Checking a wing’s frequency is still a valid inspection technique.
I am looking at a document from HPH titled “Ground Vibration Test
Report”, dated 11 April 2000 it refers to the checking of a new 304CZ.
It lists the air temperature, tire pressure and a brief outline of the
method used. As far as I know this test is still done by the other
glider factories as well.

Tomorrow when the owner and I assemble this 304CZ for a weighing, I
re-check the wing frequency and enter that number on the W&B sheet.

The most common method used is to raise the glider up off the tires
both main and tail. You want to check the wing frequency not the spring
rate of the tires or landing gear suspension. I excite the wing at the
tip and a helper keeps track of the count I make in one minute. I do
this three times and almost always at least two counts are identical
with the third being off by 2 or 3. If the count is off by more than
10% from the last number then it is time to do some very serious
investigating. So far I have never found a count so far off and I hope
I never do.

The HPH documents clearly states the do the test with the main wheel on
the ground but the tail raised to the level position, an interesting
and unusual variation.

These test are useful for both wood and composite wings, and for all I
know even metal ones.

Often the original factory paperwork is lost and so too is the original
oscillation number, or the owner and his mechanic simply do not know
where to look in the paperwork for the number. Or do not appreciate the
importance and helpfulness of this check.

Robert Mudd
Moriarty, New Mexico