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

Date:         16 Sep 99 16:42:56 
From:         "robert wright" <>
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>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

    These interactions can be a biggie.  Four engine transports behave very
differently from twins in most flight regimes.  I would assume that the
structures also change quite a bit, as the mass is now distributed
differently along the wing, which would almost certainly affect vibration
modes and flutter.  Does anyone how the A330/340 wing deals with the
different characteristics of two big engines in close or four medium ones
spread out?  (Total engines, I know there are only one or two on each wing.)

>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.

    At Boeing, when I worked on some studies for a new large aircraft to
possibly replace the 747, we looked at twins, tris, and quads.  Trijets
always came out worst in terms of operating economics by a long way, largely
due to the CG issues and added weight of shielding around the engine to
protect airplane systems in the event of a rotor burst.

>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.

    The GE-PW Engine Alliance is offering (threatening?) to build a 72K,
101-inch fan engine.  I think the proposed RR Trent 900 is of similar size,
but with more shafts (3, as opposed to 2).

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

    Also, of course, a twin will need more overall thrust than a
similar-sized quad.  Of course, a quad may need a bigger vertical fin and
rudder to trim out a dead outboard engine, due to the much greater moment
arm.  This would be a very interesting parametric study.  I kind of wish I
had some weights or structures experience....