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.