Stall speed/altitude

[FlyboyZR1]

Does the stall speed of an aircraft increase with altitude? If so, why?

[Dugie8]

Indicated Airspeed, for the most part, no.

[JAM]

No. Well...maybe. Actually, yes. It really just depends.

Seriously, it should stay the same indicated, but increase in true. And even then, not always.

[fish314]

As previously posted, it does increase when you are talking about TRUE airspeed, but not when you are talking about indicated airspeed. Here's why:


First, stall speed is the speed at which the aircraft stalls under a given set of conditions, so let's nail down the conditions that we are concerned with first. For this discussion we will be considering aircraft flying in straight and level, unaccelerated flight. The only variable we will change is altitude.

Let's look at straight and level flight at a low altitude first. In all straight and level flight (regardless of altitude) the force that balances the weight of the aircraft is called LIFT. Lift is calculated via the following formula:

L=1/2 CsubL rho V^2 S.

Here are what the terms mean:
L: Lift

CsubL: the coefficient of lift. Normally this is written as a C with a subscripted L next to it, but this browser doesn't seem to allow subscripts. CsubL is a dimensionless number that describes the lifting properties of the airfoil, wing, or airplane, etc. It is based on a number of factors, but two of the most important are the shape and the angle of attack of the wing. CsubL increases as angle of attack increases, until you get to a maximum angle of attack, called the stall angle of attack or critical angle of attack, at which point it drops of suddenly. This is a stall.

rho: The greek letter rho (which looks like a squiggly p) is the symbol for air density.

V^2: V stands for velocity, and it is measured as TRUE airspeed. This speed is squared in the lift equation (multiplied by itself).

S: Surface area of the wing. In this discussion we will treat this term as constant, and ignore devices like slots, flaps, slats, and others that can actually change the size of the wing.

Back to the Lift Equation:
So we see from the equation above that there are only two terms that pilots can control, the velocity and the coefficient of lift. Pilots control the velocity by speeding up or slowing down, and they control the CsubL by changing the angle of attack of the aircraft. A greater angle of attack yeilds a greater CsubL, until you reach the stall angle of attack. This means that regardless of altitude, a particular wing has a maximum angle of attack at which it will produce lift, and it will have the maximum CsubL at this angle of attack. This is the angle of attack the wing will always be at when it is at it's stall speed. ALWAYS.

So what happens when we change altitude?
First imagine we are flying at a low altitude at our stall speed. Lift equals weight, and CsubL is at its maximum, because the wing is at it's critical angle of attack.
Now imagine that we start a climb to a high altitude and try to slow again to our stall speed. What happens? Well, when we change altitude, density (rho) decreases. Since it decreases, lift would decrease if we kept everything else the same. In order to fly straight and level, however, lift MUST equal weight, so therefore something has to get bigger to compensate for the decrease in density.
Since CsubL was already at it's maximum, we can't increase it any more, so the only term left that we can increase is velocity. For this reason, it takes a greater velocity (TRUE AIRSPEED) to fly at a higher altitude. Or another way of saying it is that an aircraft will stall at a higher true airspeed at a high altitude than it will at a low altitude.

So that covers TRUE AIRSPEED, but what about INDICATED AIRSPEED?
True airspeed is a measure of how fast the airplane is moving compared to the air molecules that it is flying through. INDICATED airspeed is an indirect measure of the speed of the aircraft. It works by measuring the force of the air molecules being shoved down the pitot tube.
Indicated airspeed is greatly affected by the density of the air that you are flying through, so at a lesser density it takes a higher true airspeed to register the same airspeed on the airspeed indicator. That is an important concept! As you increase altitude, the same indicated airspeed equals a greater and greater true airspeed all because of the change in density of the air.

So what about stall speed? Well as it turns out, even though the TRUE AIRSPEED at which the aircraft stalls will increase as altitude increases (due to changing density), the airspeed that you read on the airspeed indicator (indicated airspeed) will be absolutely independent of altitude. It will stay the same. Or in other words, as long as nothing else changes (weight, bank angle, G-loading, etc.) altitude has no effect on INDICATED stall speed.

[meritflyer]

fish, you kill me!

[fish314]

I just can't seem to write a short post!

[seagull]

All of which ignores "mach effect", of course!

[Flysher]

Good post fish. Aerodynamics for naval aviators also has a fairly good explanation for this, although I think you hit it better.

[meritflyer]

psst.. he is a military aviator

[Flysher]

i know, that was obvious from his avatar, i was mentioning the book for the original poster.

[adreamer]

any chance to buy a book you read? Fish


adreamer

[fish314]

Depends on what you're looking for. For a pretty good series of books about all sorts of aviation topics try The Proficient Pilot, by Barry Schiff. I've only read vol. 1, but he explains complicated concepts with very simple explanations and very little math. He does kind of jump around at times, though and he covers some concepts that many pilots might find esoteric or of little value (like pressure pattern navigation). Overall, a very good book.
If you are looking for a more in depth look, I get some of the information that I post here from looking through my old textbooks (I was an aero engineering major way back when), and some of it I get through my old ground school stuff from USAF pilot training and from my ground school in Tankers and Tweets.
And then, of course, I also reference the old standby's of the FAR/AIM, the Airplane Flying Handbook, the Instrument Flying Handbook, and the Pilot's Handbook of Aeronautical Knowledge. These are awesome, but aren't always the best written or the clearest. But since these are the ones written by the FAA they're must have references.

[meritflyer]

I just ordered Vol. 1, 2, and 3.

[seagull]

The Flightwise series by Chris Carpenter is far superior to the other aero books out there.

[B767Driver]

Originally Posted by seagull

The Flightwise series by Chris Carpenter is far superior to the other aero books out there.

I'm reading this one now. The first 5 or 6 chapters were pretty dry but now it's become a really good reference.

[tgrayson]

Originally Posted by FlyboyZR1

Does the stall speed of an aircraft increase with altitude? If so, why?

For light (read, "slow") aircraft, you won't see a change in IAS for the stall at high density altitudes.

However, the stall speed is affected by both the Reynolds and Mach numbers, so there will be some variation in faster, higher-flying aircraft. The stall speed initially goes down at moderate mach numbers, then increases at higher machs.

[tgrayson]

Originally Posted by fish314

As previously posted, it does increase when you are talking about TRUE airspeed, but not when you are talking about indicated airspeed. Here's why:

Nice analysis. However, your calculations assume that CLmax is constant throughout the flight regime, but this isn't necessarily the case. Higher Reynolds numbers can increase CLmax, whereas higher machs will decrease it. You can see how that would affect your answer.

[fish314]

True. If you are talking about a stall speed in a "fast mover" like a fighter at a high altitude, you might start seeing some variations. Might be noticable in airliners also, but I'm not sure. I flew 707's (actually KC-135's) for about 5 years, but we never flew close enough to stall speed (especially at high altitude) to ever notice it. For us, the big number that we looked at was gross weight, as opposed to altitude.

[comet605]

Originally Posted by JAM

No. Well...maybe. Actually, yes. It really just depends.

Seriously, it should stay the same indicated, but increase in true. And even then, not always.

Thats the most informative and confusing explanation i have heard yet.

[Tinstaafl]

'Mechanics of Flight' by A.C. Kermode (ISBN 0582237408) is an excellent reference. Not quite so mathematically based as 'Aerodynamics for Naval Aviators' but still manages to explain the concepts.

[seagull]

Originally Posted by Tinstaafl

'Mechanics of Flight' by A.C. Kermode (ISBN 0582237408) is an excellent reference. Not quite so mathematically based as 'Aerodynamics for Naval Aviators' but still manages to explain the concepts.

Ah oh, here comes TG! Better listen to him too, he's right!

[Tinstaafl]

Who's TG?

[SteveC]

Originally Posted by Tinstaafl

Who's TG?

tgrayson

[tgrayson]

SteveC, don't you have any secret powers to re-age this thread?

[SteveC]

Originally Posted by tgrayson

SteveC, don't you have any secret powers to re-age this thread?

"Re-age"??




Oh...holy cow. I didn't realize it was a 2006 thread resurrected!


Well in that case I guess I can lock it up, and if anyone has comments or questions regarding the topic they can start a new thread. Multi-year conversations can be pretty confusing.