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than a turn or two. The thermal strength used to determine
the height band should be an average achieved
climb. Many electronic variometers have an average
function that display average climb over specific time
intervals. Another technique involves simply timing
the altitude gained over 30 seconds or 1 minute.
Theoretically, the optimum average speed is attained if
the MacCready ring is set for the achieved rate-ofclimb
within the height band. To do this, rotate the ring
so that the index mark is at the achieved rate of climb
(for instance 400 fpm) rather than at zero (the setting
used for maximum distance). A series of climbs and
glides gives the optimum balance between spending
time climbing and gliding. The logic is that on stronger
days, the extra altitude lost by flying faster between
thermals is more than made up in the strong lift during
climbs. Flying slower than the MacCready setting does
not make the best use of available climbs. Flying faster
then the MacCready setting uses too much altitude
between thermals; it then takes more than the optimum
amount of time to regain the altitude.
Strict use of the MacCready ring assumes that the next
thermal is at least as strong as that set on the ring and
can be reached with the available altitude. Efforts to
fly faster must be tempered with judgement when conditions
are not ideal. Factors which may require departure
from the MacCready ring theory include terrain
(extra height needed ahead to clear a ridge), distance to
the next landable spot, or deteriorating soaring conditions
ahead. If the next thermal appears to be out of
reach before dropping below the height band, either
climb higher, glide more slowly, or both.
To illustrate the use of speed-to-fly theory, assume
there are four gliders at the same height. Ahead are
three weak cumulus clouds, each produced by 200-fpm
thermals, then a larger cumulus with 600 fpm under it
as in Figure 11-11 on the next page.
• Pilot #1 sets his ring to 600 fpm for the anticipated
strong climb under the large cumulus, but
his aggressive approach has him on the ground
before reaching the cloud.
• Pilot #2 sets his ring for 200 fpm and climbs
under each cloud until resetting the ring to 600
fpm after climbing under the third weak cumulus,
in accordance with strict speed-to-fly theory.
• Pilot #3 is conservative and sets his ring to zero
for the maximum glide.
• Pilot #4 calculates the altitude needed to glide to
the large cumulus using an intermediate setting
of 300 fpm, and finds she can glide to the cloud
and still be within the height band.
By the time Pilot #4 has climbed under the large cumulus,
she is well ahead of the other two airborne pilots
and is relaying retrieve instructions for Pilot #1. This
example illustrates the science and art of faster crosscountry
soaring. The science is provided by speed-tofly
theory, while the art involves interpreting and
modifying the theory for the actual conditions.
Knowledge of speed-to-fly theory is important as a
Figure 11-10. Example of the height band.
11-12
foundation. How to apply the art of cross-country soaring
stems from practice and experience.
The height band changes during the day. On a typical
soaring day, thermal height and strength often increases
rapidly during late morning, then both remain somewhat
steady for several hours during the afternoon. The
height band rises and broadens with thermal height.
Sometimes the top of the height band is limited by the
base of cumulus clouds. Cloud base may slowly
increase by thousands of feet over several hours, in
which case the height band also increases over several
hours. Thermals often “shut off” rapidly late in the day,
so a good rule of thumb is to stay higher late in the day.
[Figure 11-12]
It is a good idea to stop and thermal when at or near the
bottom of the height band. Pushing too hard can lead to
an early off-field landing. At best, pushing too hard
leads to lost time at lower altitudes because you are trying
to climb away in weak lift.
Another way to increase cross-country speed is to avoid
turning at all! A technique known as dolphin flight can
be used to cover surprising distances on thermal days
with little or no circling. The idea is to speed up in sink
and slow down in lift while only stopping to circle in
the best thermals. The speed-to-fly between lift areas is
based on the appropriate MacCready setting. This technique
is effective when thermals are spaced relatively
close together, as occurs along a cloud street.
　
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