The picture you show and numerous others like it imply a rotating air mass, but wait, this effect is caused by lowered pressure in a highly localized area in the center of the tip vortice, which reduces the temperature below the dew point. The visible moisture is carried rearward in the boundary area between the slipstream and the surrounding, stable air, until it disapates because it looses the rotational velocity(of the vortice) to maintain the low-pressure/low temperature zone. The air in the slipstream is not rotating like the tracing of the vortice, but rather resembles a paper tube being extruded from a machine that wraps a flat piece of paper to achieve the tube. The location and direction of travel of each element of visible vapor is more directly behind the location of the prop tip at the instantaeous moment when that vortice portion was formed. In short, I don't see these pictures as proof for or against any spiral slipstream effect.

I didn't follow your statement that the apparent slipstream goes the wrong way to support the theory. Doesn't the apparent spiral, following the rotation of the standard prop, move rearward surrounding the extended crankshaft and perpendicular to the center of the prop disk? (in the situation where the prop disk is perfectly aligned with the airflow to the prop) If it does, then (the argument goes) it would strike the left side of the vertical fin, and absent another, equally effective (area x mean distance from center of mass on the longitudinal axis) surface below the center of mass, one result would be a net force moment to the right at the tail. (nose left) Obviously, everything over-simplified, but assuming other factors to net zero.

Regarding the prop imparting some energy in the direction of rotation, I definitly agree, again going to the wing airfoil analogy, in stable flight, the prop's slipstream moves rearward after passage of the aircraft, and the vortices generated by the wings move generally downward but also slightly forward. This implies generation of a rotational component by a rotating prop in the immediate area of the prop and cowl, and for some distance behind it. My Question relates to whether or not the rotation continues, and if so, how so? It seems that the laws of physics tell us that "a body (element) in motion will continue in the same direction and velocity unless acted upon by an outside force". What outside force changes the direction of our air particles after the prop has passed through them? Curves or spirals require an outside sustaining force, as air particles have both mass and velocity at all times relevant to this discussion.

Can you help me on this?