What is the immediate cause of stall?

Question

The direct cause of stall is unclear to me.

• I heard about exceeding maximum angle of attack (around 40°) ;
• I heard about reaching the stall speed in the current configuration (flaps, etc).
• I heard about load factor which can lead to a stall.

Are these three parameters both direct causes of stall? By "direct", I mean for example that pitch is not a direct cause to stall, since pitch let AoA vary which causes stall.

It's unclear because there is a relation between angle of attack and speed. When we increase the angle of attack, we lower speed. But at the end, what will be the cause of stall?

If both causes are direct causes, are they independent? - It should be possible to fly very fast (faster than stall speed) and have an angle of attack of 40°. - It should also possible to have a very low speed (lower than stall speed) and have an angle of attack which produces lift.

What parameter will win and how are they related together?

When an aircraft is on the ground, taxiing at 10kt, the speed alone makes the plane stall, regardless of the angle of attack, am I right?

I would like to know necessary conditions to stall, and sufficient conditions to stall (all conditions being direct causes).

Answer 1

The immediate cause of a stall is the detachement of the airflow from the wing:

^{Image from NASA}

This happens when your Angle of Attack is too high.

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How can the AoA become too high?

--For level flight:

• Given a certain velocity, you will have a certain AoA that will provide the lift needed for level flight. The lower the velocity, the higher this AoA.

• Given a certain wing (configuration) you will have a certain maximum allowed AoA (in the picture is the AoA for wich you have the highest value of the plot):

^{Image from Wikipedia}

Given these two tidbits of information, you can see what is the answer to your question:

• If the velocity is too low, to fly straight you need an AoA that exceeds the capabilities of your wings (or a vectorial thrust that exceeds the weight of your veichle, but I would say that it is not quite common)
• If the AoA is too high, well, is quite evident that you have stalled.

--For curved flight:

During a turn you need to use a component of your lift vector to actually turn

^{Image from here}

As you can see the lift you need to turn is larger that the lift you need to fly level (given the same speed). And to have a higher lift you need a higher AoA.

And once again back to the AoA we are: if you want to turn too much (high load factor) you need a lot of extra lift, meaning that you have to increase your AoA.

If you exceed your wing limits, you stall

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Dynamic stall

This is not exactly my cup of tea, but reading from the already linked wikipedia article:

Dynamic stall is a non-linear unsteady aerodynamic effect that occurs when airfoils rapidly change the angle of attack. The rapid change can cause a strong vortex to be shed from the leading edge of the aerofoil, and travel backwards above the wing. The vortex, containing high-velocity airflows, briefly increases the lift produced by the wing. As soon as it passes behind the trailing edge, however, the lift reduces dramatically, and the wing is in normal stall.

I would say that once again we fall in the "too big AoA" case, even if for different causes.

Answer 2

There is not much to add to the two good answers, but I'll try to refine them. The main reason for a stall is flow separation and, accordingly, lift loss beyond the angle of attack of maximum lift. Unfortunately, this is not a fixed number, but it depends on a range of parameters. The three most important ones are Reynolds number, rate of pitch increase and Mach number.

The Reynolds number characterizes the ratio of inertial and viscous forces in a fluid. In other words, friction has more influence on airflow at lower speeds and smaller dimensions. Friction is the main reason for flow separation (please read the linked article to get an explanation), and the slower the wing moves through air, the smaller the stall angle of attack is. In a taildragger taxiing at 10 knots, the wing is certainly stalled.

In flight you need to factor in how much lift you demand from the wing. This is determined by the product of aircraft mass and load factor. If you fly a parabola with zero g, you need no lift, and the aircraft's wing will not stall at any speed. Note, however, that the elevator might need to create substantial lift in order to keep the wing close to it's zero-lift angle of attack, so the elevator might stall at low speed. You will notice when this happens, because your parabola will come to a sudden end.

On the other end, flying a steep turn might cause a stall even at high speed, if you pull too many gs for the given speed. This is just the same as a stall in level flight at low speed. Due to Reynolds number effects, the stall angle of attack might be a few degrees higher, but details depend on the particular aircraft and it's airfoils. Generally, your stall speed in a turn goes up with the inverse of the square root of the cosine of the bank angle. In a 60° turn, your stall speed is 1.41 of your level stall speed, and at 75° it will be almost double your level stall speed.

The pitch rate can have a dramatic, but short-lived influence. See this post for details. In tests, the maximum lift could be increased by 50% simply by quickly pitching up. If the stall AoA is approached rapidly, the boundary layer over most of the wing has still the characteristics which go with the low AoA that prevailed when that parcel of air flowed around the wing's nose. Once that boundary layer has been washed away, the airplane is deeply in stall territory and needs to pitch down a lot to recover. Pitch fast enough, and normally benign aircraft can show dangerous stall characteristics. This is fun to try, but make sure you have enough altitude below you to recover.

And now for the influence of Mach number. Again, this post has more details (scroll down to the lower five paragraphs). Once part of the flow over the wing becomes supersonic, maximum lift suffers and the stall angle of attack is lowered dramatically. This is a high-speed stall, and it can become hard to recover. Stalling means a loss of lift, so the aircraft will pitch down, picking up more speed. This will drive it deeper into the high Mach region with severe shocks on the wing, so by accelerating it will make the stalling condition worse.

Especially high altitude aircraft can get into a condition where they fly right between the low-speed and the high-speed stall. Decelerating means increasing the angle of attack above it's maximum, and accelerating means the shocks on the wings get worse, reducing lift and forcing the aircraft into a prolonged shallow dive, until air density is sufficient for recovery. U-2 pilots called this the coffin corner of the flight envelope.

Wing sweep increases the maximum angle of attack, and with leading edge sweep angles of 60° and more, flow separation at the leading edge creates a vortex which increases lift with higher angles of attack, so that a normal stall does not occur. Of course, pitch high enough and the vortex will become unstable, but increased drag, separated flow around the vertical tail and vortices on the forward fuselage will limit how high you can pitch. The F-4 Phantom II has a maximum angle of attack at just 23°, where the wings will happily produce a positive lift gradient with angle of attack. But the vertical tail rapidly becomes ineffective beyond those 23°, and the aircraft will violently yaw out of control if that angle is exceeded (nose slice). Effectively, aircraft like the F-4 never stall, they just go out of control if you pitch up too much.

Answer 3

Stall means that the wing has exceeded its critical angle of attack. Nothing more, nothing less.

Is it possible to fly very fast beyond critical angle? Sure. Look up Pugaychev's Cobra. Or look at the Saturn V rocket. Both aircraft are stalled, yet they continue to fly. My previous example was flawed, but the sentiment stands. Put enough thrust behind a brick outhouse and you can get it to fly. (Thanks newmanth!)

What about flying slowly beyond stall? That won't work; beyond critical angle you develop less lift the more you increase AoA. For any given airspeed / wing / air density configuration, there is some maximum lift value which cannot be exceeded. So the more slowly you fly, the more down you go.

Your example of an aircraft taxiing at 10kt is an excellent one. Just because the airplane is moving slowly does not mean that the wing is stalled. There is 10kt of relative wind passing over the wing, and the angle of attack is close to 0. The wing isn't stalled, it's just not generating enough lift to fly.

To reiterate, stall means that the wing has exceeded its critical angle of attack. That's it.