Wing Bending Oscillation

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

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?

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

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

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.

"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

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

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

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.

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

-John