Sat Oct 21, 2017 6:37 pm
I typed a long answer earlier which disappeared! This is from memory of my reading, so ...
On a winch launch the wings need to generate lift to equal the weight of the glider + the downforce from the cable's pull. Max winch is set to keep the wing loading at a safe margin below the wing's structural limits. À weak link in the system should break if that safe loading is exceeded substantially, but before the wing is at risk of structural damage. I don't know how the margins are set, but you could reverse engineer them via the formula for calculating max winch after a glider is reweighed.
Max aerotow is a mystery to me, but I know from experience that when being towed near that max the controls require more force to operate than when the glider is in free flight at the same airspeed. Guess this might be because the glider is climbing rather than descending through the airmass, and thus the wing and tailplane are operating at a higher AoA, but I don't understand that relationship (you might though). As a further guess, either this speed is related to max manouvering speed, or that beyond that speed the controls require excessive force to control the aircraft.
A 15 min phone chat with the BGA Chief Technical Officer would probably give the information rather more reliably!