Re: Engine Design

From:         chuanga@iia.org (Andrew Chuang)
Organization: International Internet Association.
Date:         16 Mar 95 14:38:56 
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Jim Anderson (James.C.Anderson@williams.edu) wrote:


: I have a question about the diffenence in engine design between say, GE/PW and
: RR.  Corrent me, please, but don't the GE and PW engines have a central "spike"
: coming out the back of them and the RR engines don't?  The Rolls engines have 
: look of the "pre-high-bypass" days (jt8d seris(?)).  What's the difference?

I believe the "spike" you are talking about is the tail cone.  To the
best of my knowledge, all turbofan engines have tail cones.  The reason
that you don't see tail cones on low-bypass engines is because they are
enclosed in long engine/fan ducts.  In the long duct, the bypass flow
and core flow are mixed before exhaustion.  Why are the two streams
of flow mixed?  It's for improving fuel consumptions and reducing noises.
For high-bypass engines, the engines are much larger.  With a mixed-flow
configuration, the extended fan duct will add a lot of weight to the
engine.  Therefore, it is not widely used.  With their three-spool design,
the R-R engines are usually shorter and smaller than the American ones
(both GE and P&W engines have two spools, only).  As a result, R-R engines
are better candidates for mixed-flow configuration.  Currently, the
RB.211-524G/H (for the 747 and 767), -535E4 (for the 757), Trent 700 (for
the A330), V2500 (for the A319/320/321 and MD90), and the CFM56-5C (for the
A340) have the mixed-flow configuration.  R-R called it "integrated exhaust
nozzle", while GE/SNECMA called it "long-duct mixed-flow" (LDMF).  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.  I think the Trent 800 does not have an integrated exhaust
nozzle (but I can be wrong).  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)!!!  CFMI originally planned to have the LDMF
for the CFM56-7 (which will power the B737-6/7/800), but I believe they
have decided against it.

BTW, the tail cone is not always visible even on the non-mixed-flow engines;
for example, the PW4000 on the A310 and the B747, and the CF6-80 on the A310.

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         |  H Andrew Chuang    chuanga@iia.org  |
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