On Saturday, September 9, 2017 at 7:46:30 AM UTC-4, Martin Gregorie wrote:

Reasoning: as the glider flies into the thermal it enters a rising air
mass and the further it moves into this air mass, the faster the vertical
movement becomes. The effect on the glider is that its effective AOA is
reduced by the air's vertical velocity (draw a vector diagram and this
becomes obvious)...

Seems the opposite to me. The rising air hits the wings from below, and thus at a higher AOA. This increases the lift on both the wing and the tail, but more so at the tail. This is what positive stability means. The glider then behaves just like after raising the nose momentarily and then letting go of the stick: it pitches nose down. It's not the trimmed speed it is seeking per se, it is the trimmed AOA. Thus in the thermal entry it pitches nose-down despite the speed being normal. Once the glider accelerates upwards due to the rising air, the vertical motion relative to the air returns to normal, and the normal pitch attitude relative to the horizon will be restored on its own. The more pitch-stable the glider (and yes this is called "longitudinal stability" IIRC?), the stronger this pitching effect should be. Maybe that's why I haven't noticed it in my gliders, since I have the CG near the rear end of the range, i.e., weak stability.