This question may have been considered before...Has anyone considered the
idea of using a rotary engine with a speed increaser to drive a low-pressure
fan similar to one in front of a high bypass turbofan engine?  Simply put,
the engine would run at aprox. 6000 rpm and the fan would turn at 10,000
RPM or so.  Then the airflow would be necked down via a funnel output to
create a vectored thrust.Any past articles, threads or comments on this
one?Thanks!

Kevin in Denver

That is called a ducted fan.

Perry Mick experimented with that for about ten years
as I recall.

http://mickaircraft.com/duckt.htm

A half a dozens medium speed ducted fan airplanes have been built
by others and they all failed.

It is not until you get over 400 MPH that ducted fans become
more efficient than a un ducted propeller. That too is now being
challenged with new high speed propeller designs.

Putting it another way.... if you have enough HP to push the airplane
faster than 400 MPH than by all means use a ducted fan.

Lets say you get the drag down to the equivalent of a two square
foot flat plate. The dynamic pressure at 400 MPH is 416 pounds
per square foot. So the drag is 832 pounds.

400 MPH is 587 feet per second. The HP required is therefore
587 X 832 or 488,000 pound feet per second.
One HP is 550 pound feet per second.

Therefor you will need 887 net HP out of the duct. You might be
able to pull it off with a 1200 HP turbo charged 3 or 4 rotor Wankel
engine just to go 400 MPH. If you wanted to go 450 MPH you would
need a lot more HP as top speed is proportional to HP cubed.
With a 1200 HP rotary engine the fuel burn would be about
600 pounds per hour or about 100 gallons an hour.
A turbo compound version would do 20% better.

A Cessna Citation jet that flies about the same speed
(352 Kts) burns about 140 gallons an hour.
http://www.jetadvisors.com/aircraft-specs/Description-CessnaCitationCJ2-SmallCabinLightJets.htm

Airliners also use ducted fans but modern high bypass turbo jet engines
are in the range of 50,000 HP and up.

Paul Lamar