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Are there any advantages or disadvantages to having the engines closer or farther to the center of the plane?

How is the ideal engine spacing determined length or span wise for performance?

Related: How is the center of mass controlled in planes with multiple tail engines?

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    $\begingroup$ The main reason they are usually kept close to the fuselage is probably their weight that the wings have to carry. $\endgroup$ – rauberdaniel Jul 6 '19 at 18:39
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    $\begingroup$ Hmmm. Weight? As far as I learned here, placing engines on the wing reduces the load on the wing, which is created through the lift. Even on two engine planes the engines are separated from the fuselage by a quite large distance. $\endgroup$ – Peter Jul 6 '19 at 18:45
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    $\begingroup$ Further out -> reduces wing bending in flight, but increases wing bending and also obstacle collision risk (signage etc.) on ground. Also makes asymmetric thrust harder to control. May reduce fuselage influence on engine intake flow. $\endgroup$ – Cpt Reynolds Jul 6 '19 at 19:18
  • $\begingroup$ For military aircraft like the A-10, having the engines separated from the body helps in situations where the engines “Go explodey on you”, in the words of an A-10 pilot. $\endgroup$ – CourageousPotato Jul 6 '19 at 21:47
  • $\begingroup$ Lengthwise or span wise? $\endgroup$ – Koyovis Jul 7 '19 at 6:39
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the further apart the engines are, the worse becomes the yaw in an engine-out situation. but spacing them (and their fuel tanks) further apart along the span of a wing allows more even distribution of load stresses.

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  • $\begingroup$ edited just now. $\endgroup$ – niels nielsen Jul 8 '19 at 5:46
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One of the mysteries in general aviation is that twin engine airplanes have more accidents than singles. One of the factors is asymmetrical thrust after an engine failure. As such, centerline thrust is considered to be a virtue because pilots have an easier time with it. It also allows all the engine's power to be used for thrust, instead of wasting some on rudder.

So the closer to centerline the engine is, the easier it is on the pilot.

However, the nature of propellers makes it difficult to put two propellers on centerline. You either need a "pod" fuselage with a twin tail, like a Skymaster... or stacked contra-rotating props with one engine per prop. Of course, the trend toward electric props is going to improve that, allowing prop placement now not possible.

Of course in complex aircraft and jetliners, the autopilots relieve much of the workload and opportunity for error. Those will usually be there to assist, because there's little chance of those automated systems also failing at the same time. That reduces the safety impact of widely separated engines.

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  • $\begingroup$ in case of an engine-out in a light twin, you can always rely on the second engine to deliver you precisely to the crash site! $\endgroup$ – niels nielsen Jul 30 '19 at 4:17
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Ideally you would like the engines as close to the aircraft centerline as possible to eliminate asymmetrical thrust loading caused by failure of an engine. In multi-engine aircraft, this is not always possible to do due to fuselage configuration, empennage, fuel tanks, wheel wells, etc in the wings. When multiple engines are all turning and burning, there is little difference between a centerline thrust twin and a conventional twin. But the difference becomes problematic in the event of an engine failure.

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