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High Speed Stall

Old 10th May 2009, 10:49
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High Speed Stall

Can anyone be kind enough to explain the high speed stall to me please?

I imagine it's pretty much the same as what I know of as ' departure ', with the airflow going straight over the wing rather than behaving nicely and keeping in the laminar flow ?

How does one cope with it, stick out an airbrake or is there something better...

This is for academic interest only, rest assured I'm not about to fly anything fast, but do get to play / talk with an amateur simulator at a museum, so I try to be ahead of smart-ass cadets !
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Old 10th May 2009, 11:21
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Can anyone be kind enough to explain the high speed stall to me please?

It occurs whenever the angle of attack of a wing reaches the stalling AoA at a speed above the st and level unaccelerated stall speed. This ocurs when pulling 'g' as in a turn or other pitching manouvre.

I imagine it's pretty much the same as what I know of as ' departure ', with the airflow going straight over the wing rather than behaving nicely and keeping in the laminar flow ?

Well, my understanding of 'departure' is 'departure' from controlled flight, so I'm not sure of your idea.

How does one cope with it, stick out an airbrake or is there something better...

Very simply - relax the 'g' that caused it - a bit difficult when the windscreen is full of green and brown bits, however....
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Old 10th May 2009, 11:35
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BOAC, Thanks.

I can think of someone I knew who had ' his windscreen full of green & brown bits ' and didn't make it; whether he was in a high speed stall is a moot point, the fact is he did the classic loop too low mistake - the aircraft impacted with a high sink rate, 27 degrees nose up.

He was a very good Test Pilot - he couldn't have let off the G, as speed & height were already critical, and I suspect pulling the handle and binning a one-off aircraft was unthinkable to him.

The sad thing is, another prototype was made fairly quickly; not the pilot though.

As for departure, I was thinking of separation of the airflow from the wing, so I imagine we're sort of talking the same language - full departure being mayhem.
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Old 10th May 2009, 20:05
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BOAC 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.

The recovery requires, as always with a stall, reduction of the angle of attack in order to get the flow back into the attached flow condition. But depending upon the exact conditions, other techniques which also help in recovery - such as adding power and building speed margin to prevent secondary stalls and regain additional control power for the low speed stall - may be inappropriate at higher speeds.
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Old 13th May 2009, 17:35
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Thanks Mad,

That funnily enough makes more sense to me !

DZ
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Old 14th May 2009, 07:53
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Is this all a version of the 'coffin curve' ?
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Old 14th May 2009, 10:18
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I suspect the coffin curve relates to power/drag. At normal cruising speeds the drag and the power are in equilibrium and constant speed is achieved. If you reduce power, the drag slows the aircraft, but a slower aircraft has less drag and you achieve a slower cruise (and a slightly increased angle of attack). Eventually, as you reduce power and you try to keep the aircraft flying, the drag actually starts to increase as the airspeed decreases. At this point you've got a high Angle of Attack (and hence are about to stall) and you're in trouble unless you're a few inches above a runway or a few hundred feet in the air - and hence the coffin curve. (There are other threads describing how to land, and the concept of stalling the aircraft onto the ground with which you may or may not concur.) At "normal" flight speeds the stall occurs at a fixed AoA irrespective of speed.
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Old 14th May 2009, 13:09
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Is this all a version of the 'coffin curve' ?
Do you mean "coffin corner"? That is the term used for the high-altitude situation, achievable by some jets, where you are at the low-speed (i.e. 1.0g) stall and the limiting mach (and the associated buffet and shock-induced stall) at the same time. The term was used by, I think, B-47 crews who got into that situation and found it very difficult to escape it.
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Old 15th May 2009, 01:14
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And, as an additional curiosity (minor for fixed wing but perhaps not so for rotary), there is a phenomenon described as high speed stall associated with VERY high pitch rates (scratchy memory suggests in excess of around 70 deg/sec). A stable vortex can form along the leading edge and angle of attack values significantly in excess of "normal" stalling angles are possible.

As I recall I first read of this in an RAeS tech report ?
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Old 15th May 2009, 09:28
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Originally Posted by john_tullamarine View Post
And, as an additional curiosity (minor for fixed wing but perhaps not so for rotary), there is a phenomenon described as high speed stall associated with VERY high pitch rates (scratchy memory suggests in excess of around 70 deg/sec). A stable vortex can form along the leading edge and angle of attack values significantly in excess of "normal" stalling angles are possible.

As I recall I first read of this in an RAeS tech report ?

The limited research done on tumble aerodynamics of tailless aircraft has shown something like this - the vortex tends to form above the leading edge which is pitching nose-down (or vice-versa), with a nominal diameter around 2/3 of the root mean chord. In the tumble it tends to travel towards the trailing edge sustaining the pitching motion.

This certainly occurrs at pitch rates in excess of 200/s (tumbling aircraft have been observed to show over 400/s), but intuitively lower pitch rates in the order of 60/s will almost certainly do the same - indeed this is probably part of the tumble initiation mechanism.

If you can handle a 5Mb pdf, this contains some pretty pictures of it about pages 6-10.

G
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Old 15th May 2009, 11:52
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kenparry -
xxx
The "coffin corner" name was coined in the days of the 707 and DC8...
Was for the smart "aces" who avoided FL370 CB tops by going to 390.
The 747 is a pussycat when it comes to low speed and high speed buffets.
xxx

Happy contrails
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Old 15th May 2009, 16:11
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Question coffin corner

There used to be a legend, in the days of the de Havilland Vampire fighter, that there was a "secret host" of lost Vampires in orbit at the height at which "high" (for those days) Mach No. loss of control and the stalling speeds coincided, so they couldn't go up, and couldn't get down ...

But then there was also the legend that steep turns in a Tiger Moth were dangerous because the "W" arrow, lengthening under the acceleration, would get too long and puncture the tyres ...
At least that showed that us cadets had been following the instructor's drawings on the blackboard
PS 'Tis a light-hearted day when you can see the sun for once through the clouds ...

Edited by Jig Peter @ 1849 CET - Hairy Dai Nammicks (and, at 1851: spelling)

Last edited by Jig Peter; 15th May 2009 at 16:51.
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Old 15th May 2009, 16:45
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Drag arrow would get too long and puncture the tyres
- indeed, Jig, and of course the same reason why very low-level steep turns are dangerous if the arrow gets caught in trees or buildings.
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Old 15th May 2009, 16:54
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@ BOAC : them too - and HT cables and similar droopy thingies ...
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Old 15th May 2009, 18:21
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Canberras demonstrate the classic 'shock stall'. Flying straight and level, once the Mach No gets near M 0.84'ish things start to rumble, take it further and suddenly lift quits. Recovery is to throttle back, air brakes out and wait 'till the beast decelerates.

A lot of height can be lost!

lm
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Old 15th May 2009, 19:24
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You can do a high speed stall a glider. A mandatory part of learning to fly gliders is spin recovery. Exiting from the spin you can end up pointing nose down and with a fair turn of speed. There is a temptation to pull too hard to try and recover the situation and that can provoke stall buffeting. The solution is just to ease off the back stick a bit
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Old 19th May 2009, 21:32
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Lost Vampires

Jig Peter,

That sounds a bit like the book ' The Sheppard ' where a ghostly Vampire helps out a pilot in trouble ?

After my and other's experiences I can't rule anything out, then again I wouldn't count on it ( help ) at the exact moment necessary either.
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Old 20th May 2009, 13:19
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Without (I hope) being seen as pedantic the term 'high speed stall' is best used in conjunction with straight and level flight at high speed where mach effects have caused loss of lift/trim changes/buffet etc (as has been described above).

A stall at a speed above the normal 1g stall speed caused by manoeuvring is best described as an 'accelerated' stall. This can occur in any aircraft even if the aircraft cannot fly fast enough to experience mach effects.
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Old 20th May 2009, 16:09
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Absolutely correct, John, as usual, but my excuse is I guessed at what I thought '00' wanted to know and tried 'KISS'..................looks as if I picked the wrong answer
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Old 20th May 2009, 20:44
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BOAC,

Thanks, Keep It Simple Stupid generally works for me - and some ex-non pilot colleagues who have yet to realise it ! - You'll know them when you see them.

Cheers,

DZ
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