Engine Efficiency

Date:         28 Mar 98 14:21:48 
From:         "P. Wezeman" <pwezeman@blue.weeg.uiowa.edu>
Organization: The University of Iowa
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	"Jane's All the Worlds Aircraft" lists the Rolls Royce RB211 524H
high bypass turbofan as having a specific fuel consumption in cruise of
.570 pounds of fuel per pound of thrust per hour at mach .85, which I
think is about 500 knots. This figures out to be about 35 percent total
efficiency. Other modern engines have similar performance.
	The total efficiency is the product of the engine's thermal
efficiency in converting chemical energy in the fuel into kinetic energy
in the exhaust stream, and the propulsive efficiency in converting the
kinetic energy in the exhaust stream into useful thrust horsepower.
This propulsive efficiency is a function of the ratio of the speed of
the exhaust to the speed of the aircraft. "Jane's" does not list the
information I need to calculate the propulsive efficiency.
	I would appreciate it if someone could post the cruise exhaust
velocity of the RB211 524H or other engine of similar performance (the
Trent 700 and 800, some versions of the CFM56, and others are listed
with s.p.c. in the .575 range). Alternately, if I had the cruise mass
flow through the engine I could calculate the exhaust velocity by
dividing the cruise thrust by the mass flow.

                          Thank you in anticipation,

                        Peter Wezeman, anti-social Darwinist

                             "Carpe Cyprinidae"