On May 14, 2:34*pm, wrote:
Dear Stealth (and the Group)

We may be looking it this the wrong 'way 'round.

There are plenty of engines which offer excellent power-to-weight
ratios. *Unfortunately, they do so at rpm's which make them
impractical for slinging a prop UNLESS a PSRU is used.

A good case-in-point is the Rotax. *The Rotax engine is only 1300cc
but it is designed to operate near 6000 rpm. *What makes the engine
successful is the PSRU between the engine and the prop.

There are two obvious conclusions we can draw from this. *The first is
that the engines themselves, despite any practical combination of cam
& cooling, are simply too small to be used with the propeller mounted
directed to the crankshaft. *(This leads to another series of
questions worthy of discussion but which I will leave untouched at
this time.)

The second point is that the PSRU, which does NOT enjoy the same TBO
as the engine itself, has been designed specifically for this
application, taking advantage of the engine's torque & power curves,
and including mechanical features that make it suitable for the
mounting of a propeller; mounting the engine to an airframe and so
forth.

It may then be argued that we are wasting our time by focusing on the
ENGINE; that we should be devoting our energies to a suitable PSRU
that may be attached to a WIDE VARIETY of engines.

Having devoted most of my attention to the VW engine, I have little to
offer the Group should the discussion turn to PSRU's but it would seem
that the hand-maiden of these light-weight, powerful engines MUST be
an automotive TRANSMISSION having similar features of light-weight and
power-handling capacity. *Here again, I lack the background and
experience to do more than mutter; there are aspects of PSRU's,
transmissions and torque converters about which I know nothing at
all... other than the fact they must exist (since the engines exist).

At the very least, I know the GEARS must exist.

Were I in Western Australia, rather than curse the darkness (and wish
for a Corvair to suddenly appear on my doorstep) I think I would light
a single candle by diving into whatever came my way in the form of
light-weight engines and trannies.

I suppose there has to be a clutch in there somewhere, so that means
I'm probably looking at a flywheel as well... fate stacking the weight
against my urge to fly. *But perhaps some of those powerful, light-
weight engines ARE large enough to be able to drive a prop directly,
even if I had to find someone to grind me a new cam.
Re. PSRU's

There is a gear reduction unit that handles massive torque and power
the size of a a one-pound coffee can - it's the planetary gearset from
an automatic transmission. I have two in my Jeep Grand Cherokee
'airport car' transmission that have lasted 300,000 miles - so far.
These things are built to very tight tolerances and are VERY tough.

If you want still tougher, speed shops sell replacement planetaries
that can handle 1500HP or more. Ask one to handle only 100HP and
they should last forever. You can specify just about any reduction
ratio you want.

All you have to do is machine a nose case from billet aluminum to hold
the planetary gearset and the thrust bearing.