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In the process of understanding the aerodynamics of spins, I'm having a hard time understanding why one wing drops. I understand that the for spins to occur, a stall + a yaw must occur. I also understand that both wings are stalled, but one is more stalled.

Let's say we're doing stall practice and a student puts in too much right rudder. The nose will swing to the right, causing the left wing to accelerate and the right to decelerate. This will result in more lift on the left and less on the right. What I'm having trouble understanding is why the right (inner) wing will exceed the critical angle of attack first. Why does one wing exceed the critical angle of attack further than the other?

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Think about the free airstream velocity vector of the two wings. Not just the magnitude, (airspeed) of the air, but the relative direction (AOA) of the air molecules when they hit the leading edge. Just like the rotor on a helicoptor, molecules hitting the leading edge of the aft moving wing are impacting the wing at a higher AOA than the ones on the forward moving wing. This is because the velocity vector of the air molecules, relative to the airfoil, must be determined based on the velocity of the aircraft through the airmass, and the velocity of each portion of the wing, relative to the aircraft, caused by the yaw rate. This second velocity is not aligned with the chord line of the wing (it is at a significant AOA), so the vector sum effectively tilts the relative velocity vector downwards, increasing AOA.

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