I reported here that my 912iSport, at 430 hours, had for a year or so, odd vibrations at an intermediate rpm range. Oil analysis at every change (50 hours - I run avgas) did not show any accelerated wear. I had done a dynamic balance and was really puzzled.
Now, at 470 hours, the mystery has been unpleasantly solved. Flying at 10500' over precipitous, snowy, isolated alps, at 5500rpm, a very heavy vibration suddenly began - something was coming very loose! I throttled down somewhat - the vibration got worse! But eventually I found a narrow rev range where the vibration seemed less likely to precipitate a sudden failure. But at those revs I could not maintain altitude without stalling. Gradually descending I flew down an alpine river valley to an isolated agricultural strip I knew of, preparing to do a precautionary landing, arriving there at 7000'. Making regular calls of situation I figured I would make it to a serviced field, as in the denser air I was no longer descending. After landing I had a mechanic help me search and we found an almost invisible surface crack in the gel-coat of the spinner. Removing it showed witness marks of vibration. With the nose-cone off I had the prop pulled, and, with the back-plate, cleaned, replaced and torqued, then dynamically balanced WITHOUT the nose cone.
The difference in balance, with and without the nose cone, indicated that the fibreglass nose cone might not have been laid up precisely evenly, and had been out of balance from the very start, compensated by the weights added to the fibreglass backplate.
But the imbalance had, at various speeds, been flexing the backplate (which also had witness marks to that effect) thus changing the radius of the implied 'lump' in the nose cone, and so defeating the dynamic balance somewhat when that happened, resulting in mysterious vibrations at other rev ranges.
So far, without the nose cone - which turned out to be moving towards delamination and potential (and perhaps fatal) total failure in flight, there are no mysterious vibrations, but if that should happen I will investigate and provide another report. Yes, the plane isn't so pretty without the nose cone, but it feels a lot safer.
I do not believe, in my case, that there is a gearbox issue.
