Morgans wrote:
"Frank van der Hulst" wrote
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.

The whole point of these wheelmotors is that they are smaller than
equivalent DC motors, and turn at low speeds. The numbers I've seen are
110-150rpm. See http://www.smartmotor.no/products/quiet_traction/ --
this talks about wheelmotors in relation to wheelchairs, and claims
twice the efficiency over traditional DC+gearbox systems.

You could think of the generator/motor combination as an electrical
rather than mechanical gearbox.

Another interesting site is http://www.killacycle.com/ -- an
electrically powered drag bike that gets 9.450 seconds @152.07 mph over
the 1/4 mile. Still a long way from the 5.895 @ 238.22 nitro powered
world record though.

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.

But the diesel only ever runs at one speed... it's most economical. I'm
assuming that there will be significant weight savings in that. I may be
wrong in that assumption.

I do know that having a car petrol engine which can potentially deliver
a lot more horsepower means that it uses more fuel at lower horsepower
output. Maybe this relationship doesn't apply to diesels.

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.

No. I'm saying that when descending, less power from the
engine/generator goes to the rotor, leaving more to go to the battery.