Radio Control Soaring - 2006/01
Darwin Barrie [[email protected]]
How to recognize and fly in thermals
Congratulations to everyone who competed in the 2005 Nationals. A special thanks to Jim Bacus who kept us informed as the event progressed. This was a huge commitment of time and energy after flying all day in the heat, humidity, and often tough conditions.
On the topic of tough conditions, this month I’m going to write about those pesky rising columns of air we call thermals. On occasion they are our friends, and on others they vex us with their size and unpredictability. I won’t go into a technical description of a thermal, but I will cover the techniques for recognizing and then flying them.
In Arizona, and probably most parts of the country that have large expanses of dirt, there are visible thermals we call “dust devils.” Throughout the years I have observed these to have three basic forms, with little variance:
- A thin shape resembling a #2 pencil. These dust devils are often tall, extending quite high.
- The typical tornado shape. This kind is often lower than the thin type in altitude but can also be extremely high.
- A wide cylindrical appearance. From my observations this variety does not share the altitude of the former two.
Understanding these basic forms will assist in recognizing and then setting up to fly them.
Recognizing a thermal can be a tricky proposition. Not only is the recognition influenced by your model’s altitude, but also by the type of thermal.
From level cruising flight, a disruption of the aircraft is your first indication that you have possibly encountered a thermal. A rise of a wing panel, lifting of the tail, yawing, or a pitch-up of the nose is a strong indicator. In some cases this reaction can be violent. I have experienced thermals that have completely rolled the airplane inverted or pitched the nose into near-vertical orientation. This makes the decision about what to do next easy; in other cases it is not quite as easy. If the disruption is not so pronounced, you’ve got some decisions to make.
I try to plan that no one else in the air is going to be of benefit to me. But I’m not stupid either, so if there is a gaggle of airplanes clearly going up, I’m there.
My general technique is to fly crosswind from the launch in cruising mode and watch for a change in attitude. If it is not one of the obvious violent signs, I will turn downwind from the attitude change and fly back to the area. The thermal should be going downwind, right?
When I encounter it again, I begin a turn and adjust as necessary to stay in the thermal. Every effort should be made to ensure that the turn is as coordinated as possible, to maximize the model’s efficiency. This often requires using more or less rudder than you have in a preset mix. Adjust your turns as necessary to stay in the thermal. You should be aware of the wind conditions and factor those into your adjustments.
During the process, your timer should be scanning the area for other activity in case the thermal your model is in dissolves. Remember the cardinal rule of thermal flight: “Never leave rising air.” The caveat to that is that if you are at the edge of your visual range, better to be safe than sorry. You gotta get back!
That was my basic technique. I have enlisted the help of five-time World Champion Daryl Perkins to discuss his theory of thermaling. Following are some of his thoughts.
Daryl Perkins — Thermal entry and technique
“If you use any one of these techniques, I guarantee you’ll make your times more consistently.”
Darwin asked me to do a little write-up on thermal entry, what I look for in the air, reading the model, etc. I’m going to base this on contest flying and optimizing a contest-type flight. I’m not going to go into ground signs or how to read the air prior to launch (enough has been written about that).
At most of the contests we fly in there are already airplanes in the air. These airplanes are tools for the contest pilot. We can use these to decide where we want to fly—and sometimes where not to.
Let’s assume we’ve just come off tow, and we see a gaggle of sailplanes. Let’s even assume they’re high. Do I go straight for the gaggle and try to fly in with them? Nope. I will always go downwind of the gaggle and come back toward them.
Why? Two reasons. Most TD (Thermal Duration) pilots really don’t thermal very well and even most of the top-rated pilots tend to fly upwind out of thermals. So I’ll usually go downwind and pop the good stuff. This works probably 90% of the time.
Now if I go downwind of the gaggle and I don’t pop, I can then come back straight toward them. Coming in from behind the thermal, I can see all the airplanes in that thermal, read them, and pick and choose the stronger air. I will use this technique whether there is one sailplane in a thermal or 20. It works very well.
Now, what do we do if we don’t see anyone buoyant? We don’t like the air that anyone has? Very simple: don’t fly there! I often see people come off tow and go to where someone else is already struggling—safety in numbers I guess. Big mistake!
If there is a bunch of airplanes or one airplane coming down, go the other way! Go 180° from where the bad air is. You’ll be surprised how often you’ll pop just by doing so.
Okay, now we’ll address every sandbagger’s and contest flier’s nightmare: no airplanes in the air, we have to pick our own air, and we have no idea which direction to go. The tendency is to come off tow and go straight upwind. I typically don’t like this. It’s hard to read a model upwind.
I’ll come off tow and pick a direction usually between 45° and 90° off of the upwind direction. Left or right—doesn’t matter, but I’ll be flying somewhat crosswind. I’ll just let the model go. Trim it for a little bit of cruise, not maximum L/D necessarily, and not minimum sink, but somewhere in between. Stay off the sticks and let it fly. The airplane will tell you where the air is.
Ever notice how sluggish a model feels in sink? Conversely, it gets just as lively in lift. As we fly in one particular heading, it becomes very easy to read the model. We can see the wings wobble or the tail yaw in a particular direction.
Hint: If we are abeam a thermal, the vertical fin will be pulled toward that thermal. So if the fin gets pulled to the left, roll slightly toward the left (not 90°, just slightly) and see what happens.
The biggest mistake I see here with most contest fliers is they come off tow and wallow to the left, wallow to the right, airspeed increases, they then slow up, sailplane wallows some more. How the heck are you supposed to read a model when flying like this? You’ll notice all the top pilots pick a heading, stick to it, pick an energy, stick to it, and let it fly.
Okay, now we’ve bungled our way into a thermal. What do we do now?
There have been some articles written about flying all the way through a thermal to define it. No. If I’m going up, I’m happy. But I won’t get complacent.
I’ll slow up and start to circle, usually a fairly large circle. I’ll do this to read the energy all the way around. I will constantly extend toward the stronger part of the thermal.
When I say extend, I rarely straighten out and fly straight once I’ve started circling. I’ll just open up the circle in the direction of the better air, maybe one-quarter diameter, maybe half diameter. Remember, the goal in a contest isn’t to get high, but to make your target time.
As I get more and more comfortable and I’ve decided the thermal is going to hold together, I will then begin to define the really strong parts, or the core, of the thermal. And I’ll optimize my climb accordingly.
Most pilots tend to have one diameter thermal turn they tend to fly. They bank over at 45° no matter what the air is doing and keep it circling. This isn’t the way to optimize our climb. Every thermal is different and needs to be flown differently.
The key is how to fly so we climb the fastest: big, lazy circles and shallow bank angles? Small, tight circles staying in the tiny core of a thermal? Or somewhere in between?
Ever notice how the top pilots climb right through you? Once in a thermal, they’re defining the strong spots and optimizing their climb rates. They’ll fly almost as slow as the model will comfortably fly (minimum sink), with a constant bank angle and constant energy.
If you use any one of these techniques, I guarantee you’ll make your times more consistently. Use them all; you still won’t beat me. I didn’t tell you everything I know ...”
That’s it for this month. Next month I’ll carry this discussion a little further and then begin the “after the thermal” process and setup for landing.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
