Sorry guys -

In reflecting my earlier post, I now think my earlier reasoning is
invalid. Yes there is a second harmonic (80 Hz) yaw moment generated
within the engine, but the rotating propeller will also change the
reflected yaw inertia at 80 Hz.

Therefore, now I don't see how this coupling could make a 40 Hz shake.

Can somebody shed some more light on this? I know the orientation is
critical on a 172M, and I've seen a couple of homebuilts that had
similar prop orientation problems too.