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This question refers to large passenger jet aircraft only, which I define as being able to carry at least 100 people.

There have been many 2-, 3-, and 4-engined planes in this category. Some of the twinjets have both engines on the wings. Other twinjets have them both at the tail. Some trijets have all engines at the tail, and other trijets have 2 on the wings and 1 at the tail.

But most quadjets (again, only 100+ passenger jet airplanes) put all of them on the wings. Why not put 2 on the wings and 2 at the tail?

Such a configuration would save weight on the wings, because the wings wouldn't need to be so strong to support 2 outboard engines far away from the fuselage. If twinjets can be configured with both engines at the tail, then I see no reason why a quadjet cannot.

In fact, I know of one old quadjet that put all engines at the tail---the Ilyushin Il-62. So I do not think balancing the center of mass and center of pressure is a problem. Some trijets put all engines at the tail, whereas what I'm talking about would only put 2 engines at the tail (and 2 on the wings).

So is this a good idea? Why not do it this way?

EDIT: It was pointed out that exhaust from the wing engines would interfere with the rear engines. If so, then can't we borrow the trijet design for the intake? A large intake at the vertical stabilizer splits in half and feeds both rear engines.

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    $\begingroup$ @ymb1 Interesting, but if trijets can do it then i don't see why quadjets cannot. For the complex fuel routing, I opened a separate question about it for trijets: aviation.stackexchange.com/questions/31522/… $\endgroup$ – DrZ214 Sep 13 '16 at 23:17
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    $\begingroup$ Related $\endgroup$ – Pondlife Sep 13 '16 at 23:26
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    $\begingroup$ Other quadjets: Lockheed Jetstar L-1329 aircraft $\endgroup$ – wbeard52 Sep 14 '16 at 0:50
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    $\begingroup$ Don't forget the VC10 $\endgroup$ – cavver Sep 14 '16 at 10:23
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    $\begingroup$ "the wings wouldn't need to be so strong to support 2 outboard engines far away from the fuselage" -- is that really true? On the ground, it is true that wing-mounted engines must be carried by the wing spars, but in flight having the engine on the wing places it right where the lift to keep it up is generated and right where the thrust needs to apply to counter the induced drag. WIth a fuselage-mounted engine these forces need to be transmitted to the wings in flight. $\endgroup$ – Henning Makholm Sep 14 '16 at 15:05
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Such a configuration would save weight on the wings, because the wings wouldn't need to be so strong to support 2 outboard engines far away from the fuselage.

You seem to be under the impression that the fuselage supports the wings. The reality is the other way around (in flight, at least.) The wings are the most efficient place to hang weight, as they are what is providing most of the lift. This is why the main fuel tanks are usually located in the wings, for example. The strength required to support an outboard engine while the plane is sitting on the ground should be relatively small compared to the strength required for the wings to support the weight of the entire fuselage (including your proposed tail-mounted engines.) Aircraft with tail mounted engines actually need a heavier wing spar, not a lighter one.

It's also important to note that aircraft with 4 engines nowadays are usually very large aircraft with very large engines, such as the 747-8 and A380. Engines of this size would be very difficult to mount from the tail, especially with the modern high-bypass turbofans and their enormous diameters. Think of a GEnx with 2.8 m / 111 inch diameter fan, for example. This would require strengthening the fuselage as well as the wing spars and would also wreak havoc on the center of gravity. Having twin tail-mounted engines would also probably require converting the plane into a T-tail, which causes yet more engineering problems for an aircraft that large. The bending moments on the engine mounts would be enormous, too.

Can you imagine this hanging from the side of the fuselage? Note the size of the engine compared to the size of the van parked beside it.

GEnx
GEnx hanging from a 787 wing Source: Wikipedia

It is usually only much smaller aircraft that have had tail-mounted engines. This is done largely to allow them to keep the wings low to the ground. For larger aircraft, there's usually already enough clearance under the wing to mount the engines anyway or, at least, the increase in wing clearance needed to wing-mount the engines is not as much as would be needed in a smaller aircraft. Of course, for high-wing airplanes, clearance is also not an issue and most of these do indeed wing-mount the engines, even on small regional jets. For example, the Avro RJ-100:

Avro RJ-100 with wing-mounted engines
SWISS Avro RJ-100 with wing-mounted engines Source: Wikipedia

The question Why do large aircraft have their engines mostly on the wings, while smaller ones tend to have them in the tail or the tip? provides more detail on this. As usual, Peter's answer is informative and there are other good answers, too.

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  • $\begingroup$ You seem to be under the impression that the fuselage supports the wings. The reality is the other way around (in flight, at least.) I know the wings support the fuselage in flight, and the fuselage supports the wings on the ground. I know the load relations are reversed. But you have to engineer for both, don't you? Otherwise the wing would snap off on the ground! Putting engine weight farther out places higher torque on the wing and also doubles the length of the strong spar, adding weight. $\endgroup$ – DrZ214 Sep 14 '16 at 23:31
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    $\begingroup$ @DrZ214, yes, for both, but the loads in flight are far greater than those on the ground. The wing must support the whole structure (including engines when mounted on the fuselage), all times the maximum load and safety factor (~ x3). It is a well known basic fact in airplanes engineering that wing-mounted engines save weight. There are (military) designs where engines (or fuel tanks) are even placed at the wing tips. $\endgroup$ – Zeus Sep 15 '16 at 0:40
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    $\begingroup$ @DrZ214 Yes, you need to engineer for both, but the fuselage usually weighs more than the wings already and you don't have to deal with sudden changes in the forces due to turbulence when you're parked on the ramp vs. when you're flying through a jet stream. Gravity doesn't tend to fluctuate much on the ramp, whereas lift can fluctuate substantially in flight as a result of turbulent airflow (or AoA change in general.) The spar already has to be long in order to use the lift from the outer part of the wing without snapping it off in flight. $\endgroup$ – reirab Sep 15 '16 at 3:54
  • $\begingroup$ Otherwise the wing would snap off on the ground! -- that would only happen in a plane where the wing is significantly heavier than the fuselage. Any wing that would snap off on the ground would definitely snap off in flight. $\endgroup$ – slebetman Jun 19 '17 at 14:06
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enter image description here

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The unsteadiness and temperature of the exhaust from the under wing jets will greatly reduce the performance or even stall the jet engines mounted on the aft fuselage walls.


Can't we borrow the trijet design for the intake? A large intake at the vertical stabilizer splits in half and feeds both rear engines.

If the engines are still wall mounted, the routing of the air won't be as easy as the s-duct of an L-1011, the extra twisting of the duct will take from the energy available from the air, plus an added noise and stress factor.

By twisting I mean taking in the air, then directing it down, forward, outboard, and aft again for the wall mounted jets.

enter image description here

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  • $\begingroup$ If the intake is located more forward, the S-duct wouldn't have to be so bendy. However, I understand this will add weight and drag. The intake will also need, presumably, 2x as much intake area in order to feed 2 engines. Another idea is to have the rear engines more elevated. The real question is, would all this be a bigger penalty than the extra weight/strengthening the wings to hold 4 engines instead of 2? $\endgroup$ – DrZ214 Sep 14 '16 at 2:13
  • $\begingroup$ @DrZ214 wall mounting is as hard. And relative to airplane structures, a jet engine is very light. Another issue is maintenance access. Less is more, in art and engineering :-) $\endgroup$ – ymb1 Sep 14 '16 at 13:25
  • $\begingroup$ @ymb1 The engines aren't that light. For a 747-8, as an example, roughly 11% of the empty weight is the engines. That said, as my answer points out, tail-mounting the engines is a weight penalty, not a weight savings. $\endgroup$ – reirab Sep 14 '16 at 16:21
  • $\begingroup$ @reirab I understand, I wrote in the comment above about wall mounting. Anyway 3% per engine (all four 11%) is still considered light. +1 on your answer. $\endgroup$ – ymb1 Sep 14 '16 at 16:29
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reirab provided a good answer, and I'd just like to summarise it in simple terms.

The essence is: such configuration has all the disadvantages and very few benefits compared to the traditional design (or all-tail design for that matter). It also adds its own problems like exhaust interference mentioned by ymb1.

If you have two inboard engines under the wing, you already must provide enough ground clearance for them (with the associated drawbacks of heavier landing gear, maintenance etc.) What's the point of having the other two engines at the back then? It will substantially complicate the fuel system, CG/load management, and, as explained before, will add weight.

This weight penalty comes not only from the unfavourable load distribution, but also because wing-mounted engines work as anti-flutter weights (this is why they are protruding forward rather than being just under the wing). The Ilyushin Il-62 you mentioned has heavy steel weights all along its leading edge.

When you have all engines at the back, at least you can benefit from the 'aerodynamically clean' wing and low ground clearance. For trijets, you'd want to mount the 2nd engine symmetrically, so tail is the only reasonable option - otherwise designers would happily hang it on the wing. (That said, I remember reading that DC-10 (or L-1011?) designers considered mounting the tail engine on one side of the fuselage).

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