Mongrel 747s (Re: Development of MD-11?)

Date:         11 Feb 98 04:23:52 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>> That's fine if they happen to use the same airplanes in the same places,
>> but that may not be the case.  United uses their 747-400s across the
>> Pacific, whereas their 777s fly Atlantic and Latin routes.  If they
>> were to have a hypothetical 747-500/600 stuck in Sydney with a problem
>> in a PW4084, it wouldn't be much consolation that they hard parts and
>> expertise in London (and in the US).
>
>They can keep PW4084 parts and expertise in Sydney also. It would
>be no harder than keeping the parts and expertise available for
>the alternative all-new engine for the hypothetical 747-500/600.
>As long as *one* of the engines on the mongrel 747 is in common with
>other planes that airline has in the same place, the airline
>is no worse off compared to flying 747s with an entirely new engine.

You're assuming they fly other types to the same place, which is a
totally unreasonable assumption.  Continuing with the United/Sydney
example, UA only flies to SYD from LAX and SFO, both comparable
distances.  They currently fly both routes with the 747-400. If they
needed a hypothetical 747-500/600 on one of the routes, they'd very
likely need it on the other one, too.  Therefore, they'd have to have
support for both engine types for just the one aircraft type.

This is hardly unusual.  AA flies nothing but the MD-11 across the
Pacific (and will transition to the 777 exclusively).  Ditto for DL.
UA is moving towards having nothing but the 747-400 in the Pacific
division for passenger service (Hawaii is Domestic, not Pacific), and
already flies only the 767-300(ER) and 777 in the Atlantic and Latin
divisions.  There are plenty of other examples.

In all of these cases, a mixed-engine design forces the airline to have
support for two engine types in far-flung locations, where a new engine
would allow them to support only one engine type in those locations.

>I would have thought that putting the big engines outboard would
>be preferable from a weight point of view - the big issue is not
>supporting the engines when the plane is on the ground, but supporting
>the fuselage when the plane is in the air. I would have thought that
>putting the big engine outboard would allow a weaker (lighter) wing
>between the two engines.

Huh?!  The entire wing generates lift, not just the wing root.  The
fuselage and payload weight vastly more than even the heaviest 777
engines, so where you place the heavier engines is little bearing on
how strong the wing has to be at a given location to lift a given
fuselage and payload weight.

Whether you like it or not, you *do* have to support the engines when
the plane is on the ground.  Putting a heavier weight further out means
the wing has to be stronger, since on the ground the support is coming
from the wing root (where the main landing gear attach).  In addition,
the wing also has to convey the thrust forces of the engine to the rest
of the airframe -- in flight, the engines are essentially dragging the
rest of the plane along with them.  Those forces require a significant
amount of structure, too, and putting the higher thrust engines outboard
again increases the necessary structure.

--
Karl Swartz	|Home	kls@chicago.com
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"The average dog is a nicer person than the average person." - Andrew A. Rooney