This article comes from SAM Speaks #115 January-February 1994, no author was stated. It provides an interesting introduction to thermal soaring which is a skill that is fundamental to our hobby/sport.
THE ART OF R/C SOARING IN THERMALS
Not too long ago when a fellow who had been flying power for years became interested in soaring, he requested aid in the art of locating and riding up on thermals. He has since derived so much pleasure from soaring that he suggested expounding on a “thermalling primer”.
What’s a thermal? A thermal is, in the simplest language, a batch of hot rising air-an updraft. Damn its origin and all that unnecessary technical stuff; all we care about is what it “looks like” and how we can find it, recognize it when we do, and how we can make the most of its lift. Here’s a method that works. Good pilots may have variations that differ but only slightly. This method has had me up in excess of one hour a number of times.
The basic skills required are the ability to launch to at least 300 feet, and to turn smoothly.
Proper preparation is important. Your sailplane should be balanced properly. The correct balance point will vary, not only with the design, but with the wind conditions and the skill of the pilot. That is, the balance point must be moved forward on “floater” type sailplanes to penetrate in wind, and the less skilled pilot will find that a rearward C.G. makes a smooth turn sheer luck. I have found that most pilots have the controls set too sensitively, thus magnifying the natural tendency to over-control. If you are having trouble making smooth turns, DESENSITIZE YOUR CONTROLS. Where you go in a thermal is not nearly so important as how smoothly you get there. The trim should be set for the optimum glide angle when not turning. Then, when in a thermal, the pilot will move the rudder trim for the turn radius desired and feed in up-elevator trim to maintain the proper glide angle.
The Search is not just a matter of luck. Unless someone else is going up like a homesick Martian, or if you have some other good reason to head for a known spot, you must perform a logical search pattern to reduce the chance of bad luck. The search pattern should take into account the recognition problem.
It is easier to recognize lift when the flight path is perpendicular to a line between the observer and the aircraft. A favorite method is to fly a pattern as shown below:
Because of altitude limitations, the downwind leg will usually not be as long as shown above, but the basic idea is to keep the model on a straight, smooth course, perpendicular to the pilot’s line of sight. Another excellent method, especially when the wind is up, is to fly a series of left-to-right-to-left zig zags upwind, being careful not to cover the same “ground” (air) again.
You may develop your own, maybe better, search pattern, but keep the salient points above in mind.
Recognition is the most difficult part of thermalling. The real difference between being able to go up in thermals and just getting umpteen 3-minute (or less) flights every Sunday, is the simple (so it may seem) ability to recognize lift when it happens. I’ve got to say it again! Nobody can recognize lift when he’s jerking the elevator up and down. Keep your hands off the stick!
Picture in your mind’s eye the normal sink rate of your machine. Now-when you see that downward line become zero, or better yet, an upward line, you’re in LIFT. Even if the sink rate only becomes zero, you’re in lift. Many times I have seen expert flyers max out for 10 minutes, never getting any higher than the launch. ZERO SINK! On occasion I have been in zero sink for 2 or 3 minutes, only to have the embryonic thermal develop into the fable of the week, taking my model to the limits of visibility. Don’t throw away the “zero sinkers”.
Measure the diameter of the thermal to get the most out of it. This is an important aspect that many otherwise good pilots miss. Because the normal sink rate of any glider goes up as the radius of turn goes down, it is a superior technique to fly the largest circle that lets you remain inside the thermal. Thermals vary in diameter, not only from thermal to thermal, but within the thermal. As the height increases, so does the diameter.
Do not turn the instant you recognize lift. Continue straight until the lift has been passed. Now do a 180 deg. At the previously determined center turn 90 deg. and fly to no lift. Turn 180 deg. and repeat. Now you know its depth, width, diameter and exact location. You know how big a circle you can fly and where its center should be. The knowledge thus gained is worth hundreds of feet, and will have cost you less than 50 feet. Many thermals are lost because the pilot never quite knew exactly where they were. Many feet of altitude are lost by turning in a tight spiral in a big thermal. I have frequently noticed another pilot in a thermal, joined him, measured it and then by flying with this knowledge (which he never bothered to get) flew right up past him. When someone flies up through me it embarrasses the hell out of me. I won’t let it happen if I can help it-will you?
Fly smoothly! Second only to recognition, smoothness is the most important aspect of flying thermals well. Learn to turn without losing altitude. Learn just how sharp you can turn your particular model without tip stalling. Practice this until you can turn as tightly as possible without diving or tip stalling.
Make the largest circle you can and still stay in the thermal. This will result in the lowest relative sink rate and therefore the greatest net rising velocity.
Drift with the lift. Did you ever notice a “whirlwind” or a “dust-devil”? They move DOWNWIND! So does a thermal, but generally, not as fast. Therefore, it is nice to find a thermal upwind and stay in it drifting downward until you feel it is wise to return upwind and find another. To fly an hour, you are likely to fly in 10 or more thermals, yet never move from the launch area.
When is the best time? I have seen days when all the best lift was over before 10:00 AM, and have flown in good lift when it was too dark to fly a block away, but generally, the best lift will be between 10 AM and 3 PM, mean sun time. Generally speaking, before 10 AM, there is insufficient heating of the ground by the sun for good lift, and by 3 PM, the air has heated to the point where good lift is less likely.
Where? Everywhere south of 90th parallel, except over water and sometimes even over water. Ever see a flat-bottom cloud? It’s sitting “on-top” of a thermal. Ever see a “dust-devil” or a “whirlwind”? Those were thermals. I flew a Drifter clean out of sight in a “dust-devil” once. Having spent most of my life in the East, I can tell you the lift is good from coast to coast.