Why hasn't there ever been a large 4-engine commercial airplane with 2 engines on the wings and 2 engines at the tail?

Question

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.

Answer 1

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 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:

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.

Answer 2

Source

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.

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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.

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Answer 3

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).

Answer 4

Like the answer posted above, the hot air from the front engines would decrease the efficiency of the rear engines. I think it could probably help balance out the weight, but it would be too complex and too innefficient. (this might just be me, but the throttles would be confusing. most planes have left->right as 1->whatever. with this design, there would be more difficult things.) overall, just too many small issues from solving one problem (if weight distribution is really that bad)