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A New “Open Rotor” Jet Engine That Could Reduce Fuel Consumption
NASA and GE Aviation are testing a new “open rotor” jet engine with a different design that puts the fan blades on the outside of the engine.
The jet fuel consumption will be reduced by more than 30 percent.
In the 80s, NASA and GE designed the engine named GE36, and developed it into a product, but because of falling oil prices the product was never commercialized.
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Yet this year, in the face of rising fuel cost, they have decided to to revive the jet engine design and at NASA’s Glenn Research Center, where GE36 wast tested, and plan to start wind-tunnel tests in early 2009.
According to the European Climate Action Network, the exhaust from kerosene-burning jet engines is already responsible for 4 to 9 percent of the climate change impact because it releases high up into the atmosphere. Many air travel industry companies are trying to reduce emissions. There are also energy companies trying to use biofuels instead of the classic kerosene, such as Sapphire Energy, Solazyme and Aquaflow Binomic.
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PROPFAN
NASA/GE Unducted FanAn unducted fan or propfan is a modified turbofan engine, with the fan placed outside 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. >
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A first problem - propeller blade tip speed limit Turboprops have a fairly optimum speed 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 propeller is spun fast enough that the blade tips near the speed of sound, even if the aircraft is motionless on the ground.
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The most effective way to counteract this problem (to some degree) is by adding more blades to the propeller, allowing it to deliver more power at a lower rotational speed. This is why many World War II fighter designs started with two or three-blade propellers and by the end of the war were using up to five-blades in some cases as the engines were upgraded and new propellers were needed to more effectively absorb that power. The major downsides to this approach is that adding blades makes the propeller harder to balance and maintain and the additional blades cause minor performance penalties (due to drag and efficiency issues). 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). >
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A solution to the first problem
Swept propellerA method of decreasing wave drag was discovered by German researchers in World War II — 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 propeller 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 practice that was first used as far back as 1909, in the Chauviere make of two-bladed wood propeller used on the Blériot XI. |
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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 Douglas DC-9, propfans reached a 30% improvement over the OEM turbofans. >
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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 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 free turbine. The turbine rotors drive one propeller, while the other propeller is connected to the free 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.
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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, California, 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 demonstrator aircraft concluded in 1988, exhibiting a 30% reduction in fuel consumption 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. > |
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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-70The 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 canceled. However, the An-70 remains available for further investment and production. |
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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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Green sky thinking - carbon credits and the propfan comeback?
Will spiralling fuel costs and the prospect of carbon credits signal the return of the propfan? Engine makers are dusting off some old concepts.
On display in Boeing's Future of Flight centre in Everett near Seattle sits a curious, multi-bladed contraption that often has visitors scratching their heads. Easily mistaken for a Henry Moore sculpture or a Heath Robinson egg-beater, this is one of a few surviving propfans and a potential bellwether of 21st century aerospace.
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Although no work of art, this engineering achievement is the Pratt & Whitney/Allison 578-DX unducted fan, which, along with the similar General Electric GE36, successfully demonstrated the fuel miserly ultra-high bypass (UHB) propfan concept in the late 1980s. >
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Advantages of alcohol engine.
Less aggression to the environment;
Motor runs cooler;
Possible extension of the Engine Overhaul (TBO);
Increased power;
Considerable decrease of operating cost.
Assuming a fleet of 600 aircraft Ipanema, the alcohol engine:
Eliminates demand of 16.8 million gallons of avgas per year;
Generates demand of 21.6 million liters of alcohol per year;
Leads to reduction of $ 13.5 million per year operating cost of the fleet of Ipanema.
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JRLucariny EMBRAER 150 PropFan
DIMENSIONS
Wingspan 85 ft 4 in 26.00 m
Length Overall 98 ft 1 in 29.90 m
Height Overall 32 ft 4 in 9.85 m
Fuselage Width 9 ft 11 in 3.01 m
Fuselage Height 11 ft 0 in 3.35 m
Power 2 General Electric's GE36 Unducted Fan
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Drawings
“JRLucarinyModels - JRExpCom01” |
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