Re: Trijet engine mounts

From:         kannan91@iastate.edu
Date:         04 Feb 93 02:36:33 PST
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In an article on the S-duct Karl Swartz writes:

> 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.

I would just like to add one point in favor of the S-Duct particularly
on the L-1011. The use of an S-Duct as opposed to the straight
intake philosophy of the DC-10 is that allows a much greater
rudder area and consequently more rudder effectiveness. This
becomes of significance particularly in the event of the failure
of either engine #1 or engine #3 on the DC-10 wherein a greater
rudder deflection is necessary to counter the yawing moment
produced by dissimilar thrust. It may be noticed that the two
wing-mounted engines on the DC-10 are more inboard when compared
to the wing-mounted engines on the L-1011. I remember reading
that it was to reduce the rudder deflection in the event of an
engine outage that McDD decided to mount these engines futher 
inboard. As a consequence, bending moments produced on the wing
of the DC-10 will be more thereby the necessity of using a 
stronger wing structure. So, although the design of the DC-10 duct
itself was simpler, higher costs had to be paid elsewhere.