Nice writeup, Angel!
I'm a little confused about what the motor rotation frequency represents, or rather what influence it has. To my layman's understanding, and assuming a well-balanced motor/prop combo, doesn't most (?) of the vibration happen due to a prop blade passing over the arm (or other obstruction)? Which would be 2x the RPM, assuming it's even relevant. I've definitely seen what you describe in terms of some RPM ranges setting up horrible resonant frequency vibrations elsewhere on the MR, often in the props themselves, as well as the arms and LG like you say. Seems to happen most at RPMs below normal hover/50%, but that's based on very limited observation.
Can any of this help determine the right material/application to use for dampening something like a camera, or perhaps even the FC itself? Or isolating the arms/LG from the body. Seems like a lot of that stuff is guesswork/trial & error at this point in our hobby, with many attempts just making things worse. Is it "just" a matter of analyzing the most common/disruptive frequencies, say during a flight? And we even have this data in the AQ ACC logs, but need a way to compute frequency averages? (Is lack of sleep catching up with me?
)
Cheers,
-Max