When I went to the Farnborough air show in 1988, P&W or GE
(I forget which) was demonstrating a radical new engine mounted on an MD
-80 in place of one the usual engines. The engine was radical because it featured an unducted, contra-rotating fan that had six or eight blades per section. The fan was mounted at the rear of the nacelle. This, apparently, was the next leap forward in engine technology, and I believe that most of the major engine manufacturers came up with similar concepts. So, what I am curious to know is whether the unducted fan concept has been dropped, and if so why. Were there too many technical barriers for example? Are they too noisy?