On Monday, April 24, 2017 at 3:45:05 PM UTC+3, Chris Rollings wrote:
A large proportion of the tow-plane upsets, including most of the fatals,
happen very shortly after take-off, before there is an opportunity to get
into low tow. Also, descending into the prop-wash can be a trigger to the
momentary loss of control that leads to the upset, if the glider is on a C
of G hook.

Do these upsets *only* happen with a C of G hook?

I've done a fair bit of flying in a C of G hook Janus, including on days with wave and pretty bad rotor. Never had a problem.

Does an upset really happen because the glider pilot simply flies out of position? It seems unlikely to me. You need not only to be high but also with significant pull on the rope. All the times I've gotten high it's because the towplane suddenly ran into sink (of at least out of lift), and the result has been a slack rope, not the tight rope required for an upset. Simply maintaining station (even though high) until the slack starts to come out, and then descending back into position with a constant amount of bowing (i.e. low tension but not slack as such).

Without having actually been there myself, I suspect that upsets may be caused not by being out of position, but by having a slack rope suddenly come tight. With a C of G hook this causes a reasonably large nose up pitching moment which may not be able to be countered by the elevator. The glider can get significant angle of attack and lift and upwards and (as the angle increases) backwards acceleration. If you're towing at 68 knots (or more) with a stall speed of 40 knots then you can generate 3 G of acceleration. With 3 G of acceleration you've only got to get to 10 degrees nose up to tension the rope to half the glider's weight, 20 degrees nose up to put a full 1 G of force onto the rope, and 30 degrees for 1.5 times the glider's weight in tension.

The glider being out of position is most likely the very quick *result* of the glider going into winch launch mode, not the cause of it.

I can't see that low tow would make any significant difference to this. Slack being taken out violently (but not quite enough to break the rope) will have exactly the same effect as in high tow.