GREAT PLANES Spirit Manual de instruções - Página 15
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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 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.
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 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 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 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 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.
When encountering sink, immediately turn and fly 90
degrees to the direction of the wind (towards you if
possible). Apply a little "down elevator" and pick up some
speed to get out of the sink as fast as possible. Every
second you stay in the sink is precious altitude lost.
POINTERS FOR CONTEST FLYING
Pay Attention! – Pay close attention to the sailplanes flying
before you, watch them and try to establish where and when
the thermals are being formed. Thermals are often formed in
cycles and can be fairly regular, so if you keep track of the
time intervals you will have a pretty good idea of when and
where a thermal may be generated.
Watch The Birds! – Thermals suck up small insects that
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
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