Propfan
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re-disambig 'compressor' - should point to [[axial compressor]].
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Image:GE-36 UDF Domke.jpg A propfan is a modified turbofan engine, with the fan placed outside of the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high by-pass (UHB) engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop.
Turboprops have a fairly strict sweet spot at speeds below about 450 mph. The reason is that all propellers lose efficiency at high speed, due to an effect known as wave drag that occurs just below supersonic speeds. This powerful form of drag has a sudden onset, and led to the concept of a sound barrier when it was first encountered in the 1940s. In the case of a propeller this effect can happen any time the prop is spun fast enough that the tips of the prop start travelling near the speed of sound, even if the plane is sitting still.
This can be controlled to some degree by adding more blades to the prop, using up more power at a lower rotational speed. This is why most WWII fighters started with two-blade props and were using five-blade designs by the end of the war. The only downside to this approach is that adding blades makes the propeller harder to balance and maintain. At some point though the forward speed of the plane combined with the rotational speed of the propeller will once again result in wave drag problems. For most aircraft this will occur at speeds over about 450 mph.
Image:Swept-propeller.png A method of decreasing wave drag was discovered by German researchers in WWII: sweeping the wing backwards. Today almost all aircraft designed to fly much above 450 mph (700 km/h) use a swept wing. In the 1970s, NASA started researching propellers with similar sweep. Since the inside of the prop is moving more slowly than the outside, the blade became progressively more swept toward the outside, leading to a curved shape similar to a scimitar. Image:Antonow An 70 prop detail rvb jno MACS 2001 0037.03.jpg The propfan concept was intended to deliver 35% better fuel efficiency than contemporary turbofans, and in this they succeeded. In static and air tests on a modified DC-9, propfans reached a 30% improvement. This efficiency comes at a price, as one of the major problems with the propfan is noise, particularly in an era where aircraft are required to comply with increasingly strict Stage III and Stage IV noise requirements.
General Electric's Unducted Fan (UDF) is a variation on NASA's original propfan concept, and appears similar to a pusher configuration piston engine. GE's UDF has a novel direct drive arrangement, where the reduction gearbox is replaced by a low speed 7 stage turbine. The turbine rotors drive one prop, whilst the other prop is connected to the 'unearthed' turbine stators and rotates in the opposite direction.
Boeing intended to offer GE's pusher UDF engine on the 7J7 platform, and McDonnell Douglas were going to do likewise on their MD-94X airliner. Both airliners were to use rear-fuselage mounted General Electric GE-36 engines. Similar was the Antonov An-180, powered by two rear-mounted Progress D-27 propfans and planned for a 1995 introduction. Another Russian design was the Yak-46. None of the projects came to fruition, mainly because of excessive cabin noise and low fuel prices.
During the 1990s, Antonov also developed the An-70, powered by four Progress D-27s in a tractor configuration; the Russian Air Force placed an order for 164 aircraft in 2003.
Propfan engines
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