I have a vibration that began at approximately 30 hours. The sudden onset suggested that it was the result of some change brought on by a maintenance event, however I couldn't think of anything that preceded the event. The vibration occurred when the airplane reached cruise airspeed. Usually after the airspeed reached or exceeded about 90 KIAS at engine RPM's of 5400 and above. Climbs, yaw, roll or slips would not stop the vibration. Only slowing the airplane to 80 KIAS or below or reducing the RPM to below 3000 or 4000 would stop the vibration. Increasing the throttle to MAX would not change the vibration. Changing the prop pitch didn't change the vibration airspeed trigger- only the engine RPM (i.e.- the vibration still began at about 90 KIAS the RPM was lower/or higher).
All the customary fixes were done- balancing wheel/tires, making sure wires and hoses were isolated, balancing the propeller (.029 isp), downloading engine diagnostics and fault/error codes, checking control surface rigging and cable tension,
At 71 hours I pulled the gearbox and had it serviced. The only fault found was a 1mm under-shim and adjustment of the friction torque. The vibration persisted after reinstalling the gearbox.
Propeller experts advised me that airspeed related vibrations are usually caused by harmonics of airframe parts and not by the propellers. I embarked on a long list of possible sources. I installed a video camera on the tail and on the wing to view the main and nose landing gear, control surfaces, antennas, and sheet metal. I then installed a video camera in the empennage to observe the strobe, elt, and rudder/stabilator controls. Without any obvious signs visible, I added some stiffeners to some belly skins. I added supports to the battery box and oil tank. The vibration persisted. I rebalanced the nose gear tire/wheel and adjusted the linkage (nose gear steering) and tightened the nose gear bushing. I individually removed the wheel pants thinking that the change in mass or aerodynamic load would change or eliminate the vibration. Still the vibration persisted. I finally came to the belief that the vibration has to be in the engine/propeller and not any other part of the airframe. The vibration is a "BUZZ", felt mostly in the rudder peddles. Imagine if a vibrating sander or orbital sander was attached to the nose or tail of the airplane- that is what it feels like. The airplane has nose wheel steering and has a direct linkage to the rudder peddles. The nose wheel is part of the steel tubing frame that the engine is mounted to, so any vibration transmitted from the engine through the mounts would be felt in the rudder peddles and airframe. The engine mounts (new airplane/new mounts) don't appear to have any faults and obviously do a good job of isolation until the airplane reaches about 90 KIAS. There is only one other airplane model like mine with a 912 iS in the U.S. and the owner says that he doesn't have any vibration similar to mine. The only difference between mine and his is the design of the cowling. Visual observations don't seem to identify the cowling as the source of the vibration.
I was faced with replacing the Warp Drive propeller or doing more diagnostics on the engine. Downloading the diagnostics in flight didn't show anything unusual. I did notice that the vibration can usually be "triggered" by turning OFF lane B. Once the vibration starts it usually can be stopped by turning OFF lane A? (usually but not always)
After speaking with the service center and with Eric Tucker during Heavy Maintenance training, they advised me to pull the gearbox again for replacement of the gear set and dogs. So with about 150 hours on the engine I pulled it again and sent it to the service center.
