Wing Bending Oscillation

Reminds me of two situations where natural oscillation
of a glider component has surely saved me from potential
harm.

A few years ago I was a staff instructor at a club
here in the UK with a couple of K13s as the main school
fleet.
One day after a good few flights we were walking back
up the field to the take off point with the student
walking the wing, he commented that the wing 'felt
strange'. A good visual inspection showed nothing out
of order. However a gentle shake at the wingtip revealed
a broken spar about 4 ft in from the tip - result of
a poor repair several years previously.

Note *gentle* shake - we're not trying to snap off
the tip...just feeling the natural resonance, maybe
a degree of mechanical empathy is needed. But I think
if you are shown what 'feels right' for a particular
glider then anyone would notice if things were amiss.

The other somewhat scarey situation was visiting another
club and being given an Astir to fly. Ok that's scarey
enough, but seriously...
I had been shown a long time before how to check for
tailboom damage using the same method - a gentle push
back and forth at the top of the fin.
In this case the oscillation check revealed serious
internal damage from an unreported ground loop - again
there was no external visual clue!

I now always check both, after rigging my Std Cirrus
and everytime I fly anything else.

Rowan
Cirrus C2




At 11:48 19 February 2006, Jcarlyle wrote:
Yikes! I guess the moral to that story is, if you're
going to shake
them then do it before flight!

-John

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

wrote:


The most common method used is to raise the glider up off the tires
both main and tail.

This is the method shown in my Schleicher ASH 26 E manual, with the
front suspension point about where the fuselage dolly is and back point
just ahead of the tailwheel.

snip


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.

My factory measured number is on the original weight and balance sheet
(126 per minute). I've never measured it, but I'll try it next time I
assemble. It won't match the factory value because winglets were added
since then, but it will give me new baseline.

Or do not appreciate the
importance and helpfulness of this check.

My manual stresses this measurement after a hard landing. Sounds like a
good idea!


--
Change "netto" to "net" to email me directly

Eric Greenwell - Washington State, USA

www.motorglider.org - Download "A Guide to Self-launching Sailplane
Operation"

Rowan,

[Astir]
I had been shown a long time before how to check for
tailboom damage using the same method - a gentle push
back and forth at the top of the fin.

could you please explain exactly how you do that?

Thank you,
Michael

wrote:
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.

Robert's post is most interesting, because it indicates that HPH
considers wing frequency measurement part of a Ground Vibration Test.
The GVT, which is usually done with instrumented hammers or shakers and
a large number of accelerometers, is a form of modal analysis.

One of the major purposes of a GVT is to help determine an aircraft's
susceptibility to flutter. If glider manufacturers consider checking
the wing oscillation frequency a means to forestall flutter, then we
should all be doing this test annually!

Robert, can you provide a URL to the HPH report you mentioned? Or could
you scan it and post it on the web? I, for one, would be most
interested in seeing it!

-John

In message , Michael Huber
writes
Rowan,

[Astir]
I had been shown a long time before how to check for
tailboom damage using the same method - a gentle push
back and forth at the top of the fin.

could you please explain exactly how you do that?

With the glider rigged and with one wing on the ground push and pull at
the top of the fin. You will hear bad problems as creaks or unusual
movement in the fin top. It can help to have someone with their ear
close to the bottom of the fin around the rear wheel and possibly the
front of the fin where it fairs into to boom.

Robin

Thank you,
Michael



--
Robin Birch

Hans-Georg Berger, the Schempp-Hirth factory manager,
described to me last summer how exactly the same GVT
procedure that you describe is used for flutter analysis
of their prototypes and I guess that the same will
apply to all the main manufacturers.

Unfortunately sophisticated analysis of more modalities
of resonance than just the longitudinal wing oscillation
frequency would be required for ongoing evidence of
flutter resistance during routine airframe checks -
you would effectively need to do a full airframe GVT
which would be a bit impractical.

Stick to the typical glider manual advice and regard
measurement of the wing oscillation frequency as
simply one of the ways of uncovering structural wing
damage rather than proof against flutter susceptibility.

John Galloway




At 12:00 20 February 2006, Jcarlyle wrote:
wrote:
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.

Robert's post is most interesting, because it indicates
that HPH
considers wing frequency measurement part of a Ground
Vibration Test.
The GVT, which is usually done with instrumented hammers
or shakers and
a large number of accelerometers, is a form of modal
analysis.

One of the major purposes of a GVT is to help determine
an aircraft's
susceptibility to flutter. If glider manufacturers
consider checking
the wing oscillation frequency a means to forestall
flutter, then we
should all be doing this test annually!

Robert, can you provide a URL to the HPH report you
mentioned? Or could
you scan it and post it on the web? I, for one, would
be most
interested in seeing it!

-John

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

It would take a significant structural defect to show
up on a simple stop watch wing oscillation test.

John Galloway



At 15:30 21 February 2006, Jcarlyle wrote:
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