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

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

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

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 -

In looking again at the only glider flutter video I have it appears
that there is control surface involvement. But wing bending and
tortional masses and elasticities are also part of the equation. This
raised a question in my mind - if you experience wing flutter doe it
matter whether you hold onto the stick or let it go? Holding on to the
stick would provide some damping of control deflections. If it does
matter, which should you do? My instincts say hold on, but that may
just be my personality at work...

9B

Here's a reference to complement the DG video and also a discussion of
changes in certification requirements w.r.t. flutter. An interesting
read.

http://www.dg-flugzeugbau.de/dg1000-flattern-e.html

Some of the wording is a bit ambiguous, but the way I read it there
are two conclusions that are relevant to this discussion:

1) Holding on to the stick tends to damp out one mode of wing flutter
(and perhaps other controls too). It is a mode that is exacerbated by
the fact that when the wing flexes up an unbalanced aileron will tend
to deflect downward and vice versa.

2) Adding water ballast can decrease the flutter speed. If I read it
right the DG-300 had it's Vne reduced due to the test depicted in the
video.

9B

On Jan 5, 4:44*pm, wrote:
Here's a reference to complement the DG video and also a discussion of
changes in certification requirements w.r.t. flutter. An interesting
read.

http://www.dg-flugzeugbau.de/dg1000-flattern-e.html

Some of the wording is a bit ambiguous, but the way I read it there
are two conclusions that are relevant to this discussion:

1) Holding on to the stick tends to damp out one mode of wing flutter
(and perhaps other controls too). It is a mode that is exacerbated by
the fact that when the wing flexes up an unbalanced aileron will tend
to deflect downward and vice versa.

2) Adding water ballast can decrease the flutter speed. If I read it
right the DG-300 had it's Vne reduced due to the test depicted in the
video.

9B

I'm reaching way back here but I remember flight test aircraft
equipped with dampers in the control system, similar to small shock
absorbers. The dampers would stiffen up if a control surface started
to flutter. The idea was to let the test pilot note the airspeed at
onset of flutter without letting it become destructive. The controls
felt like they were in molasses but the aircraft was still flyable for
the purposes of the test.

That might still be a workable strategy for those pushing the envelope.

On Jan 6, 10:49*am, bildan wrote:
On Jan 5, 4:44*pm, wrote:





Here's a reference to complement the DG video and also a discussion of
changes in certification requirements w.r.t. flutter. An interesting
read.

http://www.dg-flugzeugbau.de/dg1000-flattern-e.html

Some of the wording is a bit ambiguous, but the way I read it there
are two conclusions that are relevant to this discussion:

1) Holding on to the stick tends to damp out one mode of wing flutter
(and perhaps other controls too). It is a mode that is exacerbated by
the fact that when the wing flexes up an unbalanced aileron will tend
to deflect downward and vice versa.

2) Adding water ballast can decrease the flutter speed. If I read it
right the DG-300 had it's Vne reduced due to the test depicted in the
video.

9B

I'm reaching way back here but I remember flight test aircraft
equipped with dampers in the control system, similar to small shock
absorbers. *The dampers would stiffen up if a control surface started
to flutter. *The idea was to let the test pilot note the airspeed at
onset of flutter without letting it become destructive. *The controls
felt like they were in molasses but the aircraft was still flyable for
the purposes of the test.

That might still be a workable strategy for those pushing the envelope.- Hide quoted text -

- Show quoted text -

Another question about high altitude gliding - My understanding is
that the potential energy available to the sailplane is height times
weight. The potential energy would not vary with altitude. The drag,
however, would be less because of the thin air. Therefore would the
sailplane travel farther for a given amount of potential energy
used?? I have very limited time in the cockpit of jets, but it
appeared to me that the fuel flow was much less at altitude while the
true airspeed stayed high. More miles for a given amount of energy.

Also any comments on the post reporting different indicated air speeds
(at different altitudes) (in the flight manual) to achieve best L to D
in a jet. I would haved guessed that the best L to D would always
occur at the same indicated air speed.

On a dual wave flight at 25,000 feet I was warned about the danger of
high true air speed at altitude. We were cruising at a about 60 knots
IAS. I calculated that was a TAS of about 90 knots. I ask the
instructor if he thought our sink rate was what you would expect for a
Grob going 90 knots? He said no.

6W

wrote:
Another question about high altitude gliding - My understanding is
that the potential energy available to the sailplane is height times
weight. The potential energy would not vary with altitude. The drag,
however, would be less because of the thin air. Therefore would the
sailplane travel farther for a given amount of potential energy
used?? I have very limited time in the cockpit of jets, but it
appeared to me that the fuel flow was much less at altitude while the
true airspeed stayed high. More miles for a given amount of energy.

The sailplane will travel farther for a given amount of potential energy
used at a given speed, if that speed is relatively high. To put it another
way, the thinner air means less parasitic drag, but it also means that the
wings have to work harder to produce lift, so it means more induced drag.
Whether this is a net gain or a net loss depends on where you are on the
polar. If you're going faster than best L/D, then increasing altitude
pushes you closer to best L/D, allowing you to cover more distance for
each piece of altitude. If you're already at or below best L/D, then it
starts to hurt instead of help.

If you vary your true speed with altitude to keep a constant indicated
speed then the ground you cover for your altitude stays constant, although
you'll cover it faster when you're higher.

On a dual wave flight at 25,000 feet I was warned about the danger of
high true air speed at altitude. We were cruising at a about 60 knots
IAS. I calculated that was a TAS of about 90 knots. I ask the
instructor if he thought our sink rate was what you would expect for a
Grob going 90 knots? He said no.

Ask him instead if the sink rate is what you would expect for a Grob going
60 knots, multiplied by 1.5. You're still at the 60-knot mark on the
polar, so your L/D is still right around your optimum. But you're sliding
down that hill 50% faster than normal. If your sink rate is, say, 2kts at
60kts at sea level then you'd expect to see 3kts at 25,000ft (as opposed
to the maybe 5kts you'd see at sea level at 90kts indicated).

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon