I'm not with he P & Gyro factor. I mentioned tip weight some posts ago and I'm still going for that. Here's the deal based on what I've experience with a meticulously laterally balanced aircraft; Perfect hand of vertical verifying the right thrust but a peculiar slight yaw (or what appeared to be yaw) to the left then pulling or pushing. As mentioned, this was cured by the addition of 15g of right wing tip weight. Expecting some noticeable trim changes on the first flight out, I experienced non, nothing, nada... except the yaw problem had vanished. Now I know the typical problems you get with a heavy wing and the tests, and cures to fix it, but as mentioned, this aircraft was perfectly balanced previously and here I am now flying and aircraft that is distinctly unbalanced... yet now flew superbly! Why?
Here's what I think: The natural effect of the engine torque is to rotate the airframe in the opposite direction, to the left when viewed from the back. 99,99 % of the time this is unnoticeable, the wings/physics countering this and holding the aircraft in equilibrium. Now for some reason, could be wing shape, prop choice, cg or what ever, that equilibrium I can be very slightly disturbed during a pull/push causing the airframe to slightly roll left. We're talking minuscule amounts... but it's enough to appear that the aircraft is yawing left. Why this happens, how 15g of wing tip weight can result in no noticeable trim changes is as mentioned, a can of worms I never bothered opening.