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This question focus on jet airliners.

The De Havilland Comet's engine were mounted inside the wing, the Concorde's engines were mounted underwing (as many jet engines) but without pylon. It seems jet airliners' engines are now mounted on pylon. That assumption goes wherever the engine is mounted (underwing or at the rear of the aircraft).

What are the advantages of doing so (aerodynamic, maintenance, safety,...)? Is there some drawback (must be minimal as this design is dominant)?

Note: I don't want discussion about where to mount engine (underwing as for the B747, overwing as for the VFW-Fokker 614, at the rear as for the MD-80, ...), I'm curious about why we use pylons for all this designs.

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  • $\begingroup$ possible duplicate of Why not mount airliner jet engines above the wings? $\endgroup$
    – GdD
    Commented Jan 28, 2015 at 10:25
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    $\begingroup$ @GdD It isn't. The answer of the linked question discuss the location of engine, not the fact that we use pylons. The word "pylon" is used only 2 times, both discuting mounting engine over the wing (nothing about pylons itself). $\endgroup$
    – Manu H
    Commented Jan 28, 2015 at 10:35
  • $\begingroup$ I'd say the answers in there together give all the information you need, however if others disagree I'm sure you'll get explicit answers here. $\endgroup$
    – GdD
    Commented Jan 28, 2015 at 10:46
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    $\begingroup$ @GdD I still disagree (otherwise, I wouldn't have ask this question). No answser discuss the absence of pylon (espacially on the 2 aircraft I cite, Comet and Concorde), but only the position of the engine (underwing, overwing or at the rear). I'll edit the question to make it more clear. $\endgroup$
    – Manu H
    Commented Jan 28, 2015 at 10:57
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    $\begingroup$ I think it's a good question. I'm sure there are many aerodynamic reasons, but I'm guessing it's also related to engine failures. Sioux City happened due to an uncontained engine failure in the tail of the DC11 severing all hydraulic lines of all three systems. Furthermore, the pylon offers an opportunity for the engine to break away cleanly rather than taking the wing with it. Although such attempts have been unsuccessful in the past (e.g. El Al 1862, AA191), it could potentially allow the plane to continue fly. $\endgroup$
    – falstro
    Commented Jan 28, 2015 at 11:11

5 Answers 5

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Engine placement was a tricky question when jets were introduced, and much has been learned since then. The initial idea was to mount them inside the fuselage or wing root. When they grew too big, they were closely mounted to the wing ("underslung" position, like in the Me-262, the Il-28 or the early Boeing 737).

However, the underslung position has two disadvantages:

  1. More of the leading and trailing edges is obstructed, blocking space that would otherwise be used for slats and flaps. Consequently, an underslung position reduces maximum lift.
  2. The center of gravity of the engine is too far back. Moving it more forward helps to place the center of gravity of the wing ahead of its elastic line. Bending will now create an opposing inertial moment, which is very helpful to dampen flutter.

Starting to move the engine downwards away from the wing initially increases drag, because now a narrow slot opens between both with a tendency to separated flow. Only when the engine is moved downwards enough, the drag of the wing-engine arrangement reaches its minimum. This placement was chosen in the next generation of jets like the B-52 or the Boeing 707.

Moving it still further down makes the pylon longer and thus increases the total drag again.

The big diameters of modern high-bypass ratio engines make it impractical to move it down enough to reach the drag minimum, but by moving it forward, the engine can again be lifted up, so a shorter landing gear becomes possible. Vortex generators are added if that pesky separation between engine and nacelle cannot be avoided.

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  • $\begingroup$ If over the years we learned underslung wasn't the best, why did Boeing go from more separation on the 707 to less on the 737 (which came after)? $\endgroup$
    – cpast
    Commented Feb 4, 2015 at 4:12
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    $\begingroup$ @cpast: Because the initial design was with small JT8D engines and then the low landing gear forced them to put the engine high enough. Covered also here. $\endgroup$
    – Peter Kämpf
    Commented Feb 4, 2015 at 7:16
  • $\begingroup$ I remember reading somewhere that the forwards placement helps with bending loads on the wing, since then the torque from the thrust and the engine weight cancel each other out. Can't find a source for it right now, though, but it makes sense on paper. $\endgroup$
    – Sanchises
    Commented Oct 4, 2015 at 8:43
  • $\begingroup$ @sanchises: The bending relief comes from placing the engines out at the wing. Their mass directly counters lift, regardless of their lengthwise position. Torque from thrust would cause a torsion moment, not a bending moment in the wing. Now this torsional moment does indeed run counter to the torsional moment from the engine mass, but would be rather variable over a whole flight. $\endgroup$
    – Peter Kämpf
    Commented Oct 4, 2015 at 9:18
  • $\begingroup$ @PeterKämpf Sorry, I indeed meant torsion relief. My bad! $\endgroup$
    – Sanchises
    Commented Oct 4, 2015 at 9:30
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Mounting the engine on a pylon rather than integral to the wing, allows the aircraft to accept different engine types (eg, Rolls Royce, P&W, or GE) or different sizes of engines. For example the 757 had three different engines, each made by a different manufacturer:

Launch customers Eastern Air Lines and British Airways selected the RB211-535C turbofan built by Rolls-Royce, which was capable of 37,400 pounds-force (166 kN) of thrust.[19] This marked the first time that a Boeing airliner was launched with engines produced outside the U.S.[8] Domestic manufacturer Pratt & Whitney subsequently offered the 38,200 pounds-force (170 kN) thrust PW2037,[19] which Delta Air Lines launched with an order for 60 aircraft in November 1980.[8][20] General Electric also offered its CF6-32 engine early in the program, but eventually abandoned its involvement due to insufficient demand.

The pylon not only mounts the engine, but allows the different engine designs to adapt to the airframe.

Secondly, by disassociating the airframe from the engine, the engine can be repaired and replaced without disassembling the wing.

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The passenger jet engine mounts (pylons) are designed to break away due to excessive vibration (a type of engine failure) because you would rather drop an engine than have your wing break off and drop like a brick.

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    $\begingroup$ Could you expand your answer (direction and intensity of vibration at which the engine is dropped) and add references? $\endgroup$
    – Manu H
    Commented Oct 4, 2015 at 7:51
  • $\begingroup$ Things were edited out, I have been trying to find the original designer of the concept. $\endgroup$
    – user11565
    Commented Oct 7, 2015 at 3:06
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The foremost reason is, the engine is isolated from critical structural components in the wing, enhancing safety. If there were a fire, the fire would not be likely to burn the wing off since the airstream would ensure the flames avoided the wing, in which the fuel is housed in modern jet designs. Also, if the engine were to explode in an uncontained failure, the shrapnel would be less likely to ignite a fuel source, damage a wing spar, or damage flight control surfaces. The hanging pod design surrounding the compressor and turbine sections helps contain catastrophic failures, and the pod design also enables bypass air to envelop the engine on all sides, an important enhancement for hot-section cooling and fuel economy. Additionally, a pylon-mounted engine does not interfere with the airflow over and under the wing to the same degree as a wing-integral design would. Another less important advantage is, the engine is more accessible for maintenance purposes. But a disadvantage is the engine is more vulnerable to picking up foreign object damage, being closer to the ground.

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The real answer: pylons make engine maintenance easier and are a more practical solution for current high bypass turbofan engines.

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    $\begingroup$ References and links to support your answer and to provide further reading would be welcome to improve your (tiny) answer. $\endgroup$
    – Manu H
    Commented Oct 17, 2016 at 12:39

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