Effect of speed and altitude? on stall AoA
Moderators: FrankM, el, Dmmoore
Effect of speed and altitude? on stall AoA
Interesting stuff there that I was not aware of.
One thing I am struggling with is the significance...with AirFrance, the dude actually BURNED a ton of airspeed, so I'm asking if the AoA was really all that reduced...Still the TAS might have been pretty fast and is it TAS that affects the AoA or the thin air, or both?
Also interesting were the AoA vs. speed cartoons. I'm ok with some artist drawing pretty lines, but dang, you think they might come close to a few realistic datapoints.
Anyway, thanks to Gabiee, and the parlour talking aggie ass hat learns some more stuff he will never use except on MSFS (PS, I never noticed particularly unusual stall behavior other than a little bit of coffin corner stuff on MSFS).
One thing I am struggling with is the significance...with AirFrance, the dude actually BURNED a ton of airspeed, so I'm asking if the AoA was really all that reduced...Still the TAS might have been pretty fast and is it TAS that affects the AoA or the thin air, or both?
Also interesting were the AoA vs. speed cartoons. I'm ok with some artist drawing pretty lines, but dang, you think they might come close to a few realistic datapoints.
Anyway, thanks to Gabiee, and the parlour talking aggie ass hat learns some more stuff he will never use except on MSFS (PS, I never noticed particularly unusual stall behavior other than a little bit of coffin corner stuff on MSFS).
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
I don't understand the question, but perhaps this helps?is it TAS that affects the AoA or the thin air, or both?
The slope of the curve CL vs AoA steepens with higher Mach numbers, meaning that, other things being the same, you have more lift at higher Mach numbers. Or, conversely, since normally you will have the lift that you need to have (usually = weight), other things being equal, at higher Mach you will need a lower AoA to achieve the desired lift.
The CLmax (max lift, the lift at the point of stall) goes down at higher Mach because the stall AoA is lower, but not as much as one may originally think, because the steepening of the slope partially compensates that.
The Mach number is TAS/c (c = speed of sound)
The speed of sound c is a function of the temperature of the air (it is proportional to sqrt(T), where T is in an absolute scale like Kelvin, not C or F)
And the temperature goes down with altitude. At sea level ISA, c = 662 knots. At 37000 ft ISA c = 574 knots.
So at the same TA,S you have a higher Mach at higher altitude. But the TAS tends to be higher at higher altitude, until the point where the speed reference is changed from IAS to Mach typically somewhere around FL300.
For example, a climb profile might be 270 knots (IAS) until you reach M 0.74 and then M 0.75 until cruise altitude. All the way up to the point of transition to Mach, when flying at a constant IAS, your TAS has been increasing, since the IAS is the speed that would give you the same dynamic pressure that you have if the air density was the standard sea level one. Since you are climbing, the density is going down (thinner air) and hence you need a higher TAS to produce the same dynamic pressure (i.e. to keep the IAS).
For example, in a standard atmosphere, the 270 knots IAS will be 270 knots TAS at sea level and the Mach will be 270/662 = 0.41.
At about 30000 ft you will reach the transition point, you will still have 270 kts IAS but your TAS will be 441 kts and the Mach will be 441/590=0.75 (not that not only the numerator increased but the denominator, i.e. the speed of sound, diminished).
From that point you are going to keep not the IAS but the Mach number.
There is an altitude at which the temperature stops decreasing and remains essentially constant as you keep climbing. The standard atmosphere model established this point at 36100 ft.
When you reach 36100ft, your Mach will be 0.75, but since the speed of sound went down to 574 knots (because the temperature still went down from FL300 to FL361), the TAS is now a bit lower than at FL300: 574*0.75 = 430 knots. Now, not only the TAS went down a bit, but the density also went down. Compared with FL300, at the same Mach but now lower TAS and lower density, your IAS is 234 knots.
If you keep climbing, the temperature will now remain constant (according to the ISO model), and hence the speed of sound will remain constant, and therefore if you keep the Mach number you will alos keep the TAS. But watch your IAS because the density will keep going down as you climb.
At 37500 ft (the altitude at which AF 447 stalled), a Mach of 0.75 still means 430 knots, but now your IAS is 226 knots.
Your AoA will be not as high as you would need to hold 226 knots IAS at sea level because the slope of the lift curve steepened with the higher Mach. But then your stall AoA will also not be as high as it was at sea level. In fact, at the same IAS, you would not be able to pull as many Gs at high Mach number (high altitude) as you would be able at low Mach numbers (low altitude).
Re: Effect of speed and altitude? on stall AoA
Gabiee in black font
3BSee in red.
I don't understand the question, but perhaps this helps? Not sure.
you have more lift at higher Mach numbers. Yes, so to do an accelerated stall where you blow past the critical AOA will result ins some significant Gs
CL Lay off the bad acronym, which also means center line, or even "center of lift" which shifts backwards at high speeds
The Mach number is TAS/c (c = speed of sound)
Blah blah blah blah
Ok too many trees...3BS wants to understand what causes the stall to happen at a lower AoA….why the flow separates...
Is it because you are going so dang fast the molecules cant follow the wing surface downward?
Plus what does it mean...If you go way up high, but try to go slow, your TAS will still be high, while your IAS is lower due to reduced pressure. If you are way up high going as slow as you can, will the stall AOA be "more normal" or is it still reduced.
Maybe both factors come into play- maybe only one factor comes into play, but the other correlates because they are related mathematically- your IAS at altitude is not all that high even though you are around 80% the speed of sound.
So you are hitting fewer air molecules, then again, there's a linear relationship to the speed of how you hit the molecules right? If you could somehow double the speed, but half the number of molecules you hit, lift would double....
Trying to understand if it's the speed or the thinness of the air that causes the flow over the wing to be disrupted.
3BSee in red.
I don't understand the question, but perhaps this helps? Not sure.
you have more lift at higher Mach numbers. Yes, so to do an accelerated stall where you blow past the critical AOA will result ins some significant Gs
CL Lay off the bad acronym, which also means center line, or even "center of lift" which shifts backwards at high speeds
The Mach number is TAS/c (c = speed of sound)
Blah blah blah blah
Ok too many trees...3BS wants to understand what causes the stall to happen at a lower AoA….why the flow separates...
Is it because you are going so dang fast the molecules cant follow the wing surface downward?
Plus what does it mean...If you go way up high, but try to go slow, your TAS will still be high, while your IAS is lower due to reduced pressure. If you are way up high going as slow as you can, will the stall AOA be "more normal" or is it still reduced.
Maybe both factors come into play- maybe only one factor comes into play, but the other correlates because they are related mathematically- your IAS at altitude is not all that high even though you are around 80% the speed of sound.
So you are hitting fewer air molecules, then again, there's a linear relationship to the speed of how you hit the molecules right? If you could somehow double the speed, but half the number of molecules you hit, lift would double....
Trying to understand if it's the speed or the thinness of the air that causes the flow over the wing to be disrupted.
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
I am assuming that "causes the flow over the wing to be disrupted" you mean "causes the stall to happen at a lower AoA".Trying to understand if it's the speed or the thinness of the air that causes the flow over the wing to be disrupted.
So is it the speed or the thinness of the air?
Neither. It is the compressibility i.e. the Mach number i.e. the ratio between TAS and speed of sound.
Now, you can do an experiment where you have the same density (thinness), the same TAS and IAS, and different temperatures (hence different speed of sound) and the stall will happen at a lower AoA in the test where the temperature is colder, hence the speed of sound is lower, hence the Mach is higher for the same speed.
So there you have, same speed, same thinness, different stall AoA.
Last edited by Gabriel on Thu Sep 26, 2019 9:07 pm, edited 1 time in total.
Re: Effect of speed and altitude? on stall AoA
Double post. Don' we have a "delete post" feature in this forum????
Re: Effect of speed and altitude? on stall AoA
No.Double post. Don' we have a "delete post" feature in this forum????
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
I'll have to go back and read your compressibility dissertation there.
My brain is at the molecular level and wondering why air molecules behave differently at Mach 0.8 vs 120 knots in a 172.
Air molecules average 767 MPH in speed (at the proverbial standard conditions) as they bounce all over creation in a relatively random motion...but that's forward, backwards, up, down, with some going near 0 and some probably pushing 1534 MPH...so what's the big deal with the wing slicing through all that at 613 MPH vs 306.5 MPH? (With air molecules varying speed by 3000 MPH (some going forward, some going back)).
Trying to understand why "the flow separates"...sadly, my simplistic thinking is "because the wing is going so damn fast, it cant follow it down fast enough".
My brain is at the molecular level and wondering why air molecules behave differently at Mach 0.8 vs 120 knots in a 172.
Air molecules average 767 MPH in speed (at the proverbial standard conditions) as they bounce all over creation in a relatively random motion...but that's forward, backwards, up, down, with some going near 0 and some probably pushing 1534 MPH...so what's the big deal with the wing slicing through all that at 613 MPH vs 306.5 MPH? (With air molecules varying speed by 3000 MPH (some going forward, some going back)).
Trying to understand why "the flow separates"...sadly, my simplistic thinking is "because the wing is going so damn fast, it cant follow it down fast enough".
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
Gabieee, there:
By the way, all those MS Paint - like graphs are all wrong even from the conceptual point of view.
They show the curve as displaced to the left and shorter, thus with a lower stall AoA and a lower CLmax, but with a higher CL at any AoA lower than stall.
But actually what changes is the slope of the curve.
In a first-order-of-magnitude approximation (which is directionally approximate-ish for mid Mach numbers, say up to 0.6 or 0.7, well below the critical Mach), the effect of compressibility can be understood in terms of frame of references, in a similar way that Einstain general relativity, and not surprisingly arrive to a similar formula. There is a somehow complex theory to get to that formula, but we can explain it simplified from a conceptual point of view, and it is still more or less accurate.
From the wing frame of reference, compressinility makes that the air molecules come packed closer together in the direction of motion. Imagine the drawing of an airfoil in a graph paper. Now imagine that the paper starts to move relative to the table, but by some sheer magic the airfoil stays stationary relative to the table. In other words, we have the graph paper (representing the air) passing by the airfoil. As the Mach increases and the compressibility effect starts to kick in, you can think that the vertical lines (but not the horizontal lines) in the graph paper will start to get closer one to the next, so the little squares in the paper now become little rectangles, narrower than tall. So more vertical lines fit in the length of the airfoil now. Now let's shift to the graph paper frame. From this perspective, the paper is static and the squares are still squares, but since the airfoil now covers more squares, it looks longer (but not thicker). In other words, it looks like a more slender airfoil. This has a couple of effects:
1- The chord appears to be longer. If you make the Cl vs AoA curve taking into account that chord, the slope will look the same as usual, but the lift (where you need to multiply the Cl times the chord) will be higher due to the longer apparent chord. Now, for the Cl vs AoA curve, when calculating the Cl we don't divide the lift by this "aerodynamically longer" chord but by the real shorter chord, resulting in a higher Cl. The result doesn't change at zero lift (zero divided by any chord is still zero), so the zero lift AoA is still the same. But as the AoA increases the lift, and hence the Cl, will be higher than normal by a factor "apparent longer chord / real chord", thus making the slope of this curve steeper, i.e the Cl and hence the lift will increase faster per each extra degree of AoA.
2- The airfoil will seem distorted from the graph paper (i.e. air) point of view, by stretching it only chordwise. Obviously the relative thickness of the airfoil will go down (i.e a 16% thick airfoil may look like a 12% thick one). Let's see what happens with the leading edge. Let's see, just for simplicity, that the leading edge was half a circle. Now this half a circle will aslo be stretched in the chord direction resulting in a ellipse where the long axis is horizontal and longer than the original circle's diameter, and the vertical an shorter axis will steel be equal to the original circle's diameter. I hope you can visualize that the radius of the leading edge is smaller now, resulting in a sharper leading edge. And do you know what sharper leading edges do? They stall at lower AoA's.
As a side note, the factor of the apparent stretch of the airfoil is 1/sqrt(1-M^2) or 1/sqrt(1-v^2/c^2), where v is the airspeed and c is the speed of sound. Interestingly, c is also used for the speed of light, and if you do that and use the same formula you just got the factor for the length contraction (only in the direction of motion), time dilation and mass increase in Einstein's special relativity theory.
I will try to reconcile this to bouncy/darting gas molecules.
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
- flyboy2548m
- Posts: 4391
- Joined: Sat Feb 02, 2008 12:32 am
- Location: Ormond Beach, FL
Re: Effect of speed and altitude? on stall AoA
What?
"Lav sinks on 737 Max are too small"
-TeeVee, one of America's finest legal minds.
-TeeVee, one of America's finest legal minds.
Re: Effect of speed and altitude? on stall AoA
I can't tell you much about molecules. ALL of aerodynamics and fluid mechanics, including the proverbial Navier Stokes equations assume that the fluid is a continuum, not made up of little pieces. Which of course is not true, but boy how well it works!!! Subsonic, supersonic, hypersonic in the high atmosphere (like the re-entry of the Space Shuttle), honey flowing from a jar, cavitation in the propeller of a speedboat, how a fly flies, you name it.
I never studied or used a model that considers that a fluid is anything other than a continuum.
The simplified explanation I used in what you quoted is based on a continuum too, so if you think it at molecular level it will make no sense.
I never studied or used a model that considers that a fluid is anything other than a continuum.
The simplified explanation I used in what you quoted is based on a continuum too, so if you think it at molecular level it will make no sense.
Re: Effect of speed and altitude? on stall AoA
Yes.What?
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
Ideal gas law?I never studied or used a model that considers that a fluid is anything other than a continuum.
Re: Effect of speed and altitude? on stall AoA
PV=nRT? What does it have to do? It is still taking the gas as a continuum (even if you have the n for the number of moles, which you can easily replace by the mass and molecular weight). Actually this law is integral to Navier Stokes equation when considering compressible flow in ideal gases, which is what we do in aerodynamics. And we still take infinitesimal volumes of air as a continuum to do that.Ideal gas law?I never studied or used a model that considers that a fluid is anything other than a continuum.
Re: Effect of speed and altitude? on stall AoA
Word mincing:PV=nRT? What does it have to do? It is still taking the gas as a continuum (even if you have the n for the number of moles, which you can easily replace by the mass and molecular weight). Actually this law is integral to Navier Stokes equation when considering compressible flow in ideal gases, which is what we do in aerodynamics. And we still take infinitesimal volumes of air as a continuum to do that.Ideal gas law?I never studied or used a model that considers that a fluid is anything other than a continuum.
You "never studied a theory that got into discrete numbers".
Well, you DID study THIS theory which DOES get into discrete numbers...
BUT
Concur, 3BS's lame attempt to understand the effect of high speed on critical AOA with an individual molecule mindset is not helped by PV = nRT…
I do understand that once you reach a few bazillion of those molecules, you can kind of gloss over the individual behavior with a very high degree of accuracy.
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
How's that?THIS theory DOES get into discrete numbers...
Re: Effect of speed and altitude? on stall AoA
You said it yourself:How's that?THIS theory DOES get into discrete numbers...
Pressure * Volume = THE DISCRETE NUMBER OF MOLECULES * a unit-appropriate constant * A temperature scale (where 0 = absolute zero)
I see where you can sometimes substitute other numbers for the NUMBER of molecules...but, the gas law is absolutely based on the number of molecules since H2 gas weighs a good bit different than CO2 gas....
Commercial Pilot, Vandelay Industries, Inc., Plant Nutrient Division.
Re: Effect of speed and altitude? on stall AoA
Moreover, the ideal gas law is based on the approximation that a gas is made up of individual non-interacting particles. If you relax that assumption, it doesn't work.
Who is online
Users browsing this forum: No registered users and 7 guests