I shall take advantage of having some clearly well informed people taking an interest. So here goes with a couple more observations.
To overcome the downforce of the near vertical cable at the end of the tow, the glider must be producing (I estimate) 3 or 4 G of lift. Is that not a pretty teeth jarring kick in the backside when the cable is released?
The winch must start off fast with low torque while the glider is on the ground, but must rapidly slow down with increasing torque as the glider climbs.That sounds like a job for an electric motor, but the winch is clearly a combustion engine. How is that geared as the torque and speed requirements of this particular usage sounds like a perfect recipe for it to stall?