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Is a spin a by-product of a stall? So if a airplane stalls is a spin guaranted sooner or later?

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If you are in a spin your wings are stalled but not all stalls are spins.

This description puts it pretty nicely:

If a stalled aircraft is subjected to a sufficient yaw rate, it will enter a spin. The yaw can be induced by improper use of the rudder or can be a result of the wing drop characteristics of the aircraft in question. In a spin, both wings are in a stalled condition but one wing will be in a deeper stall than the other

You can keep an aircraft stalled without spinning it, and the spin will only occur when some kind of yaw aggravation induces it. But to dig into that statement a bit there are two things to note "improper use of the rudder" and "wing drop characteristics" these things can be common in lots of scenarios which is why stalls are a maneuver to be done with caution and at least one eye on the ball at all times. Spins are not "guaranteed" as long as you are in proper control of the yaw axis.

If you want some examples, scenarios like that which AF447 encountered lead to a stall that did not become a spin. More benign, all student pilots are taught stalls and stall recover generally fairly early on in training, broadly these dont lead to spins or at least should not if the instructor is paying attention.

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You can think of a spin as a special case of a stall, in which one of the wings is mostly stalled and the other is mostly not stalled. the wing that is still mostly flying and generating lift rolls the airplane up on its side and points the nonflying wing down and the plane then swings nose-down and begins rotating around the roll axis with its nose pointing steeply towards the ground.

This means it is a common occurrence to enter a spin right after experiencing a stall if one wing resumes flying before the other does.

Since the plane's propeller is swirling the air around the fuselage and wings in one direction, the propwash encounters one wing from below and the other wing from above. This results in a plane that "wants" to spin more easily in one direction than the other, and can be brought out of a spin more easily in one direction than the other.

So... a spin starts with a stall that then develops asymmetrically. Depending on the design of the plane, it may be possible to control the plane through a stall so a spin does not develop during the stall break or the stall recovery. In other planes it takes skill to avoid a stall-spin event and even greater skill to successfully recover from one.

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A stalled airplane is moving straight ahead and the airflow disruption is more or less the same on both wings, mostly separated but with some attached flow near the tips giving aileron control and still providing some lift and driving the plane forward somewhat, in a straight line.

If you hold back one of the stalled wings and make the airplane yaw, the held back wing's AOA rises, so it stalls more and becomes a sort of anchor, and the opposite wing moves ahead, so its AOA decreases and partially un-stalls a bit more.

The unstalled part of the wing is still making lift and the lift vector is producing a thrust component that drives it forward, as when gliding straight, so, the thrust being on one side only, the autorotation starts.

One wing is trying to glide forward, but the other wing is kind of stuck in the mud, so the gliding wing circles around the stuck one.

The center of rotation in a spin is generally around 1/3rd span on the fully stalled wing, so the inside tip might actually be moving backwards a bit.

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