GREAT PLANES Spirit Gebrauchsanweisung - Seite 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