to the bottom of each tank. The valves are controlled from inside the cockpit. The tanks can be fully or partially drained while the glider is on the ground to reduce the weight of the glider prior to launch, if the pilot so desires. The ballast tanks also can be partially or completely drained in flight—a process called dumping ballast. The long streaks of white spray behind a speeding airborne glider are dramatic evidence that the glider pilot is dumping water ballast, most likely to lighten the glider prior to landing. The filler caps are vented to allow air to enter the tanks to replace the volume of water draining from the tanks. It is important to ensure that the vents are working properly to prevent wing damage when water ballast is drained or jettisoned. [Figures 5-21 and 5-22]
It is important to check the drain valves for correct operation prior to flight. Water ballast should drain from each wing tank at the same rate. Unequal draining leads to a wing-heavy condition that makes in-flight handling, as well as landings, more difficult. If the wing-heavy condition is extreme, it is possible the pilot will lose control of the glider.
Ballast drains should also be checked to ensure that water ballast drains properly into the airstream, rather than leaking into the fuselage and pooling in the bottom of the fuselage. Water that is trapped in the fuselage may flow through or over bulkheads, causing dislocation of the CG of the glider. This CG dislocation can lead to loss of control of the glider.
The flight manual provides guidance regarding the length of time it takes for the ballast tanks to drain completely. For modern gliders, it takes about 3 to 5 minutes to drain a full tank. When landing is imminent, dump ballast early enough to give the ballast drains sufficient time to empty the tanks.
Use of water ballast when ambient temperatures are low can result in water freezing the drain valve. If the drain valve freezes, dumping ballast is difficult or impossible. If water in the wings is allowed to freeze, serious wing damage is likely to occur. Damage occurs because the volume of water expands during the freezing process. The resulting increased volume can deform ribs and other wing structures or cause glue bonds to delaminate. When weather or flight conditions are very cold, do not use water ballast unless antifreeze has been added to the water. Prior to using an antifreeze solution, consult the GFM to ensure that antifreeze compounds are approved for use in the glider.
A glider carrying large amounts of water ballast has noticeably different handling characteristics than the same glider without water ballast. Water ballast:
- Reduces the rate of acceleration of the glider at the beginning of the launch due to the increased glider weight.
- Increases the length of ground roll prior to glider liftoff.
- Increases stall speed.
- Reduces aileron control during the takeoff roll, increasing the chance of uncontrolled wing drop and resultant ground loop.
- Reduces rate of climb during climb-out.
- Reduces aileron response during free flight. The addition of large amounts of water increases lateral stability substantially. This makes quick banking maneuvers difficult or impossible to perform.
Water ballast is routinely dumped before landing to reduce the weight of the glider. Dumping ballast:
- Decreases stall speed.
- Decreases the optimum airspeed for the landing approach.
- Shortens landing roll.
- Reduces the load that glider structures must support during landing and rollout.
The performance advantage of water ballast during strong soaring conditions is considerable. However, there is a down side. The pilot should be aware that water ballast degrades takeoff performance, climb rate, and low-speed handling. Before committing to a launch with water ballast aboard, the pilot should review operating limitations to ensure that safety of flight is not compromised.