A stall occurs when you exceed the critical angle of attack, but apparently something called boundary layer separation is a part of that. So, does the boundary layer help generate lift? I get Bernoulli; I’ve read that the boundary layer forms around the airfoil because air has viscosity; I’ve read about the laminar flow, transition point, turbulent flow, etc.; but I don’t get the relationship between the boundary layer and stalls, specifically the relationship between boundary layer separation and stalls and if the boundary layer actually assists in generating lift.
Please explain this so a chimp can grasp it, and please try to explain it in a manner directed towards pilots. I’ll read an encyclopedia-length post if you take the time to type it.
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Thank you.
In: 4
Under normal conditions the air flowing over the top of the wing stays close to the wing and the pulling the air down contributes to lift.
At a high angle of attack the flow is disrupted. Instead of a smooth flow the boundary layer separates from the wing, the flow becomes turbulent, and because the air is no longer being directed downward there is no lift. There’s a nice photo [here](https://en.wikipedia.org/wiki/Flow_separation)
Like other matter with mass, air has inertia. Air curves around an aerofoil as it moves because atmospheric pressure holds it against the surface. The [breaking a ruler under a newspaper experiment](https://youtu.be/UvMmfacVA24) demonstrates this effect. With an intact boundary layer, the net movement of air above and below the wing is down, which draws the wing up.
As angle of attack increases, the curve that moving air must follow to remain “attached” to the aerofoil’s top surface becomes sharper. Eventually, the air’s inertia means ambient pressure is no longer sufficient to hold the airstream in place. As the airflow separates from the wing, air from below the wing “leaks” around the trailing edge and is drawn into the void left by the now separated flow. This creates turbulence, drag, and loss of lift because less air moves down than before the stall.
The reason a stall happens at the same angle regardless of speed is that inertia is proportional to speed, so that at slow speed the differential pressure between the top and bottom surfaces is be very small, and so too is the air’s inertia and the force needed for it to separate. Likewise, as speed increases, so to does differential pressure *and* the force needed to separate flow.
Lift is mainly generated by difference of pressure between bottom and top of wing.
Airplanes (when flyng below sound speed) are mailly “aspirated” from top. It is not the air directed downward that gives lift.
Boundary layer is the part of air between the wing surface, where speed is 0 (molecules directly in contact with wing surface are still) and the distance where relative speed between air and wing equals the plane speed. So its thickness varies with speed.
When boundary layer is broken and separated from wing back because of turbulence, caused by excessive attack angle or other cause like a storm, wing and all the plane is no more aspirated upward, lift collapses and stall occurs.
The shape of the wing is very important to creating lift.
Now think of what happens if you try to force your hand palm first through the water vs side first through the water. Side first, your hand will slip through the water and always remain in contact with water. Palm first and you will create a pocket of air behind your hand ( the water does not remain in contact with your hand.
When the boundary layer separates its like forcing your hand palm first through water. Rather than remaining smoothly in contact with the air over the whole wing, part of the wing creates a pocket of “messy air” that doesn’t follow the shape of the wing, which as we first said is very important to create lift.
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