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high-performance gliders necessitates development
of checklists and discipline during glider assembly
and disassembly. Other considerations for gelcoat
care include extreme cold soaking. There is evidence
that flying a composite glider with a gelcoat finish to
very high and cold altitudes followed by a quick
descent to warmer levels can seriously reduce the life
of the gelcoat. Composite gliders appear to be more
susceptible to flutter than metal gliders. Flutter is a
function of true airspeed, thus the GFM/POH of composite
gliders sometimes present a table of the indicated
VNE for different heights. For instance, a popular
two-seat composite glider shows 135 knots as the sea
level VNE, 128 knots at 10,000 feet MSL, 121 knots at
13,000 feet MSL, etc. Read the GFM/POH carefully
and abide by the limits.
Many high-performance gliders have the capability of
adding water ballast. A heavier glider has a higher
minimum sink rate and speed; thus it has a larger circling
radius. A glider with water ballast achieves the
same glide ratio but at a higher speed than the same
glider without water ballast. To maximize average
cross-country speed on a day with strong thermals,
water ballast can be used. The gain in speed between
thermals outweighs the lost time due to slightly slower
climbs with water ballast. If thermals are weak, ballast
should not be used. If strong thermals become weak
the water ballast can be dumped. In any case, water
ballast should be dumped before landing because
heavy wings are more difficult to keep level on the
ground roll and a hard landing is more likely to lead to
damage with a heavier glider. Dump times vary but are
typically between two and five minutes.
Water ballast is carried in the wings in built-in tanks or
water bags. The latter works well but has been known
to have problems with leaks. Filling and dumping systems
vary from glider to glider, and it is vital to be
familiar with the ballast system as described in the
GFM/POH. Filling without proper venting can lead to
structural damage. Care must be taken to ensure that
both wings are filled with the same amount of water. If
one wing has a few extra gallons, it can be lead to
ground loops and loss of control on take-off, especially
in the presence of a crosswind.
Water unfortunately expands when going from liquid
to solid state. The force of the water ballast freezing
can be enough to split composite wing skins. If anticipating
flying at levels where the temperature might be
below 0°C, follow the GFM/POH recommended additive
to avoid freezing.
Some gliders have a small ballast tank in the tail as
well as ballast in the wings. Tail ballast is an effective
means to adjust for a CG that is too far forward. It
should be used with caution, however, since the position
of the tail ballast tank gives it a long arm aft of the
empty CG. Acareless calculation can lead to too much
water in the tail tank and a flying CG that is aft of the
limits.
CROSS–COUNTRY USING
OTHER LIFT SOURCES
Many world distance and speed records have been broken
using ridge or wave lift. Under the right conditions,
11-17
these lift sources can extend for hundreds of miles from
sunrise to sunset. Ridge or wave lift is often more consistent
than thermals, allowing long, straight stretches
at high speed.
Cross-country on ridge lift poses some special problems
and considerations. Often the best lift is very
close to the ridge crest where the air can be quite turbulent.
Great concentration is needed over several
hours close to terrain in rough conditions for longer
flights. On relatively low ridges, for instance in the
eastern United States; ridge lift may not extend very
high, so the pilot is never too far from a potential offfield
landing. These are not conditions for the beginning
cross-country pilot. In milder conditions, gaps in
the ridge may require thermalling to gain enough
height to cross the gap. Ridge lift can provide a place
to temporarily wait for thermals to generate. For
instance, if cumulus have spread out to form a stratus
layer shading the ground and eliminating thermals, a
wind facing slope can be used to maintain soaring
flight until the sun returns to regenerate thermals.
Wave lift can also provide opportunities for long
and/or fast cross-country flights. Most record flights
have been along mountain ranges with flights in excess
of 2,000 km having been flown in New Zealand and
along the Andes. In the United States, speed records have
been set using the wave in the lee of the Sierra Nevada
　
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本文鏈接地址：Glider Flying Handbook(160)