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| Thermals |
| Free Lift |
By Fletcher Anderson
Most modern gliders sink through still air at less than 150 feet per minute, and glide at ratios of at least 28 to 1. Any time the air is rising at better than that rate, you can soar. Once you are high enough, this flat glide ratio allows you to head out cross-country towards the next source of lift.
Any source of rising air will work, but there are three basic things to look for: ridge lift, wave lift and thermals.
Ridge lift is the wind deflected up the face of a ridge. You stay aloft by surfing back and forth on the incoming wind. You don’t necessarily get very high or go very far, but the lift is very dependable. The glider duration records have all been set in ridge lift, generally at ocean side cliffs with incoming sea breeze. These records are no longer recognized because flights in excess of 72 hours were getting too dangerous due to sleep deprivation.
Wave lift produces the most phenomenal altitude gains and unbelievable distances. Mountain waves, if you could see them, would look like standing waves of water in a river. The water flows over a rock, sinks down, then rises up again, usually forming a series of stationary waves downstream from the obstacle. Wind blowing over a range of mountains does the same thing, particularly when you have relatively high winds and stabile air. The current altitude record for gliders is in excess of 47,000 feet in wave lift east of the Sierras. The longest cross-country distance ever flown by a glider is over 1600 miles, flown last year in wave lift east of the Andes in Argentina.
 Thermals, however, are the bread and butter of non-powered flight. Contrary to what many people think, sunlight does not heat the air to any significant degree. Rather, sunlight shines right through the air and heats the ground. The warm ground then heats the air by convection. The surface of the ground is not uniform. Some ground is snow-covered and reflects sunlight back into space with very little heat exchange. Conversely, black, bare rock hillsides soak up and release considerable heat. Open fields soak up heat, while brush covered fields don’t. Air rises over the hot ground and sinks over the cold.
Envision a pot of water boiling on the stove. This is a good analogy for the air on a hot, high-pressure day. Bubbles form on the bottom of the pot. When enough energy is built up, the bubbles release and rise up to the surface. Flying on one of these days, you are hit by a series of short duration tight little bullets of lift. On a day with unstable air – such as you find with low pressure and an approaching cold front – in place of tight little bullets, you are more likely to encounter broader continuous plumes of rising air. Once the bubble or plume of warm air begins to rise, its lower density warm air causes it to rise ever faster through more dense cooler air around it until it gradually dissipates by mixing with the surrounding air. All forms of rising warm air are called thermals.
Thermals are generally small and tight near the ground. They increase both in diameter and strength as they rise, until they reach an altitude where their energy is spent and they begin to fade out. There are as many methods of flying thermals as there are pilots. Techniques change depending on the speed of the glider and the size of the thermals. Here is an encounter with a typical midsummer thermal:
You are flying slowly, more or less straight towards what you hope is a source of lift. It might be a rocky mountain ridge. It might be a bare open field out on the prairie surrounded by growing crops. It might even be an asphalt parking lot. (The one at the hospital in Aspen is my favorite, you couldn’t ask for a better spot to crash land if you sink out!)
Suddenly you hit just a little turbulence and your sink rate increases. Then, just as suddenly, you are gaining altitude. Quickly, very quickly, you crank the glider around in a steep bank. A 45-degree or steeper bank is normal – less bank than that and at 50 knots you just fly right out the other side of the thermal. At the same time, you also pitch the nose up slightly to get a minimum sink speed of something like 45 knots, depending on the glider you are flying. Two or three full circles is more than enough to tell that lift is much stronger on one side of your circle, so you fly a short little straight segment to move the center over.
For the first couple of turns, you might only be climbing at a hundred feet per minute or less, but as you adjust your circle, the rate of climb very gradually increases. A few more circles, a few hundred feet higher, and it is much easier to keep your circling in lift and your climb rate increasing. Four or five more circles and you are one thousand feet higher, and climbing at one thousand feet per minute.
By now the ground is dropping away and the once tall mountains are spread out below you like a carpet. The precise source of the thermal is no longer apparent, but the tops of nearby thermals are – a puffy fair weather cumulus cloud marks the top of each. Circle up to the cloud base, then head out at best glide to get under the next cloud.
It’s early still. These clouds will ultimately overdevelop into afternoon thunderstorms. But for now, the sky is no longer the limit and you have hours and miles of unmotorized flight ahead, courtesy of nature’s thermals.
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Fletcher Anderson lives in Telluride, Colorado, and is the operator of Telluride Soaring (970-728-5424), the world’s highest glider operation, and Mountain Aviation Services (970-728-1728), the second highest flight school in America. He has over 4000 hours mountain flying experience and has given over 1000 hours of mountain flying instruction.
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