Stall speed changes with altitude.

[FlyBieWire]

I have read many places that the IAS for a stall is the same at SL as it is at at say 10,000 feet. That would be for say a Cessna 172. I can understand that because while the air going over the wing is thiner at altitude, the air going into the pitot tube is proportionally thinner at those same altitudes. Thence, same IAS stall no matter the altitude.

If this is all true, as I have read it to be the case, why is it that jets flying at high altitude have and increased indicated stall speed at those high altitudes? Don't the same principles of pressure apply to jets too. This is a mystery to me. Can someone please explain this.

 

[MauleSkinner]

If this is all true, as I have read it to be the case, why is it that jets flying at high altitude have and increased indicated stall speed at those high altitudes? Don't the same principles of pressure apply to jets too. This is a mystery to me. Can someone please explain this.

The same principles apply to jets up high that apply to a 172 down low.

Without the reference that is the basis for this question I can only guess, but I'm assuming that you're referring to the "coffin corner" that jets see at high altitude, the high speed end of which is mach buffet, and possibly tuck.

The low speed end is often referred to as a "stall", but it's really a low-speed mach buffet...As the angle of attack increases, the acceleration of airflow over the top of the wing increases to the point where, due to the high angle of attack, airflow accelerates to the speed of sound. The result is a shock wave that can interfere with airflow over the tail, and a tremendous drag increase that may require the airplane to descend for an increase in speed, as it doesn't have enough excess thrust to overcome it.

Hope that answers your question.

Fly safe!

David

 

[VNugget]

The low speed end is often referred to as a "stall", but it's really a low-speed mach buffet...As the angle of attack increases, the acceleration of airflow over the top of the wing increases to the point where, due to the high angle of attack, airflow accelerates to the speed of sound. The result is a shock wave that can interfere with airflow over the tail, and a tremendous drag increase that may require the airplane to descend for an increase in speed, as it doesn't have enough excess thrust to overcome it.

Hm, then what limits the high end? I thought that the high end limit was the shock stall, and the low end limit was the plain old AOA stall we all know. (Honest question, no smartassery here)

 

[Flyerjosh]

The high end is limited by several factors, with each aircraft having different limiting issues. Here are a few from the list that I can think about that limits the upper end:

Mach Tuck/Center of lift change
Control surface flutter
Sonic disturbance over control surfaces
Engineering stress load limits

 

[Stby One]

There are basically two effects. Both due to compressibility at higher mach numbers.
The first is as has allready been explained the separation of airflow due to the shockwave being created over the wings at transonic speeds.

The second is simply that the IAS and CAS will overread due to the compressibility effect of the airspeed indicator.

Add the two up and your IAS and CAS stall speed will increase with altitude (EAS too but then only due to the separation over the wings).

 

[UndauntedFlyer]

The same principles apply to jets up high that apply to a 172 down low.

Without the reference that is the basis for this question I can only guess, but I'm assuming that you're referring to the "coffin corner" that jets see at high altitude, the high speed end of which is mach buffet, and possibly tuck.

The low speed end is often referred to as a "stall", but it's really a low-speed mach buffet...As the angle of attack increases, the acceleration of airflow over the top of the wing increases to the point where, due to the high angle of attack, airflow accelerates to the speed of sound. The result is a shock wave that can interfere with airflow over the tail, and a tremendous drag increase that may require the airplane to descend for an increase in speed, as it doesn't have enough excess thrust to overcome it.

Hope that answers your question.

Fly safe!

David

This is a great explaination of high and low speed buffet (I think). It is simple and easy to understand. Nice job MauleSkinner.

 

[MauleSkinner]

This is a great explaination of high and low speed buffet (I think). It is simple and easy to understand. Nice job MauleSkinner.

Don't ask how I learned it

Fly safe! (definitely fly safer than when I learned it)

David

 

[Bjammin]

The low speed end is often referred to as a "stall", but it's really a low-speed mach buffet...As the angle of attack increases, the acceleration of airflow over the top of the wing increases to the point where, due to the high angle of attack, airflow accelerates to the speed of sound.

So you're saying that at high altitude a jet buffets at low speed due to the airflow going the speed of sound over the wing at the low speed stall airspeed?

The problem I have is that in flying the T-45 on test flights at high altitudes and at low airspeeds I find this not to be the case.

Airplanes stall at a specific angle of attack. As we all know INDICATED stall speed changes with weight and TRUE stall airspeed changes with altitude, pressure, humidity, etc. but, an aircraft will always stall at the same angle of attack.

The T-45 has an angle of attack indicator and at high altitude the buffet occurs at the same angle of attack as it does at low altitude.

If your explination were correct I would expect to have buffet at a lower angle of attack at high altitude then at low altitude.

I also notice that the min INDICATED airspeed bar on the 747-400 EFIS does not change with altitude and we fly as high as FL430. I would think it would if the above were correct.

I'm truly not trying to be argumentitive, I just have not seen anything in practice or in books to support this.

I can see this being the case at extreamly high altitudes where stall and mach buffet are only seperated by a few knots.

 

[bubbers44]

Mauleskinner said it just as it explains in "Flying the big jets". I had this same question 30 years ago and found the same answer there. I could not find an explanation anywhere else. Low speed buffet being caused by supersonic airflow over the wing because of the angle of attack at high altitude is not exactly the same as stall buffet. Without really studying it in depth it doesn't make sense why altitude would lower the IAS stall speed.

 

[bubbers44]

Sorry said it backwards, meant why higher altitude increases IAS stall speed.

 

[bubbers44]

Also the book said the low speed buffet caused by supersonic airflow over the wing reduced the efficiency of the wing meaning you would have to increase the AOA to compensate therefore reaching stall AOA at a higher IAS.

 

[Bjammin]

I can buy that, but again we have to be talking when there is only a few knots between stall and mach buffet. I'm sure this only happpens ABOVE the max altitude for the airplane. Except for the U-2.

 

[MauleSkinner]

I can buy that, but again we have to be talking when there is only a few knots between stall and mach buffet. I'm sure this only happpens ABOVE the max altitude for the airplane. Except for the U-2.

But we (or at least I) am NOT talking about "stall buffet"...There is a point where, due to increasing angle of attack, airflow over the wing goes supersonic, creates a shock wave (more accurately probably an expansion wave) that causes buffet...it's not a stall...depending up on the airplane, you may or may not be able to hold altitude, and you may or may not be able to increase power to fly out of it.

As an example, the Falcon 10 will do this at FL390 and Mach .69 (about 210KIAS) if you encounter turbulence or increase the g-load slightly above 1 g. The airplane will buffet, the buffet will continue after you reduce the load or leave the turbulence, and at least in the ones I flew, you will have to descend 3-400 feet to get past the drag divergence. Again, no stall, just low-speed mach buffet. Well below the 45,000-ft max altitude for the airplane.

Note also that this is the low-speed end of "coffin corner", and with an Mmo of .87, the upper limit is over 270KIAS, for about a 60-knot spread.

Fly safe!

David

 

[Bjammin]

Mach .69 seems like way to low of an airspeed.

At FL400 (Max is FL410) I don't get any buffet above the correct AOA for stall buffet and this correlates to a specifc speed at a certain weight. I am usually paying attention to AOA here and not trying to get a specific speed or mach number. On my next functional check flight I will get these numbers for you.

As I said, if your theory were correct for all cases I would expect buffet (from the low speed condition supersonic flow) at an angle of attack less then that for actual low speed stall buffet and I don't experience that.

Wing shape will also have alot to do with these conditions as well. A high speed, swept back wing will have less speed difference between the top and bottom of the wing and will thus not experence this condition until way higher.

The T-45 has a MMO above mach 1, but you still get mach buffet at about .86 and tuck at .88

 

[bubbers44]

Last time I was in low speed mach buffet was in an MD80 flying a one hour flight to LAX from SFO with a new captain on the airplane. We were light but he got slow trying to get to 370 to get on top of the clouds. At 500 to go I said we are getting slow so he selected altitude hold to accelerate but unfortunately that pathetic airplane doesn't fly well at high altitude with out close attention so it slowed down some more and as the buffeting got worse and the flight attendants wanting to know what was wrong we had to descend to keep from going deeper into buffet. After that experience I never let an airplane get slow at altitude again.

 

[MauleSkinner]

Mach .69 seems like way to low of an airspeed.

Depends on the airplane...for a Falcon 10 it is...for a Citation it's not...for a Beechjet it's about right for the climb.

At FL400 (Max is FL410) I don't get any buffet above the correct AOA for stall buffet and this correlates to a specifc speed at a certain weight. I am usually paying attention to AOA here and not trying to get a specific speed or mach number. On my next functional check flight I will get these numbers for you.

As I said, if your theory were correct for all cases I would expect buffet (from the low speed condition supersonic flow) at an angle of attack less then that for actual low speed stall buffet and I don't experience that.

Again, it depends on the airplane...I've never seen it in the Hawker, although we do have charts for it. The airplane just becomes a serious slug long before you get to that speed/AOA (still well above "stall") that I've never been tempted to get close.

Fly safe!

David