Christopher Tidy wrote...

I'm trying to figure out if there is any benefit in adding a flywheel to
a rotary phase convertor.

A flywheel would reduce, not increase, the idler's ability to respond to
load changes. When the electrical load on the idler increases, the
idler's rate of rotation falls (I.e., the slip increases). This raises
the current draw from the single phase source. The higher winding current
increases the strength of the rotating magnetic field in the idler, which
pushes the generated third leg voltage up. The upshot of all this is that
the response rate of the third leg voltage to electrical load changes is
inversely related to the inertia of the idler's armature.

That's my understanding. Perhaps one of the old regulars can explain it
better. Is Fitch still around? I seem to remember his doing some tests on
this very thing a few years back.

Jim