Brent,

Let me distill a couple of your comments.

Resonant frequencies. Obviously unknown at present, but a question arises as to measurement. Is there an instrument that can be attached to the engine somewhere that will measure frequencies to assess likely nodes of "don't run there?"

The fatigue limit for steel vs aluminum. Given that the major aluminum item in my setup is the engine block, with a secondary unit (PSRU case) of aluminum, it would seem that the fatigue life for steel is best tested at 50+ hours, and that's the major concern for me.

Helical thrust loading: That's why Timken helicopter experts were involved in designing the gearbox. As for cooling, we haven't been able to get the PSRU oil temperature above 120 degrees on the ground, even during a 5 minute full power run. That doesn't seem to be a high probability failure area. Fatigue failure in that unit? That seems to be the most likely, if at all, aluminum failure possibility, and that would be case cracking. Something I'll have to keep inspecting for a while.

Ted Noel