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Would installing 6 engines on an Airbus A380 and or Boeing 747-800 achieve a shorter take off distance? Would that result in excessive acceleration and harmful or uncomfortable G-forces? Would the extra thrust and leverage exert excessive force on the wings? How about braking,would the braking be shorter if all engines were fitted with thrust reversers

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    $\begingroup$ You know that only the two inner engines on the A380 are fitted with reversers? The issues you have adding 2 more engines are structure to hold them (weight), burning more fuel, higher drag, etc. Yes, this will reduce take-off distance (why not just use disposable JATO bottles?), but the expense in fuel consumption and reduced range isn't worth it. These aircraft can't take off from smaller fields anyway (too wide), so why worry about distance? $\endgroup$ – Ron Beyer Feb 1 '18 at 16:11
  • $\begingroup$ Yeah why not rockets? $\endgroup$ – user3528438 Feb 1 '18 at 17:05
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    $\begingroup$ I'd like to suggest you ask such questions differently, e.g. for this one you could just ask "does adding engines allow shorter takeoffs" rather than "if the A380 had 6 engines..." Why? Because if you want to ask for a basic principle, you'll get good answers with extensive details, if you ask for hypothetical changes that will never happen, the question will be frowned upon, because it's missing the counterparts: Added weight and drag, redesign of the wing and tanks, etc. As I assume you're not redesigning the A380, the direct basic question would be better. $\endgroup$ – mins Feb 1 '18 at 20:34
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    $\begingroup$ A last word: Designing a plane, and anything else actually, is not a matter of having the best solution for each requirement, this is not possible (.g. you may not have the most fuel efficient plane if you also want the most silent engine). There is one major requirement: To be economically viable (you sell the plane to airlines). It's a matter of finding the best compromise to fulfill all requirements. Improving a plane is possible, but an improvement means a better compromise, not a better takeoff distance alone. $\endgroup$ – mins Feb 1 '18 at 20:45
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    $\begingroup$ The A380-800 as it is has a rather short takeoff capability already; as its wing was designed with the (never built) larger siblings-900 and -1000 in mind, it is very big for the current version. This allows takeoff and landing speeds much (!) slower than the 747. $\endgroup$ – Cpt Reynolds Feb 1 '18 at 21:48
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Would installing 6 engines on an Airbus A380 and or Boeing 747-800 achieve a shorter take off distance?

Yes in theory, more thrust is faster acceleration.

No, because engine/wing interaction is not simple:

  • For the 747: the flaps will need an additional thrust gate, which will reduce the flaps capability.
  • For the A380: since it doesn't have a thrust gate, the additional thrust will create extra drag and pitch down moment, making it harder to lift the nose.
  • Bolting on engines is unrealistic, as it will require a new wing to take into account the extra forces on the wing.
  • The extra weight of the engines and their associated systems will increase the weight, which in turn will reduce the acceleration due to the increased normal force on the tires.
  • The extra engines will intake some of the air meant for the wings, so alongside the reduced capability of the flaps, there goes the full potential of the wing.
  • After a successful higher-speed takeoff, the extra engines will draw more fuel than needed (jet engines run less efficiently at lower-power). Which will result in a diminished range capability.

Would that result in excessive acceleration and harmful or uncomfortable G-forces?

No, you can use the engines at low thrust if needed.

Would the extra thrust and leverage exert excessive force on the wings?

Idem.

How about braking, would the braking be shorter if all engines were fitted with thrust reversers.

Yes, in theory, this could be shorter.

No, with the decreased lift, the airplane will land at a faster speed, so no, not necessarily.

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