very high bypass

very high bypass

An engine with a bypass ratio in the region of 15:1. These engines may be used for reverse thrust. This type of configuration is known as cold reverse thrust, as opposed to hot reverse thrust, which is provided by hot exhaust gases.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
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[Note that the open rotor engine fuel burn does not include a debit for installation effects associated with the weight of required local hardening of the aircraft for blade loss which is difficult to evaluate for this level of comparison.] As one might anticipate, the open rotor engine is a clear winner at low flight speeds, but loses this advantage as flight speed is increased beyond a flight Mach number of 0.7.With current cruise speeds in the range of Mach 0.8 and higher, the very high bypass ratio engine the scale tips in favor of the very large bypass ratio turbofan.
He concluded: "The aim is to achieve a very high bypass ratio engine that can meet the noise and specific fuel consumption goals on ACARE without the penalties of drag and weight normally associated with low specific thrust engine".
Many new turbofan engine designs just coming into production also use very high bypass ratios for increased fuel efficiency, but the propfan is still about 25 percent more efficient than these, Hanson says.

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