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

Date:         27 Aug 99 16:25:25 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>Are we not getting to a point where conventional jet engines are
>reaching the top of the bell curve and any improvements will cost more
>and more ?

I've not noticed any signs of that.  Compare an airframe from the late
1960s to one from the late 1980s -- before Boeing put a new wing on the
737, it's performance lagged the A320 a bit but not by a huge amount
given comparable (CFM56) engines.

Now compare engines of roughly similar vintages -- the JT9D is a weak,
thirsty, unreliable beast compared to the PW4000.  The GE90 is likewise
a huge leap beyond the PW4000, in only about ten years.

>On the other hand, by using 4 engines (or even 3), it allows you to use
>much more conventional technology which is not only cheaper  for R&D but
>has greater market and hence more competition.

Conventional *engine* technology, but there is added complexity to the
airframe, not to mention higher operating costs.  Our aerodynamicists
can go into detail but having two engines on a wing instead of one
greatly increases the difficulty of the R&D for a four-engined aircraft
compared to a twin due to aerodynamic interactions between the adjacent

The center-line engine required for a tri-jet introduces its own share
of problems.  There are all the usual CG and structural issues that come
with any aft-mounted jet, plus concerns about safety when you put so
many critical systems in such a small location.  Besides the engine,
there are all the hydraulics for the rudder(s) and elevators.  Wing-
mounted engines are a lot easier to isolate.

>Also, in the theroretical A3XX plane, what sort of power is expected out
>of each of the 4 engines ? Would they be using 777-class engines (90k
>and above) or 747 class engines (about 60k pounds, right ?) ?

Neither.  The numbers I've seen have been in the 70k-75k range.  Pratt
and Rolls both have engine families which straddle that range, so you
could presumably re-fan a PW4000 or RB.211/Trent and get an engine in
the right class, but Airbus wanted an all-new engine so they'd get
higher efficiency than they'd get from a derivative.

>Also, is there a significant difference in weight for 4 medium vs 2 huge
>engines ?

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.

Karl Swartz	|Home
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