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High Altitude Stall

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High Altitude Stall

Old 19th Apr 2001, 13:17
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Wilfred
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Question High Altitude Stall

The books say that the stall speed increases with altitude. Have I got the reason right?

1/2RhoV2SCl. For a stall in a given configuration, S will remain the same, and Cl(a combination of wing shape and incidence) will be the same. Stall always occurs at a certain incidence. So if air density (1/2Rho) decreases with altitude, then the airspeed at the stall would have to be higher to balance the equation.

Please can you put me straight folks.
 
Old 19th Apr 2001, 14:37
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Smurfjet
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Its the same reasoning I came to, regarding stall at increased weight. I'd be interested in more input for both cases.

SJ

[This message has been edited by Smurfjet (edited 19 April 2001).]
 
Old 19th Apr 2001, 16:10
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supermunk
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An aircraft will stall at the same IAS irrespective of height (more or less). As you go higher, for a given IAS, TAS increases due to the reduction in air density. In extreme cases (the TR1 is one of them) you can have only a very small speed range to aviate in between stall amd mach buffet.
 
Old 19th Apr 2001, 17:24
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Smurfjet
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Ah! True but irrelevant You're forgetting the basics, Lift = weight and air density
 
Old 19th Apr 2001, 17:53
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kabz
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Not so fast smurfjet, supermunk has it right. IAS (indicated airspeed) will show a similar speed approaching the stall, to that at lower altitudes, simply because the amount of air the wing sees near the stall and the amount the airspeed indicator sees are both similar to lower altitudes. Higher TRUE airspeed takes care of the lower air density...

Lots of books cover this stuff, and go into the real detail of angle of attack, load factor and why planes suddenly start to feel really unsteady towards a stall.

Here's a real good reference:
http://www.monmouth.com/~jsd/how/
 
Old 19th Apr 2001, 19:12
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Smurfjet
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Right, I mixed my TAS and IAS again, arrgh
Now thats irrelevant

SJ
 
Old 19th Apr 2001, 21:14
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fireflybob
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Aren't you forgetting compressibility effect?
If you stall at (very) high altitude then although the EAS (Equivalent AirSpeed) will be the same at Sea Level, the IAS will be somewhat higher - least that's what I seem to recall from a long time ago!

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Old 19th Apr 2001, 22:34
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Golf-Kilo Victor
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I thought Wings stalled at the same Angle of Attack, not Airspeed..

Of course your all going to laugh at me now, cause you are all aeronautical engineers....at leaste that's what it sounds like...
 
Old 19th Apr 2001, 22:42
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kabz
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Talking

GKV you're dead right. I think people above were assuming a gradual pull back of the stick with power off whilst maintaining level flight, in standard student manner.

If you rip the controls back, yup, it'll stall at the critical angle of attack, no ifs no buts.

I keep meaning to try it, but you should be able to get a recognisable stall out of a kite, if you jerk the string(s) hard enough.

... ahh "Peter Powell Stunt Kite, where are you now ??"
 
Old 19th Apr 2001, 23:08
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Smurfjet
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At the critical angle the aerofoil (wing) is stalled.

Just before the critical angle I have maxLift. For a given weight = maxLift, I'm still flying level.

Now this maxLift is = 1/2 rho S V^2 Cl

Increase my weight by 20% gives me weight > maxLift.
Brings me to the conclusion that unless I increase IAS (1/2 rho V^2) I will not be able to maintain level flight. I'd say as a pilot I would call this a stall? Bearing in mind I have no power left to increase my IAS.

Yank the stick backwards and you are beyond the critical angle and the aerofoil (wing) is stalled.

So from this I will try to answer the original question...

Well maybe not, there is still a missing link somwehre

Ball in your court gentlemen, time to dig mechanics of flight
 
Old 20th Apr 2001, 01:47
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fly4fud
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Wink

and as a practical example, when flying heavy at hi level (say 410) with the A-310, I remember having seen the coffin corner as small as 7 kts! You then just pray for calm air...

------------------
... cut my wings and I'll die ...
 
Old 20th Apr 2001, 09:59
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static
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Fly4fud,
These 7 knots are between your "yellow bands", so that`s not taking into account your 1.3 margin over Vstall and machbuffet.
The coffin corner will actually be much, much bigger.
But in these circumstances I`m also hoping for still air.
 
Old 20th Apr 2001, 12:24
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Wilfred
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GKV nope, just a pilot!

DP Davies writes in Handling the Big Jets (or the Bible)that the indicated stall speed increases for two reasons.

1. Due to compessibility effects causing a larger difference between EAS and IAS. That I can cope with. But...

2. "The actual EAS stall speed increases due to Mach No. effect on the wing. At very high altitude the EAS stall speed occurs at a significant Mach No.(180 knots = 0.61 Mach No., for example); the pressure pattern is disturbed and a higher stall speed results"

Could someone explain, in easy words writ large for pilots, the second one please? Ta.

Also, was my opening assumption incorrect; nobody actually said for sure.
 
Old 21st Apr 2001, 01:46
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BOAC
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Wilfred, you know that a stall develops when the airflow can no longer flow smoothly over the wing upper surface. Remember the airflow speeds up over this part. As you climb, the increased speed of the air gets towards the point where compressibility starts to have an effect, and once shock waves start forming the airflow begins to separate from the wing = stall. Used to be known as a 'shock stall'.
It becomes easy to stall a wing in a turn in these conditions, as the airflow speed further increases due to the increased lift generated by the wing.
 
Old 21st Apr 2001, 22:05
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John Farley
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Wilfred

Another way to look at your para 2 point is to say that in general compressibility effects are seldom good news. They reduce aerodynamic efficiency in various ways. In this particular case the stalling angle of attack of a wing in compressible flow is a little less than the same wing in incompressible flow. This lesser angle is naturally reached at a slightly higher speed.

JF
 
Old 22nd Apr 2001, 12:50
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alapt
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BOAC, I do not think Compressibility, unless you are at the critical mach number will enter the equation here. (Remember that is why we have SWEEP to delay the onset of compressibility) At high and low altitudes, the pressure distribution will in effect move the Aerodynamic center on pressure forward. (Just past the max Lift/Drag curve) At that critical point, flow separation will occur and you will eventually stall. The main thing to know is the difference between IAS, TAS, and EAS. Very simple I know, but the details would take three pages.
 
Old 22nd Apr 2001, 13:06
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Wilfred
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BOAC and John Farley, thanks, but are you talking about the high speed stall situation? The quote from HTBJ in my last post here refers to 0.61 Mach; would sinificant shock waves be developing at such a low Mach No.?

I was under the impression that the plain old "there ain't enough air flowing over me wings" stall speed actually increases with altitude.

I get the feeling now that I am missing something so bl***y obvious. Don't lose patience with me please. Ta.

Wilf.
 
Old 22nd Apr 2001, 20:06
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BOAC
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Let's not get into sweep or we'll be here all day talking about thickening wing-tip b-layers/tip stall/pitch-up etc!

Wilfred, cannot answer the question about 0.61M - it depends on how much the air accelerates over a particular thin or thick section. It only requires LOCALLY sonic conditions to cause separation. Mach 1 is always REACHED somewhere in the airflow around an aircraft when the actual physical Mach is below 1.0. Pulling 'g' aggravates the compressibilty effect. Ask any FJ pilot who has dropped a boom in combat while 'subsonic'! Try the B737 at 0.8 Mach and a steep turn (in the sim, of course!) - you'll get shock buffet.
 
Old 22nd Apr 2001, 20:59
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John Farley
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Hi Wilfred

If BOAC has sorted you please ignore everything after this!

I think your comment about “high speed stall” might have given me a clue as to what is bothering you.

In my book a “high speed stall” is simply one that happens at more than 1g. It can still be wings level (say at the bottom of a loop) or it could be at the normal cruise speed for the type and involve a lot of bank on and pulling hard. It has nothing to do with “high speed” as in compressibility or Mach number effects. It just means the pilot made the aircraft stall at a speed that is higher than the wings level 1g case. Indeed the proper term for what we all refer to as a “high speed stall” is an accelerated stall.

So doing an ordinary stall (wings level with speed reducing in level flight) at say 40K feet does not mean you have experienced a high speed stall even though the Machmeter may well be some way round the dial at the time you stalled.

Just in case the above does not unlock the issue for you, I will spell out my understanding of the subject from the top (forgive the sucking eggs bit but I don’t want to miss out any step just in case that is the one giving problems.)

There are two types of mathematical approach to aerodynamics, one treats the air as if it is an incompressible gas and the other assumes (correctly) that it is compressible. This latter correct approach has the disadvantage that it is much more complicated.

The difference in numerical results between the simple theory and the complex one is very small indeed at low speeds - say below 150 kts - and is usually ignored in normal life because the scatter in results from a whole host of other factors tends to be greater than the compressibility effect. (By these “other factors” I mean inaccuracies in flying, effects of turbulence, instrument and sensor calibrations and errors and so on)

The fact that air IS compressible does effect some aspects of the simple (incompressible) theory more than others. What we are interested in here, namely the loss of the maximum available lift coefficient available from a wing (i.e. the amount of lift we can get before the flow breakdown that we call the “stall” happens) is probably affected as much as anything.

What I am talking about is the breakdown in the simple relationship between stalling speed and applied G. Simple (incompressible) consideration of the lift equation (L = Cl x ½ x density x wing area x speed squared) tells us that if we have a wings level stalling speed of say 100 kts, then at 141.4 kts (or whatever the square root of 2 is) we should be able to pull 2g, ie just manage a 60 deg bank steady level turn at the stall.

But somehow one can never quite achieve this and the shortfall at 173.2 kts is even greater (root 3 when we would expect 3g from simple theory).

Now I know we are taught that a wing always stalls at the same angle of attack (and nobody goes around saying THAT more than me) but it is actually only a true statement if the Mach number at the stall is roughly constant. Numbers for the Harrier in conventional flight are locked in even to my failing memory, so forgive me quoting type specific numbers to make what is a general point:

In a metal wing Harrier at low level the angle of attack you see in a 1g stall wings level is around 12 deg (clean configuration, flaps up) That number will not change even if you double the aircraft weight (although the IAS at which you reach it will be much higher) and equally if you turn too hard you will stall at the same 12 AOA regardless of weight or bank angle in use.

BUT, if you go up to 20K feet and spiral down at say .8 Mach in a high G turn, pulling harder and harder until it stalls, you will probably not see more than 9-10 degrees on the AOA gauge. This reduction in the max lift available has happened because of compressibility effects.

Hope that helps, if not get back to me as there is no such thing as a bad student only a poor instructor (or as my doctor says - there is no such thing as an impotent man only and incompetent woman - advice which I find very comforting)

JF

 
Old 23rd Apr 2001, 00:13
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BOAC
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Nice to see you still quoting the 'bona jet', JF.
PS Have you got your doctor's phone number?
Mike
 

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