Re: Unducted fans?

From: (Carl Nordstrom)
Organization: Electrochemical Engine Project
Date:         07 Jan 94 02:44:45 PST
References:   1 2
Followups:    1
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In article <airliners.1994.828@ohare.Chicago.COM>, kls@ohare.Chicago.COM
(Karl Swartz) wrote:

> >what happened to unducted fans?


> >I seem to recall that somebody even bolted a prototype onto one side
> >of a DC-9 and flew it around.
> The prototype MD-80, actually.  I once saw it at Mojave, with an
> extra-long pylon but no #1 engine at that point.  Boeing did the
> same thing with a 727.  Seems to me the MD-80 had a GE engine while
> the 727 had a Pratt, but I wouldn't bet on it.
There were in fact two "competitors" during this time - GE and Pratt &
Whitney/Allison Gas Turbines.  There were significant differences between
the two approaches.  Both engines had counter-rotating highly swept blades
(to reduce noise and increase prop efficiency at the high rotational
speeds).  The P&W/A approach was to arrange the powerplant (front to back)
inlet, engine, gearbox, prop.  This approach was traditional, and turned
out to be very trouble-free (no vibration or performance restriction
problems that were noteworthy).  I think the set-up flew up to .7 Mach, a
typical cruise speed for a passenger jet.  The GE approach was to attach
the prop rows (2) directly to the LP turbine section of the engine, with
one row attached to the turbine blades and the other row to the turbine
vanes.  The advantage of this approach was simplicity.  I think (please
correct me if I am wrong) that the weight was a wash with the PW/A approach
because of the structure required to support the blades.  The efficiency
was also not quite as good, since the blades turned faster than optimum,
and the LP turbine turned slower than optimum (fixed 1:1 speed ratio).  GE
had some developmental problems with vibration, I'm relatively sure due to
the large rotating structures which may have had dynamic AND aerodynamic
problems.  Their design was much more gutsy (and advanced), and potentially
could be superior in some important ways with a lot of development.

Carl Nordstrom