On Jan 3, 9:02*am, Frank Whiteley wrote:
On Jan 2, 7:13*am, Andy wrote:





On Jan 1, 4:52*pm, wrote:

One interesting experiment would be to deflect the wings on the ground
and release them - with and without water - and measure the difference
in the frequency of the oscillations.

That would be of interest if the flutter limit speed was set by
primary wing structure, *Is it, or do the control surfaces flutter
first.

In my experience in transport aircraft flight test the flutter testing
is always done with maximum allowable free play in control linkages.
Do glider manufacturers do that, it not, does flutter speed reduce as
control links wear?

Andy

I think the flutter mode which occurs first may change with altitude,
the generation of glider, and wear, excluding the pilot induced mode.
Since the optimization of structures for operating under 6000m, I
would suspect dynamic flutter to occur first at lower altitudes, but
elastic flutter to occur first at higher altitudes, say above 8-9000m,
as the center of pressure shifts. *Dynamic pressures are more directly
related in IAS, rather than TAS. *Elastic modes are related to TAS.
IIRC, spar placement in modern designs is not as resistant to elastic
twisting at higher altitudes.

Frank Whiteley- Hide quoted text -

- Show quoted text -

You should be able to do something structurally to reduce the bending/
tortional coupling. NASA built the X-29 with a carbon fiber wing that
had forward sweep to show exactly that. Forward sweep has always been
known to have performance and handling advantages in transonic jets,
but "structural divergence" kept designers away from it in practice.

http://www.nasa.gov/centers/dryden/n...-008-DFRC.html

Excerpt: "Construction of the X-29's thin supercritical wing was made
possible because of its composite construction. State-of-the-art
composites permit aeroelastic tailoring, which allows the wing some
bending but limits twisting and eliminates structural divergence
within the flight envelope (i.e., deformation of the wing or breaking
off in flight)"

The past few generations of composite sailplanes would appear to have
greater aeroelastic stability by virtue of swept back leading edges
and (perhaps) spars that are further back in the chord.

Here is the sailplane wing flutter video I was referring to:

http://www.youtube.com/watch?v=kQI3AWpTWhM

You can see the flutter is symmetric with several waves from tip to
tip. It looks to me like you can see the twist increase at the tip as
the wing deflects upward - there may also be some aileron involvement,
but from the frequencies involved I would think this is secondary to
the main flutter mode. In reflecting on this a bit I recall that
control surface flutter is typically at much higher frequencies (often
described by pilots as making a buzzing sound). While this may destroy
the control surface itself or the hinges and control circuits, it
seems unlikely that it would activate the resonant frequency of the
associated primary structure (wing, horizontal/vertical stab). That's
not to say that losing you elevator is any less cause for concern than
losing your wing. I think wing flutter by design occurs at the lowest
airspeed. By virtue of the smaller forces on control surfaces it
should be easier to damp out control surface flutter mechanically -
unless your control circuits are out of spec. Going back to the
original question about water ballast, it would appear that ballast
might help damp out or delay the onset of the bending/twisting flutter
mode - although in the video the amount of deflection isn't that great
where the ballast tanks would be located so who knows how favorable an
effect it would be.

I'm not totally sure, but it kind of feels sensible to me.

9B