Howard Wilkinson wrote:

Vance:
    My thinking is that you have nothing to lose.....  that is over a simple
duct fan.  Any energy which is not imparted to the second and larger prop is
direct thrust.... it all exhausts out the back of the duct.  The more torque
you can impart to the large diameter blades the more you gain.  Efficiency
would not be on par with a gear redrive, but better than a small duct alone.
Losses would be no more than bearing drag and aerodynamic drag of the duct
which you would have with a duct fan.
    The system would be lossy indeed if you depended only on the thrust
generated by the large diameter prop.  Conventional duct fans have not
proven to be successful for most applications, redrives create all kinds of
problems... with this you have no concern for the torsional vibrations,
lubrication, cooling, etc you have with a redrive..... how much is lost???
I suspect less than with a simple duct system designed for a 6000 RPM prop.
(I say simple with tongue in cheek).

                                                                        H.W.

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