A stall occurs when the angle of attack of a wing or other airfoil becomes so high that the airflow over the upper surface of the wing separates from the wing, rather than remaining attached to it; this causes the wing to produce less lift and more drag, making it harder to maintain level flight and more sluggish to respond to control inputs.

However, shouldn't the airflow separating from the wing's upper surface result in a large area of low pressure above the wing, and, thus, greatly increase lift? What am I missing?

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    I suspect (but don't know, which is why I'm commenting and not answering) that the salient point is that the airflow is separated, meaning it doesn't follow the nice contours it should -- but it's not a vacuum. So you still have air (and thus pressure), it's just not doing what you want it to. – yshavit Sep 18 at 14:29
up vote 21 down vote accepted

For a parcel of air to generate a lift force as it flows over the wing requires the wing to tip that air parcel's momentum vector downwards slightly; the reaction force that the wing experiences as it does this is what we measure as lift.

In the case where the airflow over the top of the wing separates from it, the parcels of air flowing by do not get their momentum vectors redirected downwards anymore and the wing hence "stops flying".

Meanwhile, the region of separated flow constitutes a zone of turbulent air which forms a stirred-up wake in the rear-facing "shadow" of the wing at its high angle of attack and the only work performed by the wing in this case is to stir up that wake- and that constitutes lots of drag.

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    Long ago I read an article where the writer demonstrated that the package of air redirected extended about half a span above the wing and did calculations that showed quite a staggering amount of air mass redirection was involved in generating action/reaction force. It was the low pressure distribution created by the airfoil curvature that induced air above to move down so the wing was doing more than simply deflecting downward the air directly below. The stall disrupts this "suction induced" air movement so that only the air below is being deflected, killing off most of the lifting force. – John K Sep 18 at 12:35
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    Because an aircraft in level flight is in equilibrium with gravity, a sudden reduction in lift can have a dramatic effect. A rapid sink and nasty shuddering as stall turbulence rocks the aircraft. However, checking the lift to AOA curve shows the plane still has significant lift in stall, but not enough to match gravity. Exacerbating the situation, the high AOA lift vector now has a rearward component, essentially "putting the brakes" on forward motion. AOA increases as you sink, moving towards deep stall. Fortunately, as you sink, the (rear) H stab will try to push the nose down. – Robert DiGiovanni Sep 18 at 22:33
  • @RobertDiGiovanni: It only moves towards a deep stall if your aircraft has a T-tail; deep stalls are physically impossible in aircraft with conventional tails. – Sean Sep 19 at 3:29
  • CG too far back will create same condition, as will holding full up elevator. Please notice the commonality here, insufficient force to lower nose. Swept wing designs also create the same condition, here because now lift too far forward. The Sabre jet may have benefited from larger H stab! – Robert DiGiovanni Sep 19 at 13:16

Shouldn't the airflow separating from the wing's upper surface result in a large area of low pressure above the wing?

Intuitively you might think so, but it's false. In subsonic fluid dynamics, slow air = high static pressure, and fast air = low static pressure.

So the way low pressure is created above the wing is by having fast flow speed, for that to work it needs to flow smoothly, but once the flow is no longer attached, it is no longer fast flowing above the wing.

When a plane climbs it does so against the flow of air through the wing, and it changes altitude while crossing through different air pressures and air in different directions, hence airplanes slow down during lift (relatively).

You do forget to consider the air that resists the fuselage as well.

As @MichaelHall below mentions, A stall only occurs when the critical angle of attack is exceeded. This can occur in a climb, but it can also occur straight and level, in a turn, or while descending.

A Stall Generally means the plane's wings are getting reduced Air flow on them to maintain the stability in speeds. It can also occur when it climbs too steeply with the wing getting resisted more during lift. This is when the airplane loses lift or it loses the ability to maintain the plane on the air:


Source, modified by me


Source

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    Nice pictures, but your explanation is not clear. For example: "A Stall occurs when it climbs too steeply." is not correct. A stall only occurs when the critical angle of attack is exceeded. This can occur in a climb, but it can also occur straight and level, in a turn, or while descending. – Michael Hall Sep 18 at 18:03
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    The horizontal stab is in the turbulent flow. Is this jet's bad day about to get much, much worse? – Wayne Conrad Sep 18 at 18:59
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    @WayneConrad It can actually happen, is called "deep stall" and this is not a good thing. In fact if the turbulent air is in the path of the control surfaces, the controls are not working anymore so you cannot get out of the stall and yes, that means you should think over the question: "Did I update my last will?" – Thorsten S. Sep 18 at 23:18
  • @ThorstenS: Technically, it is possible to escape from a deep stall by rolling to a high enough bank angle that the horizontal tail leaves the "shadow" of the wings (since you still have at least some aileron control authority). Still a very bad day for any occupant(s) of that plane, though. – Sean Sep 19 at 3:28
  • Thank you, i have added the source. @MichaelHall True, i forgot to mention that part, and it is because the aircraft is not able to generate enough lift right. It may be because of the engine and sometimes due to the pilot . . . Sean, it may be possible, however in the pilot's manual it is suggested to drop your nose down to generate enough speed to get lift again . . . – Ani Sep 19 at 5:22

However, shouldn't the airflow separating from the wing's upper surface result in a large area of low pressure above the wing, and, thus, greatly increase lift? What am I missing?

Well the total aerodynamic force DOES indeed increase, but as it is now pointing almost backwards most of it is decomposed as drag, and virtually none of it is left as lift.

By def. the rearwards component of the total aerodynamic force is drag.

Starting off with a little physics: An airfoil (wing) generates lift due to pressure variations created by the shape of the wing. Bernoulli's equation which models the laminate flow of a fluid (or in this case a gas) though a restriction is: P+(rho*V^2)/2 = n (Pressure + (density x Velocity squared) divided by 2 = some constant. I'll spare you the entire derivation but the end result is the equation for lift which is:

Lift = CI * (rho*V^2)/2 * A Where: CI = Coefficient of lift rho = Air density V = Air Velocity A = Area of the wing

In essence this shows that the faster air is forced over the top of the wing, the lower the air pressure becomes above the wing. Since the air pressure under the wing remains high due to a relatively flat surface, and that nature will look to fill a void, the wing is sucked up into the low pressure (or, if it is easier to wrap your head around the high pressure below the wing pushes it up into the low pressure above the wing (Newtonian view)) thus, we have lift. (By the way, the Newtonian view and the Bernoulli's view are not mutually exclusive, they are two different approaches that show the same thing... but I digress and that is a whole other ball of wax)

Now, to the meat of your question: As we increase the angle of attack(AOA), lift increases due to a higher coefficient of lift. This would seem to suggest that your initial supposition is correct after all, higher AOA mean greater coefficient of lift but if we look back at our equation we see that the velocity of the airflow over the wing is critical to lift. If we take an idealized look at the airflow as it separates from the wing(Critical Angle of Attack), the air velocity over the wing drops to zero which means we have: Lift = CI((rho*0.0)/2)*A = 0.0, no lift which causes a stall.

As to the second part of your question, why does the drag increase? A fairly straight forward application of Newton's laws tells us what is going on. As the AOA increases and increasing large surface area of the wing is exposed to the relative wind. Combine that with increasing vorticities behind the trailing edge of your wing, creating a low pressure area, and you have increasing drag.

Hope that helps and best of luck in your flying.

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    The lower surface of a wing is not necessarily flatter than the upper surface. Many aircraft types have symmetric airfoils, and it's possible to create lift with a completely flat airfoil, such as a kite. You might like to read our question on how wings generate lift. I think to really explain the stall you need to focus on why the flow separates. – Dan Hulme Sep 19 at 7:40
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    You are correct, there are symmetric airfoils, there are also any number of variations on wing designs, swept wing, delta wing, rectangular wing, tapered wing, rotary wings. Supersonic aerodynamics is a good time and of course, a wing will still 'fly' even when inverted. None of that has anything to do with the price of tea in China.. Bernoulli's principle still holds (the magic happens in that CI variable). Have a gander at NASA, MIT Aeronautics or Embry Riddle's webpages. I'll spare you the alphabet soup associated with my name, it's meaningless on the web anyway. – Aaron Sep 19 at 16:35

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