We have had a little niggle with our SkyRanger for since the beginning of the year and it is really making us scratch our heads! After 15 minutes of flight, we have noticed a vibration if we throttle back between 3,800 and 4,200rpm; outside those RPMs the aircraft is smooth.
So, we went about replacing the spark plugs, balancing the carburettors, etc., balancing the propeller, but the vibration still remains. We had a Rotax technician over to check the engine and I took him for a flight to demonstrate the issue. He dropped each magneto in turn, but it didn't make any difference. His suggestions was that the needles in the carbs may have been slightly worn making the mixture slight rich when throttling back, so we went about replacing the needles.
Another test flight followed, but the vibration was still there!! Our engineer has double checked everything on the engine and is convinced that the vibration is airframe related and not caused by the engine!
Our next step was to check the engine mounting so see if they were worn allowing too much movement. The lower mounts seem to have more movement in them than the top mounts, so we replaced them. There is very little movement in the mounts now. Unfortunately, we have not had flyable weather since that work, so we will have to wait to see if that has cured it!
If you have any ideas, let me know!!
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