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I've noticed that the nacelle outlet of the Lockheed Tristar is situated below the engine itself. It is at the end of the fuselage. The nacelle outlet of the tail engine of the DC-10 on the other hand, is at the same height as the engine. Why did Lockheed choose to lower the nacelle outlet? Perhaps this picture will help show you guys what I mean.

enter image description here

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    $\begingroup$ given the shape seen in the photo, I suspect that is the inlet to be above the engine, not the engine to be above the outlet. nice question nevertheless. $\endgroup$ – Federico May 19 '15 at 19:10
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    $\begingroup$ The Boeing 727 has the same arrangement. It used to be called an S-duct. Anecdotally I was told that after the the 727 Boeing decided that never again would they use an S-duct. $\endgroup$ – Terry May 19 '15 at 19:40
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    $\begingroup$ The S-duct was actually more common than the straight-through design, as the DC-10 and MD-11 were the only major designs to use it $\endgroup$ – SSumner May 19 '15 at 22:05
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As Federico points out, the engine is located directly in front of the exhaust portion. This is true for almost all engines with ducted arrangements. It's much easier to duct the cool, slow intake air to the engine than duct the hot, fast exhaust air away from it, for both temperature and drag reasons.

In this cutaway you can see the intake duct leading down to where the engine is:

L-1011 Cutaway View
Source

Putting the exhaust nozzle lower allows more room for rudder above it. Putting the nozzle higher, like on the DC-10 series, means the rudder is pushed higher up the tail.

This arrangement keeps the rudder as close to the vertical center of gravity as possible, which reduces the stress on the tail and limits the rolling motion induced by the rudder. It also keeps the engine forces close to the vertical center of gravity. This configuration also allows the supports to tie in to the existing structure at the rear of the fuselage. Placing the engine further up the tail would add surface area (and therefore drag), and require additional structure to support the engine above the fuselage.

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    $\begingroup$ Thank you for the answer. But why didn't McDonnell Douglas use the same feature on the DC-10? $\endgroup$ – Madhav Sudarshan May 20 '15 at 2:37
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    $\begingroup$ Simplicity, increased usable fuselage volume, reduced vibration transfer to the cabin, easier access to the engine without taking the back of the airplane apart... these are only a few good reasons I can think of not to use an S-duct design. $\endgroup$ – KeithS May 22 '15 at 0:01
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    $\begingroup$ I believe that if you ask anyone that has worked on a DC-10/MD11, 'easier access to the engine' will not be high on their list of pros for this engine placement. $\endgroup$ – Sports Racer Jun 8 '15 at 0:04
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    $\begingroup$ You might add that the lower engine location will cause less pitch trim change with thrust changes. $\endgroup$ – Peter Kämpf Jun 23 '15 at 15:30
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    $\begingroup$ @PeterKämpf, not necessarily, because of the two other engines under the wing. I remember reading that DC-10 was remarkably pitch neutral to thrust changes. But you would be right for the more common tri-jet configurations like 727 or Tu-154. $\endgroup$ – Zeus Jun 30 '16 at 1:48
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This arrangement of placing the engine intake duct above the fuselage is called S-duct (Thanks Terry).

The answer of why would be it is how it was designed, probably as a differentiating feature. This is vaguely mentioned here:

... a small vertical fin [is] between the bottom of the middle engine intake and the top of the fuselage.

From the pictures below (images' source), it appears that what Federico stated appears to be true.

Picture 1

Picture 2

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I have discovered some more information regarding the S-duct design in an L-1011 marketing book.

S-Duct Payload Benefits

S-Duct Payload Benefits The S-duct center engine location benefits payload by its lower fuselage weights. It allows a longer constant fuselage section thereby adding seating space. The more effective rudder allows the engines to be located further outboard on the wings and provides an optimized center cabin door location. The exhaust efflux provides an effective fineness ratio increase and hence reduces drag.

Center Image Position

Center Engine Position In the S-duct arrangement the engine is 10 ft. lower than a fin mounted engine. This permits easier access for maintenance or engine changes.

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The 'nacelle outlet' you speak of is actually Engine #2. It is fed air via an 'S' shaped duct running through the tail section of the aircraft, starting at the dorsal intake just forward of the vertical fin.

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