There are some plane designs where the engines are placed quite far away from the fuselage, one example is the SR-71. enter image description here

This amplifies the effects of any asymmetry of the engine thrusts, the worst case being zero thrust on one side. What are the reasons that justify or force those designs?

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    $\begingroup$ What makes you think the engines on the SR-71 are far from the fuselage? They are similar to the 737 which is about the same size. $\endgroup$ – fooot Jun 19 '15 at 16:05
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    $\begingroup$ If you want to drop paratroopers from the forward doors, it helps when the propellers are far enough away to not shred them once they leave the plane. $\endgroup$ – Peter Kämpf Jun 19 '15 at 16:35
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    $\begingroup$ @PeterKämpf You just gave me a mental image of paratroopers being dropped from an SR-71. I strongly suggest you work together with Michael Bay. $\endgroup$ – Sanchises Jun 19 '15 at 17:02
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    $\begingroup$ One other reason would be cabin sound level - the further the engines, the less acoustic dampening necessary $\endgroup$ – user2813274 Jun 19 '15 at 19:14
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    $\begingroup$ The asymmetric yaw from a dead engine can be extremely large, take for instance a 777...yet that plane handles fine OEI simply because it has plenty of rudder authority to let the pilot deal with that yaw. $\endgroup$ – UnrecognizedFallingObject Jun 20 '15 at 0:59

From Wikipedia:

Placing engines on the wing provides beneficial wing bending relief in flight. The further the engines are away from the fuselage the greater the wing bending relief so engines buried in the wing root provide little relief. Almost all modern large jet airplanes use engines in pods located a significant distance from the wing root for substantial wing bending relief.

One could imagine that, in flight, the wings are holding up the aircraft, but this causes a massive bending moment at the wing roots (just visualize an airplane suspended by ropes from the wings). As such, is favourable to have the airplane body be as light as possible, and put as much mass (e.g., fuel) as possible on the wings, where the lift forces coincide (or at least, are closer to) the weight of the engine, reducing bending moment on the wing roots. This allows for smaller (and thus more aerodynamically efficient) wing structures.

As noted in the comments, placing the engines further away of course increases downwards bending moment when the plane has landed. However, this will be 1g by definition (at touchdown, the wings still provide lift (generally)), whereas during the flight, an airplane is designed to take up to 2.5g (and, I suppose not completely coincidentally, -1g).

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    $\begingroup$ On the other hand you have more stress on the wings while standing or touching down. There must be an optimum distance considering this, but I guess that is not too far from the fuselage as in many designs the engines are attached directly to the fuselage. $\endgroup$ – DarioP Jun 19 '15 at 15:29
  • $\begingroup$ On-body engines are generally due to other design considerations, such as ground clearance (which is why small jets have tail-mounted engines so that you can have a door-sized staircase) But as far as your 'optimum distance' concerned: that distance would be almost exactly the distance that is used on modern jets - it is inconceivable that airliner manufacturers have not optimized this. Note that engine placement is also dependent on aerodynamic effects (e.g., wing flutter), not just weight. $\endgroup$ – Sanchises Jun 19 '15 at 15:36
  • $\begingroup$ I totally agree with your considerations on (boring) airliners. I was just wondering about less common designs where the engines looks quite far away :) $\endgroup$ – DarioP Jun 19 '15 at 15:45
  • $\begingroup$ @DarioP, I think for the SR-71 case Victor Juliet's answer is partly correct - aerodynamics around the turbine inlet. Not a safe design if one stopped though (some Blackbirds were lost this way). $\endgroup$ – Andy Jun 19 '15 at 15:51
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    $\begingroup$ @DarioP I tried to keep the answer general, but in the case of the SR71, I can make two guesses. First, I think the engines need to be ahead of the shock wave made by the nose, while still keeping the aircraft sleek (i.e. elongated), see VJ's answer. Secondly, I think the SR71 is capable of pulling more than 2.5g, and look at the massive engines compared to the flimsy wings. $\endgroup$ – Sanchises Jun 19 '15 at 15:51

There are a couple of reasons which drive the need to place the engines at optimum distance from the fuselage. In case of Blackbird SR-71, the engines have to be placed at a distance such that they can ingest optimum amount of air for proper combustion. Placing the engines too close to the fuselage would hinder the air flow, and hence the engines might not work properly. The CONS of having such a placement, as mentioned in the question can be done away by having proper damping, and design techniques to reduce the effect of any kind of asymmetry.

  • $\begingroup$ The SR-71 was a pretty extreme example - I've read it was very nasty if one engine stopped. Maybe damping, etc. would be OK for less extreme aircraft though. $\endgroup$ – Andy Jun 19 '15 at 13:51

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