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# Mach Number on Stall-Speed

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# Mach Number on Stall-Speed

6th Oct 2023, 14:25

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Mach Number on Stall-Speed

I'm wondering if there's any generalized rules of thumb regarding the effect of mach number on stall-speed? Instinctively, I would assume supersonic aircraft would have the least effect, and subsonic aircraft the most.

With that said, what variations are there for an aircraft like an F-86 which was supersonic in a dive at low and high altitude? I was told the F-4 had a variation of around 15 knots from subsonic speed at low altitude and high subsonic-speed at around 35000 feet if I recall.
6th Oct 2023, 22:40

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Salute!

It ain't speed, it's angle of attack subsonic and complicated shock wave patterns above the mach.

Subject is beyond the scope here unless moved to our aero study hall.

Gums sends...
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7th Oct 2023, 00:45

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Or perhaps a wider question about ‘coffin corner’ when the Mach limit becomes the same as the stall limit as experienced in the Canberra, U2/TR1 when in a turn one wing can be at one limit when the other hits the ‘other’ limit but that is as far as my aerodynamics knowledge goes lol
7th Oct 2023, 08:17

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Mach number represents a flow condition.
Stall speed represents a dynamic pressure condition + geometry effect.

The flow condition is based on the compressibility which in turn is based on the ability for gas molecules to get out of the way which is based on how fast they are going which is based, in part, on the temperature of the gas. The compressibility is notable when going more than 80% of the local speed of sound. The lower the temp, the lower that speed is.

Dynamic pressure is based on the square of the speed and the density, usually by how deep one is in the ocean of atmosphere, but also from humidity and temperature.

To give a feel for this, at the pressure and temperature on the surface of Mars one might hardly notice a breeze blowing by at Mach 1. The actual speed is low so Mach 1 is low. But the density is so low that even at Mach 1, the dynamic pressure is low. It might not push an empty paper cup from a picnic table, were NASA able to put both there, hence the problem of getting fine dust off the solar panels.

The main intersection is that dynamic pressure is from the square of the speed times the density. When a shock wave forms the speed and density change. Per conservation of mass, density times velocity will be a constant. But notice the dynamic pressure is related to the square - if I am getting this right, the dynamic pressure drops as it passes through a shock wave. This means that whatever balances were in place before reaching Mach 1 will be out of balance when the shockwave forms.

This is not a stall problem but part of the function of a wing is to produce downwash; if the shockwave change of the dynamic pressure reduces that effect the horizontal stabilizer sees less downwash, produces less down force, and the nose tucks under, increasing descent rate, increasing speed, increasing the strength of the shockwave and increasing the chance of being aluminum parade confetti if the condition isn't dealt with.

It appears to me the coffin corner is the intersection between having too little airspeed to produce enough dynamic pressure and getting too much airspeed and having a drastic change in controllability. Look at the NASA Space Shuttle that re-enters at around Mach 40 (flow condition) and nearly no dynamic pressure, it can be a surmountable problem - as long as the rest of the maneuverability envelope becomes that of a bread loaf.

Last edited by MechEngr; 7th Oct 2023 at 22:21.
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7th Oct 2023, 08:58

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Mac Number on stall-speed

No just for Military bods. Interesting topic for us Civiest Can I pick your brains Mech, too ?

High level, civil transport, asked to hold at high level where hold speeds in the QRH are given both in Mach and IAS. Fellow Captain complied but went very quickly fo manual speed select and wound to speed (rather than Mach) and selected about 210kts rather than the equivelant mach. I , of course gave birth and was reluctant to over-ride a Senior Company (new on type) Commander where I was just a mucky contract guy..

An attempt to advise that Mach gave a flow picture at high altitude and became more significant than IAS fell on deaf ears as round and round we went at high alt at selected 210kts.

Now I was really confused.

Later, bigger transport type, I was offered Max altitude for type in cruise or ,due traffic, descent to much lower alt. Checked the books,graphs and within limits for weight, up we went to 43000ft. Sat there with FMS engaged and although nicely settled on Mach, IAS was terrifyingly low. Couple of minor turbulent bumps left me in no doubt; down we went to the lower level, insufficient fuel at that level to continue and diverted for gas.

I get the theory but practice left me bemused. Tea & bics with CP over the diversion led to to shared confusion over Mac/IAS and he just hunched his shoulders. "Never stop learning-eh?" was his parting shot and I drove home not knowing whether to sit at 70kph or 70mph !
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7th Oct 2023, 09:21

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MechEngr.
It's been a long time since I studied this, I'm sure I was taught that Mach tuck was caused by the CofP moving rearward as the shock wave forms on the wing.
Happy to be corrected though. 👍
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7th Oct 2023, 11:21

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The way I think of it is Mach No is the speed of sound,. In simple terms, below Mach 1, the air molecules can start to get out of the way before the ac arrives, above it they can't.
The speed of sound is a function of air density, the greater the density, the faster the speed of sound and vice versa, As you go up, the density decreases to the speed of sound decreases to so Mach 1 occurs at a lower airspeed so the Mach effects occur at a lower airspeed. Do I have it aright?
7th Oct 2023, 11:46

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Originally Posted by Ninthace
The way I think of it is Mach No is the speed of sound,. In simple terms, below Mach 1, the air molecules can start to get out of the way before the ac arrives, above it they can't.
The speed of sound is a function of air density, the greater the density, the faster the speed of sound and vice versa, As you go up, the density decreases to the speed of sound decreases to so Mach 1 occurs at a lower airspeed so the Mach effects occur at a lower airspeed. Do I have it aright?
Not really.
In fact, assuming an ideal gas, the speed of sound c depends on temperature and composition only, not on the pressure or density (since these change in lockstep for a given temperature and cancel out). Air is almost an ideal gas.
- Wikipedia, Speed of Sound.
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7th Oct 2023, 11:50

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Some further information:
For a given ideal gas the molecular composition is fixed, and thus the speed of sound depends only on its temperature. At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly.
- https://en.wikipedia.org/wiki/Speed_of_sound
​​​​​​​
7th Oct 2023, 12:46

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Victor SR

During the IRT in a Victor SR2 we were required to do Max Rate Turns at 50,000. Limiting Mach number was recognised by a light buffet, pre stall buffet similarly. The gap between those two at 50,000 ft was 18 kts. Max angle of bank achievable was 15
degrees. I understand that the gap between those speeds in a U2 at its max altitude was a mere 2 kts. Flying with finesse!

Originally Posted by GreenXCode
Or perhaps a wider question about ‘coffin corner’ when the Mach limit becomes the same as the stall limit as experienced in the Canberra, U2/TR1 when in a turn one wing can be at one limit when the other hits the ‘other’ limit but that is as far as my aerodynamics knowledge goes lol
7th Oct 2023, 15:57

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Originally Posted by gums
It ain't speed, it's angle of attack subsonic and complicated shock wave patterns above the mach.

Subject is beyond the scope here unless moved to our aero study hall.
We have an aero study hall?

That said, I think I follow what you said: Mach number determines how the airflow behaves; equivalent airspeed determines the amount of airflow going over the aircraft; AoA for a given airspeed and mach number correlates to a given amount of lift with the critical angle being the point where a stall occurs, and when 1g equals the critical mach number: A stall results.

Since critical AoA lowers as mach number increases, to hold 1g (or any g-load), one would have to fly at a higher speed to stay above the stall, correct?
7th Oct 2023, 16:10

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Look up the term equivalent airspeed—CAS corrected for compressibility. The holding Mach is there to protect you from stalling. KEAS is what to watch, but not displayed anywhere unless you use an E-6B.
7th Oct 2023, 22:06

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Salute!

Close, Zipper......
Plenty of aero refs on the 'net and in your local libberry.
Pprune has the Tech Log and that is where the serious stuff can be found.
PLZ stop thinking about the mach and stall correlation. Ditto for speed and gee.
You can stall a plane at close to it's max recommended or tested speed that rips the wings off or fries the motor... go see "accelerated stall", and all the basics about AoA.

Supersonic is different than subsonic and there are also wings that can handle subsonic mach numbers better than others. The phrase "sound barrier' came up was when some of our WW2 planes encountered the shockwaves at high mach....not even 0.7M in some cases, but due to the wing some air was at the mach ( I use that for 1.0, and it's an old habit). The pressure behind the shock wave changed the basic aero stability of the plane and we saw phenomena like roll reversal and aileron flutter and nose tuck and........ The T-37 trainer USAF used for a half a century had a problem above 0.6M or so, and I saw it.

So this forum may have lottsa war stories like mine and others, but you best visit the liberry on line or downtown and read a book.

Gums sends...

Last edited by gums; 8th Oct 2023 at 13:30. Reason: advice and Tech Log ref
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7th Oct 2023, 22:16

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Originally Posted by HOVIS
MechEngr.
It's been a long time since I studied this, I'm sure I was taught that Mach tuck was caused by the CofP moving rearward as the shock wave forms on the wing.
Happy to be corrected though. 👍
I would not dispute that description - plenty of effects happening and CoP is a composite description to make other calculations easier. It can be derived by measurements on the balance on a wind tunnel which is convenient.
8th Oct 2023, 08:43

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Mach NUmber on Stall-speed

OOOOh, delighted it's not just me but some worthy peers too.

Fave book of mine was" Flight without formulae" by, I think, AC Kermode. Aerodynamics lecturer at Oxford Air Training School wa ex development Guy from Concord. He could not explain anything without resort to algebra. Bloody nische bloke though.

New T-shirt ordered. On the front it says "I survived". On the back, it says "Coffin Corner".
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8th Oct 2023, 09:39

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Mach number etc

Back in the early 60s when we were still on a steep learning curve, there was a 'rule of thumb', as requested in the first post, to help the young jock deal with this anomoly.
Yep, even my groundschool notes were changed during a course, to provide new theory on the production of shockwaves around a 'chisel' leading edge ! - F104 ....
We were told just to remember that, at 40000 feet, TAS was double the IAS.
So, even manoeuvring at a high Mach number, the lower IAS would risk pitch up, double flame out and, in our case an instant doubling of G to maybe out of limits. No worries cruising at 90knots these days !

Last edited by Sleeve Wing; 8th Oct 2023 at 10:06.
9th Oct 2023, 02:20

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if the shockwave change of the dynamic pressure reduces that effect the horizontal stabilizer sees less downwash, produces less down force, and the nose tucks under, increasing descent rate, increasing speed, increasing the strength of the shockwave
According to "Aero for Naval Aviators" and NASA the normal CoP is generally at about 25% chord, supersonic it moves to generally about 50%, that being the case I think CoP movement would have far greater effect than the change in downwash on the tail, hence effects at the tail generally don't get mentiponed.
9th Oct 2023, 09:21

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Some useful information in this Airbus article, written by their then Chief Test Pilot. https://safetyfirst.airbus.com/high-...manual-flying/
9th Oct 2023, 16:13

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Originally Posted by megan
According to "Aero for Naval Aviators" and NASA the normal CoP is generally at about 25% chord, supersonic it moves to generally about 50%, that being the case I think CoP movement would have far greater effect than the change in downwash on the tail, hence effects at the tail generally don't get mentiponed.
If it only were that simple. For one, the effect of "downwash on the tail" acts on much longer lever, assuming that this is a conventional layout. For another, it presumes a certain wing shape and a particular profile (or lack of) washout. As the 2nd figure in the above Airbus doc shows, the movement can be exactly in the opposite direction.

This is a very rich topic if you are excited by this sort of stuff, and you just made me recall that I have on my shelf "Mach 1 and beyond" by Larry Reithmaier who tries to cover it all without any formulas. Just a lot of diagrams and pictures. Thanks for reminding me - it's due for a re-read.
10th Oct 2023, 04:26

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If it only were that simple
Horses for courses I think. Mach tuck became a big thing when the first Lears came out and folk were including secret overspeed warning cut out switches and hence learning about Mach tuck first hand, it lead to a number of accidents and incidents, being a T tail personally wouldn't expect it to be much influenced by down wash.In any event, because the contribution of down wash on the tail is so rarely mentioned I would assume its contribution is minimal.