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to fly away from thermals. [Figure 10-7] As the glider
flies into the first thermal, but slightly off center, the
stronger lift in the center of the thermal banks the glider
right, away from the thermal. It then encounters the
next thermal with the right wing toward the center and
is banked away from lift to the left, and so on. Avoid
letting thermals bank the glider even slightly.
Sometimes the thermal-induced bank is subtle, so be
light on the controls and sensitive to the air activity. At
other times there is no indication on one wing or
another. In this case, take a guess, CLEAR, then turn.
As a note, new soaring pilots often get in the habit of
turning in a favorite direction, to the extreme of not
being able to fly reasonable circles in the other direction.
If this happens, make an effort to thermal in the
other direction half the time—being proficient in either
direction is important, especially when thermalling
with traffic.
Optimum climb is achieved when proper bank angle
and speed are used after entering a thermal. The shallowest
possible bank angle at minimum sink speed is
ideal. Thermal size and associated turbulence usually
do not allow this. Large-size, smooth, and wellbehaved
thermals can be the exception in some parts of
the country. Consider first the bank angle. The glider’s
sink rate increases as the bank angle increases.
However, the sink rate begins to increase more rapidly
beyond about a 45° bank angle. Thus, a 40° compared
to a 30° bank angle may increase the sink rate less than
the gain achieved from circling in the stronger lift near
the center of the thermal. As with everything else, this
takes practice, and the exact bank angle used will
depend on the typical thermal, or even a specific thermal,
on a given day. Normally bank angles in excess of
50° are not needed, but exceptions always exist. It may
be necessary, for instance, to use banks of 60° or so to
stay in the best lift. Thermals tend to be smaller at
lower levels and expand in size as they rise higher.
Therefore, a steeper bank angle is required at lower
altitudes, and shallower bank angles can often be used
while climbing higher. Remain flexible with techniques
throughout the flight.
If turbulence is light and the thermal is well-formed,
use the minimum sink speed for the given bank angle.
This should optimize the climb because the glider’s
sink rate is at its lowest, and the turn radius is smaller.
As an example, for a 30° bank angle, letting the speed
increase from 45 to 50 knots increases the diameter of
the circle by about 100 feet. In some instances, this can
make the difference between climbing or not. Some
gliders can be safely flown several knots below minimum
sink speed. Even though the turn radius is
smaller, the increased sink rate may offset any gain
achieved by being closer to strong lift near the thermal
center.
There are two other reasons to avoid thermalling
speeds that are too slow: the risk of a stall and lack of
controllability. Distractions while thermalling can
increase the risk of an inadvertent stall and include, but
are not limited to: studying the cloud above or the
ground below (for wind drift, etc.), quickly changing
bank angles without remaining coordinated while centering,
thermal turbulence, or other gliders in the thermal.
Stall recovery should be second nature, so that if
the signs of an imminent stall appear while thermalling,
recovery is instinctive. Depending on the stall characteristics
of the particular glider or in turbulent thermals,
a spin entry is always possible. Glider pilots should
carefully monitor speed and nose attitude at lower altitudes.
Regardless of altitude, when in a thermal with
other gliders below, maintain increased awareness of
speed control and avoid any stall/spin scenario.
Controllability is a second, though related, reason for
using a thermalling speed greater than minimum sink.
The bank angle may justify a slow speed, but turbulence
in the thermal may make it difficult or impossible
to maintain the desired quick responsiveness, espe-
Figure 10-7. Effect of glider being allowed to bank on its own
when encountering thermals.
10-6
cially in aileron control, in order to properly remain in
the best lift. Using sufficient speed will ensure that the
pilot, and not the thermal turbulence, is controlling the
glider.
Soaring pilots’ opinions differ regarding how long to
wait after encountering lift and before rolling into the
thermal. Some pilots advocate flying straight until the
　
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本文鏈接地址：Glider Flying Handbook(138)