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

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.

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

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

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

Andy wrote:


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



Height = AGL
Altitude = above MSL


Jack

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.

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 he

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.

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 |

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

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