Yes. These also cause the lateral and fore/aft vibrations in the non-rotating plane which come through at (# of Blades +/- 1)*RPM.
Yes, that appears to be the case
That is interesting. I looked at all three axes and they all showed the same peak around 660 RPM. The other thing that makes this more difficult to pinpoint is that you have to rotors and their vibrations could add and subtract to make new frequencies.
Yes that would be the rotor or drivetrain resonating with the structure. This is the stuff that should scare you. I had a drivetrain that I built for an RC V-22. During my first run up I had a pretty bad vibration where the rotor was coupling with the tube that connected the two nacelles together. I had to go back and use a bigger tube to be able to get through that vibration to a speed where the vibration went away.
I've been using LOG_BITMASK = 397310 which gives me IMU data at 50 hz. There is 655358 which gives raw IMU but I'm not sure what rate that is. Remember that the placement of the Pixhawk can affect the vibrations it sees because of the aircraft structure. Also the Skookum is great but at the end of the day, the vibrations seen by the Pixhawk are what counts.