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
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.
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