Re: Superfan

Date:         09 Dec 97 03:54:24 
From:         Chuanga@cris.com (H Andrew Chuang)
Organization: Concentric Internet Services
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In article <airliners.1997.2834@ohare.Chicago.COM>,
Matthew Willshee  <96mjw@eng.cam.ac.uk> wrote:
>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.

I agree with your general statements here.  I would also like to add
that it may also depend on the stage length of a trip.  A heavy engine
with low thrust specific fuel consumption (TSFC) may use more fuel on
a short trip than a light engine with high TSFC, but less fuel on a
long trip.  I believe that's a reason why most of the GE90 orders are
for the B777-200IGW.

Nevertheless, based on various data I have seen, I believe the GE90,
some 3,000 lbs heavier than the Trent 800 per engine, is slightly more
fuel efficient than both the PW4000 and the Trent 800 (not just the TSFC).