Mike I don't think the effect has anything to do with standard vs
flapped gliders. I've owned three flapped ships and this phenomenon
has perplexed me on each.

I had a tow in my ballasted ASG-29 yesterday and I'd told the tow
pilot to maintain minimum 80 mph. Several times he got down to 70 mph
(60 knots IAS) and I was fighting to maintain control of the glider,
at a speed that would be just a couple knots below best L/D were I in
free glide.

I've learned to ask for a speed that will keep the glider happy at
neutral flap setting (think standard class) and then lower the flaps
when the tug is flying to slow, and I have to do this at some point on
darn near every tow.

2NO