GREAT PLANES Fling DL ARF Buku Petunjuk - Halaman 15

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THERMAL FLYING

Thermal soaring is one of the most intriguing of all aspects

of flying and the Fling DL ARF was designed to excel at

thermal soaring even in the hands of a novice. It can be hard

for the average person to understand how a plane can fly for

hours and gain altitude without a motor!

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 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