Very exciting, certainly worth going for.
Just remember, because it's stationary, it my not prove conclusive.
There is certainly something about the aircraft "flying into" the slip stream/ prop vortex/ prop tip air disturbance (or what ever it is) that needs to be understood.
If the plane is fixed and only allowed to rotate around the vertical axis/CG the gyroscopic effect of the rotating prop mass will possibly give a false picture. A quick test with my indoor foamy hung by a tread at the CG shows a rotation to the left, as throttle increase and when the throttle was increased further and the plane established a 65-80 deg vertical angle (still held stationary on the thread) the continued to rotate left but this is almost certainly due to a rotation opposite of the motors rotation/trust and not the yaw effect we're discussing.
In hover the plane naturally rotates left(torque rolls) but in forward flight, remained horizontal.The engine torque being easily countered by the wing.
The left yaw seems to be some momentary distubance left from the prop tip that effects the fin... as it flys into it!
As mentioned in the orther thread, it's opposite when the aircrafts inverted/fin at the bottom.
Good luck and post the results.