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Mountain thermals are often small, turbulent, and powerful. We are often working them close to terrain. To work these thermals safely and efficiently we must fly accurately, maintaining a steady attitude and a small circle.
The circle size is a balance of speed and angle of bank. In a turbulent thermal, or close the the terrain, you are better off fast and steep than slow and shallow.
The optimum speed for thermalling is the minimum sink speed for the glider. This will give the best rate of climb.
The minimum sink speed varies, increasing in a turn due to the load factor. The load factor varies with the angle of bank.
The optimum angle of bank for thermalling varies, but the sweet spot is around 45 degrees. At 45 degrees, the load factor is approx 1.4. All the load related speeds (stall, min sink, best glide) go up by approx 20%.
For the DG1000, dry, one up, the min sink speed is approx 43 knots. At 45 degrees, that becomes approx 52 knots. An extra pilot would bump that up a few knots.
A reasonable target thermalling speed would be 55 knots.Some gliders need a bit extra to maintain good control. Then add a bit for turbulent air, just plus five knots. If close to terrain, add another five knots. When the wind blows, add approx half the windspeed.
For the example DG1000, close to terrain, use 60 knots. If it looks to be turbulent, start with 65 knots. Plus a little for wind. In a ten knot breeze we're up to 70 knots. At this speed bank more than 45° to keep the circle small. As I climb I might slow down if the air isn't rough, and as I gain more terrain clearance. It is very important to have all the safety margins at the start and only pull them in once you have the feel for the conditions.
I generally start with 45° as my angle of bank. If those additional safety margins of speed add up I steepen up to maintain a small circle. If your circle is less than 200 meters in diameter you are doing well. If it is more than 250 meters you need to work harder. At more than 300 meters you are unlikely to climb unless you're in a really generous thermal.
To judge this angle, note that the mounting screws for the round instruments are traditionally 45° off vertical/horizontal. This means when you're in a 45° bank the horizontal mounting screws will be lined up with the horizon.
When circling beside terrain it is essential to have a safety margin. You need to be flying a small, regular consistent circle. The angle should be steep, but not as steep as you can fly. If you make a mistake you need to be able to steepen up to tighten your turn if you need more clearance. Moderately steep, not as steep as you can manage.
Circle size is available from some flight computers. XCSoar based flight computers (XCSoar, LK8000, Top Hat) can provide it.
You need to fly a circle so that you stay in the thermal. A steady attitude (speed) and a steady angle of bank. If you are struggling to control the glider you should back off a notch until you smooth out. Keep your safety margins at all times. Once you are stable, tighten up a little at a time. Steep is good, but consistent is essential.
We like to carry a little slip in the turn. A correctly coordinated turn will show a little slip on the yaw string because the string is ahead of the center of rotation of the glider. A little bit more will help stabilize the glider in the turn and make the glider less likely to drop a wing if it gets a bit slow. I'd think ten or twenty degrees of slip showing on the yaw string would be about right.
To clarify: Slip is when you are turning with not enough rudder into the turn. The nose lags in the turn. The yaw string free end is high or outside the turn.
Skid is the opposite. You are turning with too much rudder into the turn. The nose leads in the turn. The free end of the yaw string is low or inside the turn.
A skidding turn is setting up for a spin entry.
This is tricky. Often books say you should not do this. That is good advise. If you don't need to circle close to terrain, don't. You can often work thermals up the side of a mountain by working it like ridge lift, doing beats back and forth through the area that has the thermal going up. This might compress down to doing 'figure eights' in a smaller thermal or be many hundreds of meters in a anabatic flow. Always remember ... You must always have adequate terrain clearance. You must always have a clear escape.
A 'normal' thermal turn is 200 - 250 meters across. So I would consider half a turn to be close. Call it 100 meters. Five wingspans for a Duo, six and a bit for a little single seater. Always turn away from the hill first, so a regular circle will be over 100 meters away from the terrain when you come round the first time. Even a little wind drift will still leave room to move in one turn.
How for away is enough to ignore the hill and turn toward it? With a 200 - 250 meter circle and a 100 meter margin, 350 meters minimum. Realisticly, more like 500 meters (half a kilometer) before I'd be getting relaxed about that. Many pilots turn closer to 300 meter diameter, and they probably need a bigger margin, so for a less experienced pilot I'd say 500 meters minimum and more like a kilometer before I'd relax.
Steep slopes are generally safer than shallow slopes. Your escape is easier beside a steep slope. Just fly away and you get plenty of ground clearance quickly. A shallow slope may have you sinking at the angle of the slope and getting no extra ground clearance as you run out to the flats. People sink onto terrain below them as often as they fly into terrain beside them.
You are better off above a small hill than beside a big hill. If you can retreat to a hill you can be above the top of you will probably get a better climb and a safer climb there.
If there is a spur sticking out from the hill you can figure eight around to work both faces you get an easier and safer climb than in a gully where your escape is more difficult and you're closer to the terrain for more of each turn.
If that is not doing it for you and you need to climb, if you are going to circle close beside the terrain you must do several things. In this situation the penalty for error is severe. You must be able to fly a consistent circle without reference to a good horizon. You must make a conscious decision each time you turn back to the hill that you have room to complete the turn. You will only get this wrong once, so be sure.
Briefing prepared by Phil Plane.
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