OK, a bit of background first.
My 3DR Y6 had been sitting in the attic for a couple of years, so I dusted it off and got it back in the air. I still need to tune it, but I flew it first in front of my house (just hovering), and then decided to take it to a field near by and do an auto mission. I should point out that I reset everything, but the drone is flying OK (as I said, I still need to run Auto Tune).
Anyway, last flight lasted a bit under 10 minutes, and I am checking everything now. When I looked at the vibrations, I am OK in X and Y, but rather high on Z.
My question is how do I get rid of vibrations on the Z axis? I would imagine X and Y are the props, but what causes Z vibrations?
I had this happen to my copter after a benign modification. I have very well balanced props and they are checked on a Dubro balancer. I changed motor mount because I thought they would be more secure, but they added 20 g and extended the arm length. After getting vib warnings I reviewed some logs and saw that my z vibes greatly increased after the mod. I checked the fft plot in MP and there was a frequency peak consistent with my prop rpm(from the charts) * 2, two blade prop. If the resonant frequency or harmonic coincides with this frequency then the is a tuning fork effect.
In my case I reverted to the old hardware config and all has been well.
To do an fft on a log hit Ctrl f in MP and it will take you to the secret functions ;)
You could try different props and thus rpm's and see if this has an effect. Or change the arm length or end mass.
The part that I do not understand is that before (a couple of years back, when it was flying), it was OK in the vibrations (of course, MP now changed the way they are measured, so I am not sure that is not the reason). I was thinking maybe there is one (or more) props that is balanced, but that is not sitting perfectly parallel to the ground?
The other thing is that it says "Maximum acceptable values appear to be below 60m/s/s" on the documentation website (http://ardupilot.org/copter/docs/common-measuring-vibration.html), and this is actually below 60, so I do not know if it is OK or not. It does say that you should "graph the Clip0, Clip1 and Clip2 values which increase each time one of the accelerometers reaches it’s maximum limit (16G). Ideally these should be zero for the whole flight but low numbers (<100) are likely ok especially if they occur during hard landings. A regularly increasing number through the log indicates a serious vibration problem that should be fixed", and mine are well under 100 (3 is the highest value) but one does increase to 3 and another to 2...
By the way, what do you mean by FFT? I will try to look it up tonight...