Howard Wilkinson wrote:

Paul:
    I've often wondered about a duct fan redrive system for a pusher
application.  That would be a drive and a driven fan running in the same
duct with the air driven fan having long blades which extended out to normal
prop dimensions in a section of duct which rotated.  The engine driven fan
would use a different pitch than the fan driven by the airflow from it
thereby giving a speed reduction.  The exhausted air from the duct would
contribute to thrust of course.  Such a system would be 100% immune to
torsional vibration problems which plague gear redrives.
    The main problem I see is structural..... the support of the large prop.
The small fan perhaps 30" would be bolted directly on the crankshaft and run
in a stationary duct.  The the air driven fan / large prop would be on a
shaft which would be mounted to the crankshaft via a "pilot" bearing so that
it wasn't driven, and would require a second support / thrust / gyroscopic
load bearing which would have to be supported via struts which of necessity
would pass through the prop's airstream.  The engine mounted fan would have
flatter pitch than the prop mounted fan (air driven) to accomplish the
desired reduction.
    Pretty bizzare I suppose, but it might work.... an air torque convertor
so to speak.... anybody who's worked with wind driven props knows that flat
pitch develops speed & course pitch develops torque... The proper
combination might even achieve decent efficiency.

                                                        H.W.

 
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