"Frank van der Hulst" wrote

It *might* be possible that diesel-electric system would be lighter than
pure electric or pure diesel.

I'm from Oklahoma. Show me!

A small diesel could be optimised to run continuously at its most
economical RPM,

It would already have to have a gearbox for a heli, so it can run whatever
RPM you want. An electric motor would also have a gearbox. Yes I know,
there are low speed electric motors, but they are huge and heavy.

producing only a little more electricity than needed to
drive the rotor in level flight.

There will always be losses, driving a generator, then a motor. You will
have to have at least 10% more power output by the diesel motor, just to
break even, and more than that to charge batteries.

So lets see, we have to have a gearbox both ways, no change in weight
between the two. We will have to have a bigger diesel to take care of
losses, so penalty goes to electric. We would have to have batteries for
electric, so more penalty for electric. Since our electric has gotten
heavier, we will have to have a bigger engine, and more fuel, so BIG penalty
for electric.

For additional power, electricity from the battery would also drive the
rotor. When descending, the battery gets charged faster.

What you are talking about is like an autorotation. The blades in an
autorotation have to go to negative pitch, just to make the blades maintain
RPM, and to get more power to generate electricity than a plain
autorotation, they would have to be at even more negative pitch. You would
have to descend so fast to get the rotor to produce electricity, your
passengers would freak!
--
Jim in NC