Editing Turbofan (section)

==Extreme bypass jet engines==

In the 1970s, Rolls-Royce/SNECMA tested a [[M45SD-02]] turbofan fitted with variable-pitch fan blades to improve handling at ultralow fan pressure ratios and to provide thrust reverse down to zero aircraft speed. The engine was aimed at ultraquiet [[STOL]] aircraft operating from city-centre airports.

In a bid for increased efficiency with speed, a development of the ''turbofan'' and ''turboprop'' known as a [[propfan]] engine was created that had an unducted fan. The fan blades are situated outside of the duct, so that it appears like a turboprop with wide scimitar-like blades. Both General Electric and Pratt & Whitney/Allison demonstrated propfan engines in the 1980s. Excessive cabin noise and relatively cheap jet fuel prevented the engines being put into service. The [[Progress D-27]] propfan, developed in the U.S.S.R., was the only propfan engine equipped on a production aircraft.