I don't recall anybody saying the stock 2 rotor shaft is not up
to 10,000 RPM.  The other problem
is the single rotor torque pulse goes negative once per revolution
just like a piston engine.

IMHO it is better to change the airplane :)

Paul Lamar ...No rotor no motor.

Paul,
I agree with all you say. but the inference was that a cut down twin
shaft would not be up to the task; and would need to be cut from billet
or a special forging. There is no history to suggest this and if we
start with a twin shaft, half the machining has been done already.
The single shaft strength and balance questions are minor compared to
the torque pulse. Even 4 cyl Jabiru engines can tear the center out of
carbon fiber props.
IMHO the torque pulse is the real problem, yet so far, I know of nobody
who wants to address it.    Elastomeric couplings  have had marginal
success and have their own vibration problems.
BR, Dave McC

I agree. Absolutely nothing wrong with a cut down shaft.
The torsional resonant frequency of the Elastomeric couplings
must be half of what it is for a two rotor. That is a soft
coupling indeed.

A four cylinder engine still goes negative unlike a two rotor.
Probably about the same as a one rotor.

Paul Lamar ...No rotor no motor.

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