John, having witnessed GVTs I agree that simple wing oscillation is too
simplistic for checking flutter susceptibility. That's why I was
questioning it. So we agree on this.

Where I still have a problem is with using measured wing oscillation
frequency to uncover structural wing damage. As long as we're talking
gross damage (see Rowan's post of a failed repair or serious torsional
damage; Robin's post about listening for torsional damage; Shawn's post
about odd ripples or bulges) I don't have any qualms about shaking a
wing to find this type of damage. But for finding small damage (say on
the order of a inch or so) through oscillation frequency changes, well,
I believe the physics just isn't there.

I know you believe that the slight change in flexing of the Czech wing
of your Discus BT was indicative of sub-critical amounts of spar
damage. Considering that we're talking about a hand-made wing built by
two different factories, though, I think you were just seeing material
variations (density and modulus differences). The spar problem arising
later is, I think, just coincidence. I'd be more inclined to agree with
your view if the original German wing frequency had changed and then
spar damage had been discovered in it.

Nevertheless, as reported in this thread, Eric's ASH-26E manual,
Robert's HPH 304CZ report, and your 2005 glider manual all recommend
checking the wing oscillation frequency after hard landings to see if
it has changed. What I'd like to know is - is there anything from a
glider manufacturer that states the type and size of damage one could
expect to find by checking wing oscillation frequency with a stopwatch?

-John