Re: Superfan

Date:         01 Dec 97 02:33:50 
From:         Matthew Willshee <96mjw@eng.cam.ac.uk>
Organization: University of Cambridge, England
References:   1 2 3 4
Followups:    1
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On 29 Nov 1997, H Andrew Chuang wrote:
> The bypass ratio of the GE90 is around 9 to 10, much higher than its
> competitors (the PW4084/90/98 and Trent 800 [bypass ratio of around 6]).
> (Various "superfan"/UHB designs have bypass ratio of around 12 to 15.)
> The GE90, even though a heavier engine, does have a slightly better fuel
> efficiency.

Be careful here.  Engine weight doesn't come into fuel efficiency
calculations.  Specific Fuel Consumption is measured in pounds (of fuel)
per hour per pound force (of thrust).

You can't really compare engines of different types with this because the
fuel efficiency of the engine aircraft combination also depends on the
engine's weight, the nacelle and the integration with the airframe.  SFC
is important, but is not the only factor.

If you improve SFC by putting a huge fan on the front of the engine you
will make the nacelle bigger and heavier.  You will then have to burn more
fuel to cope with the extra weight and nacelle drag.  The end result might
be better or worse aircraft fuel burn depending on how everything balanced
out.

____
Matthew Willshee                               E-Mail: mjw44@cam.ac.uk
Churchill College, Cambridge, CB3 0DS                : 96mjw@eng.cam.ac.uk