"Dale Scroggins" wrote

It's going to need a starter; diesels are hard to hand prop. And the
engine is not likely designed to handle thrust and gyroscopic loads.

Many auto engine cranks have done well with these loads, and no extra help.

So, because of starter placement

Alternate starter schemes may be developed, and might save weight.

and need for a well-supported prop shaft, a nose-piece casting with
bearings and a prop shaft will be needed.

Not a given, IMHO.

Incorporating reduction gearing (or belt) and a device to smooth power
pulses

Smoothing power pulses results in heating up the device, which means it is
robbing power and efficiency. Double or triple bad thing. Weight, power,
and reliability.

wouldn't add much additional weight.

I've heard that so many times, and it all adds up to an overweight airplane.

The torque curve isn't flat to maximum power.

True, but how much extra power will have to be made, to haul around the
extra weight?

If a fixed-pitch prop is installed that is efficient at low RPM, the engine
may not be able to make take-off RPM without a bit of mechanical
advantage - or variable pitch, which leads back to the PSRU.

PSRU's are HEAVY! Why add one, if it is not absolutely needed? Extra
weight has to be well justified, as does the possibility of extra components
causing possible extra failures. It is hard to imagine that hauling around
the extra weight could be justified, with a torque curve like that engine.
It would need to make a lot of extra HP to justify a PSRU, to me.

Since Subaru is hinting that a 200 hp version is on its way (giving even
better hp/weight ratio), a variable pitch prop
should be considered.

Why?

With conventional aircraft engines, they make best power and torque at up
around 2700 RPM. If you have to run too big of a prop to utilize 200 HP,
then you need constant speed to keep from breaking supersonic.

If you have an engine running best power (or close to it) and best torque at
23 or 24 hundred RPM, you can run a bigger prop, and keep the tips slow
enough, without the extra weight and complexity of constant speed props.

Don't get me wrong. I'm not against PSRU's in all cases. I am against
extra weight and complexity when it is not absolutely necessary. I don't
think a clear case has been made, for this engine.

Wait until it is being used, and see how well it does, is my opinion. It
will let us know, by how it does after it has been eXperimented with, I
believe. You may be surprised. I might be, too. Until then, keep an open
mind.
--
Jim in NC