DIJKKIJK
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Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 2:20 am

What is the lowest speed a fully loaded jetliner like the Boeing 744 can attain in level flight, and still remain airborne?


Thanks

[Edited 2006-12-05 18:20:47]
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N231YE
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 2:56 am

VS0 would be my guess, but I am probably incorrect. I am not familiar with the 747, so I do not know the actual value of VS0.
 
SlamClick
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 3:19 am

Quoting N231YE (Reply 1):
I am not familiar with the 747

Only about five percent of aviation professionals are - it is a rare airplane. Hint: Unless you really need very specific data, leave the question parameters more open. There are, maybe two guys on this forum who've flown the 747. There are a hundred or more who fly all the other planes in the various fleets.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 6:02 am

Quoting N231YE (Reply 1):
VS0 would be my guess, but I am probably incorrect.



Quoting DIJKKIJK (Thread starter):
What is the lowest speed a fully loaded jetliner like the Boeing 744 can attain in level flight

At Vso most aircraft would typically be descending unless the nose was held high and loads of power could be applied. High performance aircraft can maintain level flight at minimal airspeeds (below stall). I would guess for a 747 flown "normally" that a speed somewhat above touchdown speed would be the minimum level-flight speed in the landing config. However, in a clean config. I believe the speed would be considerably higher--a while ago on this forum 747's at MGW were said to have a minimum recommended cruise speed above 200 kts. and therefore could not accept a speed restriction of 200 kts.(of course there would be a considerable safety margin there). Anyway, at MGW I'll guess 160 kts. dirty and 190 kts. clean. Any other guesses?
"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
 
SlamClick
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 7:01 am

Okay, since "guesses" are starting to appear, it is time to jump in.

VS is "stalling speed or minimum steady flight speed" which is actually contradictory. If you are at a speed at which the stall occurs you are no longer in steady flight. Among the published stall/minimum steady flight speeds VSO is in the "landing configuration"

If our flight is "steady" in the landing configuration that means that we can still maintain altitude or even climb. So, yes, a jet airliner will climb at VSO.

So - at what speed? That depends on some factors - in sort of descending order here:

1. Airplane type - BAe-146 or B-747. Very large variations here.

2. For a given type - configuration, flap setting etc. For jets power-on or power-off does not change anything. For straight wing props it does.

3. For a given type in a specified configuration - gross weight. Big variation with increasing weight.


So I reached out blindly and my hand landed on a 727-200 manual for an airline I did not fly for. Here are some samplings of the numbers for just this one airplane. This manual actually does show a table of stall speeds (and stick shaker speeds which are just a few knots higher) which is kind of unusual. Normally we just have a table for VREF which we know to be 1.3 VSO

Again, B-727-200 series with -9 engines (not that that matters)

At 170,000 pounds, which is fairly heavy for this type, stall speeds range from 163 knots clean down to 107 knots at flaps 30.

At flaps 30 the stall speeds range from 107 knots at 170000 lbs gross weight down to 80 knots at 100000 lbs gross weight. A hundred thousand pounds would be a very light weight for this airplane - very little fuel, very little payload.

So the distilled answer for this one airplane is a charted range from 80 to 163 knots depending on configuration and gross weight.
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N231YE
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 7:12 am

Quoting SlamClick (Reply 4):
Among the published stall/minimum steady flight speeds VSO is in the "landing configuration"

In the Cessna 172, you can accomplish near VSO (33kias for the -R) while in flight, aka slow speed configuration, hence my guess. However, you are correct, other than this training manuver for student pilots, the only other time you get near VSO is in landing configuration. But that is with the C172, a very different airplane from an 727 or a 747.
 
CF188A
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 9:32 am

Well... an aircraft can fly backwards. I have done it at 3000 ... and I am sure hundreds on here have also done it one one time or another in the slow level flight flying lesson. You actually can move backwards. You can also do it on the tarmac?... obviously not for long periods of time but it is possible. Other than that just hovering pointlessly above the earth in a Cessna does the trick to lol .

[Edited 2006-12-06 01:33:47]
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citationjet
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 10:44 am

Quoting CF188A (Reply 6):
Well... an aircraft can fly backwards.

You are talking about ground speed. All other posts are talking about airspeeds.
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hangarrat
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 11:21 am

Quoting CitationJet (Reply 7):
You are talking about ground speed. All other posts are talking about airspeeds.

I feel a rehash of the conveyor belt runway discussion coming on.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 11:26 am

Hey Slam. Will a Cessna 172 with a standard engine at MGW climb at Vso. I've done a lot of power on entries into stalls (a few knots above stall) and I believe the airplane is losing altitude--and that's at a weight well below MGW. It won't climb. Also, are you suggesting a modern jetliner at MGW could take off at Vso (full flaps, slats) (better bring the gear up) and climb out at that speed. Sounds impossible to me without some BIG engines. I don't mean to doubt your knowledge, but I can't believe it.
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Curmudgeon
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 11:55 am

At Vso in a jet transport, the drag would very likely eat up most of the excess thrust available for climb (even at low altitude). Vmu (minimum unstick) speed is above Vs, and in many types will involve a tailstrike, so a takeoff at Vs would be impossible anyway. (Not to mention the stick shaker, and in some types, stick pusher)

Typical climb is conducted at V2 + 15-20 kts, which is approx 1.5 Vso.

I have lent my 744 manuals, so I can't provide you with a definitive answer to the OP question. Normal minimum clean for a 744 at 396 tonnes would be around 260 kts, as I recall*. (its been awhile)

Since Vso is defined as a zero thrust stall speed, slower flight would be (theoretically) possible by selecting maximum thrust at high pitch attitudes-the vertical component of the thrust would yield a significant reduction in apparent mass, hence the slower stall speed.

BTW, Vref is 130% of the stall speed for the flaps selected.

[Edited 2006-12-06 04:00:18]
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SlamClick
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 2:14 pm

Quoting Curmudgeon (Reply 10):
Since Vso is defined as a zero thrust stall speed

Where is it so defined?
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2H4
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 2:33 pm



I think the lowest speed attainable in flight for any fixed-wing aircraft would occur in ground effect at high power settings, way behind the power curve. What that particular speed is, of course, depends on the factors outlined by SlamClick.


2H4

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Curmudgeon
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RE: Lowest Speed Attainable In Flight.

Wed Dec 06, 2006 6:01 pm

Quoting SlamClick (Reply 11):
Quoting Curmudgeon (Reply 10):
Since Vso is defined as a zero thrust stall speed

Where is it so defined?

Hi SlamClick...

If I had been paying attention when I scrolled down through this thread, I would have seen your post the first time, and not bothered with my own, because you covered it cold.

I can't find a definition right now in my books, except for a citation in "Handling the Big Jets" which defines it as "The (not more than) zero thrust stall speed for the maximum landing flaps". this would have been an old C.A.A. definition, broadly equivalent to the FAR's of the 70's.

In flight test (Not that I did much), the V speeds were determined by approaching the predicted stall speed from about 1.5Vs at a rate no greater than 1 kt/sec. Power was "zero thrust", which necessitated a shallow descent to achieve the 1kt/sec delta V. Later tests were conducted at idle (for prop planes), and were considered equivalent to zero thrust only if the difference in tested stall speeds was less than 1/2 knot.

Jet stall speed determination is conducted at idle, since it yields a higher Vs.

I don't have a copy of the FAR or JAR regs (or the MIL Spec) here at home, but the airline, military and flight test texts don't mention thrust in their Vs definitions.

Regards

Cur.
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SlamClick
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 3:26 am

Quoting 2H4 (Reply 12):
I think the lowest speed attainable in flight for any fixed-wing aircraft would occur in ground effect at high power settings

I think I'd agree and there are probably more than one or two reasons for this. My personal best example occurred in a BAe-146.

For some dumb reason one plane would not come all the way back to flight idle on a visual approach. I had no trouble (love that tail-mounted speed brake) making the touchdown area but in the flare was carrying more thrust than I wanted and even with the levers back against the stop the N1 was a few percent too high.

Result was that even after an on-speed flare, I floated a thousand feet or more down the runway "feeling for the ground" like a damn Cessna. We touched down at flight idle with about 70 knots indicated, when stall speed should have been 89 knots. Also did not get stick shaker/pusher.

Then in best BAe fashion the problem went away and we could not duplicate it in three more legs.

It occured to me that this particular type might get some synthetic ground effect with engine thrust getting bottled up under the wings, ahead of the flaps, but I don't know.

In his autobi, Chuck Yeager claims to have landed the delta-winged XF-92 at 67 MPH (58 knots) on his first flight in it.

But then he is Chuck Yeager and we are not.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 6:41 am

Slam, lets say at 1kt. below Vso the aircraft is stalled and dropping like a rock. And, at a few kts. above it is climbing a bit. Wouldn't there be a point in between where the aircraft would simply hold altitude? From what you've stated the aircraft is either stalled or climbing. No matter how technically-well you support your argument, it simply is not logical. IMHO.
"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
 
SlamClick
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 7:26 am

Quoting Redcordes (Reply 15):
No matter how technically-well you support your argument, it simply is not logical. IMHO.

Not logical if...

Quoting Redcordes (Reply 15):
at 1kt. below Vso the aircraft is stalled and dropping like a rock. And, at a few kts. above it is climbing a bit.

...was what I said. But it isn't. Here...

Quoting SlamClick (Reply 4):
If our flight is "steady" in the landing configuration that means that we can still maintain altitude or even climb.

...is part of what I said.

Steady flight. Controllable flight. So anywhere below the angle of attack that is represented by an indicated airspeed of VSO the plane is "steady" and "controllable" per the regulations.

Now if my flight is steady and controllable when I am exactly, spot-on VSO then I can climb, descend, or maintain level, even level turning flight.

I can climb or descend as I always do - with basic pitch and power management. I can turn normally as long as I don't exceed the bank angle that would increase my G-load and therefore my effective weight & stall speed. I'd be comfortable with 15° angle of bank as that is considered to have nil effect on airplane performance.

Does that make more sense?
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redcordes
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 8:30 am

Maybe for commercial jet airliners Vso is not really the absolute threshold of a stall, but a calculated number that keeps the aircraft a bit above a fully stalled condition that ensures safe operation. The next time I take up a 172 I'm going to try to fly with full power just above an actual Vso stall (which will be below the MGW Vso due to a lesser load) and watch the VSI. I'm quite sure the VSI will be well in the negative range. With full flaps in the Vso config. I imagine a speed about 10-15 kts. above Vso would be necessary to even hold altitude. If true, how do you explain that the aircraft won't climb even though the wing is not stalled? The more excess power probably the slower this hold-altitude speed would be. Also, the original question asks for the lowest speed to simply hold altitude. From your answers you are implying there is no such speed; and, that the aircraft either will be able to climb or not. I'm sorry this seems ridiculous on the face of it. Forget about the "steady" and "controllable" per the regulations obfuscations. My apologies.
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SlamClick
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 8:52 am

Quoting Redcordes (Reply 17):
Forget about the "steady" and "controllable" per the regulations obfuscations. My apologies.

Okay, silly me. I thought FAR 1.2 Abbreviations and Symbols was a pretty good place to get the meaning of VSO.

I have to admit, my command of English is so questionable I also thought that if you were unable to control your rate of descent your flight regime might not be described as "steady."

I'll butt out now.
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Bellerophon
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 9:15 am

DIJKKIJK

...What is the lowest speed a fully loaded jetliner like the Boeing 744 can attain in level flight, and still remain airborne?...

148 kts


B747-436 Stall Speeds, Sea Level, Forward CG. Clean / Flap 30° + Gear Down.


MTOW.................396,890 kgs / 875,000 lbs
  • Clean.........208 KIAS
  • Flap 30°.....148 KIAS

MLW...................285,760 kgs / 630,000 lbs
  • Clean.........172 KIAS
  • Flap 30°.....124 KIAS

Typical LW...........250,000 kgs / 551,000 lbs
  • Clean.........158 KIAS
  • Flap 30°.....116 KIAS

Regards

Bellerophon
 
redcordes
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 9:18 am

Quoting Bellerophon (Reply 19):

...What is the lowest speed a fully loaded jetliner like the Boeing 744 can attain in level flight, and still remain airborne?...



Quoting Bellerophon (Reply 19):

B747-436 Stall Speeds, Sea Level, Forward CG. Clean / Flap 30° + Gear Down.

Does stall speed equal minimum level-flight speed? I very much doubt that it does!
"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
 
Bellerophon
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 9:29 am

Redcordes

... Does stall speed equal minimum level-flight speed? I very much doubt that it does!...

Just out of interest, why?

Regards

Bellerophon
 
N231YE
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 9:38 am

Quoting Bellerophon (Reply 19):
DIJKKIJK

...What is the lowest speed a fully loaded jetliner like the Boeing 744 can attain in level flight, and still remain airborne?...

148 kts

B747-436 Stall Speeds, Sea Level, Forward CG. Clean / Flap 30° + Gear Down.

MTOW.................396,890 kgs / 875,000 lbs
Clean.........208 KIAS
Flap 30°.....148 KIAS

Was my guess of VSO correct?

[Edited 2006-12-07 01:48:08]
 
3DPlanes
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 11:11 am

Quoting Redcordes (Reply 20):
Does stall speed equal minimum level-flight speed?

You seem to be hung up on level-flight vs stall... If you have enough excess power - or low enough drag, I don't see why you can't climb at Vso (okay, technically just a hair on the high side of it). Since lift increases with alpha, isn't the wing generating maximum lift just before the stall speed? Jet fighters can certainly climb at Vso (arguably, even below it), given all the power they have.

On a 172 (which I've yet to fly <-- noob), I understand that FULL flaps are almost never used as the plane (as you seem to describe) can't maintain altitude, let alone climb. I've read a number of accident reports where a 172 went in to the trees because they tried to go around without pulling in the full flaps.

In a Warrior or Archer, I can fly around all day along at Vso without losing altitude. And I have to pull a little power to do it, so I imagine I could likely eek out a bit of climb... I'm thinking of it like its just a slower (and less efficient) version of Vx or Vy.

Something else to try the next time I'm up, and not in a hurry.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 12:25 pm

Quoting 3DPlanes (Reply 23):
Since lift increases with alpha, isn't the wing generating maximum lift just before the stall speed?

Lift is also dependent on airflow/airspeed. That is why maxium lift is probably generated at Vy not Vso.

Quoting 3DPlanes (Reply 23):

You seem to be hung up on level-flight vs stall.

Yes, because that was the question posed in the initial post.

Quoting 3DPlanes (Reply 23):
Jet fighters can certainly climb at Vso (arguably, even below it), given all the power they have.

I agree and mentioned that in an earlier post.

Quoting 3DPlanes (Reply 23):

In a Warrior or Archer, I can fly around all day along at Vso without losing altitude. And I have to pull a little power to do it, so I imagine I could likely eek out a bit of climb.

I think you'll need full power and a very nose-high attitude if you've got full flaps (Vso) and I'd be surprised if you have done it for more than a brief time. Also, unless you're at MGW, you'll need to be several knots below Vso as published for MGW. I'll try it in a 172 and you can do it in the Warrior. I would suggest experimenting to find what the true stall speed is for your given weight and balance and then trying to fly at a few knots above that speed to see if you can climb. Should be fun.
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Fly2HMO
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 1:00 pm

Quoting Redcordes (Reply 9):
Will a Cessna 172 with a standard engine at MGW climb at Vso.

Sure will, but you'll be lucky if you get anything over 200fpm.

Quoting 2H4 (Reply 12):
I think the lowest speed attainable in flight for any fixed-wing aircraft would occur in ground effect at high power settings, way behind the power curve.

That would be one way. Another way to do it would be reducing the amount of G load on the plane. Vso is considering flight at a 1G condition. But if you fly at, say .5G, you could slow down below Vso without stalling the plane. It's tricky but not impossible.

Quoting 3DPlanes (Reply 23):
can't maintain altitude, let alone climb.

Oh yes it will, been there, done that. Big grin
 
redcordes
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RE: Lowest Speed Attainable In Flight.

Thu Dec 07, 2006 2:09 pm

Quoting FLY2HMO (Reply 25):
Oh yes it will, been there, done that.

If you could climb at Vs for a specific configuration--let's say 30 deg. flaps--in an averagely loaded 172 (I'm guessing the stall speed would be about 40 kts.) then you should be able to take off at that speed (ground effect would help). I will never believe you could get the aircraft airborn and climb out at 40 kts. Why would you think that an aircraft can climb when the airflow is separating from the wing, stall buffeting would be occurring and the wing is right on the verge of a full stall. I've been there, and there's no way you're climbing out. I think you guys have sharpened your pencils a little too much this time.

[Edited 2006-12-07 06:09:40]
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SlamClick
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 12:33 am

Quoting Redcordes (Reply 26):
I've been there

Well that settles it.
The rest of us are out of our league presuming to argue with you.

Especially when you reject verbatim FAA definitions as "obfuscation" and ignore the most obviously correct definition of "steady" and continue to post guesses and decline to answer a direct question from our resident Concorde pilot.

So, the original question:

Quoting DIJKKIJK (Thread starter):
What is the lowest speed a fully loaded jetliner like the Boeing 744 can attain in level flight, and still remain airborne?

has been answered with authority right here:

Quoting Bellerophon (Reply 19):
148 kts

Therefore I am recommending that this thread be closed before any potentially dangerous misinformation gets posted.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 1:04 am

Quoting SlamClick (Reply 27):

has been answered with authority right here:

Quoting Bellerophon (Reply 19):
148 kts

Slam, 148 is the Vso speed. You and others have stated that typically an aircraft will climb at this speed, therefore it is clearly not the slowest speed at which level flight can be maintained. Sorry, just doesn't make sense that slightly above a particular speed an aircraft will climb, and below that speed it will descend. For some reason you keep avoiding answering this question. Why? Isn't it likely that there is a speed that an aircraft can neither climb or descend. That is the question and has nothing to do with regulations, etc. Like I said, I'll do some flying myself in a 172 and see what happens in that aircraft--it would be different than in a jetliner, but the fundamentel aerodynamics might be similar. As far as closing the thread on your opinion that it might be dangerous... not sure I'm with you on that.
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2H4
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 1:24 am




Quoting Redcordes (Reply 28):
For some reason you keep avoiding answering this question.

For some reason, you keep avoiding anwering this question:

Quoting Bellerophon (Reply 21):
Quoting Redcordes....Does stall speed equal minimum level-flight speed? I very much doubt that it does!...

Just out of interest, why?

Regards

Bellerophon

....Why?


2H4


Intentionally Left Blank
 
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vzlet
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 2:02 am

Quoting Redcordes (Reply 28):
148 is the Vso speed. You and others have stated that typically an aircraft will climb at this speed, therefore it is clearly not the slowest speed at which level flight can be maintained.

What may be getting lost here is the critical role of angle of attack. To fly more slowly than VSO requires an increase in AoA. That's what results in a stall. If an aircraft has sufficient available power, it can climb while maintaining VSO because that does not require an increase in AoA.
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SlamClick
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 2:04 am

Quoting Redcordes (Reply 28):
You and others have stated that typically an aircraft will climb at this speed, therefore it is clearly not the slowest speed at which level flight can be maintained.

Okay, more complication than the original question required but airplanes stall at an angle of attack - NOT an airspeed. Your experience with a Cessna 172 is misleading you. Airspeed indicators in low-end airplanes like that are not very accurate and I have seen less than 25 knots on a 172 ASI on occasions. I know damn well that my actual airspeed, if we could solve backwards from TAS, would have been higher than 25. I've even seen zero airspeed in flight in another light single. I knew I was moving, so I knew the ASI was not accurate.

But there is no fooling the angle of attack.

Angle of attack, to review, is the angle between the chord line of the wing and the relative wind. Since you cannot actually see the relative wind it needs to be stated that angle of attack has nothing directly to do with pitch attitude, the location or position of the horizon, or even my intentions at the moment. You cannot look at an airplane and have any real idea what the angle of attack is.

As for VSO and whether it is the "stalling speed" or it is the "minimum steady flight speed" both being stated in FAR 1.2 let us take the position that it is an infinitely fine point, it is the black null space between two pixels. Below it, the angle of attack is too high and the wing quits flying. We depart from controlled flight. Above that null we are okay, we are controllable. This must mean, it has to mean, and it does mean that the primary flight controls are effective. At one G, the airplane is fully controllable with all that implies.

So I am flying at that razor's edge, the pixel above the divide, and my plane is controllable. If I squeeze off a tiny bit of thrust and relax a bit on the back pressure I will start a descent. Now the relative wind is coming from forward and just below, so the angle of attack is still the same.

I level off - I add back the bit of thrust I'd taken away and I think one increment of back pressure onto the yoke, and I am holding this new altitude.

I can climb. I can add a bit of thrust and pull ever so carefully. Now we are climbing so the relative wind is coming from ahead and slightly above our present position so the angle of attack is the same.

In the descent the deck angle, the pitch attitude is lower than in level flight.
In the climb the deck angle is higher than in level flight.
But the angle of attack is identical.

So why do we climb or descend with the same angle of attack?

Effective weight reduction by thrust vectoring.

In the climb, the jet thrust is angled slightly more downward than in level flight and there is a bit more of that thrust.

In level flight what is the sum of lift and thrust?
Exactly equal to your weight and drag.
Add one pound of lift or one pound of downward thrust vector and you climb.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 2:54 am

Ok Slam. From what you have said, I guess there is no speed and attitude whereby a 747 and many other aircraft at full power could only hold altitude and not climb--even on the back side of the power curve. Obviously, if there is such a speed and attitude (and I believe there is) it would be a very tenuous position to be in and not something that would ever be done in an airliner. I do believe this maneuver can be done in high-performance aircraft as others have mentioned in this thread. Why would it be impossible in a 747? Is there a fundamental difference in the aerodynamics that makes this impossible, and if so, what is this difference? Is there no excess power available? I believe it would simply be a matter of how far behind the power curve the aircraft can be flown depending on the available excess power. And, if the aircraft has thrust equal to its weight it could "fly" at 0 kts. Or, are you saying a 747 or jetliners in general aerodynamically can not be flown on the back side of the power cuve? As you can see, there are four question marks that follow four questions. Please BRIEFLY answer these four questions. Thank you for your patience and knowledge.
"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
 
DIJKKIJK
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 3:12 am

Thanks for the all the replies , chaps!

Happy Holidays!
Never argue with idiots. They will bring you down to their level, and beat you with experience.
 
N231YE
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 4:59 am

Quoting SlamClick (Reply 31):

I am having euphoria of the mind at this moment...I learn something new everyday...
 
SlamClick
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 5:07 am

Quoting Redcordes (Reply 32):
Please BRIEFLY answer these four questions.

You can ask me to answer a question or you can specify HOW the question is to be answered but you cannot do both.  Smile

* * *


I'm not being flippant here but I want you to look up the word "steady" in any good dictionary. Understanding of the concept is crucial to these questions.

With the definition firmly in mind consider a jet fighter without any sort of thrust-vectoring nozzles, in an extreme nose-up attitude, "standing on" its prodigious thrust and see if you think that mode of flight is "steady" or whether you think the pilot may be mentally reviewing his ejection seat envelope at this moment.
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redcordes
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 8:49 am

Quoting SlamClick (Reply 35):

I'm not being flippant here but I want you to look up the word "steady" in any good dictionary. Understanding of the concept is crucial to these questions.

Ok, I looked up "steady". Now maybe you could look up "answer".

The original post said "level" not steady. Good pilots can fly on the back edge of the power curve, and as I said it is tenuous but is certainly done. And forget about my requesting how I asked you to answer the questions because the answers are obvious. The real question is why you won't--maybe because the answers contradict some of your statements earlier in the thread. I'm surprised you weren't able to answer even one of the previous four questions with a concise answer.
"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
 
Pihero
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 9:22 am

I think SlamClick has tried to explain his position well.
Problem is that people keep going out of topic, but first let's make one thing clear :
One achieves minimum sustainable flight at the AoA of Clmax.Period. And it's not open to discussion. Using an enormous engine takes you out of the subject as we've seen Harriers do stationary flights, but the physics are not the same.
To be perfectly clear, SlamClick did not mention that a climb or descent flight path, though at Clmax doesn't respond any more to the 1g requirement as the load factor changes to the Cosine of the flight path (granted it's a minute difference but a difference regardless).

Now for the out-of-topic questions :

Quoting Redcordes (Reply 32):
I guess there is no speed and attitude whereby a 747 and many other aircraft at full power could only hold altitude and not climb

Wrong,that is called here sustentation ceiling, better known as "the coffin's corner" at which however powerful your engine is you cannot achieve sustained flight...but this phenomenon is due to compressibility . By definition, Vs1 is demonstrated outside Mach effects. All aircraft have such a ceiling, whether high performance combat aircraft or airliners;
That answers three of your questions, Redcordes.

Quoting Redcordes (Reply 32):
are you saying a 747 or jetliners in general aerodynamically can not be flown on the back side of the power curve?

I'm not quite sure what a backside of a powercurve is (unless that's the name of that cute Brazilian top model Big grin ) but if you refer to second flight regime flying, of course we could do it. Problem is we have to cope with a very unstable speed keeping and use an awful lot of power / thrust that negates airline's economics.That's the reason why we fly long range instead of max range.
Hope it helps.
Cheers
Contrail designer
 
SlamClick
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 9:32 am

Quoting Redcordes (Reply 36):
The original post said "level" not steady.

Well excuse me.

I didn't know you meant "level" as in pitch attitude. I thought you meant it the way most pilots would - as in "level flight" as in maintaining altitude five seconds from now and thirty seconds from now. To accomplish this last feat your flight must be "steady" a fact I think everyone but you comprehended.

So okay, here is an airplane flying LEVEL.


The short part of a second later it was a ball of fire rolling down the runway at MUO but as of this moment it is level.

Now I ask you, is it STEADY?

As for your other four questions I think you know why I did not address them. Disregarding the snotty attitude on your part, some answers just take as many words as they need. Some concepts do not fit on a bumper sticker.

I think I have been very generous with my knowledge and experience, even after your obtuse refusal to accept authority (The FAA and other professional pilots' not mine) began to piss me off.

Anyone other than Redcordes think I've been evasive here?
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YYZAeroEng
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 9:41 am

Quoting Redcordes (Reply 32):
And, if the aircraft has thrust equal to its weight it could "fly" at 0 kts.

That right there doesn't make sense.
With an airspeed of 0 kts there is no lift generated and you can't fly.
Also your thrust and lift vectors point in different directions. In level flight their components don't lie in the same plane.

As for the your other questions, I think I've got a vague idea of what you're asking, so here goes nothing....

The only way an aircraft can climb is if the lift being produced is greater than the weight of the airplane.

Your lift is governed by:

L = [Cl]*(.5*rho*(Vinf^2))*S
Where L is your lift
Cl is your lift coefficient
rho is your air density
Vinf is the free stream velocity
S is wing area

At steady level flight your Weight is equal to your Lift, and your angle of attack (AoA) is 0 degrees. Since we know your AoA we can find Cl from a graph of airfoil data. If you isolate Vinf and solve with the data you have, you'll be able to figure out what speed you need to maintain steady level flight.
Any change in Cl, S, rho, or Vinf is going to change your lift.
So you can reduce speed, and maintain steady level flight as long as one (or all) of the other variables is increases to compensate.

Hopefully this answers your question, if not let me know

[Edited 2006-12-08 01:43:49]
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Bellerophon
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 1:51 pm

YYZAeroEng

...The only way an aircraft can climb is if the lift being produced is greater than the weight of the airplane...

No.

A common misconception. During a climb, the aerodynamic lift being produced will be less than the weight. An aircraft climbs because of an excess of thrust not an excess of lift.


...At steady level flight your Weight is equal to your Lift, and your angle of attack (AoA) is 0 degrees....

No.

In straight and level flight, where there is no load on the tailplane, Lift will equal Weight; however the AoA will (almost certainly) not be zero.

(An aircraft with a cambered wing, at high speed, can still generate some lift, when the AoA is technically zero.)


Regards

Bellerophon

[Edited 2006-12-08 06:13:07]
 
YYZAeroEng
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 2:13 pm

Quoting Bellerophon (Reply 40):
A common misconception. During a climb, the aerodynamic lift being produced will be less than the weight.

An aircraft climbs because of an excess of thrust, not an excess of lift.

It really isn't a misconception. The lift acting at the aerodynamic centre of the your airfoil will be acting perpendicular to your chord line. Thus a component of lift will be acting opposite to your thrust vector. This will slow you down. Excess engine power is needed to increase your speed. The engines don't generate lift. The wings generate lift an that's what keeps you aloft and climbing. In a climb the vertical component of your lift vector plus the vertical component of your thrust vector must be greater than your weight.
A simple free body diagram should be able to clear up and confusion you have.

Quoting Bellerophon (Reply 40):
No.

In straight and level flight, where there is no load on the tailplane, Lift will equal weight, but the AoA is not zero.

From Aircraft Performance and Design by John D Anderson Jr
"For steady level flight, the weight is equal to the lift"
Additionally if you're at steady level flight your AoA will be zero.
For a cambered airfoil lift can be generated a 0 AoA.
If there is a washout or and incidence angle, things are a little different, but I figured that was too complex for this discussion.
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Bellerophon
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 3:11 pm

YYZAeroEng


...The only way an aircraft can climb is if the lift being produced is greater than the weight of the airplane. ...

That statement is, and remains, factually incorrect.

I assume you now tacitly agree, because you go on to say:

...In a climb the vertical component of your lift vector plus the vertical component of your thrust vector must be greater than your weight...

Not necessarily greater than the weight, equal to it will do for a steady climb. If you were perhaps now to draw ...A simple free body diagram... perhaps that might ...clear up any confusion you have....  Wink


...Additionally if you're at steady level flight your AoA will be zero....

That statement is, and remains, factually incorrect.

I accept that a cambered wing can generate some lift at zero AoA. It may be possible, for all I know, for some such aircraft to fly straight and level at zero AoA. If it can, it will only be able to do it at one particular airspeed, alter that airspeed and the AoA will change with it.

However, to make a general statement that any aircraft in steady level flight, regardless of airspeed, must be at zero AoA, is, with respect, nonsense, regardless of who said it.


Regards

Bellerophon
 
redcordes
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 3:47 pm

Bellerophon, your answers make a lot of sense. However, the original post asks: "...lowest speed...level flight...., and still remain airborne". I read the " still remain airborne" as at the edge of the absolute flight envelope--not within typical operating guidelines. Therefore, I believe there is a speed below the Vso of 148 kts. (which I suppose is a flight idle speed) at which level flight could just be maintained with the right attitude and full power. Of course, this would never be done in normal operation. So, in respecting Slam's concerns, I must caution any readers here: Please do not attempt this when flying your airliner. And, for those of you who fly aircraft other than airliners, make sure you have plenty of altitude! Thank you.

[Edited 2006-12-08 07:53:48]

[Edited 2006-12-08 07:58:13]
"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
 
BAE146QT
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RE: Lowest Speed Attainable In Flight.

Fri Dec 08, 2006 7:08 pm

Quoting SlamClick:
Some concepts do not fit on a bumper sticker.

At the risk of sounding like a fanboi, I reckon that would make a great signature line.

Quoting SlamClick:
With the definition firmly in mind consider a jet fighter without any sort of thrust-vectoring nozzles, in an extreme nose-up attitude, "standing on" its prodigious thrust and see if you think that mode of flight is "steady" or whether you think the pilot may be mentally reviewing his ejection seat envelope at this moment.

As I was reading this thread, but before I got to your post, this example sprang to mind. I would have asked if you hadn't mentioned it already.

So. Plane's got no airspeed. But assuming enough thrust, it ain't falling. So technically it's not stalled, (though maybe it did to get into that position in the first place?), but I wouldn't call it stable. All it would take is a slight asymmetry of thrust, or a gust of wind, and that plane is going over. In what I imagine would be a stalled condition with no control authority.

I reckon the pilot would be wanting to find a way out of this situation in a hurry.

Am I right?
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Bellerophon
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RE: Lowest Speed Attainable In Flight.

Sat Dec 09, 2006 12:13 am

Redcordes


...Therefore, I believe there is a speed below the Vso of 148 kts. (which I suppose is a flight idle speed) at which level flight could just be maintained with the right attitude and full power...

A wing stalls when its critical angle of attack is exceeded.

What the aircraft is doing at the time is broadly irrelevant, as is the amount of thrust applied (disregarding prop wash effect over the wings on propeller driven aircraft). I often used to stall the wings, on various aircraft I flew, whilst in a vertical climb, vertical descent and various lines in between, with all sorts of thrust settings from idle to full power.

The wing always stalled at the same angle of attack, and nothing I did ever allowed me to exceed that angle of attack without stalling.

In 1g unaccelerated flight, the speed associated with this critical angle of attack is called the stall speed. In the landing configuration this stall speed is designated Vso. Go below this speed and you will exceed the critical angle of attack. Exceed the critical angle of attack and the wing will stall, even if you were climbing at 1,000 fpm with full thrust applied the moment before.

I wonder if perhaps you might see the contradiction in what you are saying?

If you could maintain level flight at a speed below Vso, then, by definition, Vso can not have been correct. If Vso is correct, then you can not maintain steady level flight at a lower speed.


... I read the " still remain airborne" as at the edge of the absolute flight envelope...

As did I, hence the answer I gave. Vso.  Smile

You might, if you were foolish enough to try, sustain steady flight above Vne or above the flight envelope maximum altitude; but you won’t sustain steady flight below Vso.

The stall speed is the hard edge to the flight envelope.


...not within typical operating guidelines....

If you think that typical airline operating guidelines permit operation anywhere near the stall speed, let me show you some typical airline SOP minimum operational speeds:

B747-436, MLW 285,760 kgs / 630,000 lbs, Calm, Sea Level, Fwd CG, Clean / Flap 30° + Gear Down.

  • Min Clean............233 KIAS
  • Stall Clean..........172 KIAS
  • Min Flap 30°........158 KIAS
  • Stall Flap 30°.......124 KIAS

...I guess there is no speed and attitude whereby a 747 and many other aircraft at full power could only hold altitude and not climb...

That depends entirely on the drag characteristics of the aircraft concerned, coupled with the stall characteristics of the wing concerned.

An aircraft, with a low stall speed, but on which the drag rises steeply with decreasing airspeed, can easily reach a speed where full thrust only just equals the drag, long before it gets near its stall speed.

This speed is know as Vzrc (Zero Rate of Climb speed), and at this speed, an aircraft will have no excess thrust available to climb with or to accelerate with, but will, just, be able to maintain level flight. The only way to accelerate away from this speed is to descend, trading potential energy for kinetic energy. Delta wing aircraft are (generally) a good example of this.


Regards

Bellerophon
 
SlamClick
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RE: Lowest Speed Attainable In Flight.

Sat Dec 09, 2006 12:25 am

Quoting BAe146QT (Reply 44):
So technically it's not stalled, (though maybe it did to get into that position in the first place?), but I wouldn't call it stable.

I think the assumptions you make surrounding that statement are substantially correct.

I posted the F-16 crash picture to make this point: If you are willing to accept conditions of very little controllability, or even of impending disaster, and you are willing to accept a very brief snapshot of time then a "fully loaded...744" can probably (briefly) be at a position removed from the earth's surface by some distance, at zero indicated airspeed. There is a second or two at the top of a hammerhead or other extreme vertical maneuver when Sean Tucker is maintaining altitude to private pilot standards. Is this flight or ballistics?

I took the thread-starter's question to assume sustained flight and controllable flight and not just a temporary bit of airplane mishandling. The answers I gave were for a condition that could be sustained as long as the fuel lasts. Since this thread seems to have been a drive-by on the part of its creator, we will not know if the conditions he intended have been satisfied unless he comes back.

There is, indeed, a regime of "aloftness" (I cannot call it flight) below the established stall speed, where a combination of forces may keep you airborne. I've been there myself.

I once flew a Doyn-conversion C-172, rather like a Cessna 175, and we could not stall the airplane by conventional approaches. The problem was not an excess of lift performance by the wing, but a lack of elevator authority to get you into the attitude. When it slowed below a certain speed you just could not lift the nose any higher and you'd begin a descent. So we dove to gain speed and pulled from near redline until we had it about thirty degrees nose up. We were able to hold that as speed decayed and it did stall. Oh yes! It did stall.

The airspeed indicator pretty well went to sleep at some point during that little exercise. Published stall speed? No idea!

So, sustained, controllable "flight" I think the answer is VSwhatever.

Brief period of separation from the planet? At the risk of sounding like Clint Eastwood: "do you feel lucky?"
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xv408
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RE: Lowest Speed Attainable In Flight.

Sat Dec 09, 2006 1:29 am

Quoting Bellerophon (Reply 42):
I accept that a cambered wing can generate some lift at zero AoA.

In my aero classes (a long time ago) a typical zero lift AoA would be -4 degrees or so. Depends on camber. But most subsonic wings have camber.

Quoting Redcordes (Reply 32):
I guess there is no speed and attitude whereby a 747 and many other aircraft at full power could only hold altitude and not climb

Correct me if I'm wrong, but isn't that what defines the absolute ceiling of an aircraft? (The service ceiling being defined as when the maximum climb rate is below a certain ammount, say 100 feet/min.) And if so, every aircraft has such a set of circumstances?
 
BAE146QT
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RE: Lowest Speed Attainable In Flight.

Sat Dec 09, 2006 2:35 am

Quoting SlamClick (Reply 46):
Is this flight or ballistics?

Taking the maneuvre as a whole, I would say "both". Flight either side of a ballistic movement.

The tricky part (for me at least) is deciding when one ends and the other begins. I'm probably misusing the phrase, but I imagine it's when the wings go beyond the critical AoA and aren't doing anything. The aircraft is just moving under inertia and eventually, gravity, (which is what I understand "ballistic" to mean)
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Bellerophon
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RE: Lowest Speed Attainable In Flight.

Sat Dec 09, 2006 4:07 am

Xv408

...But most subsonic wings have camber....

Agreed.


...a typical zero lift AoA would be -4 degrees or so. Depends on camber...

I'm happy to go along with that, I would have suggested about -2°AoA for zero lift, but as you say, it all depends on the aerofoil.

However, I would also point out that, at 0°AoA, the lift coefficient (CL) for such a generic, subsonic wing is typically only around 10%-15% of CLmax.


...isn't that what defines the absolute ceiling of an aircraft?...

Possibly, although not necessarily.

Like many aircraft limits, it will be the most limiting of several factors, and other factors may have become limiting before the engines run out of excess thrust.

Two obvious such factors are:
  • Maximum cabin differential pressure
  • Coffin Corner
Maximum cabin differential pressure will determine the maximum aircraft altitude, because the cabin altitude must not exceed the maximum permitted altitude of 8,000 ft.

Coffin Corner, because some aircraft have such powerful engines, that they will happily take the aircraft to an altitude the wings can not sustain flight.


Regards

Bellerophon

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