Minumum Sink Rate/Best L/D at 17,000 feet ?

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

wrote:


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.

When I pause the video, I can see one tip is up while the other tip is
down. Isn't that asymmetric flutter?

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

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

On Jan 3, 12:39*pm, Eric Greenwell wrote:
wrote:

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.

When I pause the video, I can see one tip is up while the other tip is
down. Isn't that asymmetric flutter?

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes"http://tinyurl.com/y739x4
* * * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" atwww.motorglider.org

Your definition is, of course, correct Eric. I looked to me like once
the flutter was established it was symmetric. I'll take another look.
I think the symmetry or assymmetry may be aside to the main points of
the discussion as it isn't clear to me that it would necessarily
indicate anything one way or the other on the issue Andy raised about
control surface interaction and certainly not on the ballast question.

9B

wrote:
On Jan 3, 12:39 pm, Eric Greenwell wrote:
wrote:

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.
When I pause the video, I can see one tip is up while the other tip is
down. Isn't that asymmetric flutter?

Your definition is, of course, correct Eric. I looked to me like once
the flutter was established it was symmetric. I'll take another look.
I think the symmetry or assymmetry may be aside to the main points of
the discussion as it isn't clear to me that it would necessarily
indicate anything one way or the other on the issue Andy raised about
control surface interaction and certainly not on the ballast question.

Looking at it again, I can see the ailerons going to full deflection
(one up, one down, of course). That might be what is driving the wing
oscillations asymmetrically. If the oscillation was symmetric, I suspect
the ailerons would not be deflecting, since they can't both go down at once.

I am curious about ballast. My first guess is it lowers the oscillation
frequency because that's what mass usually does to a resonant mechanical
system, but frankly, I haven't a clue, and don't find any mention of
it's effect in FOSD, either. Either way, it's kind of scary to see how
fast it flutters.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4
* New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more

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