Wing Bending Oscillation

[jcarlyle]

On my ASW-19B's airworthiness test report there's an entry for "rate of
wing bending oscillation". Two questions:

1. Why would a pilot want to know this frequency? Flutter prevention?
Attachment integrity?

2. How would he go about obtaining it? Accelerometer and frequency
counter? Stopwatch?

-John

[Shawn]

jcarlyle wrote:
> On my ASW-19B's airworthiness test report there's an entry for "rate of
> wing bending oscillation". Two questions:
>
> 1. Why would a pilot want to know this frequency? Flutter prevention?
> Attachment integrity?

This rate is a physical characteristic of each glider's wings. If the
frequency changes something has changed in the wing structure, for
example a delamination, crack in the skin or spar, broken aileron or
flap hinge etc. It's an easy way of assessing the integrity of the wings.

> 2. How would he go about obtaining it? Accelerometer and frequency
> counter? Stopwatch?

Stand at one wing tip and push up and down on the wing making it
oscillate (flap!) up and down at its natural frequency. Nothing
aggressive just a few inches up and down. Have a helper time how many
flaps/minute. Very simple to do and anecdotally, I've heard of people
discovering damage this way. If the frequency changes, best to get
things checked out.
I was also told to carefully look at the wing surfaces while flexing,
looking for odd ripples or bulges. Sounded reasonable to me, now part
of my pre-flight.

Shawn

P.S. Anyone know of Duo and Discus owners noting a difference after
voids were repaired?

[Bob Kuykendall]

Earlier, jcarlyle wrote:

> 2. How would he go about obtaining it? Accelerometer and frequency
> counter? Stopwatch?

The latter.

I've heard that it used to be common practice to establish the
first-order symmetrical (I think) oscillation by wiggling one wingtip
up and down, and timing the cycles. If the frequency was substantially
different from that specified, it was considered cause for concern that
the wing's structure might have been compromised.

I think that this practice might have been of merit back in the says of
wooden gliders and iron pilots, but I've not heard of it considered a
standard practice for twenty years, maybe thrity. Nevil Shute wrote of
it in his autobiography _Slide Rule_.

Thanks, and best regards

Bob K.
http://www.hpaircraft.com/hp-24

[Stefan]

jcarlyle wrote:

> On my ASW-19B's airworthiness test report there's an entry for "rate of
> wing bending oscillation". Two questions:
>
> 1. Why would a pilot want to know this frequency? Flutter prevention?
> Attachment integrity?
>
> 2. How would he go about obtaining it? Accelerometer and frequency
> counter? Stopwatch?

1. Quick control of structure integrity.

2. Grab one wing tip and move it up and down. Count number of
oscillations vs. time. I'm sure you could build something complicated to
get the same result.

Stefan

[Roy Bourgeois]

It's a throw back to the days when we worried about things like "How long
will GRP gliders last?". The idea is to check to see if there are any
structural changes happening in the wing or spar. It is checked by
assembling the glider, and "bouncing" the tips to the frequency where both
tips rise and fall together. Note the number of oscillations per minute on
your watch. Make a note in the logbook. Check it next year. It should
never change significantly (and it won't). I had an ASW-17 for 18
years. It never changed. Maybe this is just a Schleicher thing.

Roy B.

[Bill Daniels]

"Bob Kuykendall" > wrote in message
oups.com...
> Earlier, jcarlyle wrote:
>
>> 2. How would he go about obtaining it? Accelerometer and frequency
>> counter? Stopwatch?
>
> The latter.
>
> I've heard that it used to be common practice to establish the
> first-order symmetrical (I think) oscillation by wiggling one wingtip
> up and down, and timing the cycles. If the frequency was substantially
> different from that specified, it was considered cause for concern that
> the wing's structure might have been compromised.
>
> I think that this practice might have been of merit back in the says of
> wooden gliders and iron pilots, but I've not heard of it considered a
> standard practice for twenty years, maybe thrity. Nevil Shute wrote of
> it in his autobiography _Slide Rule_.
>
> Thanks, and best regards
>
> Bob K.
> http://www.hpaircraft.com/hp-24
>

I remember that with wooden gliders. The wing shaking process would
occasionally elicit questions from bystanders. To which we would soberly
reply that the purpose was to inform the termites that we were going to fly
the glider and that they should hold hands to help keep it together. :)

Wing shaking was abandoned sometime around the time that large span
fiberglass gliders were intorduced. I recall seeing someone voilently shake
the tip of a ASW22, I think. This produced a wave in the wing that traveled
to the opposite tip, where it was reflected, returning to wrench the wingtip
from the pilots grip smacking him in the chin.

Bill Daniels

[jcarlyle]

Thanks to you all for taking the time to respond. I learned something!

Initially I thought the wing oscillation rate had something to do with
flutter prevention or attachment integrity. When I was being taught how
to preflight the instructor told me he once found a wing that wasn't
properly attached by shaking a tip.

If wing frequency was being used for wing structural integrity testing
years ago, I'm glad to hear that it's been abandoned! Flaws that can
cause structural failure are much too small to affect a fundamental
resonant frequency.

I can believe that looking for odd ripples or bulges on the wing
surfaces during shaking could be useful indicators of problems, though,
so I, too, will keep it as part of my preflight. I won't bother to
measure the frequency, though.

-John

[Shawn]

jcarlyle wrote:
> Thanks to you all for taking the time to respond. I learned something!
>
> Initially I thought the wing oscillation rate had something to do with
> flutter prevention or attachment integrity. When I was being taught how
> to preflight the instructor told me he once found a wing that wasn't
> properly attached by shaking a tip.
>
> If wing frequency was being used for wing structural integrity testing
> years ago, I'm glad to hear that it's been abandoned! Flaws that can
> cause structural failure are much too small to affect a fundamental
> resonant frequency.

> I can believe that looking for odd ripples or bulges on the wing
> surfaces during shaking could be useful indicators of problems, though,
> so I, too, will keep it as part of my preflight. I won't bother to
> measure the frequency, though.

Guess I should tell the instructors that recommended it that their info
is a bit dated, 'course they are too. :-)

Shawn

[Ian Cant]

Off-topic, but related. There once was a four-jet
called Valiant, and a wood-trained iron pilot who flew
them. He had an ingrained habit, after landing and
shutting down, of getting a stepladder and climbing
it to grab and shake the wingtip. Everyone laughed,
until the day his wingtip didn't shake [it never shook
much] but came down limply a couple of feet in his
hands. Next morning the entire fleet of 150 or so
aircraft was grounded, and most of them never flew
again...

Ian







At 22:48 18 February 2006, Jcarlyle wrote:
>
>Initially I thought the wing oscillation rate had something
>to do with
>flutter prevention or attachment integrity. When I
>was being taught how
>to preflight the instructor told me he once found a
>wing that wasn't
>properly attached by shaking a tip.
>
>If wing frequency was being used for wing structural
>integrity testing
>years ago, I'm glad to hear that it's been abandoned!
>Flaws that can
>cause structural failure are much too small to affect
>a fundamental
>resonant frequency.
>

>
>

[jcarlyle]

Yikes! I guess the moral to that story is, if you're going to shake
them then do it before flight!

-John

[Rowan Griffin]

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
>
>

[[email protected]]

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

[Eric Greenwell]

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"

[Michael Huber]

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

[jcarlyle]

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

[Robin Birch]

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

[John Galloway]

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
>
>

[jcarlyle]

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

[John Galloway]

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
>
>