Great Planes Spirit 100 Sailplane Kit - GPMA0550 - Manual (Page 52)

updrafts of over 5,000 feel 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 approaches a thermal, the wing tip that reaches the rising air first
will be lifted before the opposite wing lip This causes the plane
to "bank" and turn away from where we would like the plane to
go.
TYPICAL THERMAL
W i n d causes t h e r m a l
to d r i f t d o w n w i n d
When you arc thermal soaring try to fly as smoothly and
straight as possible Trim the plane to f l y 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 l i f t ) and then start circling in a fairly light 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 l i f t Thermals
move along with the wind so as you circle you will be swept along
with it Be careful when thcrmaling that you don't get so far
downwind you can't make it back to the field to land
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 s strength diminishes Eventually the thermal stops rising and
any moisture contained in the once warm air condenses and forms
a p u f f y 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 opportu-
nity 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
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 ha.s 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 ther-
mals 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 allilude very quickly This is especially helpful if the
sailplane gets sucked into a cloud or it gets too high to see The
twirlmg action will give the sun a better chance of flashing off of
the wing and catching our attention When y ou are high enough
and want to leave the thermal, add a little down trim to pick up
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" it 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
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