Re: Are Two Engine 757 & 767 Jets Dangerous?

Date:         12 Dec 96 03:49:25 
From:         Jan Polcher <polcher@ella.ens.fr>
Organization: Ecole Normale Superieure
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> Michael Hore wrote:
> "With a pair of high-thrust turbofans, the wing could support a maximum
> take-off weight of 212t, giving the A330 its 8,800km range with 335
> passengers.  With two further engines located outboard, giving extra
> bending relief, the wing could support more fuselage weight without
> stressing the wing/fuselage join more than that of the A330.  This
> meant that an extra 41,700 litres of fuel could be carried in the
> fuselage centre-section, giving the A340 the extra range it needed."
>
> So it appears to me that the A340 would always have had 4 engines,
> regardless of what engines might have been available.  It's part of
> Airbus' philosophy of meeting 2 markets (medium range / long thin) with
> 2 designs optimized for each.  Thus I understand they claim that the 777
> is less efficient than the A330 in the medium-range role, since the
> former has to carry the structural/landing gear/etc. weight for a
> long-range capability. I'm sure Boeing will dispute this...

Very convincing argument. Does it explain the very high wings tips on the
777 ? Would this design reduce the strain on the wing/fuselage join ?

I was told once that low by-pass engines are more efficient at cruising
speeds while the high by-pass engines have an advantage in the low speed
sections of the flight. Is there hard evidence for this ?

If you look at figures you have he impression that there is something to
it : for the 330 and 777 engines you get ratios around 8 while for the 340
engine it is more like 6.5. This could explain the choice of Airbus to use
two high by-pass engines for the short-range 330 and 4 low by-pass engines
for the long and ultra-long range market.

This would indicate that the 340 will be more efficient than the 777 in
long range operations. Does it mean that the 777 is more of a
compromise than the same wing for the 330 and 340 ?

I know that I am neglecting the extra drag of 4 engines but also the extra
cost of ETOPS maintenance !! Perhaps it is to close to call without the
exact figures ?

	Jan


Jan Polcher                                              TEL: -33-1-44322243
Laboratoire de Météorologie Dynamique du CNRS            FAX: -33-1-43368392
École Normale Supérieure     24, rue Lhomond    75231 PARIS cedex 05, FRANCE
http://www.lmd.ens.fr/~polcher/                           polcher@lmd.ens.fr