Re: New Tri-Jets

Date:         16 Jun 97 21:35:40 
From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works, Menlo Park, California
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>   Presuming there was a market for it, could a large tri-jet be derived
>from the Boeing 777 by extending the fuselage forward enough to balance
>the weight of the third engine and increasing the size of the vertical
>tail to compensate for the increased fuselage area forward? Of course,
>this would be in addition to the changes needed for the increased weight
>and payload allowed by the third engine.

A much bigger problem is the additional fuselage structure required to
support both the weight of the center engine and the transferrence of
the thrust it generates to the rest of the airframe.  Aft-mounted
engines also pose safety problems in that they place concentrations of
very high kinetic energy close to critical systems.  (Witness UA 232,
the DC-10 which almost landed at Sioux City.)

Yet another problem where exactly you put the engine.  An S-duct, as
on a Trident, 727, or L-1011, has aerodynamic and structural advan-
tages, but also poses length constraints which would be especially
challenging for an engine as large as those on a 777.  The DC-10-style
straight-duct doesn't have the length constraint, but leads to greater
structural weight, plus the greater height has negative implications
for maintenance.  There have been several proposals for a split-duct
(747 tri-jet, early DC-10 study) but none have been built that I'm
aware of.  I'd guess their problems would be similar to the S-duct.

It's certainly possible -- Boeing had several studies of tri-jet 747
variants, while Lockeed and Douglas went the other way with TwinStar
and DC-10 Twin derivatives, respectively -- but it's non-trivial.  I
doubt Boeing would do it as long as they can sell 747s, and I don't
see Airbus having much interest in such a project either.

--
Karl Swartz	|Home	kls@chicago.com
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