Un-ducted Fan Engine

The un-ducted fan engine also known as the "propfan". The first concept of the propfan engine was first revealed in 1975 by Carl Rohrbach and Bruce Metzger. Carl and Bruce were both from the Hamilton Standard Division of United Technologies, but the propfan wasn’t patented until 1979. It was patented by Carl Rohrbach and Robert Cornell from the Hamilton Standard. The GE36 also known as the "un-ducted fan engine" or the "propfan" was an experiment supposed to be a hybrid in between the turbofan and turboprop. It was developed by the very known General Electric. The GE-F404 was a military aircraft turbofan that was used by General Electric as the basis of the GE36. The engine showed a very low fuel consumption, but cabin noise levels were an extreme issue even though the engines were mounted at the end of the test aircraft the noise level was too extreme. It’s a multiple bladed engine that has different thin airfoil sections with swept blades and is also integrated with a nacelle to help retard the airflow. When the air flows through the different sections and blades it reduces the compressibility loss and it also uses a single stage reduction gear which will make it high performance. At a Paris air show in 1985 General Electric introduced an engine nacelle that had two rows of propeller blades 12 feet in diameter General Electric called this the new propfan engine also known as the "Un-ducted Fan Engine (UDF)". General Electric along with Boeing proposed to create a 150 seat airliner using the new propfan engine. This design would burn half as much fuel as the Airbus A320 which was roughly the same size. The aircraft proposed was the Boeing 7J7. Some of the other aircrafts that would make use of the propfan engine are the Beriev A-40, Antonov An-70, EcoJet, and the McDonnell Douglas MD-94X. There are advantages of the propfan engine the major advantage was the amount of money saved from fuel consumption compared to the conventional engine. According to research by Dan Mikkelson who worked at NASA's Lewis Research Center in Cleveland, Ohio the fuel savings would be 30 to 35 percent. McDonnell Douglas did its own research in 1988 by removing the JT8D turbofan engine and replacing it with the GE36 propfan engine. Their conclusion was a 30 percent reduction in fuel consumption over the turbofan powered MD-80. Another advantage was that the propfan would hold its own against a jet by being able to cruise at a speed up to Mach 0.8. There were some disadvantages as well with the propfan engine. The major disadvantage was the noise the engine caused. The engine was so noisy that it would cause major discomfort for any passenger on board an aircraft with a propfan engine. Another disadvantage were some safety concerns with the propfan engine such as blade failure that could do major damage to the airframe caused from one other major disadvantage, vibration. The other major disadvantage was the vibration caused by the blades on the propfan engine. Vibration raised a few issues such as affects it would cause on the airframe structure and the system integrity, and complete discomfort of the passengers on the aircraft. Today conventional turbofans are more efficient than they were in the mid 1980s so there is little interest in high speed propellers. It is also questionable whether the prop-fan could even meet current international noise rules.

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NASA/GE Un-ducted Fan. General Electrics F-404

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Boeing’s 150-seat 7J7 concept would meld prop-fan technology and lightweight composite structure to deliver big gains in fuel efficiency

References www.wikipedia.com www.areospaceweb.org www.airspacemag.com
www.aviationhistory.com