What is "propfan"

n. A modified turbofan engine with the propeller outside the engine duct of an aircraft.

A propfan or open rotor engine is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The European Aviation Safety Agency (EASA) defines it as "A turbine engine featuring contra rotating fan stages not enclosed within a casing." The engine uses a gas turbine to drive an unshrouded (open) contra-rotating propeller like a turboprop, but the design of the propeller itself is more tightly coupled to the turbine design and the two are certified as a single unit.

A propfan is typically designed with a large number of short, highly twisted blades, similar to a turbofan's bypass compressor (the "fan" itself). For this reason, the propfan has been variously described as an "unducted fan" or an "ultra-high-bypass (UHB) turbofan". In technical papers it is described as "a small diameter, highly loaded multiple bladed variable pitch propulsor having swept blades with thin advanced airfoil sections, integrated with a nacelle contoured to retard the airflow through the blades thereby reducing compressibility losses and designed to operate with a turbine engine and using a single stage reduction gear resulting in high performance." The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. The propfan concept was first revealed by Carl Rohrbach and Bruce Metzger of the Hamilton Standard Division of United Technologies in 1975 and was patented by Robert Cornell and Carl Rohrbach of Hamilton Standard in 1979. Later work by General Electric on similar propulsors was done under the name unducted fan, which was a modified turbofan engine, with the fan placed outside the engine nacelle on the same axis as the compressor blades.