To generate thrust, it is usually more efficient to accelerate a large mass of air by a small amount than to accelerate a small mass of air by a large amount. In a turbofan jet engine, the airflow rate that bypasses the engine core divided by the airflow rate that travels through the core is called the bypass ratio. The bypass ratio, or BPR, is one of the key indicators of turbofan engine efficiency. The desire for better fuel efficiency has resulted in the evolution of commercial aircraft gas turbine engines from early turbojets (BPR=0), to low bypass ratio, first generation turbofans (BPR=1-2), to today's high bypass ratio turbofans (BPR=5-10). Now, ultrahigh bypass ratio (UHB) turbofans (BPR=10-20) are being designed for a next-generation, intracontinental commercial aircraft. This aircraft could enter service as early as 2020.

The Intercenter Systems Analysis Team, consisting of systems engineers from Glenn and Langley Research Centers, conducted an analytical feasibility study of UHB turbofans for NASA's Fundamental Aeronautics Program. With a little math, it can be shown that fuel efficiency increases along with BPR. The engine core, however, has a limited supply of power available to propel the bypass air stream, so it can be difficult to simply increase BPR arbitrarily. One path to UHB engines and better efficiency is to reduce the fan's pressure ratio, which lowers the fan's power requirement and allows higher BPRs. Our analysis team designed nine notional UHB propulsion systems for this new aircraft along a parametric design sweep of fan pressure ratio. Using advanced computational tools, these propulsion systems were analytically installed on the airframe and "flown" on missions to determine overall airplane system performance.

In addition to improving engine fuel efficiency, increasing bypass ratio also reduces engine noise due to the strong relationship between noise and the velocity of the air exiting the engine. The lower jet velocities associated with low fan pressure ratio can lead to substantial engine noise reduction. There is a practical limit to how much bypass ratio can be increased before significant penalties arise that quickly erode the benefits. UHB engines have relatively large, low-speed fans, which may require gearboxes and even variable-geometry exhaust nozzles in order to be practical. Engine weight and drag increase as well, making it more difficult to integrate the engines with the airframe.

The NASA team's results have indicated that UHB turbofans can be viable, low-noise alternatives to today's more conventional propulsion systems. The purpose of NASA's parallel, independent system studies is not to direct industry's designs, but rather to exchange data, ideas, and to provide industry with innovative NASA technologies and options.

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Glenn Contacts:
Jeffrey J. Berton, 216.977.7031, Jeffrey.J.Berton@nasa.gov
William J. Haller, 216.977.7004, William.J.Haller@nasa.gov
Douglas R. Thurman, 216.433.6573, Douglas.R.Thurman@nasa.gov
Kenneth L. Fisher, 216.433.5655, Kenneth.L.Fisher@nasa.gov

Programs/Projects:
Fundamental Aeronautics Program, Subsonic Fixed Wing Project