Re: 2 vs 4 engines: R&D costs too much ?

Date:         16 Sep 99 16:42:55 
From:         westin* (Stephen H. Westin)
Organization: Cornell University Program of Computer Graphics
References:   1 2
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kls@ohare.Chicago.COM (Karl Swartz) writes:
> Comparing the 747-400 and 777-200 seems reasonable.  The 747-400 uses
> engines of about 56k lbs of thrust, versus an average of around 84k for
> the 777-200.  That's a 50% increase in thrust.  (The smallest engine
> I've noticed on a 777-200 is a PW4084 derated to 77k, the largest is
> around 92k on an ER.)  Engine weights, in pounds:
> mfr  747-400  777-200  inc  747-400 engine  777-200 engine
> ---  -------  -------  ---  --------------  --------------
> GE     9,499   16,644  75%  CF6-80C2B1F     GE90-85B
> RR     9,470   13,100  38%  RB.211-524H-T   Trent 884
> PW     9,400   14,995  60%  PW4056          PW4084
> >From this small sample, it appears that weight is roughly propotional to
> thrust, so for a comparable total thrust, the engines of a four-engined
> aircraft wouldn't weight more than those of a twin.  This ignores the
> nacelles, pylons, and other factors which might not scale so easily.

I believe there's also a fundamental economy of scale. I remember
spending time many years ago with Aviation Week's listing of all
available engines; at that time, there was about a 4:1 difference in
specific fuel consumption between the smallest contemporary turbofans
(Williams research engines for cruise missiles etc.) and the biggest
(GE CF6, as I recall). Is that still true? I suspect it is, or Boeing
would have built a 4-engine (or 3-engine) 777 to get the same fuel
economy with less engineering investment and risk.

-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.