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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 5-19 and 5-20]
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 can
lead to loss of control of the glider.
The flight manual provides guidance as to 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 de-laminate. 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
anti-freeze solution, consult the glider flight manual to
ensure that anti-freeze 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 lat-
Figure 5-19. Water ballast tank vented filler cap.
5-15
eral 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 launching
with water ballast aboard, the pilot should review
operating limitations to ensure the safety of flight will
not be comprised.
Figure 5-20. Water ballast drain valve handles.
5-16
5-17
6-1
Operating a glider requires meticulous assembly and
　
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本文鏈接地址：Glider Flying Handbook(52)