Re: Engine Design

From:         chuanga@iia.org (Andrew Chuang)
Organization: International Internet Association.
Date:         17 Mar 95 12:58:01 
References:   1 2 3
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Karl Swartz (kls@ohare.Chicago.COM) wrote:
: >Despite what I just said about R-R engines being better candidates
: >for mixed-flow configuration, the RB.211-524 is much heavier than the
: >JT9D, the PW4000, and the CF6.

: Sure, but the added weight is mostly because of the third set of
: shafts and bearings for the third stage, with the longer duct being
: a comparatively modest increment, right?

I think the longer duct accounts for most of the difference.  Although,
the third shaft add a lot of weight, the smaller diameter and shorter
length compensate for the additional shaft weight.  (The CF6-80C2 and
the PW4000 have many more stages than the -524G/H, especially the -80C2.)
The long duct not only adds more weight, it also adds more drag.

: >Surprisingly, the Trent 800 is the lightest engine for the B777 (but
: >it has the highest specific fuel consumption).  R-R claims that the
: >Trent is at least 3,000 lb lighter than the competitor (i.e., 6,000
: >lb per aircraft)!!!

: I assume you're reading the same ad I saw in AW&ST.  Like any sort of
: advertising, read carefully:

No, please read on.

:     "The Trent 800 is also the lightest engine for the new Boeing,
:     weighing UP TO 6,500lb less PER AIRCRAFT than its competitors."
:              ^^^^^              ^^^^^^^^^^^^

: The emphasis on key phrases is mine.  They'd *really* like you to
: think their engine is 6,500 lbs less than any of the competitors, but
: like most folks you caught that.  The first set of weasel words are
: the real killers though.  RR is really only comparing their engine to
: the heaviest competitor -- the GE90, which suffers a substantial
: weight penalty on account of that enormous fan and perhaps some extra
: hardware to easily allow the fan and core to be separated for shipping.
: They in effect say nothing about the PW4000, which could only be an
: ounce heavier, or perhaps even lighter than the Trent 800!  (I don't
: know how the PW4084 compares in weight to the Trent 800, though my
: guess would be that it's a little heavier.)

My info was old.  The number that I remember is that the Trent 800 is about
3000 lb lighter (per engine) than the PW4084, and 5000 lb lighter than the
GE90.  If R-R is talking about 6,500 lb difference per aircraft between the
GE90 and the Trent 800, then my estimate for the difference between the
PW4084 and the Trent 800 is about 2,500 lb per aircraft.  That's about
10-15 more passengers.  The GE90 is significant heavier is because the
engine is at least half a generation ahead of the Trent 800 and the
PW4084.  It's close to be classified as a Ultra-High-Bypass (UHB) engine.
The much higher bypass ratio (9-10 vs. 6-7) translates into better fuel and
thermal efficiencies, however, it's also a lot heavier.  According to R-R's
number, the Trent 800 will burn less fuel on trips shorter than 4,000 nm
(or statute mile, I forgot).  (Well, I'm skeptical of R-R's number, too.
After seeing their B757 claim, I know that they have a tendency to
exaggerate.)  Only time will tell if it was wise for GE to bet on the more
advanced technology.

--
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         |  H Andrew Chuang    chuanga@iia.org  |
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