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

Date:         11 Feb 98 04:23:51 
From: (Michael Woodhams)
Organization: Princeton University Dept of AstroPhysics
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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.

> >One disadvantage of mixed engines is that I suppose all engine-out
> >performance figures would have to be figured for the worst case, with
> >one of the larger engines inoperative. This could result in a lower
> >maximum take-off weight for the same airframe.
> I'm not sure it's quite that clear.  The bigger engines would presumably
> be mounted inboard for weight reasons (as you suggested, though moving
> them outboard would solve the ground clearance problem) and, for engines
> of the same thrust, an inboard engine failure isn't as serious as an
> outboard engine failure due to the shorter lever arm resulting in less
> yaw effect.  Obviously that doesn't hold if the thrust differential is
> great enough, but it would take more analysis to say with confidence
> that the failure of a larger inboard engine would be the worst case.

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.

I assumed that putting the big engine inboard was entirely due
to asymmetric thrust considerations with an engine out. Thinking
about weight however, might balance be a problem if the big
engines were mounted outboard and one parted company with the wing?

Disclaimer - my only expertise in this field is regularly reading
this newsgroup.

Michael W.