I’m really curious in finding an answer to this issue, so I did some stone knife and bear skin testing. I’ll be the first to admit what I did isn’t the best test, and I don’t have wiz-bang measuring instruments like a research lab would have, but it is the best I can muster.

I have an indoor electric I just completed that I know has thrust set to 0/0. I verified that as best I can. I also have a lazy-susan contraption I made to help set revo mix on micro helis. I strapped the plane to the lazy susan such that movement is restricted to yaw only.

I fired up the motor and the plane instantly yawed to the left. It took about 10-15 degrees of right rudder to cancel the yaw. There didn’t seem to be a big change in rudder required based on throttle.

I then fixed thread to the top and bottom of the rudder and fixed the lazy susan so it could not move and I set the rudder back to neutral. When throttle was applied, the string at the top was blown rearward and to the right (looking from the top) and the bottom string was blown the opposite way. For this to happen there must be a pressure differential across the rudder fin that is opposite top to bottom. To me this clearly indicates a spiral slip stream with a rotation in the same direction of the prop, i.e. counter clockwise as viewed from the front. From watching the threads, I can see where with proper rudder fin design it may be possible to effectively cancel these forces such that the yaw component is null. In the case of the test plane, I’d have to increase rudder/fin area below the fuse centerline, or decrease area above.

Now my question on all this is what really caused the yaw? Is it really slip stream or something else? P-factor cannot be an issue in my test as there is no forward movement at some AOA, so the thrust symmetry is equal left/right. I’d accept a slight thrust misalignment of the prop due to my measuring instruments (I’m only good to about ¼-1/8 degree), but even if we assume I’m out a full ¼ degree, would this require 10-15 degrees of rudder to correct? Somehow I don’t buy that. My gut keeps coming back spiral slip stream. I believe my test showed there is a yaw effect and a spiral slip stream within the confines of my test (zero airspeed, etc.) But are they related? This is really messing with me. Can anyone find something wrong with my conclusion?

Cheers.