What a “Stall” is in aerodynamics and why it’s an emergency for airplanes
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It’s a situation where a wing can no longer do its job, and can’t you know, keep the plane in the air, it should be very obvious why this is an incredibly dangerous scenario
as to why this happens, well air has to move smoothly over the wings to properly create lift, however in some situations, especially when at low speed and with the plane pitching up, the air can no longer flow smoothly, so the wings start producing turbulence and stop producing lift
A wing gains lift from the pressure difference between air flowing over the top and bottom of the wing, low pressure on top, high on the bottom. A stall is basically when the flow of air is disturbed or reduced to the point that the pressure difference isint enough to create lift.
Its an emergency because when the wing stalls, you stop flying and fall out of the air.
Plane wings work by taking force from the plane moving forward through the air, and turning it into force that pushes the plane upward. If the plane isn’t moving forward with enough force, then the wing won’t be able to keep it from falling
Planes operate on the principle that airflow over the wings generates lift, which keeps the plane in the air. A stall occurs when there is not enough airflow to provide enough lift to the plane; that means the plane can no longer maintain altitude and will begin to descend, potentially uncontrollably. If the plane simply noses over towards the ground, that might not be a huge issue.
However, if the plane falls straight down in whatever orientation it’s in, or falls down backwards, then there’s a very real, immediate risk that the dive is not recoverable. The plane is designed to be flown and controlled in forward-flight; all the control surfaces (like the rudders and elevators and ailerons) that let the pilot control the plane are only designed to work when the air is moving from front to back of the plane. If the air is moving from the back to the front, those controls may not operate correctly. If they don’t operate correctly, the pilot cannot control the plane and therefore cannot recover from the stall/dive, which means the plane **will** crash.
If the plane noses over, the pilot has the ability to recover the control of the plane. However, an uncontrolled dive can still be dangerous; as the plane flies faster and faster downwards, it will be harder for the control surfaces to operate, since they are pushing against the air to turn/twist the plane around. If the dive is too fast, the control surfaces might not have enough power to actually control the plane, and the plane will simply nosedive straight into the ground. The pilot must regain control of the plane as soon as possible to prevent the airspeed from building up so much that they permanently lose control over the plane.
A stall is a condition in which your wings no longer generate lift because airflow is disrupted over your wings. To generate lift you need air to flow with the win, when the flow separates you no longer have lift and the flight control surfaces on the wings no longer have an affect on the aircraft. This typically happens when your angle of attack is too high; that is the angle at which the aircraft’s nose is pointed up/down vs the angle at which the aircraft is traveling. e.g. when you are taking off or landing, the noise of the aircraft is going to be pointed up but the aircraft isn’t climbing at that steep of an angle.
Lift occurs because of the pattern of airflow around the wing. Stall is when that airflow gets sufficiently disrupted, causing the wing to no longer generate lift, and also typically stopping all of the control surfaces from working correctly. So you have a total loss of lift, and at least a partial loss of control, and you have only a limited amount of time to fix the issue causing the stall before the aircraft slams into the ground.
Take a spoon and open the tap,
Do what you see in this video, you can just hold the spoon with your hand https://youtu.be/AvLwqRCbGKY
What you see is the Coanda effect, air on top of a wing does the same as water on the back of the spoon.
A stall is when the angle between the wing and the air direction before the wing (angle of attack) is too big so -the air stops to follow the wing profile-.
Try with the spoon. You will see that as long as water sticks to the spoon, it will affect the spoon, as soon as it separates, the spoon is no more affected by the water.
With a wing stall you get two big problems:
1 you completely lose wing lift, you fall.
2 a deep stall may affect your ailerons, and trying using ailerons in a stall will make the wing you want to lift to stall even more, and opposite, may recover the wing you wanted to lower, so that wing will stop stalling and lift up. The result is that you left or right wing drops more than the other wing and you will spiral down uncontrollably.
The only way to get out of a stall is to change the angle of attack, usually by a dive nose down. Now, you can imagine that going nose down until the stall ends is already scary, but if the stall happens close to ground, you are left with the choice to 1fall to the ground, 2nose down to the ground, or if you use the alieron or you are already in a turn: 3you will spiral into the ground.
Good news: planes are fitted with stall detectors, a trained pilot will prevent the stall by reacting to the warning. In training, pilots also purposely stall the plane and recover it to practice with how it feels and how to do it best. Stall is one of the basic bricks to build a pilot. Also, pilots know their plane performances and fly always a good margin away from stall conditions.
Some good answers here that approximate the phenomenon. But the following should fill in the gaps:
The angle at which an airfoil meets the air is called angle of attack. As angle attack increases, lift and drag also increase, but lift increases faster than drag.
A stall is when an airfoil’s angle of attack exceeds its *stall angle*. The stall angle is where the ratio of lift to drag peaks, after which drag increases precipitously compared to lift, which may increase for a little while longer before precipitously decreasing.
This happens because past the stall angle, formerly smooth airflow that followed the airfoil’s top surface separates because its inertia overcomes atmospheric pressure holding it against the airfoil. Air leaks around the trailing edge into the cavity left by the separated flow. This leakage and turbulence both destroy lift and increases drag, effectively causing the wing to stop working.
In addition to forcing the airplane to descend, chaotic flow around the stalled wing reduces control effectiveness and can cause unpredictable departures from controlled flight, wnich for some aircraft may be unrecoverable.
[Here ](https://youtu.be/L2CsO-Vu7oc) is an example which demonstrates the hazard. This occurred during a test flight. The pilots intentionally stalled the aircraft, which unexpectedly rolled inverted and gathered an alarming amount of speed while losing an even more alarming amount of altitude. Only the pilot’s perfect recovery technique prevented the aircraft from breaking up due to aerodynamic stress or becoming an expensive lawn dart.
When you waterski, you will start sinking if you move too slow, or if the ski is sideways. The ski generates lift by moving across the water, and an airplanes wings generates lift by moving across the air. In the same way, an aircraft will start sinking if it moving too slow, and if the wings are sideways.
Normally when a plane flies it goes – roughly speaking – straight through the air and the wing deflects the air downwards, pushing the plane up. Both the bottom and the top deflect air downwards. It’s quite easy to imagine how the bottom deflects air by simply hitting it are an angle (probably not exactly accurate, but this is ELI5). The top is shaped so that it pulls a vacuum and the air rushes downwards to fill it. This only works if the wing curves gently so the air has time to deflect to follow it.
Stall is when the wing isn’t air stops flowing nicely over the top of the wing and being deflected downards. If you imagine a wing at a really steep angle to the air, like 45 degrees, the air isn’t going to follow over the top at all. From the perspective of an air molecule, it goes past the edge of the wing, and then the wing curves away from it way faster than the air molecule goes down. So the air molecule doesn’t follow the wing – it keeps moving in mostly a straight line. So the wing isn’t deflecting the air down, so the air isn’t deflecting the wing up, so the plane falls out of the sky.
Luckily pilots know how to avoid this and also how to get back to normal if it does happen.
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