UDF (Unducted Fan)
 

I seem to remember in the 80s there where tests with unducted fan engines, and various models where being planned powers by UDFs. Why havent these caught on and there are no commercial planes flying with UDFs today?
Luke

I have seen reports that the MD80 used suffered from excessive noise, especially at certain uncomfortable frequencies. The pictures I saw also seemed to suggest that the fan blades would not have been able to be feathered, giving extreme drag if one engine failed.

http://www.airliners.net/open.file?id=003461 http://www.airliners.net/open.file?id=166061

Note that it only had one UDF engine and was scrapped in 1994 (it flew at Farnborough 88 and 90)
It was built (normally engined) as the DC9-81/MD81 in 1979 - not sure when the UDF was fitted