Propeller Spiral Slipstream

Question

I know the common accepted answer is that in an airplane, the propeller pushes air back but imparts rotational flow to the air, and the rotating air around the body of the aircraft hits the back vertical tail fin, pushing the fin to the right, causing the nose to go left.

What I don't understand is why the air spirals around a propeller at all. Spiraling air has both a backwards motion and a circular motion. In order for their to be a circular motion there needs to be an inward radial acceleration. If the propeller were enclosed in a tube, I could see why the air would swirl while being pushed back because the tube would prevent the air from flying radial outward and impart an inward radial acceleration. But for the case of a propeller on a small airplane, or just a regular fan, it isn't inclose in a long tube to contain the flow, and wouldn't the surrounding air not impart enough of a radial inward acceleration to make it spiral?

I have not been able to find any good simulations or airflow videos of this phenomenon, only the poorly illustrated and overly exaggerated pictures of the propeller slipstream.

Answer 1

A propeller doesn't just push air. It is an airfoil, much like the wings are. Air flowing over the propeller generates lift in the "upward" direction and drag in the "backward" direction.

Given the orientation of the propeller, "upward" is toward the front of the plane. This lift pulls the plane forward. There is of course a reaction force that pushes air toward the back of the plane.

Drag is perpendicular to the axis of the propeller. It would slow the propeller, except the engine keeps it spinning. The reaction force pushes air in the direction the propeller is spinning.

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At any instance, the propeller's drag pushes air in a direction. The propeller spins, so air gets pushed in all directions around the axis. This adds up to a spinning column of air.

Without other forces, momentum would keep each bit of air traveling in a straight line. The spinning column would expand. This would leave less air in the column, reducing the pressure inside. The higher pressure outside then provides the centripetal force.