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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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    $\begingroup$ 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. $\endgroup$ – yshavit Sep 18 '18 at 14:29
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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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    $\begingroup$ 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. $\endgroup$ – John K Sep 18 '18 at 12:35
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    $\begingroup$ 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. $\endgroup$ – Robert DiGiovanni Sep 18 '18 at 22:33
  • $\begingroup$ @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. $\endgroup$ – Sean Sep 19 '18 at 3:29
  • $\begingroup$ 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! $\endgroup$ – Robert DiGiovanni Sep 19 '18 at 13:16
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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.

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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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    $\begingroup$ 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. $\endgroup$ – Michael Hall Sep 18 '18 at 18:03
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    $\begingroup$ The horizontal stab is in the turbulent flow. Is this jet's bad day about to get much, much worse? $\endgroup$ – Wayne Conrad Sep 18 '18 at 18:59
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    $\begingroup$ @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?" $\endgroup$ – Thorsten S. Sep 18 '18 at 23:18
  • $\begingroup$ @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. $\endgroup$ – Sean Sep 19 '18 at 3:28
  • $\begingroup$ 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 . . . $\endgroup$ – Ani Sep 19 '18 at 5:22
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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.

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It is possible the question is not being answered adequately because the question is incomplete. perhaps the continuation to the question would be,"does the pressure differential generate the lift or does the displacement of a mass of air caused by the differential in pressure of a wing in motion thru free air generate lift?"

Implying that reduced pressure above the wing is generating lift is confusing an effect of a result with the result of an effect. Great theory, not experimentally proveable. NASA tried during boundary layer research on a B-66 airframe in the 1960s. worked well providing BLC but lift? The data said no. Fluid dynamics may have the answers but be forewarned, it will be very complicated.

One does not expect to see an airframe levitate off the ramp by evacuating the mass of air above a wing surface and expect the resultant pressure differential in free air to "push" the airframe vertically upwards into the air, if only it were so. NASA did prove that holes in the wing BLC surface ducting smaller than the dust particles suspended in the ambient air showed a proclivity towards being clogged by said particles thus reducing BLC effectiveness; causing NASA researchers to move on to experiments in blown flaps. That, so far as my researches has taken me over the years, is about as close as "experimental data" has gotten to answering your question.

But I digress.

at stall, the air below a given wing is still at a higher pressure than the air above the wing and seeks the low pressure area by reversing flow above the sharp TE of the wing to "fill" the "void". All the data about lift vectors changing during stall conditions is quite true and proveable. Many theories of how wings generate lift are all seemingly correct. one needs to consider them all and will yet will still conclude that, at least in this universe, Newton is still correct. For every action there is an equal and opposite reaction.

So, stick to the official approved "doctrine" when answering test questions or oral examiners questions in order to pass the required testing. Remain open to new ideas from young minds, that is there job. Newton also commented on laws of inertia that many people hadn't considered, "inertia increases exponentially with the size of the bureaucracy".

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  • $\begingroup$ Hi and welcome to the site! Could you edit this so it is less of a wall of text and more readable? $\endgroup$ – AEhere supports Monica Jul 17 '19 at 16:39
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    $\begingroup$ @AEhere The author of this post mistakenly used single line breaks instead of paragraph breaks; I've edited the post to fix those. $\endgroup$ – Terran Swett Jul 17 '19 at 17:04
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    $\begingroup$ @TannerSwett Thanks for the fix. $\endgroup$ – AEhere supports Monica Jul 17 '19 at 18:44
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    $\begingroup$ Sadly this does not appear to answer the question at hand, and contains several inaccuracies as well, like the bit about Newton (not universally applicable, breaks down near the relativistic limit). $\endgroup$ – AEhere supports Monica Jul 17 '19 at 18:51
  • $\begingroup$ Also, I'm not (and wasn't when I asked the question, either) studying for any test or examination. I'm just curious. $\endgroup$ – Sean Jul 17 '19 at 20:39
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There is no more "large area of low pressure above the wing" in the stall. On the opposite, stall is when the "void" on top of the wing sucks in the air from the beneath the wing through the area of less resistance: around rear edge. It releases the pressure between over and under- the wing drops. One should NOT! be thinking about Bernoulli principles when concept is same as explaining the lightning strike: critical imbalance of negative and positive currents (or pressure in our case) leads to a dramatic discharge when opportunity arise.

Seeing is believing: lots of YouTube videos show people attach stripes to the wings and fly them to stall visualising the air currents. I.e.

You'll see how the story happens: in the beginning all is normal and the stripes are aligned with the headwind ok. Then another current starts from the wing rear side and pushing the stripes around. The two currents ARE fighting for the wing. You can see the headwind is loosing the combat as the battleground (see those chaotic stripes) moving forward and all the stripes behind it are turned the opposite way, not because of a tailwind (the aircraft is not flying backwards!),but because we have a lot of air escaping high pressure zone beneath the wing and filling low pressure zone over the wing. And it goes from around the back of the wing. The lift destroyed.

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    $\begingroup$ Hi and welcome to the site. While the video is a good resource, your explanation does not address stall modes where the stagnation point remains at or very near to, the trailing edge. Recirculation around the trailing edge is not a requirement for stall, iirc. $\endgroup$ – AEhere supports Monica Jul 17 '19 at 21:47
  • $\begingroup$ Can you find a video showing the "modes" you referring to? $\endgroup$ – Apatity Jul 18 '19 at 11:13
  • $\begingroup$ Not a video, but it serves my point: ars.els-cdn.com/content/image/… $\endgroup$ – AEhere supports Monica Jul 18 '19 at 11:18
  • $\begingroup$ More specifically, my point is that there is no need for the flow under the wing to move into the area above it. The flailing strings in your video are more likely to be caused by the shed vortices above the wing than by airflow coming around the trailing edge. Your explanation is easier to visualize and understand on an intuitive level, but not correct for the general stall case. $\endgroup$ – AEhere supports Monica Jul 18 '19 at 11:40
  • $\begingroup$ @AEhere "More specifically" there IS the need for air or fluid to travel from high to low pressure zones. God created the need and physics described it. The "more likely" meaning you're fantasising. So you are coming up with your theory, but as long as it is useful to others and explains some cause and effect it means nothing. Yours is nonsense because it needs a miracle to counterbalance the lift. Same as Radu094 who implies that at the 16 degree pitch wing reactive force directed mostly backwards. Sorry, if you don't like my theory, just ignore me. $\endgroup$ – Apatity Jul 18 '19 at 12:21
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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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    $\begingroup$ 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. $\endgroup$ – Dan Hulme Sep 19 '18 at 7:40
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    $\begingroup$ 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. $\endgroup$ – Aaron Sep 19 '18 at 16:35

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