Since moving to high-end motors and props I have had very low vibes in all my craft, so I was surprised to run into vibration issues in a recent build, my first X8. I did all the usual things of (re)checking all my motors and props for balance (both hub-wise and length-wise), checking prop tip alignment, etc, but still had the issue.
I thought it could be the turbulent air between the coax props that was causing it, and this thing has plenty of thrust to fly with only one prop in each pair so I did a test whereby I flew exactly the same load with fresh packs three times:
1) with both coax props
2) with only the lower props
3) with only the upper props
And voila... I see the usual low vibes with single props and much higher Z-axis vibes when running the coax props.
Coax Props (I flew this twice to be sure):
Lower Props Only:
Upper Props Only:
I'm still not sure what the general solution would be, but for now I've at least got the vibes within reason by adding damping at the motor mounts and additional damping under the Pixhawk.
When I get some time I'll try a modified motor config to spin the lower props in the same direction rather than contra-rotating, I suspect that might help (someone else suggested this would lessen the impact from pressure waves)
But for now I just wanted to point out my finding in case anyone else was struggling with the same issue.