how perfect is the flybar balance? Its very easy to forget to ballance the flybar... and that can create all kinds of vibration problems.

How well do the blades track? If the blades aren't tracking... you have a vibration from the wobble of the rotor disk.

And what no one ever checks do the flybar paddles track with each other?

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Other end of the heli... hows the balance and tracking of the tailrotor? (yes, the tailrotor can have the pitch of the 2 blades not adjusted for proper tracking.)