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.

In theory this is quite workable, but in practice -
converting shaft horsepower into air horsepower and
then back again to shaft power in any reasonably
compact package is unacceptibly lossy. A 3 hp
cmpressor cannot
continously operate a 1/4 inch die grinder - as an
example of this problem - Vance

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