Gordon Arnaut wrote:
However, the prop is a spring mass that acts to amplify the
excitations that comes from other sources...

No. Blade root bending within the plane of rotation can be modeled
as equivalent to a shaft stiffness. The prop itself has no special
ability to excite anything.

And yes, it is the inertia produced by the centrifugal force, not the
centrifugal force itself ....

Centrifugal force has nothing to do with it, period.

However I do not agree that the problem frequency will necessarily
have to fall within the operating range. Stiffness of the shaft will be
largely a function of its slenderness ratio, so using a material with a
high modulus, perhaps carbon, and a large diameter, could produce a
shaft that is light yet stiff enough to do the trick.

Overall system stiffness is cumulative. Every shaft or shaft
equivalent (crank twist, belt or chain elongation, flexible structure
between sprockets, blade root bending, whatever) contributes so that
the overall system is somewhat less than infinitely stiff. Even with a
hell-for-stiff carbon shaft I don't think you can push F1 up above the
operating range. You'll need an F1 above 220 hz to work with a 5500
RPM 4-cyl, or 300 hz for a 6-cyl with a 5000 RPM operating range. The
current average for short "hard" systems (think Blanton style) is about
50 hz. Good luck.

Dan