High speed buffet at high altitude level flight lead to a stall?

[Centaurus]

An AVFACTS article date 2000 Lesson 5, Buffet Boundaries, VMO/MMO Limits, states:

Quote: "High speed jet aircraft may suffer both high and low speed stall buffet. The high speed buffet is caused by flow
separation from the wings as occurs behind a shockwave at high altitudes and/or Mach numbers. The low speed
buffet is caused by the same airflow separation as the aircraft approaches the stall angle of attack."

I am at loss to understand the quoted term "high speed stall buffet .

I always thought the high speed buffet such as that which occurs close to Vmo at high altitude was caused by shock wave action. I had no idea this buffet preceded a stall in level flight. . Obviously operation at the barber pole at high altitude will be characterised by increasing buffet but certainly the aircraft is not going to stall? Is it?

"Coffin Corner" I think means the joining of low speed buffet and high speed buffet either possibly causing grief through a low IAS stall at high altitude and on the other hand a potentially serious handling problem beyond Vne - but not a high angle of attack stall.

Having never heard the term used of high speed stall buffet (as against a G force stall buffet in a steep turn whether at low or high altitude), then maybe some could explain to me the meaning of the term as displayed in the opening line from the AVFAST article

[G0ULI]

At high speeds the centre of lift on the wings moves progressively rearwards, so greater control forces are needed to keep the aircraft flying straight and level. If the centre of lift moves sufficiently rearwards and exceeds the ability of the control surfaces to keep the nose up, then the aircraft will suffer a high speed buffet and enter a dive. The use of delta shaped and swept wings helps to keep the centre of lift within the trim capabilities of the aircraft, but it is possible for straight winged aircraft to fly at supersonic speeds if they have an all moveable surface tailplane.

[Owain Glyndwr]

Centaurus

Quote: "High speed jet aircraft may suffer both high and low speed (stall) buffet. The high speed buffet is caused by flow
separation from the wings as occurs behind a shockwave at high altitudes and/or Mach numbers. The low speed
buffet is caused by the same airflow separation as the aircraft approaches the stall angle of attack."Seems to me that the first sentence is not very well constructed. If you omit the word (stall) the rest makes good sense, and the second sentence refers specifically to high speed buffet with no mention of stall.

That said, the wording of the third sentence makes it clear that buffet is always associated with airflow separation over the rear of the airfoil, which in turn is caused by strong adverse pressure gradients over the upper surface. At high speed these adverse gradients come from the passage of the air through a shock wave; at low speed and high AOA they are associated with recovery from the very high suctions over the front of the airfoil that go with high lift. If the loss of lift as a result of these separations is widespread and sudden then one may get the classic stall pattern.

However, there is a complication in that many modern wing designs don't have classic 'stall' characteristics. According to FAR25 stall can be defined either by classical loss of control or by buffet sufficiently severe to make it nearly impossible to fly the aircraft properly. For the aircraft I am familiar with the separations spread gradually and 'stall' at high speed is defined by the onset of severe buffet. Even with flaps down this may also be true. [Formally, the buffet limit would be +/- 1g at the pilot]

In this sense then one might have something called high speed stall buffet, but it isn't the same sort of stall as, for example, a sudden wing drop.

Not sure if this is enough of an answer, but I am trying to avoid complications :8

[John Farley]

Centaurus

I had no idea this buffet preceded a stall in level fligh

Like you I don't see how it could. However if you are cruising fairly fast (inside your normal flight envelope) and decide to pull hard into a turn chances are you will experience buffet and a high speed stall. With any aircraft.

[PPRuNeUser0190]

Hi centaurus,

Low speed stall: loss of lift due to high angle of attack
High speed stall: loss of lift due to shockwaves, sometimes also called shock stall

As altitude increases low speed stall & high speed stall become closer to each other, till the point where they are the same speed = coffin corner.

Stalling in relatively high speed due to turning or pulling up (g-force) is still a "low speed stall", because the angle of attack is the cause.

See also: SKYbrary - Aerodynamic Stall Awareness and Avoidance (http://www.skybrary.aero/index.php/Aerodynamic_Stall_Awareness_and_Avoidance)

[John Farley]

rvblyky7

I would make the following comment:

All stalls are caused by angle of attack - I think we can agree that!

The difference between the high speed stall and the traditional low speed one is caused by the mach number. At high mach numbers the angle of attack at which the wing stalls is much reduced.

With a commercial airliner being operated inside its certificated speed envelope the only way to get to the high speed stall angle of attack (even though it is reduced by the mach number) is to manoeuvre.

[gaunty]

Which is why we have 1.3G Buffet boundary reference charts. :ok:

[Brian Abraham]

Post by Mad (Flt) ScientistBOAC has already described one form of high speed stall.

Another meaning can be a stall where the flow behaviour has beome (significantly) Mach dependent. The classically simple stall occurs, as has been said, where the angle of attack exceeds a critical value, causing widespread flow separation and lift loss. It can often be assumed that for relatively low speeds the critical value is constant for a given configuration, and so the stall speed can be expressed as a single value of EAS, valid for all altitudes.

However, that's not entirely valid; as the mach number is increased the wing will likely stall at lower angles of attack, such that at some point the stall speed, expressed in EAS, will start to increase. I've heard this called "shock stall", a "Mach stall" or a "high speed stall" - all terms to differentiate it from the more common low speed version.

The flow characteristics are similar at a gross level - separation of the boundary layer and loss of lift - but the mechanism may be different - the boundary layer separation is likely to be triggered by shocks, not by the normal mechanism of an adverse pressure gradient.http://www.pprune.org/flight-testing/373416-high-speed-stall.html

[gums]

Both Farley and rvblyky7 are onto the explanation, IMHO

The swept wing jets have much different stall characteristics than the straight wing jets.

Take a straight wing up close to the "critical mach" and note aileron reversal, some buffet usually from the ailerons, loss of pitch control, etc. Then pull on the control stick/wheel. Whammo! May not resemble a "conventional" stall you get when low and slow, but an outright "departure". Think rapid roll, yaw excursions, pitch excursions, etc. i.e. a departure.

The swept wing planes have a more "gentle" entry into the departure unless you really command a large pitch/roll change when close to the critical mach. Some simply settle into a sustained aerodynamic stall such as we saw with AF447, with minor roll oscillations and such. That one was more of the basic aero stall, but there were some mach considerations at play early in the game until the jet had zoomed at a decent angle and lost EAS AND mach.

Good discussion.

[Owain Glyndwr]

@Brian

The flow characteristics are similar at a gross level - separation of the boundary layer and loss of lift - but the mechanism may be different - the boundary layer separation is likely to be triggered by shocks, not by the normal mechanism of an adverse pressure gradient.

The point I was trying to make, not very effectively it seems, in my earlier post is that a shock wave is accompanied by a very sharp increase in static pressure ahead/behind it. It is simply an extreme example of the 'normal mechanism' of an adverse pressure gradient.

[PPRuNeUser0190]

All stalls are caused by angle of attack - I think we can agree that!

Well :) The way it was taught to me I have to disagree.

For the "low speed stall" I agree. An aircraft stall at the same EAS 1G (or same angle of attack) at most altitudes, except for the high altitudes where the EAS becomes under the influence of mach effect and the stall speed becomes a bit higher (angle of attack a bit lower).

High speed stall (the way I was taught) is something else. First you get high speed buffet due to shock waves occuring. If you further increase the speed the shock waves become more significant up to the point where there is a significant loss of lift. This loss of lift was explained to us as "high speed stall" or "shock stall" as the effect is similar to the loss of lift due to high angle of attack.

Of course, I am no aerodynamics expert, just the way it was taught to us.

[Brian Abraham]

Noted Owain :ok:

Looks like its been with us for a long time Centaurus.

From History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference by Peter O. K. Krehl

The term shock stall originated in 1936 during high speed aerodynamic wing studies carried out in England at the National Physical Laboratory (NPL). In order to describe the compressibility stalling speed, Earnest F. Relf, a staff scientist at the Aerodynamic Department of NRL, chose the term shock stall as a shorter and more explicit expression than “compressibility stall” or “compressibility burbling speed”. The British aerodynamicist William Frank Hilton, creator of the term sound barrier, wrote in his textbook High Speed Aerodynamics: “The formation of a ‘shock wave’ on a wing will normally cause a separation of flow, and may be called a ‘stall’.

From Flight Dynamics Principles: A Linear Systems Approach to Aircraft Stability and Control by Michael V. Cook

The shock stall is sometimes used to describe the abrupt aerodynamic changes experienced when an aeroplane accelerating through the transonic regime first reaches the critical Mach number. At the critical Mach number, shock waves begin to form at various places on the airframe and are accompanied by abrupt reduction in local lift, abrupt increase in local drag, and some associated change in pitching moment. Since the effect of these aerodynamic changes is not unlike that of the classical low speed stall, it is referred to as the shock stall. However, unlike the classical low speed stall, the aeroplane continues to fly through the condition.

[JammedStab]

Centaurus

Seems to me that the first sentence is not very well constructed. If you omit the word (stall) the rest makes good sense, and the second sentence refers specifically to high speed buffet with no mention of stall.



I think you are correct. It would be better constructed if it said " As the stalling angle of attack is approached, jet aircraft may experience buffeting whether at low or high altitudes. The low altitude stall buffet is caused by airflow separation as the aircraft approaches the stall angle of attack. The buffet at high altitudes, which is typically at a relatively high mach number, is caused by flow separation from the wings as occurs behind a shockwave. However, in both cases stall is based on angle of attack. That being said, at high mach numbers, for a given wing, the stalling angle of attack is lower."

Confirmation by John Farley would be nice.

Seems to explain why it is so much easier to get stickshaker in the sim at high altitude for an RA maneuver(or why during a stall recovery at high altitude, it is so easy to get a secondary stickshaker activation).

[JammedStab]

Noted Owain :ok:

Looks like its been with us for a long time Centaurus.

From History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference by Peter O. K. Krehl

The term shock stall originated in 1936 during high speed aerodynamic wing studies carried out in England at the National Physical Laboratory (NPL). In order to describe the compressibility stalling speed, Earnest F. Relf, a staff scientist at the Aerodynamic Department of NRL, chose the term shock stall as a shorter and more explicit expression than “compressibility stall” or “compressibility burbling speed”. The British aerodynamicist William Frank Hilton, creator of the term sound barrier, wrote in his textbook High Speed Aerodynamics: “The formation of a ‘shock wave’ on a wing will normally cause a separation of flow, and may be called a ‘stall’.

From Flight Dynamics Principles: A Linear Systems Approach to Aircraft Stability and Control by Michael V. Cook

The shock stall is sometimes used to describe the abrupt aerodynamic changes experienced when an aeroplane accelerating through the transonic regime first reaches the critical Mach number. At the critical Mach number, shock waves begin to form at various places on the airframe and are accompanied by abrupt reduction in local lift, abrupt increase in local drag, and some associated change in pitching moment. Since the effect of these aerodynamic changes is not unlike that of the classical low speed stall, it is referred to as the shock stall. However, unlike the classical low speed stall, the aeroplane continues to fly through the condition.

Hi,

Interesting information. But I wonder if this is a separate issue with the so-called Shock Stall not being a stall at all but just a buffeting that when experienced by the folks at the time it was discovered, reminded them of the actual stall buffet and therefore named(or misnamed) this phenomenon as a shock stall. The result of which could lead to misunderstanding that one is close to stalling when in fact they are not.

[Centaurus]

To add a bit to the above. Around 1950 when the single seat Vampire FB 30 was introduced into the RAAF, RR Nene engines replaced the standard RR Goblin engine. These were the Vampire Mk 30 aircraft I flew. Two engine air intakes for the Nene were installed on the fuselage either side and behind of the cockpit. Unfortunately, high speed dive tests were not conducted otherwise the dangerous situation that led to the deaths of several Vampire pilots would have been detected.

It was discovered that at Mach numbers exceeding about 0.73, shock waves began to form over the curved surfaces of the air intakes behind the cockpit and in turn these caused loss of elevator effectiveness and the aircraft would pitch into a steep dive. It was called compressibility. The first crash occurred when two Vampires were practicing formation loops around 15,000 ft where the technique was to select air brakes out at the top of the loop to minimise speed build up in the recovery dive. The leader for some reason omitted to select his airbrakes out and got into compressibility (lack of elevator effectiveness). Thus he went into a high speed dive which went beyond the vertical. With no ejection seat fitted to the Mk 30 Vampire, the pilot was unable to abandon his aircraft by parachute.

The second Vampire pilot retracted his own speed brakes in an attempt to keep up with his leader and he suffered the same fate. Both pilots reported over the radio they were in compressibility.

Later another Vampire crashed when the pilot was conducting a practice diving attack and went straight in. The De Havilland (Bankstown) test pilot "Black Jack" Walker was directed to test a similar Vampire and he deliberately rolled inverted at 40,000 ft and dived to see what was happening. He reported complete loss of elevator effectiveness and was vertical until he was able to recover by 1500 ft. Following his test flight, it was thought the problem was in the shock waves on the upper fuselage air intakes. The aircraft was modified to place the air intakes underneath the fuselage and the problem went away. Meanwhile ejection seats were added and the Vampire became the Mark 31. We used to delight in diving the Mk 31 and instead of tucking under on reaching critical Mach it would suddenly pitch up - which was indeed Good Thing!

So although the reaching of the critical Mach caused loss of elevator effectiveness in the Mk 30 it surely could not be termed a "high speed stall" So whether a Boeing 737 reaching the high speed "clacker" Mach Number is called a high speed stall when there is no loss of control as it is popularly known, I still have difficulty understanding the "stall" reference as against loss of control, as in the early Vampires that crashed un-stalled at very high speeds..

[PPRuNeUser0190]

Hi Centaurus,

Depends on what is defined as stall. If stall means "a sudden reduction in lift" then both are "stalls". If stall is reduction of lift due to high angle of attack than only the "low speed" stall is a stall.

The mach buffet causes loss of lift due to the shock waves.
The (low speed) stall is loss of lift due to high angle of attack.

I guess that's why some refer to the mach buffet case as shock stall.

In our lessons the difference between mach buffet and shock stall was only that in the case of shock stall there is airflow separation, in mach buffeting not necessarily.
See difference between second and last picture.
File:Transonic flow patterns.svg - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/File:Transonic_flow_patterns.svg)

The effects on the aircraft are different. In our text book the shock stall effects are:
- possible roll (if it occurs assymetrically) -
- mach tuck effect

[Brian Abraham]

Both the F/A-18E and the AV-8B (JF should be able to shed light) suffered from "wing drop", "wing rock", "abrupt asymmetric wing stall", or "heavy wing", in the transonic regime. The reason? Shock induced separation causing abrupt alternating wing stall.

F-18E Wing Rock - YouTube

[HazelNuts39]

The mach buffet causes loss of lift due to the shock waves.
The (low speed) stall is loss of lift due to high angle of attack.Agreed, but it is too simplistic to separate mach buffet and angle of attack, because the shock waves vary with angle of attack.

The following picture is from NACA Technical Note 1390 "Effect of compressibility on the distribution of pressures over a tapered wing of NACA 230-series airfoil section" (1947).

http://i.imgur.com/wQvEmlY.jpg?1

[flyer101flyer]

Isn't it just a case of lousy writing? What if the first sentence were changed to "High speed jet aircraft may experience buffet due to high Mach number, and also may experience pre-stall buffet due to high angle-of-attack. In 1-G flight, the latter is associated with low IAS. At progressively higher altitudes, the window of safe airspeeds bewtween these two phenomena gets progressively smaller."

[HazelNuts39]

Perhaps it reflects the way popular textbooks simplify a complex subject by looking at one aspect in one chapter, and at the other in another.
At progressively higher altitudes, the window of safe airspeeds between these two phenomena gets progressively smaller."Yes, and at a certain altitude (popularly known as "coffin corner") the window disappears altogether. The 'low IAS' is then about 0.8 Mach, the pre-stall buffet due to high angle of attack is then buffet due to high Mach number.

[Centaurus]

Slight thread drift here but I would like to know the answer to the following question. In the Boeing 737 simulator, we practice high altitude (37,000 ft) approach to stalls (clean) and recovery, and low altitude stall in the landing configuration at 1000 ft on a typical final approach.

The former is not a terrain clearance situation obviously, while the landing configuration stall requires recovery with minimum loss of height.

The buffet preceding the stick shaker during the clean high altitude is quite severe and unmistakeable from weather related turbulence. Taking the worse case, recovery action is taken at stick shaker.

To gain an acceptably safe speed before recovery to level flight the pilot needs to know what speed to look for. The clue may be found in the FCTM under the sub heading Holding airspeeds Not Available from the FMC and states "above FL 250 use VREF 40 + 100 knots to provide at least a 0.3 g margin to initial buffet (full maneuver capability).

Typically in the B737 Classic with a VEF 40 of 130 knots, it means losing height until the IAS reaches 230 knots. This includes high thrust and a nose attitude in the dive of around zero degrees. This results in an average loss of 3000 feet providing the pilot does not allow the nose to rise during the descent and thus delay reaching 230 knots or cause a secondary stall buffet because of over-controlling by a ham-fisted pilot.

However in the case of the landing configuration wings level stall (in the simulator that is) there is no pre-stall buffet - only a stick shaker.
Question: Why is there no pre-stall buffet in the landing configuration stall? Because I don't know.

[Brian Abraham]

I wonder if this is a separate issue with the so-called Shock Stall not being a stall at all but just a buffetingJammedStab, with a spare $1,350 you can purchase "Introduction to the Abrupt Wing Stall Program" which may make a believer of you.

Pprune doesn't seem to like the link, so search the title. It gives a fair introduction to the publication.

[HazelNuts39]

Question: Why is there no pre-stall buffet in the landing configuration stall?Probably because the stickshaker has been set to operate before natural stall warning occurs.

But it is also possible that the stall in landing configuration is not preceded by buffet. The low altitude pre-stall buffet occurs when the horizontal tail enters the turbulent wake from regions of separated flow on the wing. On some airplanes in some configurations that does not occur.

High altitude buffet is an entirely different mechanism. It results from a cyclic variation of lift when a shockwave causes flow separation which then changes the position of the shockwave. It can occur at any angle of attack. The angle of attack at which it starts decreases with increasing Mach number.

With increasing altitude (i.e. increasing Mach number for the low-altitude stall-IAS) the low altitude pre-stall buffet gradually changes into Mach-related high altitude buffet. That transition starts when the critical Mach number is exceeded, i.e. when the local airflow velocity exceeds the speed of sound. For the model shown in the graph above at an AoA of 21° that is at a free-stream Mach number of around 0.4.

[rudderrudderrat]

Hi HN39,
Probably because the stickshaker has been set to operate before natural stall warning occurs.
Correct.
At low Mach Nos, the stick shaker will operate before the natural buffet. (Clean or Landing Config and low ALT).
At high Alt (hence higher Mach No) the natural buffet normally occurs before the stall warning.

Hi Centaurus,
or cause a secondary stall buffet because of over-controlling by a ham-fisted pilot. It is very difficult to judge how much "g" we are pulling in the simulator because we only feel 1g in the sim at all times. In real life I think you'd find your "ham fisted" pilots would be more gentle because of the feed back loop through their "g" sensitive bodies.

[Centaurus]

A bit superfluous I know, but thank you for all your replies. I now understand more.:ok:

[roulishollandais]

with a spare $1,350 you can purchase "Introduction to the Abrupt Wing Stall Program" which may make a believer of you
Thanks for these references. Are they other documents since 2003 ?

Thank you to Hazelnuts39 picture.

We know that low and high "stalls" are experienced by pilots as sudden modification of lift and pitch.

To simplify, could we remember that only pressure and pressure positions build the forces and the moments modifying lift and pitch.
These forces and moments are resulting from these pressure variations.
The low stall from vortices, the high stall from the pressure cliff Inside the wave shock. The wave shock is not a straight line as shematized in airline books, but a larger cliff.

Both vortices and wave shock pressures modifications are resulting from fractal structure of lift (Liebchaber) in cavitations and some resonances around speed of sound. Both allow 'continuity' in the équations of air density around the whole aircraft until steady air distance.

Critical reading is welcome

[John Farley]

Agreed, but it is too simplistic to separate mach buffet and angle of attack, because the shock waves vary with angle of attack.

Exactly.

Whatever stall you are talking about, reducing the angle of attack will reduce the severity of whatever symptoms one is talking about.

[Aluminium shuffler]

Mach stall is, as stated earlier, caused by the shock wave on the top of the wing causing the airflow to become non-laminar, much like a conventional low speed, high AOA stall. Where the confusion about Mach stall and AOA is creeping in I think is in that AOA will have a very swift and significant effect on the stall - the camber of the wing is accelerating the airlfow over the top surface - we all know that from the first principles lessons. That acceleration brings near-transonic airflow up to transonic, creating the shock wave. At that point, even a small increase in AOA will have a huge effect in further increasing that top flow acceleration and increase the localised Mach number, moving and exacerbating the shock wave and exponentially deepening the mach stall. Reduce the angle of attack ad the effect will be mitigated somewhat, though not eliminated until the Mach number is reduced to get rid of the shock wave entirely.

[roulishollandais]

the camber of the wing is accelerating the airlfow over the top surface - we all know that from the first principles lessonsBut we know now it is wrong (discussed in "theory of lift).:=

Shock wave is a local overpressure of air, and AoA is necessary to build it.

[JammedStab]

It is well written out in the 727 FCTM. I can't copy and paste it but it says that high mach affects stalls favourably. Separation starts at a more inboard location and at lower AOA leading to earlier warning and more pronounced pitchdown. It then mentions about the buffet occurring before artificial stall warning and says "Thus, the improved buffet warning and increased stall margins due to mach number effects give greater stall protection during high altitude flight"


So there you go. At least on this type, you have a greater stall margin. Then again, perhaps a lower margin for having to explain to somebody why you got buffeting in the first place.

One question. Once while cruising in a 737-200 we got above out target mach to maybe .76 but for sure it was below the barber pole. But all of a sudden we started getting some airframe vibration until the speed was reduced. Is this normal for this to happen when clearly below the barber pole?

[porch monkey]

I suspect you were way beyond the barber pole if you were at .86 in a 737-200. You meant 727, yes?

[JammedStab]

Sorry, I meant around mach .76 which was below the barber pole.

[Machinbird]

Once while cruising in a 737-200 we got above out target mach to maybe .76 but for sure it was below the barber pole. But all of a sudden we started getting some airframe vibration until the speed was reduced. Is this normal for this to happen when clearly below the barber pole? Couple of possibilities come to mind.
1. Limit cycle oscillation of a control surface being excited by the higher than customary speed.
2. Unstable shock waves as you apparently are suspecting.

For those who have not had the pleasure of going supersonic, the transition to supersonic in the bird I used to fly was easily perceived by a slight change in pitch attitude nose down and a sudden smoothing out of the aircraft (before it had been like running down a bit of a rough road at high speed). That aircraft did not exhibit noticeable wing drop in the transonic range.

The transition to subsonic from supersonic had an important characteristic. Whatever g we were pulling suddenly increased by ~50% with virtually no aerodynamic warning. Although this created overstress incidents, we did not experience stalls due to the relatively high Q. (This was in a non-FBW aircraft normally operated below FL450.)

[IcePack]

The A330 "hums/buzzes" if you get her much above Mach .83
So somewhere around the airframe the airflow is going sonic,but buffet due to airflow separation will occur at higher speed.
As we know some older jet aircraft suffered from jet upsets whilst knowledge of high altitude flight was being gained. So yes aircraft can high speed stall at high altitude & speed. Don't they teach that now in the ATPL?

[roulishollandais]

To math addicts
http://greenfluids.syr.edu/docs/GreRowSmi_2007.pdf
http://arxiv.org/pdf/1202.2989v1.pd
The 2000 Nora And Edward Ryerson Lecture | Office of the President | The University of Chicago (http://president.uchicago.edu/page/2000-nora-and-edward-ryerson-lecture)