"Ad absurdum per aspera" wrote in message
om...
I recall seeing GE tested scimitar shaped pusher prop engines, I
think
it was on a 727.

I seem to recall it being on the right engine of a DC-9. I wonder
what became
of that idea.


"Unducted fans" or "propfans" were, I believe, tested on both a 727
and an MD-80. See for instance

http://www.jsc.nasa.gov/er/seh/profan.html (contemporary article
from
midway through the program)



Thanks for these links. They all seem to think that a Mach 0.85 for a
scimitar blade contra-rotating coaxial "prop-fan" is possible and
indeed fuel efficient. Given that there was in a German 1940s study
suggesting 584 mph with twin piston engines I think its fairly
believable. I find the idea of a Mach 0.85 diesel or spark ignition
engine fascinating.

There is also the Soviet An 70
http://www.geocities.com/CapeCanaver...5/antonov1.htm I would be
interesting to read what the Russian desginers thought that they
gained with this designe of a straight jet or prop. I know that a
C-17 got bogged at a Sarajevo airport on a bit of wet grass and that
this lack of grunt is due to the use of jets instead of props.

Either way the Russians are ready to go apparently!

Another fascinating possibility is the use of SOFC (Solid Oxide Fuel
Cells). Many people beleive these will match piston engines in cars
in terms was weight. If turbo supercharged they should have an
amazing efficiency of 85%. They opperate at 800C at which point the
metal oxide membranse can conduct oxygen ions (instead of hysrgen
ions) and thus burn hydrocarbon fuels. The high temperature (still
only 1/3rd that of a pertrol engine) means that no catalyst is needed.
They are running in the lab at 60% efficient unburdened but becuase
they exhaust at 800-1000C they can be trubo-supercharged and excess
shaft power extracted.

These SOFCs should achieve the same power to weigh ratio of petrol
engines and thus be able to propell aircraft. Potentialy their engine
and cell life will be so high that they compete with gas trubines.
They would require only modes cooling and would require light
propellors since no piston torgue vibarations would be present.



I'm not exactly sure why they aren't much in use. Hypotheses I've
read include greater risk to the passenger cabin from uncontained
failures (I wonder if it is coincidence that both the testbeds were
the sort of jets with aft-mounted engines); undesirable "propeller"
image; noise; and parallel improvements in high-bypass turbofans of
the usual ducted design.

Cheers,
--Joe

I think there were some noise issues (not major), blade safety I
think could be handled (the scimitar shaped much add unusual
stresses), then there is the issue of gear boxes. These are high
maintain items. (The only geared turbo fan in sevice is that unit
(Allison) on the BAE avro regional/ BAE146 series jet I think)

It will be interesting to see if Pratt+Whitney's PW8000 geared
turbofan for airbus changes the anti-gearbox mindset.










http://www.aerospaceweb.org/question...on/q0067.shtml (survey
article; pix)


http://www.aviation-history.com/garb...g/udf-2_f.html


The goals were, I think, a combination of fuel efficiency and some
internal simplifications.