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

On Jan 2, 6:13*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 cut a corner or two on my post Andy.

The only glider wing flutter video I've seen looks like an interaction
between the main wing bending and tortional elasticities - wing bends
up, gains a little angle of attack and bends/twists more as a result
until the restoring force gets big enough to bring it back down (or
the wing breaks). This creates a symmetric wing flapping kind of
flutter. I would think if the ailerons were significantly involved
you'd be more likely to see something asymmetric, or more tortional
motion - which I'm sure can occur under some set of circumstances.

The reason I picked this mode of flutter was also its likely the one
most affected but water ballast which should up the natural frequency
in bending mostly.

Just educated guesses on my part, but that was my logic.

9B

On Jan 2, 4:31*pm, wrote:
On Jan 2, 6:13*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 cut a corner or two on my post Andy.

The only glider wing flutter video I've seen looks like an interaction
between the main wing bending and tortional elasticities - wing bends
up, gains a little angle of attack and bends/twists more as a result
until the restoring force gets big enough to bring it back down (or
the wing breaks). This creates a symmetric wing flapping kind of
flutter. I would think if the ailerons were significantly involved
you'd be more likely to see something asymmetric, or more tortional
motion - which I'm sure can occur under some set of circumstances.

The reason I picked this mode of flutter was also its likely the one
most affected but water ballast which should up the natural frequency
in bending mostly.

Just educated guesses on my part, but that was my logic.

9B

Interesting to note that the damage I've seen in practice has been two
cases of elevator flutter with folks pushing too fast at high
altitude. I suspect that ignoring speed restrictions at ~18k' was the
cause. I believe the mylar seals were in good condition in both cases
and presumably not a factor. There are alligators in these here
swamps... be careful where you tread.

Darryl

On Jan 2, 6:40*pm, Darryl Ramm wrote:
On Jan 2, 4:31*pm, wrote:





On Jan 2, 6:13*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 cut a corner or two on my post Andy.

The only glider wing flutter video I've seen looks like an interaction
between the main wing bending and tortional elasticities - wing bends
up, gains a little angle of attack and bends/twists more as a result
until the restoring force gets big enough to bring it back down (or
the wing breaks). This creates a symmetric wing flapping kind of
flutter. I would think if the ailerons were significantly involved
you'd be more likely to see something asymmetric, or more tortional
motion - which I'm sure can occur under some set of circumstances.

The reason I picked this mode of flutter was also its likely the one
most affected but water ballast which should up the natural frequency
in bending mostly.

Just educated guesses on my part, but that was my logic.

9B

Interesting to note that the damage I've seen in practice has been two
cases of elevator flutter with folks pushing too fast at high
altitude. I suspect that ignoring speed restrictions at ~18k' was the
cause. I believe the mylar seals were in good condition in both cases
and presumably not a factor. There are alligators in these here
swamps... be careful where you tread.

Darryl- Hide quoted text -

- Show quoted text -

Thanks for all the interesting replys.

I think the comment about "live" air is the most interesting. It is
difficult to detect a 30 percent change in sink rate when the air is
moving up and down at up to 1500 feet per minute. Looking at flight
logs most people fly pretty fast at high altitude. I guess if you
have it (energy) you might as well spend it?

Speaking of flutter. I believe that slop in control connections can
contribute to the on set of flutter.

Bill Snead
6W

On Jan 2, 5:40*pm, Darryl Ramm wrote:
Interesting to note that the damage I've seen in practice has been two
cases of elevator flutter with folks pushing too fast at high
altitude. I suspect that ignoring speed restrictions at ~18k' was the
cause. I believe the mylar seals were in good condition in both cases
and presumably not a factor. There are alligators in these here
swamps... be careful where you tread.

ASW-19 gliders were required to have an elevator modification that
substantially reduced the chord. The mod was required to prevent
flutter. Just one data point that suggests control surface flutter
may be more of a factor than primary structure in setting Vne.

Andy