Re: Aviation Fuel

From:         Andrew Chuang <chuanga@iia2.org>
Date:         13 Oct 95 02:06:35 
References:   1
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In article <airliners.1995.1591@ohare.Chicago.COM>, Karl Swartz (kls@ohare.Chicago.COM) wrote:
: >A JT8D burns about 1,000pph at idle and about 3,000pph at cruise. Don't
: >have any SFC numbers - those books are in storage for another month or so.

: Here are a few SFCs (at max power) with typical applications:

:     JT8D-9    0.60    727, 737, DC-9
... [snip]

:     PW2037    0.33    757, C-17
:     PW2040    0.563   757PF
                ^^^^^
The number for the PW2040 looks awfully high for sfc @ max power, it
probably should be for sfc @ cruise.


: Now, how'd you like to give us a tutorial on just what SFC means?

(T)SFC is (Thrust) specific fuel consumption, and is computed based on
fuel flow rate per unit thrust generated (i.e, lb/hr fuel per lb thrust,
or the more usual notation, lb/hr/lb).  Most engine manufacturers quote
(t)sfc for thrust at max power or take-off.  Rolls-Royce quotes sfc at
cruise (after all most of the fuel is consumed during cruise).  Most of
today's high-bypass turbo fan engines have sfc in the range of .32-.34 at
take-off.  The sfc for the GE90, I believe, is at around .28-.29 because
it has a higher by-pass ratio.  (As you can see from Karl's list, the
dramatic reduction of sfc from .60 for the JT8D which has a by-pass ratio
of 1.2(?) to .33 for the PW2000 which has a by-pass ratio of somewhere
between 5 and 6, I think.)  However, the GE90 is a much heavier engine (two
to three thousand pounds heavier per engine, or four to six thousand pounds
per aircraft, or an equivalent of 20 to 30 more passengers!).  Thus,
according to R-R (its Trent 800 has the highest sfc among the three
competitors but the lightest in weight) for trips shorter than 4,000 nm,
the Trent-powered 777 will burn less fuel than the GE- or P&W-powered 777.
Take the R-R number with a grain of salt.  If you have followed the news
related to BA's B777 delivery delay, you must be aware of GE's guarantee
of 5% less (actual not specfic) fuel consumption than the Trent.  GE
insisted that they would meet the goal.  Only time will tell.

--
  H Andrew Chuang   chuanga@iia2.org