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Peter's answer to this question leads me to another question.

How does the advancing wing in a flat spin create nose thrust?

I imagine this like some object spinning in a liquid. I would think that the motion of each wing would only create drag that acts as torque opposing the motion of the spin in the geographic plane of the spin that would tend to make the spin slow down rather than be maintained.

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A very good question, considering so many people do not draw the thrust vector for gliders.

The wing generates thrust by converting energy from falling to forward motion. From scratch building gliders (and reading) one comes to realize that a falling parachute (straight down) only uses drag to reach a constant rate of descent.

A glider, with its center or gravity off set from its center of pressure (including tail!!!) will start to move sideways (glidogenesis) and generate lift with its wing. This motion, known as "thrust" in this example, uses altitude as fuel and falling as its engine.

Notice both wings may be "thrusting" as it corkscrews down, but the outside wing, with its lower AOA, generates more lift/thrust and much less drag, sustaining the spin.

Once yawing is stopped, the wings equalize, and can be unstalled by lowering angle of attack.

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  • $\begingroup$ I see. So it would be better to envision the airplane like a falling rotor blade being struck from the bottom and pushed forward since the plane is tilted nose-down. $\endgroup$ – Ryan Mortensen Apr 6 at 4:31
  • $\begingroup$ From the auto rotation point of view, yes. You can make a business envelope glide by offsetting drag from falling and center of gravity, and nose down also makes it easier to recover. It is powered flight (gravity). $\endgroup$ – Robert DiGiovanni Apr 6 at 8:28
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I think Peter's diagram explains it well, I've repeated it here:

enter image description here

You can see that for the advancing wing (right side of the image) the vector addition of $V_\infty$ and $\omega_z \times y$ gives a resultant force R, that has a component pointing forward. This is forward thrust and increases the spin.

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