I ran the lazy susan test again, but this time I suspended the plane ½ wing span above the lazy susan with a 1” diameter tube. This gave the plane airflow which was undisturbed less the 1” tube for ½ wing span below and basically infinity for my purposes in all other directions.

There was still some left yaw, but it wasn’t nearly as large as the first test. It took about 3-5 degrees of right rudder to correct, with the most rudder needed at mid throttle settings. Aileron had no effect. However, elevator did. Up elevator seemed to not make any diff, but down elevator seemed to nearly cancel the left yaw. The threads on the rudder again showed there was a spiral flow, at about 2-3 in degrees.

Again, this is stone knives and bear skin testing, but from this I think it is safe to conclude that if the spiral effect is a factor in yaw for typically flight, it is a small one, at least on the particular model I tested. It may also be possible to conclude that the spiral effect seemed be enhanced in ground effect and/or from my test stand.

To me at a personal level, this seems to debunk spiral slip stream as a significant factor, which really only leaves p-factor. Sorry it took me so long to get to that point, but I don’t consider lack of equations proof and the reverse spiral explanation which couldn’t be true threw me off track some.

The problem I now have is similar to britbrats observations in his last post. I've flows a lot of planes, but in recent years they are mainly limited to 2M pattern ships. Reasonably fast, clean, high power. I typically need to carry 1.5 to 2 degrees of right thrust for good yaw tracking. Why? In level flight the positive AOA on the thrust like is probably about zero. Same for verts. So p-factor can't be a major player can it? If p-factor is significant then when comparing level flight (low AOA) and a loop (high AOA), I should see a significant yaw, yes?. But I don't. Are we back to spiral slip stream? This is making my head hurt.

Cheers.