How do you explain why the A/S increases on thermal entry?

In message .com, Fred
writes
Just got asked this question, didn't have a quick and easy answer. How
do you explain it?

I've always thought of it as a change in the lift drag vector. If your
glider is flying in still air the lift drag vector is pointing up and
towards the tail. If rising air is entered, which effectively increases
the lift vector the new lift/drag vector points slightly more forward
than previously. This reduces the effective drag and the glider
accelerates until everything balances out again.

This may be total rubbish but it is the model I've found easiest to
visualise.

Robin
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Robin Birch