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)


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.


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