I’ve approached my vibration issue from several perspectives, based on advice from forum members and others with similar problems. Not sure my experience will be helpful, but efforts included:
Engine induced vibration:
1 Carb balance
2 Prop pitch adjusted to be identical to 0.1 degree
3 Engine gearbox shim checked and adjusted
4 Prop static balance
5 Dynamic prop balance.
6 Spinner checked to confirm “true” while rotating
Engine vibration propagation/magnification:
1 Checked engine isolators to confirm correct (Notification 17-05-19)
2 Checked all hoses and wires that were connected to both the engine and airframe to confirm there was some play, minimizing vibration transmission between the engine and airframe
3 Checked for “oil canning”. Identified an area on both fuselage sides immediately behind the firewall that had the potential for oil canning, and installed additional stiffeners
4 Checked all engine component clearance from cowl
Non-engine induced vibration
1 Spinning nose wheel resulted in a significant vibration when tested on the ground. Attempted to balance wheel, but vibration persisted. Wheel found to be out of round, replaced, balanced and vibration associated with spinning the wheel resolved in ground testing
2 Braking did not impact vibration while airborne, suggesting spinning of main wheels was not contributing to the problem
3 Removal of wheel fairing did not impact vibration
4 Air seepage under canopy. Checked and added seals as needed.
5 Confirm/trim nose gear fairing to ensure it is not vibrating against bottom cowl
At this point it is clear the vibration is related to RPM (not airspeed), and is minimal at 4000-4800 RPM. I’m not sure which efforts were helpful, but the vibration has gradually improved to what I would now describe as a “buzzing” in the rudder pedals and canopy frame. At 65 hours total time, the issue is better, but not completely resolved. I would like to redo the dynamic prop balance (it was done at 4000 RPM with a relatively simple set up) with someone more experienced than me interpreting the results.
As your problem is intermittent, and unless you can relate it to select RPM, would be sure to check non-engine induced vibration, along with the usual carb balance, prop pitch etc.
N12BB RV12 flying since Oct 2016
Dues happily paid