Re: Trijet engine mounts

From:         kls@ohare.Chicago.COM (Karl Swartz)
Organization: Chicago Software Works
Date:         03 Feb 93 02:11:33 PST
References:   1 2 3 4 5
Followups:    1
Next article
View raw article
  or MIME structure

Phydeaux <reb@ingres.com> asks:
>My  question  about   the   L-1011 is  that   since   it   sounds like
>manufacturers go to  great pains  to not  'bend' the airflow  why does
>that rear have a duct that does just that? Why doesn't the tail engine
>mount look more like the DC-10?

The "S-duct" arrangement seen on the de Havilland 121 (aka Hawker
Siddeley Trident), Boeing 727, Lockheed L-1011, Tupolev Tu-154, and
others is popular because it has a number of advantages over the
"straight duct" design used on the DC-10.  Structural weight is one,
since the center engine in an S-duct design is an integral part of the
fuselage whereas the DC-10 design requires additional structure in the
fin to support the engine.  The S-duct arrangement also gets the #2
engine further away from all the hydraulics and cables for the tail
surfaces, a feature which would have been invaluable to UA 232, the
DC-10 flight which crashed at Sioux City, Iowa after the #2 engine's
disintigrating fan took out all three hydraulic systems.

In addition, the S-duct design provides better airflow around the tail
cone (which of course isn't really a tail cone in this case) and thus
reduces drag, perhaps enough to counter the required bend in airflow.

The biggest advantage of the DC-10's design for McDonnell Douglas was
that it was simpler and cheaper.  Another factor, though as far as I
now one not germane to the DC-10, is that it relaxes constraints on
the engine's overall length -- Lockheed indicated that using the
General Electric CF6 instead of the more compact Rolls-Royce RB.211
would require the sacrifice of two rows of seats at the rear of the
cabin.

BTW, it's always been my understanding that McDonnell Douglas has a
patent on the straight duct design.  I was thus rather surprised last
year, when reading the article on the Trident in the Spring 1992 issue
of Airliners, to encounter a sketch of an early design for the Trident
(then called the DH 121) which used a straight-through center engine
design.  It shows the #2 engine being mounted fairly far forward, with
a long exhaust duct, though.  Perhaps the MD patent is more specific
than simply a straight duct.  Can anyone in the know shed more light
on this?

--
Karl Swartz	|INet	kls@ditka.chicago.com		
1-415/854-3409	|UUCP	uunet!decwrl!ditka!kls
		|Snail	2144 Sand Hill Rd., Menlo Park CA 94025, USA
 Send sci.aeronautics.airliners submissions to airliners@chicago.com