GREAT PLANES Spectra Manual de instruções - Página 14
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Facts About Thermals
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 but of course much gentler. Most thermals have
updrafts rising in the 200 - 700 feet [60 - 220m] per minute
range but they have been known to produce updrafts of over
5,000 feet [1500m] per minute (that's over 50 miles/hour
[30km/h] 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 [600 and 1500m] high.
Thermal Soaring
It takes a lot of concentration to thermal soar effectively. An
electric 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' [120m] altitude) compared to the
rest of the sky, the sailplanes will rarely fly directly into the
thermal and start rising. Generally, the electric 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 electric 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
electric sailplane carefully and it will tell you what it is
encountering. When the electric 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 electric 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 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 electric 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 turned
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
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.
The twirling action will give the sun a better chance of
flashing off of the wing and catching your attention. When
you are high enough and want to leave the thermal, add a
little down trim to pick up some speed and fly 90 degrees to
the direction of the wind. If you are not real high and want to
find another thermal, you may want to look upwind of the
last thermal. The same source that generated this thermal is
probably producing another. Just watch out for "sink" which
is often found behind and between thermals.
As you might expect, with all this air rising, there is also air
sinking. This air is the electric sailplane pilot's nightmare that
can really make soaring challenging. "Sink" is usually not as
strong as the thermals in the same area, but it can be very
strong. Down drafts of many hundreds of feet per minute are
common on a good soaring day. These down drafts can
make a sailplane look like it is falling out of the air. Because
of this, it is important that you do not let the sailplane get too
far downwind.
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