Fully ballasted 70 knots minimum at all times (especially low down) what the manual specified. NB I interpreted that as being on the glider ASI as some tug types ASIs over-read significantly in the air. Pawnees seem to over-read by 5-7 knots probably due to not having accurate static pressure inputs so I asked for 75 knots with them. I always kept radio contact with the tug pilot.

My own club only had a 100 hp Eurofox so I never aerotowed the 21m there at more than 550kg and it wafted along happily at 65 knots.