This is just my intuition/gut feel; but looking at that wear suggests before I’d spend any time analyzing resonance vibrations, I’d look at lubrication/seal material as the cause(s) of this problem. We should know where these seals came from; and none of us should be using them in our engines.

Al

I finally got time to get my failed 13b apart. The rear rotor was missing one apex seal, the other 2 were intact but jammed due to the fragments of the broken one. The rotor housing has the classic chatter marks all around. Doing a Google of "13B apex seal chatter" results in plenty of examples of this failure mode. Apparently the chatter is from the natural resonant frequency of the seal, it eventually fatigues the seal causing failure.

The front rotor was in good shape, but the housing had the same chatter marks, but not as severe as the rear rotor. The front rotor housing was brand new 150 hours ago when I rebuilt this motor after the Grand Canyon forced landing. At that time the good rotor housing had slight chatter marks but I reused it. The front rotor was destroyed at that time due to foreign object entering the rotor.

Both rotors were missing the  corner seal rubber plugs on the side with the short apex seal segment. Perhaps the small segment vibrated more, disintegrating the rubber seals?

The first 260 hours of flying my rotary bird was with stock 3-piece seals, and I never saw any chatter marks. The last 250 hours has been flown with 2 piece seals and now I have chatter marks .

I now have Tracy's 2 piece apex seals and I wonder how they will perform, it appears that the heavier weight of the 2 piece seals puts the resonant frequency in the cruising range of 6000RPM?

Chuck Dunlap

RV6 13B