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.

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...

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

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.

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 !

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. 👍

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.
- Wikipedia, Speed of Sound.

Some further information: - https://en.wikipedia.org/wiki/Speed_of_sound
​​​​​​​

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!

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?

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.

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...

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.

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".

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 !

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.

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

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.

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.

the simplified answer has mainly been... CP shift but that is only partially true and does not move in the right direction to what mach tuck is classically described as.

At MCD for the airfoil, flow over the wing achieves above mach 1 and as the flow at the trailing edge of the wing is usually the second Kutta condition, a stagnant point, then the flow must decelerate, depending on the velocity profile that the profile of the section results in. Shortly thereafter, or sometime quite a lot later, MDD occurs where there is a drag divergence condition that is significant in drag. Looking at the pitching moment, Cm, there is not a great change in moment, and that is curious, we are taught otherwise... but... hang on... what happens when you continue to accelerate and push the airfoil along faster? at a point at higher speeds, due to a relatively low AOA to maintain level flight conditions (or 1g... whatever floats yer boat), shocks start to develop on the underside of the wing, (surprise!!!) and that results in a collapse in CL, an increase in Cd, and, yet not much Cm change. However from the little boys room at the pointy end, stuff happens, as the loss of CL for the AOA changes the trim state, and the aircraft now has a flight path that will diverge from desired for the static trim case. take out lift and the flight path will degrade quickly. what happened to the Cm, not much, what about the stab inflow change? not much change, but the nose will drop quite happily. The concern in a severe overspeed is that the condition is divergent, lower pitch attitude results in greater shock involvement, and loss of CL component.

Whenever I have flown Mdive I do not enter with anything other than a high power setting, to allow an opportunity to decelerate by thrust reduction. In this case the Cm has not caused the nose gown pitch, the rapid loss of CL component has caused the steepening pitch attitude. Speed brakes mostly will cause a pitch up, and will alleviate the lower shock condition, except on a Learjet, where that will open up your eyes promptly. In the end, it is just a wing and a moment arm from the AC, if you have a variable incidence tailplane. High mach overspeed with a fixed horizontal stab/elevator are quite a lot more sporty.

Mach Number on Stall- Speed
 

FDR: " Little boys in the pointy end"-? Ouch. Spoiled an otherwise valuable post.

I am unaware of any heavy-jet transports falling out of the sky from coffin corner because the little boys in the pointy end didn't know what to do.

Most of us are bright enough, educated enough, trained enough to face any uncomfortable situation with a simple-"I'm outa here !".

I also, as a real little boy, enjoying a black & white movie about tests flying or space-re-entrystuff where the pilot, entering a dive, speed increasing, instead of ejecting as ordered, pushed the nose forward. "No,no" me & my mates yelled but were silenced as the speed dropped and Hollywood ace saved the aircraft and the day.

Of the six of us, three wound up Senior Commanders of very heavy transport jets.

Firstly, I apologize for the enormous amount of time that's elapsed between my replies (I had a bunch of stuff going on).

Can I contact the admin/moderators and ask them to place this into the tech-log?

BTW: I put it in the military aviation forum because I figured this area of flight (high subsonic and above) would be an area that the military aviation folks were most familiar with.

Moved from Mil Forum

Senior Pilot

Senior Pilot Thank you

To give a simple answer and in the subsonic region, the wing stall at a given AoA which equates at 1G to a specific equivalent airspeed (EAS). At low Mach numbers, that is low altitude, IAS will be equal to the EAS. As we climb, that EAS remains, but compressibility requires a higher INDICATED airspeed to Produce that EQUIVALENT airspeed over the wing. As the pilot, we only see IAS, in knots, but because we need more IAS to have the same EAS at stall, it appears the stall has increased. Other than an E6B, I don’t have a rule of thumb other than….KEEP THE MACH UP.

My E6B says if your wing stalls at 145KEAS at F410, you’ll see 150 KIAS at stall.

Some dated light bedtime reading ... Effect of Mach and Reynolds Numbers on Maximum Lift Coefficient - NASA Technical Reports Server (NTRS)

The DH Vampire was of interesting in terms of pitch characteristics at high mach numbers, not really mach tuck as understood today. The RAAF early single seat aircraft had auxiliary engine air intakes (elephant ears) mounted on the top of the fuselage. A number of these aircraft crashed from high speed dives after pitching down at high mach. The fix was to change the position of the auxiliary intakes to the lower fuselage. While I don't know the aerodynamics involved, it would appear that the wake of the upper aux intakes would impinge on the tailplane at high mach.
The two seat Vampire Mk35 had a marked pitch up at compressibility buffet onset. The onset of compressibility was quite sudden, the pitch up was attributed to sudden shock stall of the wing markedly reducing the nose down lift/weight couple, with no change in the tailplane down load.
I acknowledge that the stability and control characteristics of these ancient aircraft should not be compared with even the first generation of swept wing aircraft, although these aircraft had their own handling problems.

That makes sense: It looks like it would have caused the local mach-number over those intakes to go up, which then produced stronger shockwaves, and the resulting turbulent air, which interfered with the airflow over the tail.

Compressibility error (CAS - EAS difference) has an effect, but more significant is the effect of mach number on the CL - alpha curve.
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40

Calibrated airspeed (CAS) exists because it is best way you can calibrate a mechanical airspeed indicator. With modern electronic flight displays CAS is no longer necessary. It would be logical (and result in less variation in staling speed) to calibrate a PFD speed tape to read KEAS. This would be a simple matter of re writing some code in the air data computer. The reason most modern PFDs are calibrated to read KCAS is because of historical precedent and desire for commonality with mechanical airspeed indicators.

AFIK, the Eclipse business jet has its PFD speed tape calibrated to read KEAS.

Thank you, I'll watch this!

very good, thanks

Coffin corner:Best way, stay away from that sorf of high altitude.

Most of the SR-71 airspeed limits are expressed in KEAS.

Minimum airspeed 310KEAS when supersonic, 300KEAS when subsonic above FL250, 145KIAS below FL250 unless angle of attack limit would be exceeded
Design Mach 3.2, Maximum scheduled cruise Mach 3.17, Mach 3.3 may be flown if approved by Commander if compressor inlet temperature 427°C is not exceeded
Maximum airspeed for normal operation 450KEAS, design limit speed 500KEAS