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What are the advantages and disadvantages of having an engine in the vertical stabilizer, a la the DC-10, Lockheed TriStar, and 727?

It seems that this has gone out of fashion among jetliners. Is there a reason for this?

It seems that the FAA's 60 Minute Rule was an impetus for the trijet configuration. But is that the only motivation? Is a tail engine less aerodynamically efficient?

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marked as duplicate by aeroalias, mins, Peter Kämpf aircraft-design Jan 5 '17 at 12:14

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  • $\begingroup$ A guess: satisfies the certification requirement with the fewest number of engines? Why use four when three will do? And if you're going to use three it makes sense that one must be along the centerline of the airplane. $\endgroup$ – acpilot Jan 5 '17 at 3:54
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    $\begingroup$ Think of all that structure that has to hold that engine up. Think about how tall the tail has to be. Extra hydraulic and fuel lines, etc. Then add in the incident of Flight 232. Now take into consideration that newer engines are much more efficient, powerful, and reliable than the older engines. It "went out of style" because it just isn't needed. $\endgroup$ – Ron Beyer Jan 5 '17 at 4:00
  • $\begingroup$ Just a nit, but in the interest of possible extreme accuracy, the L-1011 and the 727 might better be thought of as having the engine in the fuselage rather than the vertical stabilizer. The engine air intakes were at the base of the vertical stabilizer and fed S-ducts that delivered the air to the engine in the fuselage. The DC-10 and the follow on MD-11 had straight-through air flow, thus avoiding S-duct problems. $\endgroup$ – Terry Jan 5 '17 at 4:21
  • $\begingroup$ Partial duplicate and Related $\endgroup$ – TomMcW Jan 5 '17 at 5:00
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As you write, at the time those trijets were designed they enjoyed preferential treatment over aircraft with two engines for overwater operation. They also were more efficient than comparable designs with four engines. See here for that discussion.

However, the wing-mounted engine has some advantages:

  • Flutter damping when mounted ahead of the elastic line of the wing
  • Bending relief
  • Easier access for maintenance
  • Shorter fuel lines

The tail position was chosen because three engines were enough for the optimum size of those airliners and equipping them with four engines had lowered efficiency. If you need to put three engines on an aircraft, one has to be in the line of symmetry, and here the best compromise was on or in the rear fuselage. Had more powerful and reliable engines been available back then, you can be sure they would had used only two engines and mounted them on the wing. But we had to wait two more decades until turbofan engines were powerful enough and the certification authorities were convinced of their reliability.

The fashion of early jets to put all engines at the back was just that and had ended before the trijets were designed.

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    $\begingroup$ I have a question on that bending relief "engines provide bending relief on the wing, allowing better wing design (thinner wings): less drag". Putting engines under the wing requires reinforced wings, right? That would make wings likely bigger? So I assume we have generally bending on the wing because it creates (more) lift than the heavy center-body/cabin and the engines-pods put a downforce on the wing, which reduces this beending to some degree? $\endgroup$ – Peter Jan 5 '17 at 10:11
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    $\begingroup$ @Peter: No, the wing creates lift and can lift the heavy engines directly. If the engines were mounted on the tail, that portion of lift would need to be carried all the way to the wing root and the back of the fuselage, requiring a heavier structure. The static case (where the wing has to carry the weight without creating lift) is trivial - a wing fit for flight will have no trouble carrying the engine on the ground. $\endgroup$ – Peter Kämpf Jan 5 '17 at 12:04
  • $\begingroup$ Ah. Okay, make sense :) $\endgroup$ – Peter Jan 5 '17 at 12:05

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