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I ask this because the Boeing 747-236 had the engines close together while the 747's have them spread apart. Does it have anything to do with the amount of thrust of the aircraft?

Look at how spread apart the engines are:

747 in hangar

Now look at how close together those engines are on the Boeing 747-236:

747-268B used as movie prop

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marked as duplicate by Peter Kämpf, mins, Ralph J, Federico, fooot Aug 22 '15 at 23:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ As the original post on Airliners.net clearly states, the second aircraft is not a real one, but a prop for a movie.No real life Boeing has this configuration. $\endgroup$ – mins Aug 23 '15 at 1:39
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Short answer

It's the vastly different flow conditions from static to cruise which demand a separate placing of high-bypass-ratio jet engines. They would produce less thrust and more drag when paired.

Why were there paired engines at all?

The early jets had their engines mounted directly beneath or in the wing, and comparisons between separately mounted and twinned engines showed a slight advantage for the latter due to lower wetted surface area and less impact on the wing.

Arado built two four-engined prototypes of their Ar-234 jet, one with separate engines (V6, see directly below) and one with paired engines (V8, further down).

Arado 234 V6

Arado 234 V8

This twinned arrangement works well with turbojets and low-bypass-ratio turbofans. The Boeing B-47 and B-52 bombers, and among airliners the Illyushin 62 and Vickers VC-10 all had paired engines.

However, with the increasing airflow of high-bypass-ratio engines the interference between both will turn pairing into a disadvantage. In cruise, only the central stream tube flowing towards the engine will be ingested, and the rest will spill over the intake lip. Placing a second engine directly next to the first will block the flow of spilled air on that side and will increase spill flow on the opposite side. This will most likely cause massive separation there if the intake is not heavily modified, leading to noticeable drag increase. Also, the now asymmetric flow in the intake would reduce the efficiency of the fan - it needs very homogenous flow over the full cross section in today's engines.

Conversely, at low speed the engine sucks in air from all around and will face competition from a second engine, such that both will not be able to ingest as much air as when mounted separately. The consequence of pairing would be reduced thrust during take-off.

The initial disadvantage of separate engines, their collective impact on wing aerodynamics, is now greatly reduced by mounting them on pylons, so they are ahead and below the wing.

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    $\begingroup$ What about B-52s then? $\endgroup$ – Peter Mortensen Aug 22 '15 at 23:43
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    $\begingroup$ @PeterMortensen: The B-52 had turbojets. The J-57s were more similar to the BMWs of the Arado and less sensitive to interference compared to a modern turbofan. The pairing of eight of them (initially Westinghouse J-40s) was decided very early, coming from a long experience with four-engined aircraft. I do not know whether eight individual engines were ever tested - the pairing worked in the B-47 already and was then used in the B-52, too. $\endgroup$ – Peter Kämpf Aug 23 '15 at 5:23
  • $\begingroup$ aviation.stackexchange.com/questions/4966/… - difference between turbofan, turbojet (and turboprop) $\endgroup$ – Peter M. - stands for Monica Sep 13 '17 at 15:48
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That picture of the 747-200 you have with the engines in a pod together like a B-52 is a movie prop. http://www.airliners.net/photo/Boeing-747-236B/1070481/

All real 747s from the -100 through the -8 have their engines positioned like the first picture.

EDIT - Federico's response reminded me of another aspect of the engine position, and to answer your original question of why they're spread apart. Spacing between the 747s engines is important due to the need to maintain sufficiently clean airflow into the inlet. If the engines are too close together, they're essentially competing for the same amount of air, and at high power settings, the interaction between the two fans will disrupt their operation, potentially causing engine stall/surge. Additionally, the 747 uses thrust reversers, and the spacing between the engines helps ensure that the reverse flow from each engine doesn't impact an adjacent engine, for the same inlet distortion reason.

I assume, though I don't know the details of this, that the B-52 can have them that close together because they've shown that the inlet geometry, airflow required by each engine, whatever other magic of fluid dynamics that I know nothing about, prevents the engines from distorting each other's airflow enough to cause an issue. The 747 engines require a lot more air, and generate a lot more thrust, and are potentially more sensitive to inlet disruption than the engines on the B-52. This last paragraph is mostly a guess on my part as I'm not an aerodynamicist, but it's inferred from issues we've had to deal with on the 747 and other related airplanes.

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    $\begingroup$ Indeed; this prop is the fictional "Skyfleet S570" aircraft in the film Quantum of Solace. jamesbond.wikia.com/wiki/Skyfleet_S570 $\endgroup$ – Nate Eldredge Aug 22 '15 at 23:11
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    $\begingroup$ James Bond movie Casino Royale? $\endgroup$ – Peter Mortensen Aug 22 '15 at 23:46
  • $\begingroup$ Here is a wikipedia article on the airplane in question. The link in this answer appears to be dead unfortunately. $\endgroup$ – geisterfurz007 May 3 at 7:43
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In addition to what Stickhog's answer says, the fact that all 747s (and A380s) have their engines spread apart has to do with the "single point of failure" concept.

Consider for a moment the picture of the movie prop you posted, if one of the pylons suffer any kind of mechanical failure, you lose 2 engines. The same with one of the engines suffering explosive failure or uncontained fire.

Having the engines separated from each other reduces the risk that a failure that renders one engine inoperative will affect also another engine.

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    $\begingroup$ Then why did the B-52 use two-engine pods? Was it just that 8 engines was plenty of redundancy? $\endgroup$ – cpast Aug 22 '15 at 15:58
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    $\begingroup$ @cpast if you lose 2 engines on the same side on a 747 you will have issues with yaw control. if you lose 2 on the B52, it's like losing one on the 747. $\endgroup$ – Federico Aug 22 '15 at 16:08
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As well as the issues relating to the aerodynamics of the engine intakes and single-point-failures, the distance of the engines from the aircraft center-line affects the vibration frequencies of the wing and hence the flutter stability margin of the aircraft. That turned out to be a significant problem on the first version of the B747, to find a compromise position which could use three different engine types from the three engine manufacturers on a common wing design.

The first commercial jet airliner design addressed those problems by "twinned engines" buried in the wing root. For pictures see https://en.wikipedia.org/wiki/De_Havilland_Comet. That design persisted in the derivative Nimrod military surveillance aircraft https://en.wikipedia.org/wiki/Hawker_Siddeley_Nimrod until its recent retirement. The problems of trying to shoehorn a modern engine into that airframe configuration, and meeting modern safety standards (as opposed to 1950s era standards) was one of the problems which resulted in the proposed upgrade of the Nimrod MR2 to the MR4 to be abandoned (The MR2s were retired from service in 2011).

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