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R/C thermal soaring can be a very enjoyable and challenging form of model flying.  An average high start launch will get a glider up to about 300 feet high and then it's up to the pilot to try to maintain or increase the altitude.  Gaining altitude and getting long flight times isn't easy and this is what makes soaring challenging.  Being successful at gaining altitude and getting long flights is what makes soaring enjoyable.

Thermal soaring requires some extra skills and knowledge to be successful.  Most beginners are happy to get in a few flights without damage however as they learn to fly well they develop a desire to get longer flights.  Good flying skill will help the pilot to get longer flights however, a little knowledge of the characteristics of thermals makes a big difference.  Often the beginner finds out that there isn't much information available in the soaring columns in magazines on how to find and use thermals.  He usually needs to talk to other soaring pilots or search for one of the few books available on soaring.

Thermals are masses of warmer air that are rising through cooler air.  Thermals come in different shapes and sizes however they are invisible and we need to rely on our senses to detect them.  We primarily rely on our ability to see the effects of a thermal on our gliders to detect them.  Thermals also effect the wind direction and velocity and can be detected by observing objects like trees, flags and smoke that might be near the flying area.  We also can feel the changes in the wind on the ground and detect where a thermal might be.  A whirlwind or dustdevil is an indication of a thermal however most of the thermals we use don't produce them.

A thermal is formed when the sun shines on an area of the earth's surface that is darker than the surrounding area.  The air over the darker area is heated more than the surrounding air and it will rise because warm air rises and cool air descends.  As the warmer air rises it draws cooler air in behind it.  This cooler air is then warmed and follows the warmer air that drew it in.  This activity will continue until a cloud covers the sun, the wind blows it away or the warm air dissipates.

Cool air is denser than warm air and it's often difficult for the thermal to break through and rise.  Usually when the air gets warm enough to rise, it has built up a great deal of strength and this can lift our gliders quite high.  If the thermal isn't disturbed or deteriorates, we can get a long flight by keeping the glider in the rising air.
 
The shape of a thermal varies and this can make it difficult to determine where the rising air will be.  Wind and turbulence can distort and break up a thermal.

Most often thermals form a column of rising air from the ground to the base of any clouds that might be present.  Generally thermals are a small diameter near the ground and widen out as they gain altitude.  At 400 feet high, thermals are usually wide enough to be able to circle your glider in fairly wide turns and still remain in the rising air.  The top of column thermals tend to drift downwind from where they start so, as your glider gains altitude you will need to fly downwind to remain in the lift.

Sometimes thermals form a bubble near the ground and when they become strong enough to rise, the entire mass of warm air rises at once.  These bubble thermals don't have any rising air above or below them so you will need to keep your glider within them or you might loose altitude and won't be able to benefit from them.  Usually you will find bubble thermals at low altitudes and won't be able to gain much altitude from them until they break loose from the ground and push up through the cooler air.

Whenever warm air rises cooler air will move in to take it's place.  This cooler air will tend to be descending and can really suck a glider down.  It can get frustrating to fly around between the thermals without encountering one and end up with a very short flight.  Often this descending air is located near the ground and near the edges of the thermal.

The primary way we detect thermals is by concentrating on the behavior of our gliders as they fly along.  Naturally, our control input from our transmitters will change the behavior of our gliders.  To find a thermal we need to be observing behavior that we know was caused by the flow of air over the glider and not ourselves.

If the glider is flying smoothly and then takes a little bounce, rises slightly, then bounces again, it has probably just flown straight through a thermal.

Also, if the glider is flying straight and level and then begins to turn on it's own it has probably flown into the edge of a thermal.  The effect of the thermal on one end of the wing causes it to rise and turn away from the thermal.

Usually a glider will bounce a little as it approaches and leaves a thermal.  This is caused by turbulence that is generated by the change in direction of the air flow.  Not all bounces are thermals so watch carefully.

If you determine your glider has just flown into or through a thermal, turn the glider back and circle through the area where you began gaining lift.  If the glider continues to rise, it is in a thermal and you should continue to turn in circles so the glider will remain in the lift.  If the glider drifts out of the thermal it will most likely bounce and begin sinking.  Circle back to the thermal and remember where the edge is.

If your glider begins turning fairly hard on it's own, then it probably flew along the side of a thermal.  Try to turn the glider back and enter the rising air.  Remember, thermals will try to turn a glider away from them so you might need to turn very hard to get the glider back into the lift.  And don't forget where the edge was.

When you have established the glider in a circulating pattern in a thermal, watch it carefully to try to determine where the edges are.  Try to avoid flying outside the core or center of the thermal.

As the glider gains altitude you will be able to make wider circles and increase the airspeed.

Avoid trying to slow down the glider when it enters a thermal.  You'll probably  find you have a tendency to do this and it can lead to a stall or mush that can eliminate as much or more altitude than the thermal is providing.

If you keep this information in mind the next time you try thermal soaring it should help you to become more successful.

By the way, this information didn't come from books or magazine articles, it came from the experience and ideas of actual soaring pilots.