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 it 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, arc usually between 2000 and 5000 feet high.
TYPICAL THERMAL
Wind causes thermal
to d r i f t downwind
WIND DIRECTION
THERMAL SOARING
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 flys 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 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 circle you will be swept along
with it. Be careful when thermaling that you don't get so far
downwind you can't make it back to the field to land.
If the sailplane is flying along straight and all of a sudden
turns, let the plane continue to bank (you may have to give it
some rudder to keep it banking) until it has tuned 270 degrees
(3/4 of a full circle). Straighten out the bank and fly into
whatever turned the plane. If you encounter lift, and you
won't every time, start circling just as you did when flying
directly into a thermal.
Thermals are generated all day long, but the strongest
thermals are produced when the sun is directly overhead.
10:00 am - 2:00 pm seems to be the best time to get those
'killer'' thermals. Some of these thermals can be very large
and you may find it hard to get out of them. 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
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