Thermalling Too Slow
We want to be slow and steep to get the smallest turn radius possible. However, if you get too slow, you do not have the control authority that you need. The sink rate increases as you get too slow or too steep. Plus when the wing stalls it is very inefficient. Thermalling too slow causes lower control authority, higher sink rate, and is inefficient.
Stall recognition
Because we are thermalling slow and near a stall, this probably will happen sometime during your flight. If you have never had the glider stall while thermalling; you are either flying way too fast or did not recognize the stall.
The second you notice the wing is no longer flying you need to make sure you are recovering from the stall and potential spin entry. You potentially could make things worse quickly by using aileron to pick up the low wing. That down aileron increases the angle of attack on a wing that possibly could already be near the critical angle of attack.
You do not want to stall because it is not efficient. However, it is a good indicator that you are making the smallest turns possible. When the core of the thermal is only a few hundred feet wide, the FAA Glider flying Handbook Chapter 3 has some good examples showing bank angle and speed and how it determines the turn radius.
Neutralize aileron, forward stick, and opposite rudder.
If you are concerned about the glider no longer flying, spin recovery is probably a safe bet. You might not want to wait until after you realize the aileron is making it worse. The back pressure on the elevator is speeding up the rotation. You must be aware of what your glider will do.
One student and I were joining a thermal a little ahead and below a 1-26. As he was looking back over his shoulder, he shoved full rudder and pulled back on the stick. We had nearly completed a rotation before he looked forward again trying to figure out what happened.
Time to forget about efficiency
Gaggles can be very efficient in cruise and finding a thermal. What they are not good At is coring the thermal. You might be able to use others to see where the stronger climb is but many times you will not be able to shift enough because it will result in a conflict with another glider.
You will also need to speed up a little and give yourself a bit of cushion. A bad thing to do is stall while someone is underneath you. I guarantee if you trade paint, you will climb a lot slower and best case scenario you will only have to change your shorts.
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Garret Willat holds a flight instructor rating with over 8000 hours in sailplanes. His parents have owned Sky Sailing Inc. since 1979. He started instructing the day after his 18th birthday. Since then, Garret has represented the US Junior team in 2003 and 2005. He graduated from Embry-Riddle with a bachelor's degree in Professional Aeronautics. Garret represented the US Open Class team in 2008 and 2010 and the Club Class team in 2014. Garret has won 3 US National Championships.