I think Dick’s experience explains why the airflow on the Kaos does not exhibit spiral airflow along the fuselage area or vertical stab. I did some testing of air flow on a medium size household cooling fan yesterday (safer than playing with streamers around a running aircraft engine) and found the same thing Dick discussed. The airflow is weak and somewhat mixed up in the hub area of the blades. But strength of the spiral airflow can be observed when you move the streamer out toward the tip of the blades. With the streamer up close to the blades and the blade tip it is very pronounced and less pronounced down stream from the blade. The airflow down stream from the fan looks almost like a hollow tube, strong at the outer ring and week and mixed up in the center area.

Like many of the 60s, 70s and 80s vintage pattern aircraft the engine on the Kaos is very tightly enclosed around the crank case and tapers down to a very small spinner and has a thin fuselage section all the way back to the tail. I expect the airflow along thin fuselage is being produced by the inner part of the blades and is a weak airflow, thus it does not show any strong spiral airflow component. But think about what happens to this part of the weak airflow after the aircraft is up to speed and flying 80 MPH or faster.

I noticed a comment in an on-line aerodynamics reference for full scale aircraft that the yaw impact from spiral air flow only happens if the vertical stab is tall enough to reach into the spiral component of the airflow. Very interesting.