ORIGINAL: gearup
...Do you think the rotation speed of the column is the same as the rotation speed of the prop? Unless it is, there appears to be a transition taking place, whereby the rotation of the column slows rapidly the further it gets from the prop. I believe that we are dealing with the properties of a viscous fluid (air) here that most closely moves with the prop at the prop's boundary layer interface with the airstream, and that the spiralling component rapidly degrades due to the combination of friction losses to surrounding air, tip vortices off of the prop, vaning effects of the flying surfaces in the slipstream, and the absence of any additional rotational energy being introduced after the prop interface. That being said, I also assume that the process of degrading rotation of the slipstream is a function of several factors, among which are: 1: elapsed time after prop interface during which friction losses accumulate, 2: initial diameter and mass of the slipstream, and 3: velocity of the slipstream relative to the surrounding air. Do you think I'm on the right track here?

If a 12" prop moves 6" forward in one revolution, the sweep angle is very roughly 76 degrees off the longitudinal axis of the fuselage [90-invtan(3/12)]. I know the profile looks more like a sine wave but I've just had a bottle or more of an excellent Pinot Grigio and I figure 76 degrees is enough bigger than the observed maximum 3 degrees that we needn't split hairs and it's safe to say the rotational speed in RPM of the slipstream is only a small fraction of the propeller RPM.

If these suppositions are accurate, we can imply the following:
1: Spiral slipstream will generally have greater effect upon short aircraft, than those with longer fuselages. Probably, depends on shape, configuration and areas of empennage.
2: Spiral slipstream will have diminished effect at higher airspeeds. Definitely
3: Spiral slipstream will be diminished by smaller prop diameters. Not smart enough right now to comment intelligently on this, but I think probably so.
4: Spiral slipstream is not one of the major factors affecting most prop driven aircraft. Definitely not true, as stated, based on my knowledge of current literature..
5: Spiral slipstream likely exists near the prop, but it's effect and even possibly it's existance diminishes as we move aft. All tractor aircraft are likely affected in some way (even twins) by spiral slipstream even if it's minor (yaw and/or roll). I think spiral slipstream exists under at least some conditions for all propeller driven aircraft, but of course I offer no proof. The rotation, though slow and slowing, I think continues well past any likely fuselage length.
6: Canard Pushers are not affected at all. Don't know. Likely negligible, unless the "upwash" (inwash? suckwash?) begins to rotate before it reaches the prop disk, which seems possible, if one considers the behavior of wing upwash. It seems remotely possible there might be some interaction with a vertical stab/rudder very close to the prop..

HAPPY NEW YEAR ALL