I have lost track of this discussion..  Is it being debated whether a
rotary powered ducted fan can have comparable efficiency to a turbofan
powered aircraft?  Or that a rotary powered ducted fan can compete on
efficiency with a conventional propeller driven aircraft.  We're not
talking about a real airplane here right?  I mean, if we wanted a real
airplane, we wouldn't be talking about losing 10% fuel economy here
and/or 10% payload there.

Though the contours of a duct may lead to relatively low drag
coefficient in the duct, I don't think that buys any overall
efficiency.  On either configuration, the airframe still has to go
through the air, so you suffer drag losses of the airframe itself.  By
adding a duct, you are adding wetted area, and you are forcing all of
that wetter area to have high local airflow.  Not good for efficiency.
With a conventional propeller, there are localized areas on the fuselage
that see higher free stream velocity, but that is more than made up with
propeller efficiency further outboard on the blade span.  And by not
suffering the penalty of the added wetted area.

One more analogy..  It is demonstrable that at subsonic airspeeds, a
turboprop is more efficient than any kind of direct drive turbine.  How
are we getting around the concept of accelerating a large mass of air a
small amount above the cruise speed of the airframe.

We just poo-poo'ed the x-wing fighter guy because he wanted to build a
hollywood set piece and put a rotary in it.  Now I think we were on the
wrong track.  Let's get him back and get him to put a duct on the back
of his x-wing so that it will sound really cool...

There are people that say all of the class went out of the railroads
when steam was replaced by diesel- electric.  What we're talking about
here is like making a diesel fired steam locomotive, just so that we can
make the railroads classy again.

Don't post this if you don't want to, Paul.  I just have a tough time
with the idea of working so hard to make what is essentially an
impractical toy.


Matt-

Me too. I used to think a ducted fan rotary would work
and then I read Kucheman & Webber.

The bottom line is if you have enough HP to go over about 400 MPH
the rotary powered ducted fan is better than the prop but it will not
have the altitude advantages of a turbo fan. Yes it might burn
less fuel at 25,000 but of course the turbo-fan will burn less fuel
at 40,000 than it will at 25,000 and get better MPG in the
process.

IMHO the jury is still out on the small four place jet-fan.
Small jet-fan engines are always less efficient than large
jet-fan engines. It has to do with clearance between
the blade and the duct. Simple geometry. The leakage
area goes up with the circumference while the fan area goes
up with the SQUARE of the radius. The same principle
applies to ducted fans.

Picture of Robin's friend Denise in front of P&W engine
in the P&W museum in Connecticut.

Paul Lamar ...No rotor no motor.

Matt,

If you are going to pooh-pooh this entire thread, at least you can pay
attention to what's been said:

Your point about the outside wetted area of the duct has no relevance
because we have been talking from the beginning about a plane with a
duct buried inside its fuselage. (See attached drawing of the Jethawk).

Your objetion about getting the same thrust from a much smaller disk
area was the sole subject of my first post on this thread titled "ducted
fan efficiency." In that post I calculated that a 24inch prop is only 16
percent less efficient than a 72 inch prop at the same thrust, all other
conditions being equal.

In that comparison I used the thrust, airspeed, and altitude of the
D-jet because those are the numbers we are trying to match with the
rotorfan "pseudo-jet."

The point of comparing to the D-jet is so we have a real frame of
reference. I said from the beginning that this airplane could be scaled
down to a smaller size. If I were to build such a plane -- and I must
admit the idea is tantalizing -- I would start with a two-seater with a
2-rotor engine. Probably just a box-stock renesis.

I think it is premature to write this whole idea off as a toy. If we
don't build it we will never know. The reason I'm putting these ideas up
here is to get critique, which is constructive -- vital really -- to
figuring out the viability of this idea. Critique is one thing and
grousing is another.


Regards,

Gordon.




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