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lift has peaked. Then, they start turning, hopefully back
into stronger lift. It is imperative not to wait too long
after the first indication that the thermal is decreasing
for this maneuver. Other pilots favor rolling into the
thermal before lift peaks, thus avoiding the possibility
of losing the thermal by waiting too long. Turning into
the lift too quickly will cause the glider to fly back out
into sink. There is no one right way; the choice depends
on personal preference and the conditions on a given
day. Timing is everything and practice is key.
Usually upon entering a thermal, the glider is in lift for
part of the circle and sink for the other part. It is rare to
roll into a thermal and immediately be perfectly centered.
The goal of centering the thermal is to determine
where the best lift is and move the glider into it for the
most consistent climb. One centering technique is
known as the “270° correction.” [Figure 10-8] In this
case, the pilot rolls into a thermal and almost immediately
encounters sink, an indication of turning the
wrong way. Complete a 270° turn, straighten out for a
few seconds, and if lift is encountered again, turn back
into it in the same direction. Avoid reversing the direction
of turn. The distance flown while reversing turns is
more than seems possible and can lead away from the
lift completely. [Figure 10-9]
Often stronger lift exists on one side of the thermal than
on the other, or perhaps the thermal is small enough
that lift exists on one side and sink on the other, thereby
preventing a climb. There are several techniques and
variations to centering. One method involves paying
close attention to where the thermal is strongest, for
instance, toward the northeast or toward some feature
on the ground. To help judge this, note what is under
the high wing when in the best lift. On the next turn,
adjust the circle by either straightening or shallowing
the turn toward the stronger lift. Anticipate things a bit
and begin rolling out about 30° before actually heading
towards the strongest part. This allows rolling back
toward the strongest part of the thermal rather than flying
through the strongest lift and again turning away
from the thermal center. Gusts within the thermal can
cause airspeed indicator variations; therefore, avoid
“chasing the ASI.” Paying attention to the nose attitude
helps pilots keep their focus outside the cockpit. How
long a glider remains shallow or straight depends on
the size of the thermal. [Figure 10-10]
Other variations include the following. [Figure 10-11]
1. Shallow the turn slightly (by maybe 5° or 10°)
when encountering the weaker lift, then as
stronger lift is encountered again (feel the positive
g, variometer swings up, audio variometer
starts to beep) resume the original bank angle. If
shallowing the turn too much, it is possible to fly
Figure 10-8. The 270° centering correction. completely away from the lift.
Figure 10-9. Possible loss of thermal while trying to reverse
directions of circle.
10-7
2. Straighten or shallow the turn for a few seconds
60° after encountering the weakest lifts or worst
sink indicated by the variometer. This allows for
the lag in the variometer since the actual worst
sink occurred a couple of seconds earlier than
indicated. Resume the original bank angle.
3. Straighten or shallow the turn for a few seconds
when the stronger seat-of-the-pants surge is felt.
Then resume the original bank. Verify with the
variometer trend (needle or audio).
For the new glider pilots, it is best to become proficient
using one of the above methods first, and then
experiment with other methods. As an additional note,
thermals often deviate markedly from the conceptual
model of concentric gradients of lift increasing evenly
toward the center. For instance, it sometimes feels as if
two (or more) nearby thermal centers exist, making
centering difficult. Glider pilots must be willing to
constantly adjust, and re-center the thermal to maintain
the best climb.
In addition to helping pilots locate lift, other gliders
can help pilots center a thermal as well. If a nearby
glider seems to be climbing better, adjust the turn to fly
within the same circle. Similarly, if a bird is soaring
close by, it is usually worth turning toward the soaring
bird. Along with the thrill of soaring with a hawk or
eagle, it usually leads to a better climb.
Collision avoidance is of primary importance when
thermalling with other gliders. The first rule calls for
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Glider Flying Handbook(139)
