GREAT PLANES Spectra Instruction Manual - Page 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.