Thermals are a natural phenomenon that happen outside,
by the millions, every single day of the year. Thermals are
responsible for many things including forming several types
of clouds, creating breezes, and distributing plant seeds and
pollen. If you have ever seen a dust devil (which is nothing
more than a thermal that has picked up dust), you have
seen a thermal in action. Their swirling action is very similar
to that of a tornado but much gentler. Most thermals have
updrafts rising 200-700 feet per minute but have been
known to produce updrafts of over 5,000 feet per minute
These strong thermals can rip a plane apart or carry the
plane out of sight before the pilot can get out of the updraft.
Thermals are formed by the uneven heating of the earth and
buildings, etc. by the sun. The darker colored surfaces
absorb heat faster than the lighter colors which reflect a
great deal of the sun’s energy back into space. These
darker areas (plowed fields, asphalt parking lots, tar roofs,
etc.) get warmer than the lighter areas (lakes, grassy fields,
forests, etc.). This causes the air above the darker areas to
be warmer than the air over the lighter areas and the more
buoyant warm air rises as the cooler, denser air forces its
way underneath the warmer air. As this warm air is forced
upward it contacts the cooler air of the higher altitudes and
this larger temperature difference makes the thermal rise
quicker. The thermal is gradually cooled by the surrounding
cooler air and its strength diminishes. Eventually the
thermal stops rising and any moisture contained in the once
warm air condenses and forms a puffy cumulus cloud.
These clouds, which mark the tops of thermals, are usually
between 2000 and 5000 feet high.
As the glider approaches a thermal, the wing tip that
reaches the rising air first will be lifted before the opposite
wing tip. This causes the plane to “bank” and turn away from
where we would like the plane to go. The best way to get
back in is to continue the bank and turn 270 degrees
straight into the thermal.
When you are thermal soaring, try to fly as smoothly and
straight as possible. Trim the plane to fly in a straight line
and only touch the controls when you have to. Watch the
sailplane carefully and it will tell you what it is encountering.
When the sailplane flys directly into a thermal it will either
start rising or stop sinking. Either case is reason enough to
start circling. Fly straight until you feel like you are in the
strongest lift, then fly a couple of seconds farther so your
circle will be centered in the strongest lift. Thermals travel
with the wind, so be careful that you don’t get too far
downwind that you can’t get back. If you find yourself getting
too high, don’t dive the plane to get out of the lift. Sailplanes
are very efficient aircraft and they will build up a lot of speed
and could “blow up” in the rough air of a thermal. The
easiest way to lose altitude is to apply full rudder and full up
elevator. This will put the plane into a tight spin that will not
over stress the airframe but it will enable it to lose altitude
very quickly. This is especially helpful if the sailplane gets
sucked into a cloud or it gets too high to see.
As you might expect, with all this air rising, there is also air
sinking. This air is the sailplane pilot’s nightmare that can
really make soaring challenging. “Sink” is usually not as
strong as the thermals in the same area but sometimes can
be. Because of this, it is important you do not let the
sailplane get too far downwind.
Watch the birds! - Thermals suck up small insects many
birds love to eat. A bunch of swallows flying around in one
area may indicate a thermal. Soaring birds (hawks, vultures,
eagles etc.) are the best thermal indicators. They not only
show you where the thermal is but they also show you
where the center is. These “Masters of the sky” will often fly
right along with sailplanes.
Practice those landings! - Most thermal contests are won
or lost during the landing. Establish a particular landing
pattern and try to stick to it for all landings. Learn to shift
your pattern to account for the wind and particular flying
field characteristics. Flaps can be very useful during contest
landings. They allow you to bring the sailplane in for a
landing higher or faster than normal to guard against any
last minute sink or gusts and dump the extra altitude and
speed at the last second. They can also be used to help
THERMAL SOARING
FACTS ABOUT THERMALS
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