What are the forces on a tied down glider?

There is no hard data on tie down strength. Just plan for the worst.
PolyPro will lose most of its strength rapidly from UV. Good covered
climbing rope and locking carabiners are the best bet.

Back in the 60s someone wrote in Soaring about tying two-by-fours
along the tops of wings to act as spoilers. I've never seen it, but this
is strictly a "for at-home" solution if it works. Boards would need to
be stabilized in some way, of course.

At 23:05 13 August 2008, wrote:
There is no hard data on tie down strength. Just plan for the worst.
PolyPro will lose most of its strength rapidly from UV. Good covered
climbing rope and locking carabiners are the best bet.

I used to leave my ASW17 with a trestle under each wing, to stop them
moving and ties the tail down. I also left it in full negative flap which
meant that it would not lift.
If the wind was forecast to be more than 30 knots it went in it's box
although I was caught out a couple of times with unexpected wind speeds of
up to 60 knots, it never moved.
I suspect with speeds of 100kts you would be luck to find the glider
intact even if the tie down held, and even in the trailer it would be at
risk.