Dynaflite DYFA2018 - Manual (Page 27)

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 some dust), you have seen a
thermal in action. Their swirling action is very
similar to that of a tornado's but of course much
gentler. Most thermals have updrafts rising in
the 200-700 feet per minute range but they have
been known to produce updrafts of over 5,000
feet per minute (that's over 50 miles/hour
straight up!) 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.
It takes a lot of concentration to thermal soar
effectively. A sailplane can fly along the edge of
a thermal and unless the pilot is carefully
watching the model he may not realize the
opportunity to gain some altitude. Because
most thermals are relatively small (a couple
hundred feet in diameter or less at 400' altitude)
compared to the rest of the sky, the sailplanes
will rarely fly directly into the thermal and start
rising. Generally, the sailplane will fly into the
edge or near a thermal and the effects the
thermal has on the plane may be almost
unnoticeable. As the sailplane 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.
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 flies directly into a thermal
it will either start rising or stop sinking. Either
case is reason enough to start circling
(especially in a contest where every second
counts). Fly straight ahead until you feel like
you are in the strongest lift, fly a couple of
seconds farther (so your circle will be
centered in the strongest lift) and then start
circling in a fairly tight but smooth turn. When
the sailplane is low the turns have to be tighter
to stay in the strongest lift. As the plane gains
altitude, the turns can be larger and flatter. The
flatter the turn, the more efficient the plane is
flying, but don't be afraid to really "crank" it
into a steep bank when you are low. If you see
the plane falling off on one side of the turn,
move your circle over into the stronger lift;
Thermals move along with the wind so as you
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