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

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. 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.
If so a mixture of water plus antifreeze would be called for. Any
comments anyone?
Dave

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?

Eric wrote:

Pilots in Nevada and elsewhere that fly at 18,000' routinely stuff in
all the ballast the glider can hold, so apparently not.

On Jan 1, 10:22*am, 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. 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.
If so a mixture of water plus antifreeze would be called for. Any
comments anyone?
Dave

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?

Eric wrote:

Pilots in Nevada and elsewhere that fly at 18,000' routinely stuff in
all the ballast the glider can hold, so apparently not.

Some of the New Zealander's will use water in wave on occasion. With
significant factors against freezing apparently being thermal mass of
the water and insulation of the wing skin. When I asked about anti-
freeze the pilots who do this said they don't use it. They may also
have warmer temperatures aloft than Sierra-Nevada winter wave flights
that slow down freezing.

Darryl

On Jan 1, 10:41*am, Darryl Ramm wrote:
On Jan 1, 10:22*am, 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. 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.
If so a mixture of water plus antifreeze would be called for. Any
comments anyone?
Dave

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?

Eric wrote:

Pilots in Nevada and elsewhere that fly at 18,000' routinely stuff in
all the ballast the glider can hold, so apparently not.

Some of the New Zealander's will use water in wave on occasion. With
significant factors against freezing apparently being thermal mass of
the water and insulation of the wing skin. When I asked about anti-
freeze the pilots who do this said they don't use it. They may also
have warmer temperatures aloft than Sierra-Nevada winter wave flights
that slow down freezing.

Darryl- Hide quoted text -

- Show quoted text -

I've flown regularly flown 5+ hours at altitude with a full tail
ballast tank - in the summer time - with no indication of freezing. I
initially added antifreeze, but over time discovered that it takes a
lot of cold soaking to freeze 4 liters. 30-40 gallons of wing ballast
inside a foam sandwich structure would take a very long time to freeze
- but if I were doing one of those four-lengths-of the-Sierras cross
country wave flights I'd put some antifreeze in.

With respect to optimal ballast load versus altitude, I would think
the benefit of loading up increases at higher altitudes because the
cruise speed 'differential' between loaded and dry also goes up by 2
percent per 1,000 feet. A simple analysis shows the crossover from dry
to fully loaded occurs at around 2 kts achieved climb rate. The
determining factor for me has almost always been whether I can circle
tight enough to stay in the core of the thermal. Since circling radius
is a funtion of TAS and stall speed, you can expect much bigger
circles at higher altitudes and at higher wing loading. Fortunately,
thermals tend to spread out at altitude as well, so if I can climb at
7,000 feet I can usually climb at 17,000 feet.

9B

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

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

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