General Electric GE36 / UDF
The General Electric GE36 was an experimental aircraft engine, a hybrid between a turbofan and a turboprop, known as an Unducted Fan (UDF) or propfan. The GE36 was developed by General Electric Aircraft Engines and first run in 1986.
A General Electric F404 military turbofan was used as the basis of the GE36. The F404 mixed exhaust stream discharged through a seven-stage low pressure (LP) turbine, where each stator ring was 'unearthed' and free to move in the opposite direction to that of the rotors. One set of open rotor, variable pitch, fan blades was connected to the LP turbine rotor system, while the other set was connected to the contra-rotating LP turbine stators. The scimitar shape of the fan rotor blades enabled high flight speeds (about Mach 0.75) to be obtained. GE used a low speed, (effectively) 14 stage, LP turbine, rather than a conventional high speed LP turbine and reduction gearbox, to drive the fan rotor blades.
Although the engine demonstrated an extremely low specific fuel consumption, cabin noise levels were a problem, even though the engines were mounted at the rear of the test aircraft.