If I were doing a ducted fan this is how I would go about it.
First I would assume I would get no reasonable amount of
power out of a rotary unless I ran it at 6500 RPM.
I would also assume that by copying the airfoil on an
airliner ducted fan blade I could run the tips at 1.2 Mach.
That would give me a fan outside diameter.
By measuring a Mazda rotary engine I know I can get it
inside a 18 inch diameter tube not including the intake
manifold or exhaust. That would give me a rough hub
diameter for the ducted fan as I do not want to use
a drive shaft. If I used a drive shaft I might as well use
a prop and a gear box and forget the ducted fan.
Since all the data available suggest the ducted fan becomes
efficient at about 400 MPH I would use that as the target for
my top speed. To reduce the drag at those speeds I would
have to limit the wing area and thickness. This means I would
probably have to use fowler flaps to keep the landing
speed within reason. The airplane would probably look
like a scaled down fighter plane.
I think I could squeeze everything in to a gross weight
of 4000 pounds using carbon fiber everywhere possible.
This is just a wild guess.
Since the dynamic pressure at 400 MPH is 416 pounds per square
foot I can calculate the thrust I will need if I know the
equavalent flat plate area of my airplane.
If I assume a flat plate equavalent of two square feet for
the whole airplane as a rough first guess then I would
need 800 pounds of thrust roughly. BTW I think the RV
has an equavalent flat plate area of about 1.5.
If I assume the ducted fan is as efficient as a prop at
lower speeds (80%) than I would need the equavalent input
thrust of 1000 pounds to go 400 MPH.
Four hundread MPH is 587 feet per second.
In terms of HP that equals 587 FPS times 1000 pounds or
587,000 pound/feet-per-second or about 1000 HP rounding it off.
Two turbo Mazda three rotor 500 HP engines might do the trick.
The duct/engine systems would be mounted on the aft
fuselage much like a Lear Jet.
I suspect the airplane would have no trouble taking off
particularly if I used my patent applied for inflatable
duct lips :-)
Paul Lamar
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