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New (2010) Stall Recovery's @ high altitudes

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New (2010) Stall Recovery's @ high altitudes

Old 19th Jun 2010, 15:21
  #41 (permalink)  
 
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stall has everything to do with airspeed
a stall is a reduction in the lift coefficient generated by an airfol as angle of attack increases
so it obviously had something to do with airspeed, somebody dropped out of pilot taining school
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Old 19th Jun 2010, 15:34
  #42 (permalink)  
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Basically because it was correct! Your comment
The angle of attack at which a modern commercial jet stalls at 0.8M is something around half of the angle of attack at which it stalls at Vs1g.
is totally confusing.

Firstly 0.8M has nothing really to do with 'airspeed'
Secondly you are mixing a Mach effect at an unstated 'g' with a stall at '1g' at an unstated airspeed! Absolute confusion. It will still stall at 'Vs1G' at high level, will it not? Just a different Vs due to airflow Mach effects.

It has everything to do with angle of attack.
- is correct. Whatever the angle of attack at which an aerofoil will stall (yes, at whatever Mach number), 'airspeed' is irrelevant. I can stall the same high performance jet at sea level at 140kts straight and level 1g or 400kts in a manouevre.

However, all of this is pretty much irrelevant to the thread.
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Old 20th Jun 2010, 06:51
  #43 (permalink)  
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Originally Posted by BOAC
Basically because it was correct!
No, it's not correct. The angle of attack at which a high-performance wing will stall is a function of the speed of the air flowing over it, exactly contrary to what DFC said in hisher first sentence.

These AoAs, for different airspeeds, are established during certification of the airplane. You put a wing in a wind tunnel, and look at its behavior at different airflow speeds. The AoA at which, say, C_L drops off sharply is a function of the airspeed.

I must say I am somewhat surprised at your response. I had awaited something different.

Your comment
[PBL: The angle of attack at which a modern commercial jet stalls at 0.8M is something around half of the angle of attack at which it stalls at Vs1g.]
is totally confusing.
I am sorry you are confused. I think it is a precise statement, and I don't know how to say it more precisely. Let me try.
I can put numbers on these, for a specific wing X:
"The AoA at which wing X stalls at 0.8M" = S;
"The AoA of wing X at Vs1g" = T
S is round about half of T for wing X.
This won't necessarily be true for all high-performance wings X, but it is certainly true for some of them.
Less confusing?

Originally Posted by BOAC
Secondly you are mixing a Mach effect at an unstated 'g' with a stall at '1g' at an unstated airspeed! Absolute confusion. It will still stall at 'Vs1G' at high level, will it not? Just a different Vs due to airflow Mach effects.
Vs1g is a specific airspeed. What do you mean, "unstated airspeed"?

The AoA at which a wing will stall, and how it will stall, is dependent on Mach effects. These must be determined when certifying aircraft.

I don't agree that this is "off topic". Some posters are talking about stall recovery at high altitude, and many of them seem to be confused about aerodynamics, even what "stall" may mean. The vocabulary I am using is one which I use on an almost daily basis in my work with an aerodynamicist. I am gradually learning through these exchanges on PPRuNe that many pilots seem to be unaware of the aerodynamic phenomena which govern the behavior of their airplane. I don't think this disconnect can be healthy.

As I explained earlier, the "stall" is generally a range of phenomena playing out over a range of airspeeds. The certification-defined "stall speed" is the one at which (crudely) handling becomes compromised due to buffet. This may be, often is, lower than the speed at which C_L falls off. I don't agree with Mansfield that this is not a "practical" distinction. It becomes very practical when considering gusts at altitude. For certification, one has to take the wing to defined-stall ("uncomfortable buffet") and then subject it to gusts of a defined magnitude, and make sure nothing untoward happens, such as suddenly losing all your lift.


PBL
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Old 20th Jun 2010, 07:58
  #44 (permalink)  
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Vs1g is a specific airspeed.
- but varies with altitude, no? Sounds like a variable to me.

It really is of no significance to a pilot how a wing stalls - it is, as you demonstrate, academic and for wind tunnels and test pilots - not airline pilots. What is significant is what to do
a) to avoid it
b) to recover if it does

None of us have time to think (or worry) about little lambda feet creeping along our wing, of boundary layer shocks, of vortex formation etc in an aeroplane full of passengers. Yes, the confusion remains over what is a 'stall' as I have said, but in real terms it matters not. Avoid or recover. People like you can worry about S/T.
I am gradually learning through these exchanges on PPRuNe that many pilots seem to be unaware of the aerodynamic phenomena which govern the behavior of their airplane. I don't think this disconnect can be healthy.
- that is the essence of this. The disconnect is between those who hold a mouse and those who hold a control column, and in my opinion reasonable.
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Old 20th Jun 2010, 09:28
  #45 (permalink)  
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Originally Posted by BOAC
[PBL: Vs1g is a specific airspeed.]
- but varies with altitude, no?
Curiouser and curiouser, said Alice.

It varies with dynamic pressure. Maybe I can mention the term EAS and leave the rest to you?

Originally Posted by BOAC
It really is of no significance to a pilot how a wing stalls
What an extraordinary view. It appears to me to conflict with your earlier apparent interest in how a wing stalls:
Originally Posted by BOAC
my reply was to a post about high-level stalls where deploying LE devices will probably put you on your back........ The use of rudder to lift a stalled wing is ESSENTIAL and correct and MUST be taught. Use of aileron on a fully stalled wing will merely increase the wing drop (basic aerodynamics?) and will probably cause a spin.
Say you fly in or near a thunderstorm, and get hit with a strong gust. Is it all the same to you if the airplane suddenly pitches up more strongly than you can pitch down, or if your elevators don't appear to function any more to control pitch? Wouldn't it be preferable to have at least a modicum of understanding as to what might be going on, so that you can effect your desire
Originally Posted by BOAC
b) to recover if it does
?

Originally Posted by BOAC
[PBL: many pilots seem to be unaware of the aerodynamic phenomena which govern the behavior of their airplane. I don't think this disconnect can be healthy.]
...... The disconnect is between those who hold a mouse and those who hold a control column, and in my opinion reasonable.
You are welcome to your opinion. Sorry I misjudged you. I had understood from your previous posts that you were interested in what happens with a "fully stalled" wing.

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Old 20th Jun 2010, 14:28
  #46 (permalink)  
 
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stall recovery technique

Originally Posted by PBL;#23
So what data is it that a given manufacturer didn't have, that they would have gone back up to get because the regulators were worried again about LOC? Can anybody here say?
Perhaps you might find this article interesting: Airbus testpilot views on stall recovery training
If an airplane is equipped with a stick pusher, then, granted if you will, a stick pusher activates at the point of defined "stall". That is, you get slower and slower, up to a specific airspeed at which stick pusher activates, and over you go. At what point in that entire process can you be said to be "stalled"? A suggestion: at no point, or maybe at just one. So, if I may be permitted to insist on accuracy, you are performing a manoeuvre which can be best described as recovery from approach to stall, not as recovery from a stalled state. The pusher does not let you get into a stalled state. Isn't this obvious?
That depends. A stickpusher is usually fitted if an airplane, in some or all configurations and cg positions, does not exhibit a natural pitch down tendency at the stall. The 'firing point' of the stickpusher is usually just after the point of maximum lift, i.e. just after the airplane is stalled. The purpose of the stickpusher is not to prevent the stall, but to prevent the airplane from getting deep into the stall. Some manufacturers call their stickpusher euphemistically a Post-Stall Recovery System. For the recovery it is immaterial whether the post stall pitch-down is natural or generated by a stick-pusher.

HN39
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Old 20th Jun 2010, 16:27
  #47 (permalink)  
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Originally Posted by hazelnuts39
Perhaps you might find this article interesting:
I don't think so. I am much more inclined to cut out of the discussion. I am getting tired of expounding basic aerodynamics and being told it's all wrong.

PBL
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Old 20th Jun 2010, 16:32
  #48 (permalink)  
 
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I don't think so. I am much more inclined to cut out of the discussion. I am getting tired of expounding basic aerodynamics and being told it's all wrong
I agree, far too basic and its annoying when people tell you whats involved in your job
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Old 20th Jun 2010, 17:38
  #49 (permalink)  
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PBL - before you go 'outside' - personally I am interested, yes. For the great mass of all of the airline pilots in the world, it is not particularly relevant.

I actually spent 4+ years of my military life flying an aircraft that spent a large proportion of its time partly stalled, in pre-stall buffet, at speeds from 170kts in the circuit, through 300+kts in combat to M1.3+.

I don't do it now.
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Old 21st Jun 2010, 10:47
  #50 (permalink)  
 
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don't think so. I am much more inclined to cut out of the discussion. I am getting tired of expounding basic aerodynamics and being told it's all wrong.
stalls, as mentioned before, can occur in any attitude, at any thrust setting and at any airspeed--- also maximum loading is placed upon the wing at the stall,... that maximum loading varies with airspeed,

A stall at Va results in a limit load

and a stall above Va will usually result in structural failure due to ultimate loads!!!

Edit: so, therefore the AoA of the stall remains constant...

Last edited by Pugilistic Animus; 22nd Jun 2010 at 10:25.
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Old 21st Jun 2010, 17:15
  #51 (permalink)  
 
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Comment made in training course dealing with high altitude jet upsets - the first upset typical results in a loss of X (1-3K?) feet. The second upset results in the average loss of Y (7K?) feet.

Scenario - upset one starts. Crew is startled. Rocking and rolling, buffet, etc. Nose drops and altitude loss starts. Plane starts flying again. So far, so good.

Then the crew tries to minimize the altitude loss and the second upset starts. A perfectly flying wing, well below typical flying speed and typical AOA, is exposed to an increase in AOA/G(BOAC's point) in an attempt to reduce the descent rate. Critical AOA is reached very quickly with different results from a low altitude/high AOA event. Bam, upset 2 is entered.

The course mentioned the major injuries, and a/c damage, result from getting into the second upset. One is a basic a 1G event, while the other is an 'accelerated' stall event. As BOAC alluded to, they can be interesting as heck in fighters but they're a bad place to explore in a jet liner.
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Old 22nd Jun 2010, 08:23
  #52 (permalink)  

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Sounds like a good course.
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Old 23rd Jul 2010, 07:54
  #53 (permalink)  
 
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I have just picked up the latest amendment for my type's QRH which now includes Stall Recovery as a memory item. It looks as if they are trying to re-enforce the point that power is not to be used, particularly pertinent of the FBW Airbus where there is no pitch up with power application in normal or alternate law. If you are stalling, you could possibly be in direct law and you will get a pitch up.

IIRC, there was a B737 in the UK (Bournemouth?) which got to a very low speed because the pilot didn't handle the thrust induced pitch up. this is probably what has lead to the manufacturers re-examining thier advice and procedures.

The emphasis seems to be on the fact that the Standard Stall Recovery taught in the early lessons in light pistons is not applicable to all types, and that any recovery actions require consideration - or a type specific automatic response promulgated by the publication of memory actions.




But it's been an interesting thread, in respect of some of the advice being given such as using the rudder to keep the wings level at the stall, and that the stall is speed dependant. I thought these notions had been eradicated years ago. I'm suprised they still seem to exist!
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Old 23rd Jul 2010, 08:49
  #54 (permalink)  
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Dan - out of interest, which flight controls do you suggest you should use at the point of stall to control wing drop?

There are many, many more examples of underslung engine pitch/power near LOC than the BOH example.

You say that the latest amendment says 'no power'? For the avoidance of possible confusion, is that for all stalls - including low-level/approach? I appreciate we are on a 'high-level stall' thread but you quote a low-level example (BOH), and do I assume this is an AB amendment?
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Old 23rd Jul 2010, 10:12
  #55 (permalink)  
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Originally Posted by Dan Winterland
in respect of some of the advice being given such as ...... that the stall is speed dependant. I thought these notions had been eradicated years ago. I'm suprised they still seem to exist!
To surprise you even more, here is a recent diagram from Boeing showing clearly that C_L_Max is dependent on Mach number.

If you think that the point of C_L_Max is the point of stall (and let me caution about that, because that is not what the certification regulations say, but it is what many pilots seem to believe), then maybe you could ask yourself why Mr. Boeing is showing point of stall dependent on Mach number. Maybe because it is?

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Old 23rd Jul 2010, 16:51
  #56 (permalink)  
 
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And, to reinforce PBL's point, while that Boeing diagram may be principally thinking about Mach effect from, say, ).5 through to 0.8, there are aerofoils where the Mach effect is perceptible all the way down to very low numbers - 0.2 or lower - a speed regime where Mach effects are not normally expected.
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Old 23rd Jul 2010, 19:10
  #57 (permalink)  
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While you very learned and clever chaps crunch your LaPlaces and Fouriers, to bring this back to the OP's question - from a piloting point of view- compressibility etc has no relevance on stalling. We are no longer in the days of Canberras and B-47s and in 'uncharted territory' but the driver nowadays has guidelines of 'normal' pitch attitudes at different altitudes and does not need to know or care about shock separation, and modern EFIS has buffet margins clearly displayed anyway. As long as he/she can differentiate between high-speed and low-speed buffet and carry out the appropriate recovery action, that is enough.
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Old 23rd Jul 2010, 20:38
  #58 (permalink)  
 
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Originally Posted by BOAC
As long as he/she can differentiate between high-speed and low-speed buffet and carry out the appropriate recovery action, that is enough.
BOAC;
Sorry if this getting a bit off track, but I've been wondering for some time, in the context of loss of airspeed indication, would a pilot always be able to distinguish between high-speed and low speed buffet? Does it feel differently, and how would he 'learn' to differentiate?

EDIT:: I noticed in the BOEING paper that their stall warning schedule (fig.10) is designed to allow that distinction.

regards,
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Old 23rd Jul 2010, 22:37
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Originally Posted by BOAC
You say that the latest amendment says 'no power'? For the avoidance of possible confusion, is that for all stalls - including low-level/approach? I appreciate we are on a 'high-level stall' thread but you quote a low-level example (BOH), and do I assume this is an AB amendment?
This is for all stalls, low level included.
The procedure (as described by Airbus) is now to SMOOTHLY INCREASE THRUST AS NEEDED but only when out of stall.

Clearly, the following Note is in direct relation with Perpignan, and probably with Amsterdam (Boeing) as well :
In case of lack of pitch authority, reducing thrust may be necessary.
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Old 24th Jul 2010, 04:49
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Thanks CONFiture. What I should have said is 'no increase in thrust'. And a note at the bottom of the new Airbus procedure states that "If a risk of ground contact exists, once clearly out of the stall (no longer stall indications), establish smoothly a positive climb gradient" It is clearly meant for all levels. In the case of a stall warning at lift-off, there is another new memory items which is to select TOGA thrust and 15 degrees nose up.




BOAC asked ''Dan - out of interest, which flight controls do you suggest you should use at the point of stall to control wing drop?''

It depends on the type. In a straight wing type, you don't. The stalling exercise I learned many years ago and then later taught for seven years in the RAF training world had the pilot use rudder only to prevent further yaw. The wings were levelled with ailerons once the buffet had gone. So you don't try and control wing drop at the stall. You let it happen and sort it out after you have regained full control.

With swept wing aircraft, the recovery can include the use of ailerons. the only swept wing aircraft I regularly (or intentionally!) stalled was the Hawk, and IIRC, we used the ailerons to keep the winds level at the point of stall - although it's twenty five years ago since I last flew it. And the new Airbus memory item states:

- NOSE DOWN PITCH CONTROL..................APPLY
- BANK..................................................WINGS LEVEL



PBL. Thanks for the diagram. I had to stare at it for a long time before I could work out what it was trying to tell me. I thinks it's what I always understood - that lift decreases with compressability. I conceed that it may have a small effect, but in practical terms - it isn't that important. The main factor which affects stall speed as far as pilot handing is concerned is the load factor.



HazelNuts39 asked: "Sorry if this getting a bit off track, but I've been wondering for some time, in the context of loss of airspeed indication, would a pilot always be able to distinguish between high-speed and low speed buffet? Does it feel differently, and how would he 'learn' to differentiate?"

High speed buffet tends to be at a higher frequency. But the big clue should be the part of the flight envelope at which it occurs - for which the power setting, attitude and slipstream would all give major clues to even the least aware pilot as to which buffet he is in. However, there are some aircraft powerful enough to push themselves into "Coffin Corner" - the point at which the low speed buffet and high speed buffet meet. The U2 is in this category. And I used to fly the Victor, which at lighter weights could fly into this area.
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