> Gordon, take a look at this.
>
>
http://www.connel.com/freeware/airduct.shtml
>
> This is what you input
> Flow Rate 20000 CFM
> Velocity 12000 FPM
> Pressure 14.7 PSI
Temperature 75 F
Duct length 4 FT
Duct Roughness 0.0001 FT
These are the results
The duct diameter is: 17.48 inches
The air density is: 0.0742 pounds per cubic feet
Reynolds Number = 1779111.80
Friction Factor fa = 0.01235369
Friction Factor f = 0.01231736
loss PSI = 0.01082625
loss inches of H2O = 0.29997373
Paul Lamar ...No rotor no motor.
That's an HVAC (low velocity) calc--are you sure it's accurate at higher
flow/velocities?
Donald Willard Garrett
No I am not sure as the equations are not shown.
I did enter 12000 FPM which is 200 FPS or 136 MPH.
It calculated the duct size rather than inputing
the duct size which is rather strange.
--
Paul Lamar ...No rotor no motor.
They could get away with linear calcs at HVAC speeds bcz the curves are
nearly flat. For us, not only are the curves not flat, the airflow
cross-section isn't uniform, which is another likely HVAC assumption.
It's complex starting with what happens at the inlet (which can vary
significantly with AOA), then through the fan where we wind up with a
turbulent vortex.
I wonder how significant a disadvantage duct drag winds up being,
considering most aircraft (tractors) blow the down the fuselage, which
has to have more drag than any sensible duct design. Do you think we
could make money/cancel the drag loss using stators downstream of the
fan like in a turbine, to recover the thrust in the rotating component?
That could even things up for the duct a bit, as the rotational velocity
imparted by a prop is significant, wasted energy.
Excellent thread, Gordon. Another thought I have that may give the duct
idea an edge is what happens in larger applications. The trouble with
competing with a conventional prop on a skinny Cozy / Imp airframe, is
that the flat plate drag is so low to start with, the weight/drag
penalty of adding a reasonable diameter tube more than cancels the
benefit. If we mentally pull the engine nacelles off the wings of a
twin, making a bigger fuselage, buying some cabin width / fuel / cargo
capacity with it flat-plate wise, and the diameter of the fan in the
embedded duct can be almost the diameter of the fuselage with no added
penalty, we can get around the thrust / drag penalty of a small diameter
tube because volume increases exponentially with circumference. Throw in
some more fan blades to absorb the extra horsepower, bump it to a
four-rotor in the 7-800+ HP range, my guesstimatation says the seat /
payload cost per mile starts to look pretty good--especially considering
this puts you up in the flight levels with the jets as Gordon has noted.
Double the horsepower of a Skymaster with what--nine more inches in
length, zero more in diameter at the price I just saw for two additional
rotors?
Maybe stretches the budget for a homebuilt, but not by 2X. The same
basic build time for more than twice the airplane, what if I find a
partner or two and time share it like a flying club.
While I'm having fun with this--Paul, what would it take to shut down a
couple of rotors at cruise, way simpler than that Cadillac V-8--some
kind of clutch and redundant fuel / ignition? Would the fuel flow
numbers improve enough to make it worth the hassle? This could be
worthwhile to consider when you add in the thought that this almost
gives us a twin engine in a single installation.
Donald Willard Garrett
See
http://www.rotaryeng.net/RX8.html
Paul Lamar
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