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NASA / GE Unducted Fan

An unducted fan or 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 bypass (UHB) engines and, most recently, open rotor jet engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop.

A first problem - propeller blade tip speed limit

Turboprops have a fairly strict sweet spot at speeds below about 450 mph (700 km/h). 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 travel near the speed of sound, even if the aircraft is sitting still.

This can be controlled to some degree by adding more blades to the prop, allowing it to use 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 as their engines were upgraded and roughly doubled in power. The only downside to this approach is that adding blades makes the propeller harder to balance and maintain. But even with these sorts of measures at some point 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 (700 km/h).

A solution to the first problem

Swept propeller

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 use a swept wing. In the 1970s, NASAmarker started researching propellers with similar sweep. Since the inside of the prop is moving more slowly than the outside, the blade is progressively more swept toward the outside, leading to a curved shape similar to a scimitar.

A second problem - jet aircraft fuel economy

Jet aircraft are well known for permitting greater thrusts and higher speeds than could be achieved by conventional propeller-driven aircraft operating within the same aerodynamic envelope. However, jet aircraft are limited in fuel economy. In fact, for the same fuel consumption, a propeller-driven aircraft can produce greater thrust. As fuel costs become an increasingly important aspect of commercial aviation, aircraft engine designers continue to seek an optimal combination of jet engine thrust ratios and propeller fuel efficiency.

A solution for enhanced fuel economy

The propfan concept was developed to deliver 35% better fuel efficiency than contemporary turbofans. In static and air tests on a modified DC-9, propfans reached a 30% improvement over the OEM turbofans. This efficiency came 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 GE36 Unducted Fan was a variation on NASAmarker'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, while the other prop is connected to the 'unearthed' turbine stators and rotates in the opposite direction. So, in effect, the power turbine has 14 stages. Boeing intended to offer GE's pusher UDF engine on the 7J7 platform, and McDonnell Douglas was going to do likewise on their MD-94X airliner.

McDonnell Douglas developed a proof-of-concept aircraft by modifying its company-owned MD-80. They removed the JT8D turbofan engine from the left side of the fuselage and replaced it with the GE36. A number of test flights were conducted, initially out of Mojave, CA, which proved the airworthiness, aerodynamic characteristics, and noise signature of the design. Following the initial tests, a first-class cabin was installed inside the aft fuselage and airline executives were offered the opportunity to experience the UDF-powered aircraft first-hand. The test and marketing flights of the GE-outfitted "Demo Aircraft" concluded in 1988, demonstrating a 30% reduction in fuel burn over turbo-fan powered MD-80, full Stage III noise compliance, and low-levels of interior noise/vibration. Due to jet-fuel price drops and shifting marketing priorities, Douglas shelved the program the following year.

In the 1980s, Allison collaborated with Pratt & Whitney on demonstrating the 578-DX propfan. Unlike the competing GE36 UDF, the 578-DX was fairly conventional, having a reduction gearbox between the LP turbine and the propfan blades. The 578-DX was successfully flight tested on a McDonnell Douglas MD-80. However, none of the above projects came to fruition, mainly because of excessive cabin noise (compared to turbofans) and low fuel prices.

Progress D27 Propfans fitted to an Antonov An-70

The Progress D-27 propfan, developed in the U.S.S.R, is even more unconventional in layout, with the propfan blades at the front of the engine in a tractor configuration. Two rear-mounted D-27's propfans propelled the Antonov An-180, which was scheduled for a 1995 entry into service. Another Russian propfan application was the Yakovlev Yak-46. 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, which was subsequently cancelled. However, the An-70 remains available for further investment and production.

With the current high price for jet fuel and the emphasis on engine/airframe efficiency to reduce emissions, there is renewed interest in the propfan concept for jetliners that might come into service beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans.


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