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

[[email protected]]

Having done most of my flying at lower altitudes, I have wondered
about the contradicton between my unscientific observations when
flying at high altitude and what I would expect from my somewhat
limited knowledge of physics and aerodymanics. I certainly believe
that true airspeed increases with altitude. I use a rule of thumb of
about 2 percent per thousand. So (at 17,000 feet) a Indicated
airspeed of 42 knots becomes 56 knots true airspeed. An indicated
airspeed of 70 knots becomes 94 knots true airspeed. It just does not
feel like or the instruments don't seem to indicate sink rates (I have
made no careful observations) one would expect for the higher true air
speeds. Is there no free lunch?

[[email protected]]

On Jan 1, 5:45*am, " > wrote:
> Having done most of my flying at lower altitudes, I have wondered
> about the contradicton between my unscientific observations when
> flying at high altitude and what I would expect from my somewhat
> limited knowledge of physics and aerodymanics. *I certainly believe
> that true airspeed increases with altitude. *I use a rule of thumb of
> about 2 percent per thousand. *So (at 17,000 feet) a Indicated
> airspeed of 42 knots becomes 56 knots true airspeed. *An indicated
> airspeed of 70 knots becomes 94 knots true airspeed. *It just does not
> feel like or the instruments don't seem to indicate sink rates (I have
> made no careful observations) one would expect for the higher true air
> speeds. *Is there no free lunch?

If you are flying a modern sailplane at 70 kts IAS the theoretical
difference in sink rate between sea level and 17,000' would be
something like 40 feet per minute - from 150 fpm to 190 fpm. That
should be observable if the airmass isn't going up/down much, but if
there is the kind of lift that can get you to 17,000' you may find a
lot of noise in the readings. There is no free lunch - you are right.
The question is, do you think your casual observation is well
calibrated enough to pick up the 40 fpm difference in sink rate?

9B

[David Salmon[_2_]]

All the characteristic speeds remain the same, as indicated on your
instruments, whatever altitude you are at. That is assuming the
instruments work from pressure, or changes of pressure. So if your
sailplane stalls at 37 kts indicated at sea level, it will stall at 37 kts
indicated at 20,000 ft. Maybe not absolutely accurate, but near enough to
be practical. An analogy would be putting in ballast, for the same glide
angle, speed and sink are higher.
The reason for a reduction in one characteristic speed, VNE, as you go
higher, is flutter in elastic structures, which is a function of true
airspeed, not indicated airspeed. Eventually you get to"coffin corner"
where the increasing true stall speed meets the reducing true VNE, and you
are stuck, you can't go higher, and you can't go faster.
The "free lunch" is your ground speed, as you go higher, for the same
indicated airspeed, your ground speed increases.

Dave


At 14:26 01 January 2009, wrote:
>On Jan 1, 5:45=A0am, " wrote:
>> Having done most of my flying at lower altitudes, I have wondered
>> about the contradicton between my unscientific observations when
>> flying at high altitude and what I would expect from my somewhat
>> limited knowledge of physics and aerodymanics. =A0I certainly believe
>> that true airspeed increases with altitude. =A0I use a rule of thumb
of
>> about 2 percent per thousand. =A0So (at 17,000 feet) a Indicated
>> airspeed of 42 knots becomes 56 knots true airspeed. =A0An indicated
>> airspeed of 70 knots becomes 94 knots true airspeed. =A0It just does
not
>> feel like or the instruments don't seem to indicate sink rates (I
have
>> made no careful observations) one would expect for the higher true air
>> speeds. =A0Is there no free lunch?
>
>If you are flying a modern sailplane at 70 kts IAS the theoretical
>difference in sink rate between sea level and 17,000' would be
>something like 40 feet per minute - from 150 fpm to 190 fpm. That
>should be observable if the airmass isn't going up/down much, but if
>there is the kind of lift that can get you to 17,000' you may find a
>lot of noise in the readings. There is no free lunch - you are right.
>The question is, do you think your casual observation is well
>calibrated enough to pick up the 40 fpm difference in sink rate?
>
>9B
>

[bildan]

On Jan 1, 5:45 am, " > wrote:
> Having done most of my flying at lower altitudes, I have wondered
> about the contradicton between my unscientific observations when
> flying at high altitude and what I would expect from my somewhat
> limited knowledge of physics and aerodymanics. I certainly believe
> that true airspeed increases with altitude. I use a rule of thumb of
> about 2 percent per thousand. So (at 17,000 feet) a Indicated
> airspeed of 42 knots becomes 56 knots true airspeed. An indicated
> airspeed of 70 knots becomes 94 knots true airspeed. It just does not
> feel like or the instruments don't seem to indicate sink rates (I have
> made no careful observations) one would expect for the higher true air
> speeds. Is there no free lunch?

Some varios are accurately compensated for altitude and others are
not. My excellent altitude compensated Borgelt varios seem to be very
good at helping me find heart stopping sink. ):

Flying high is very much like carrying ballast. I wonder if there is
a performance reason to carry less water ballast when you expect to
fly high.

[Eric Greenwell]

bildan wrote:
> On Jan 1, 5:45 am, " > wrote:
>> Having done most of my flying at lower altitudes, I have wondered
>> about the contradicton between my unscientific observations when
>> flying at high altitude and what I would expect from my somewhat
>> limited knowledge of physics and aerodymanics. I certainly believe
>> that true airspeed increases with altitude. I use a rule of thumb of
>> about 2 percent per thousand. So (at 17,000 feet) a Indicated
>> airspeed of 42 knots becomes 56 knots true airspeed. An indicated
>> airspeed of 70 knots becomes 94 knots true airspeed. It just does not
>> feel like or the instruments don't seem to indicate sink rates (I have
>> made no careful observations) one would expect for the higher true air
>> speeds. Is there no free lunch?
>
> Some varios are accurately compensated for altitude and others are
> not. My excellent altitude compensated Borgelt varios seem to be very
> good at helping me find heart stopping sink. ):
>
> Flying high is very much like carrying ballast.

A LOT of ballast! To achieve the ~34% increase in TAS you get at 17,000'
would take a wing loading increase of ~84% at sea level.

> I wonder if there is
> a performance reason to carry less water ballast when you expect to
> fly high.

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


--
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[_2_]]

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.
2) Lower indicated Vne: If your Vne is say, 145 knots, then at 18K,
your indicated Vne limit is 106 knots.
3) Lower pitch angle: Your nose and control stick do not need to be
pointed down as much to get to 106 knots compared to 145 knots. Like
air flow noise, the amount you push the stick forward is less to get
to Vne.

Combine #1 and #3 and a few moments inattention to the airspeed
indicator, and you can quickly exceed indicated Vne. This is a key
risk in cross country wave flying, where, at least out west, you can
run at 17.5K at Vne for long stretches. The other case is when, again
out west, you leave the thermal at 17.5K, but then run into more lift
as you accelerate and think, "I'll run it up to Vne" to avoid busting
18K and not monitor the airspeed indicator.

Now consider if you are at 24K, not much higher than 18K, either
because you have a wave window or clearance, and now indicated Vne is
98 knots.

Just some related points to keep in mind....

Kemp

[[email protected]]

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.

Also, at 35,000 feet our indicated airspeed is around 260 knots and
our true airspeed is 450 knots at mach 0.74 when in level cruise
flight.

Hope that helps. Most of my soaring is in Arkansas below 6,000 feet
msl.

[kd6veb]

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.

[Darryl Ramm]

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

[[email protected]]

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

[Tuno]

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

[Ralph Jones[_2_]]

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

[Eric Greenwell]

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

[Eric Greenwell]

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

[Eric Greenwell]

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

[bildan]

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

[[email protected]]

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

[Andrew Wood[_2_]]

>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.

[bildan]

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.

[Andy[_1_]]

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

[[email protected]]

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

[Darryl Ramm]

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

[[email protected]]

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

[Andy[_1_]]

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

[Frank Whiteley]

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

[[email protected]]

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/news/FactSheets/FS-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

[Eric Greenwell]

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

[[email protected]]

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

[Eric Greenwell]

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

[[email protected]]

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

[[email protected]]

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

[bildan]

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.

[[email protected]]

On Jan 5, 6:44*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

[[email protected]]

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

[Michael Ash]

> 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

[Andy[_1_]]

On Jan 7, 6:36*pm, " > wrote:

> My understanding is
> that the potential energy available to the sailplane is height times
> weight. *The potential energy would not vary with altitude. *

Now you have lost me! You'll need to define how height and altitude
are independent.

Andy

[Martin Gregorie[_4_]]

On Wed, 07 Jan 2009 17:36:15 -0800, wrote:

> My understanding is that the potential energy available to the
> sailplane is height times weight.
Correct: PE = h * w

> The potential energy would not vary with altitude.
Not true as written: see above.

However if you meant that PE converted to KE for a given height loss is
independent of altitude then *that* is correct.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

[Eric Greenwell]

wrote:

> The drag,
> however, would be less because of the thin air.

Under what conditions? At the same *True* airspeed, yes - the drag is
less; at the same *Indicated* airspeed, no - the drag is the same.

> Therefore would the
> sailplane travel farther for a given amount of potential energy
> used??

No, not at the same *Indicated* airspeed. Caveat: there are some small
effects from Reynolds number changes, but we can ignore them in the
20,000' and under range (maybe even a lot higher, but I feel safe saying
20K).

> 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.

Too many other variables, like engine efficiency, and was the comparison
at the same *IAS*? Let's stay away from airplane comparisons. We have
all the information we need to discuss gliders without going there.

>
> 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.

Again, too many potential factors, so we should probably stick with real
gliders instead far bigger aircraft with engines hanging down that have
propellers in them.

--
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

[[email protected]]

On Jan 8, 4:06*pm, Eric Greenwell > wrote:
> wrote:
> > The drag,
> > however, would be less because of the thin air.
>
> Under what conditions? At the same *True* airspeed, yes - the drag is
> less; at the same *Indicated* airspeed, no - the drag is the same.
>
> > *Therefore would the
> > sailplane travel farther for a given amount of potential energy
> > used??
>
> No, not at the same *Indicated* airspeed. Caveat: there are some small
> effects from Reynolds number changes, but we can ignore them in the
> 20,000' and under range (maybe even a lot higher, but I feel safe saying
> 20K).
>
> > *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.
>
> Too many other variables, like engine efficiency, and was the comparison
> at the same *IAS*? Let's stay away from airplane comparisons. We have
> all the information we need to discuss gliders without going there.
>
>
>
> > 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.
>
> Again, too many potential factors, so we should probably stick with real
> gliders instead far bigger aircraft with engines hanging down that have
> propellers in them.
>
> --
> 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

If I am flying at 25,000 feet and want to fly at maximum L to D, would
I fly at the same indicated air speed as I would at 2000 feet? Would
glide ratio be the same, regardless of altitude?

Bill Snead

[Eric Greenwell]

wrote:

> If I am flying at 25,000 feet and want to fly at maximum L to D, would
> I fly at the same indicated air speed as I would at 2000 feet? Would
> glide ratio be the same, regardless of altitude?
>
> Bill Snead

Practically speaking, yes.

--
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

[Jack[_12_]]

Andy wrote:


> You'll need to define how height and altitude
> are independent.



Height = AGL
Altitude = above MSL


Jack

[[email protected]]

On Apr 12, 8:26*am, Jack > wrote:
> Andy wrote:
>
> *> You'll need to define how height and altitude
>
> > are independent.
>
> Height = AGL
> Altitude = above MSL
>
> Jack

My understanding is NO. Here's an example:

On the CRJ at 10,000 feet best L/D is 170 kias......then at the same
weight and ISA deviation and flap setting, stall speed at 41,000 feet
is 173. So if you pitched for best L/D for 10,000 at an altiude of
41,000 feet you would stall the airplane. I know this because I have
studied in great detail flight 3701. Those pilots just a few years
ago, stalled the crj at 41,000 feet at 173 knots. Then they glided
the 50 seat passenger jet into a neighborhood in the middle of the
night. The plane exploded and both pilots died. No passengers aboard
the flight. It was a reposition. Very sad day.

KIAS stall speed changes with altitude....so does L/D KIAS.

[Mats Henrikson (test account)]

> 41,000 feet you would stall the airplane. *I know this because I have
> studied in great detail flight 3701. *Those pilots just a few years
> ago, stalled the crj at 41,000 feet at 173 knots. *Then they glided
> the 50 seat passenger jet into a neighborhood in the middle of the
> night. *The plane exploded and both pilots died. *No passengers aboard
> the flight. *It was a reposition. *Very sad day.
>
> KIAS stall speed changes with altitude....so does L/D KIAS.

I think you'll find that you are confusing KTAS and KIAS. The NTSB
investigation of Pinnacle Flight 3701 is here:

http://www.ntsb.gov/Publictn/2007/AAR0701.pdf

See section 1.16.1.2, footnote 54, saying all airspeeds are in KCAS
(which is basically KIAS). Section 1.16.1.3 para gives the best L/D of
170 KCAS. It doesn't say anything at which altitude that was, because
L/D given in KCAS doesn't change with altitude.

I find that flying at 1,000 feet and 20,000 feet my glider will stall
at the same KIAS, you should try it yourself.