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A propfan, also called an open rotor engine, open fan engine [1] [2] or unducted fan (as opposed to a ducted fan), is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop.
The turboprop at its best flight speed gives significant fuel savings over a turbojet even though an extra turbine, a gearbox and a propeller are added to the turbojet's low-loss propelling nozzle. [24] The turbofan has additional losses from its greater number of compressor stages/blades, fan and bypass duct. [clarification needed]
On a reverse-flow turboprop engine, the compressor intake is at the aft of the engine, and the exhaust is situated forward, reducing the distance between the turbine and the propeller. [ 15 ] Unlike the small-diameter fans used in turbofan engines, the propeller has a large diameter that lets it accelerate a large volume of air.
1968: General Electric TF39 high bypass turbofan enters service delivering greater thrust and much better efficiency. 2002: HyShot scramjet flew in dive. 2004: NASA X-43, the first scramjet to maintain altitude. 2020: Pipistrel E-811 is the first electric aircraft engine to be awarded a type certificate by EASA.
Turboprop, turboshaft and turbofan engines have additional turbine stages to drive a propeller, bypass fan or helicopter rotor. In a free turbine the turbine driving the compressor rotates independently of that which powers the propeller or helicopter rotor.
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, [3] with its CFM International equal partner Snecma taking a 35 percent share of development. [4] Development was cancelled in 1989.
While a turbojet engine uses all of the engine's output to produce thrust in the form of a hot high-velocity exhaust gas jet, a turbofan's cool low-velocity bypass air yields between 30% and 70% of the total thrust produced by a turbofan system. [80] The net thrust (F N) generated by a turbofan can also be expanded as: [81]
GEnx on 747-8I prototype. As of 2016, the GEnx and the Rolls-Royce Trent 1000 were selected by Boeing following a run-off between the three big engine manufacturers. The GEnx uses some technology from the GE90 turbofan, [1] including swept composite fan blades and the 10-stage high-pressure compressor (HPC) featured in earlier variants of the engine.