The thing that jumps out at me is that the problem is worse on the 20B. Of course they have more total power, but they also have 1/3 more cyclic loading due to the extra power pulse per rev. What matters for machine design and fatigue rating is not only the magnitude but the number of cycles. A 20B would have roughly the same peak loading, but is 33% older in fatigue cycle terms than a two rotor. Any TBO issues will show up first on the 20B installations because of this.

 Monty

Good point, Monty. There are more cycles and the peaks are the same, but the cycles are of lower amplitude because of less ‘decay’ between pulses.  I’m not sure how that plays out on the fatigue rating.

The other question is fatigue a significant factor on the wear seen here, or would fatigue be exhibited in appearance of cracks in the rivets. Of course I haven’t looked at the rivets under a microscope to see if there are cracks.

Al