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

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?

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

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

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.

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

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

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.

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