Here is another drawback to a chain drive.
If you lower the engine so the chain drive thrust line to come out in
the right place you lose a lot of valuable volume in the
cowl for  the cooling system. In the planetary case you
have room under the engine for the water rad.

What is shown here is a four inch lower pulley and a 12 inch upper
pulley for a 3:1 reduction ratio necessary to get the
most amount of power out of an RX8 engine.

Paul Lamar

-HarrisonYoung wrote:

This is the most convincing argument against a chain drive I've heard.
I hadn't thought about this problem. My particular interest
is a rotary installation in a twin-engine plane. It's possible
that the drives could be offset to the side instead of top or bottom,
but that would result in some rather bizarre motor mount requirements.
You've finally convinced me to stick to planetary gears :)

Harrison Young

Harrison,

Here are some more pictures and illustrations for the light weight
hollow prop shaft that nobody has tried yet. The idea here is to
use the pinion shafts to connect the prop shaft directly to the 
rotating planet carrier. This minimize the load path and eliminates
the cost and weight of splines. The same idea is used on the prop
end. The prop drive lugs are welded directly to the flared hollow prop
shaft. These are threaded internally for the prop clamping bolts.
No conventional prop shaft steel flange is used. This in theory cuts
the weight greatly. The prop flange (spacer, back up washer) shown 
can be light weight aluminum as there is no load on it.

Since you have an NC mill and you are willing to build your own PSRU
you are the ideal person to try this idea.

I'll send some pictures of the planet gear set shortly so you can get
a feel for how small and light weight it really is.

Paul Lamar
 
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