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

I always though the rationale for not using ballast in freezing
temperatures was the danger of the dump valves freezing, not the
ballast itself. It would be a rude surprise to enter the pattern at
Ely to find out that one wing valve iced closed and the other didn't.
(Or worse, landing out in a tight field without that long Ely runway.)

On the original topic -- I can see how the sink rate has increased at
altitude, but the shape of the polar stays the same, right?

..02NO

On Thu, 1 Jan 2009 10:22:18 -0800 (PST), kd6veb
wrote:
[snip]

PS I have researched flutter without finding any really definitive
papers on the subject. It is widely said that if flutter occurs at say
200mph at sea level it will occur at the same speed at any altitude. I
find this difficult to believe. I always try to apply limit reasoning
to these kinds of problems. Say there was virtually no air would the
wing flutter in free space at 200mph. Of course not. So this reasoning
suggests to me that as the air density diminishes flutter speeds
increase. Now intuition sometimes let you down and there may be an
explanation why my take here is incorrect. Again any comments?

As a first, repeat FIRST, approximation, flutter depends on true
airspeed because it's a resonance effect.

When a wing is oscillating in torsion, the leading edge generates a
train of positive and negative pressure pulses that propagate back
along the chord to the trailing edge. If a positive pulse on the upper
surface reaches the trailing edge just as that edge is on the "up"
side of an oscillation, it will oppose the twist and tend to damp out
the oscillation; if it arrives when the TE is "down", it will
reinforce the oscillation. The relative timing depends on two things:
(1) the natural vibration frequency of the wing, and (2) the time it
takes for a pressure pulse to travel from LE to TE. The latter depends
directly on the true airspeed.

But there are a lot of other factors. For instance, the taper of the
wing means the pulse travel time differs at different spanwise
positions. The aeroelastic properties of the wing can put one part of
it on an "up" cycle when other parts are "down". The indicated
airspeed affects the amount of force the pressure pulses can
exert...and so forth.

So it's hard to say what speed really counts. Bottom line: If you fly
faster than the factory test pilot flew the machine, you're an
experimental test pilot...;-)

rj

wrote:
On the Canadair Regional Jet that I fly, the L/D best glide speed is
different at all altitudes. We have a chart that shows us what
"indicated" airspeed to pitch for if both engines shut down for each
altitude. As altitude increases, the best L/D speed also increases.
For instance, at 10,000 feet the best glide speed is around 170 knots
indicated. Then at 35,000 feet the best glide speed is around 235
knots indicated. This speed is if both engines shut down, and then
cannot be restarted.

Has anyone explained why the best glide speed increases? Is the drag of
the shut down engines less at higher altitudes, for example?

--
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

kd6veb wrote:
Hi Gang
There is a caveat to what Eric is saying. With a cranking mid summer
day around Minden it is true if you are going distance most pilots
would balast their gliders to max gross and expect to fly to FL 180.
However in wave flying where you might be close to FL 180 or above
with ATC permission for a good portion of your flight and with
temperatures perhaps around -25 degrees F you would not use water
ballast.

This is a legitimate concern, but not the "performance reasons" Bill was
asking about.

I have never known anyone to use water ballast for a wave
flight. Now having said that it may be that having the wings full of
water might reduce flutter at high speeds which could be advantageous.

That's an interesting idea. I'm not aware of a discussion of on how
ballast changes the flutter characteristics, but it seems like the
differences might be substantial.

PS I have researched flutter without finding any really definitive
papers on the subject. It is widely said that if flutter occurs at say
200mph at sea level it will occur at the same speed at any altitude.

The handbook values for "many" gliders built in at least the last 20
years or so usually have the Vne as a constant IAS up to about 10,000',
then a (mostly) constant TAS limit after that. My ASH 26 E is like that.
It's more complicated than just a TAS limit, but a TAS limit is
conservative. "Fundamentals of Sailplane Design" notes that some people
think a limit half way between TAS and IAS would be more appropriate.

--
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

Kemp wrote:
A related and very critical point that I've not seen mentioned or
written anywhere about higher altitude soaring (17K and above) is that
it is much easier to exceed Vne because of human factors combined with
thinner air.
1) Less noise: Because the air is thinner, there is less air flow
noise in the cockpit. Many pilots use air flow noise as a secondary
way to monitor airspeed.

My perception is the noise is greater, but maybe what I'm reacting to is
noise that's a higher frequency than the same IAS at a lower altitude.
Or, maybe what I'm responding to is more vent noise at higher altitudes,
not the glider airframe noise. I'll have to pay attention the next time
I fly!

--
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 1, 2:18 pm, Eric Greenwell wrote:
Kemp wrote:
A related and very critical point that I've not seen mentioned or
written anywhere about higher altitude soaring (17K and above) is that
it is much easier to exceed Vne because of human factors combined with
thinner air.
1) Less noise: Because the air is thinner, there is less air flow
noise in the cockpit. Many pilots use air flow noise as a secondary
way to monitor airspeed.

My perception is the noise is greater, but maybe what I'm reacting to is
noise that's a higher frequency than the same IAS at a lower altitude.
Or, maybe what I'm responding to is more vent noise at higher altitudes,
not the glider airframe noise. I'll have to pay attention the next time
I fly!

--
Eric Greenwell - Washington State, USA

I notice a significant reduction in noise when climbing or
descending... but when my ears pop, the noise is restored to its
previous level. ):

Seriously, a gliders sound is different in character and seems quieter
in laminar wave which I have always assumed was due to increased
laminar flow in the very smooth wave. I have also noticed that my
voice assumes a "helium breathing" character when I use the radio at
higher altitudes.

As for sound as an airspeed cue, the glider I fly is quiet enough the
ASI is needed for accurate speed control.

Bill Daniels
Wintering in San Diego, CA

On Jan 1, 2:14*pm, Eric Greenwell wrote:
kd6veb wrote:
I have never known anyone to use water ballast for a wave
flight. Now having said that it may be that having the wings full of
water might reduce flutter at high speeds which could be advantageous.

That's an interesting idea. I'm not aware of a discussion of on how
ballast changes the flutter characteristics, but it seems like the
differences might be substantial.


Yup - you would expect that increasing the mass of the wing would give
it a higher resonance frequency and therefore a higher flutter speed.
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.

9B

Bill Daniels wrote: As for sound as an airspeed cue, the glider I fly is
quiet enough the ASI is needed for accurate speed control.



The air noise seems mainly a matter of a good canopy seal. Is there a good
sealing material that anyone has found? I've tried commercial foam strip,
but it's rather too thick, and smears to a gummy mess after a while.

On Jan 1, 4:00 pm, Andrew Wood wrote:
Bill Daniels wrote: As for sound as an airspeed cue, the glider I fly is

quiet enough the ASI is needed for accurate speed control.

The air noise seems mainly a matter of a good canopy seal. Is there a good
sealing material that anyone has found? I've tried commercial foam strip,
but it's rather too thick, and smears to a gummy mess after a while.

My canopy seems to seal well without a gasket. However, I've seen an
external seal that looks like a smaller version of a aileron gap seal
and was told it's available from hardware stores.

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