Re: Unducted fans?

Date:         26 Jan 94 04:12:53 PST
References:   1 2 3 4
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>> How was this speed ratio maintained, in the GE design?
>> -dk

In both designs, the speed ratios of the props were controlled by prop
pitch.  I'm not familiar with the method GE used, but the PW/A design used
electric beta (prop angle) actuators (one for each blade if I'm not
mistaken) that rotated with the props.  I'll have to assume the GE approach
was similar.  The considerations were not only resonance, but power, prop
efficiency (something like 8-10% recovery from a counter-rotating prop
versus a single row prop, if memory serves), acoustics (prop tip speed)
and, in the case of the GE design, power turbine aerodynamic
considerations, since the prop speeds determined the turbine blades and
vanes relative speeds.  For those that are interested, the PW/A engine is
on display in the Allison museum at the Allison Engine Company (new name,
used to be called Allison Gas Turbines, division of General Motors before
being sold to a New York investment firm), in Indianapolis, Indiana.

Carl Nordstrom

P.S.  For those that aren't familiar with a propfan, the blades are
scimitar-shaped.  Up until a few years ago all prop aircraft had a
practical limit of around .5 mach or so (many have gone much faster, but
paying a high efficiency penalty due to compressibility effects).  Then
Hamilton Standard (and NASA?) developed the propfan design to be able to
get prop-like efficiency at turbofan cruise speeds.  So an efficient .7
mach then became possible.  Many aircraft have had counter-rotating props
to recover the efficiency lost by a first prop row (the Russian Bear bomber
is a notable example), so counter-rotating two propfan rows became a
natural evolution for this technology.  The first propfan to fly was the
Propfan Demonstrator.  It was an Allison 570 engine mated to a modified
Allison T-56 gearbox, driving a single propfan row.  I don't remember who
did the prop, but it was probably Ham Standard.  It flew on a Gulfstream
aircraft that was heavily modified for the task.  It seems that this was a
NASA project, if I remember correctly.
        The propfan idea would require more development to be a viable
commercial product.  The program was canceled before GE and PW/A were able
to fully develop their engines.  Areas such as dynamics (vibrations),
performance, life, materials, prop strength, acoustics, etc. were
identified as needing special attention.  These types of issues arise
during *every* engine development program, but in this case the issues
involved new technology that had never been tried before.  I do feel,
however, that with time and money none of the issues were particular