Dave,
I have tried for many years to get a straight answer from both Helicopter nad Vibe analysis manufacturers into the CAUSE of n per revs. Never received a straight answer yet.
My best answer has come from reading "Practical Helicopter Aerodynamics" by R.W. Prouty. Chap 4 discusses blade rigidity and the N per rev problem. Other chapters cover vibrations and design. It provides answers for N per Rev with IAS. I think the paper by Stephen Jacklin provides some answers for the harmonics of N per in flight regimes at lower IAS.
If you haven't already read it, would recommend it.
Hope it helps in your Quest....
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