Re: 757 highest thrust to weight ratio ?

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works
Date:         10 Dec 92 00:52:14 PST
References:   1 2 3
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In article <airliners.1992.113@ohare.Chicago.COM> (Fred Christiansen) writes:
>In article <airliners.1992.71@ohare.Chicago.COM> (Karl Swartz) writes:
>>Not surprisingly, the top 11 were all twins.

>Why "not surprisingly"?  As a layman with no background in this stuff,
>I would have tho't that manufacturers would keep the thrust-to-weight
>in some ballpark range (for economic reasons).  And that the advent
>of larger twins is because more powerful engines have become available,
>eliminating the need for a 3rd (or in the case of, say, MD-10 class
>aircraft, the need for a 4th).

The catch here is that once committed to takeoff, the aircraft must be
able to fly long enough to return for a safe landing with one engine
out.  With four engines, this in effect means you've got to carry 33%
more power than the bare minimum to keep you in the air; with two you
need a full 100%.  Note that this does not *necessarily* imply a much
higher thrust/weight ratio -- depending on how the certification regs
are written, one could figure maybe twenty minutes to dump fuel and
return and certify the engine for a substantial overload thrust quite
a bit above the normal "maximum" thrust.  But since essentially the
same engines would also be used on three- and four-engine aircraft
this probably wouldn't make much difference.

Looking at this from the view of normal operations, the engines on a
twin will normally not be worked as hard which should lead to higher
reliability and lower maintenance costs.  And, as mentioned in regard
to the 757, better climb performance when needed.

The Airbus A330 and A340 offer an interesting comparison, since they
are identical save the engines (two on the A330, four on the A340),
their wing attachments, and those parts of the controls directly
affected by the engine differences.  (Fuselage lengths differ too
though this is true even amongst A340 models.)  The A340, with four
engines, offers freedom from ETOPS restrictions for long, overwater
routes.  The A330, with two larger engines, offers the maintenance
economies of fewer engines for shorter or overland routes.

It's interesting, then, that Lufthansa chose the A340 even though
the A330 would have served most of the routes they had in mind.  As
I recall, they felt that four less-stressed engines would be cheaper
to operate and maintain than two larger engines.  Given the overall
popularity of twins this doesn't seem to be a widespread view; it may
simply reflect greater confidence in the more mature engines of the
A340 compared to the A330.

Karl Swartz	|INet		
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