I have had a couple of airplanes that would not loop straight but flew well otherwise. Turned out that the vertical stab was at a very small angle. The vertical stab can be checked for straightness by tying a string to the prop at 12 o’clock and holding it tight on the right side and then the left side of the stab. It the stab is off you can observe a small difference.

The rudder is more effective at slow speeds and at the top of the loop. Engine yaw also seems to be more pronounced at slower speeds and probably should be fixed first. Never did figure out how to fix the vertical stab without major surgery so I just flew it.

Bill