I've looked at the corkscrew type stakes and like the other poster,
thought they were a waste of time. In anything but quite soft dirt they
are imposible to screw in, and even then the smaller ones just don't
look strong enough. I ended up using "The claw" sold by Wings and
Wheels and others. I carry these as part of my landout kit. I've also
left the glider tied down for weeks at time at an airport using them.
They were nailed into hard dirt and were not going anywhere.

BTW since my glider only has tiedown locations at the wingtips I need
to be careful with the claw as the eye bolt sticking up from the top
could damage the wing underside if the wing came down on the claw.
Other tiedows including the screw in type, have the same problem. I
solve that at least at the airport by sitting a old aircraft tyre over
the top of the claw and running the tie down rope up through the centre
of the tire.

Darryl