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I recently visited the Boeing factory outside Seattle, WA and noted in the distance a new 777x being built with, what looked like very large winglets.

However, on closer inspection they're not - the ends of the wings are designed to fold up.

enter image description here Source: aviation today

I didn't get a chance to ask our tour guide, so I was wondering if anyone here can explain why this aircraft has been designed with folding wingtips. I'm presuming this is only for ground operations, and my guess is something to do with size - but are they really so big that they need to fold up? I would have imagined that other large wide body jetliners would have similar dimensions.

Please note that I'm not asking about whether this should have winglets, or if winglets provide any benefits, I'm asking specifically why the ends of these wings fold up rather than just fixed as per previous 777 models.

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    $\begingroup$ I think (which is why this isn't an answer) is that they need to fit into certain stands, otherwise they could use larger (more expensive) stands that have less density. Here is a video about the 777x folding wingtips. $\endgroup$ – Ron Beyer Dec 17 '18 at 16:38
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    $\begingroup$ Closely related and might even answer this? $\endgroup$ – Pondlife Dec 17 '18 at 20:25
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    $\begingroup$ Only way it would fit through the factory doors ;) $\endgroup$ – Harper Dec 17 '18 at 21:51
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The Boeing 777X website states that this is to enable a more efficient wing (read: wider span) while maintaining the airport gate and taxi footprint of the classic 777 (which ensures airlines can use the 777X on roughly the same airports and intermix operations with the classic fleet).

Website Screenshot

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Many modern aircraft have been designed with winglets, and older ones have been retrofitted with them. They allow a wing to produce more lift with less drag. However, the benefit is even greater if the wingspan is simply extended.

The downside of increasing wingspan is that wingspan affects many aspects of airport infrastructure, especially taxiways and gates. FAA Advisory Circular 150/5300-13 covers airport design, and includes an "Airplane Design Group" categorization, from I to VI. Group V has an upper cutoff of 214 feet (65 m) wingspan.

The 777-200LR and 777-300ER have the largest wingspan of the existing 777 family, at 212 ft 7 in. The 777X will have a wingspan of 212 ft 9 in when the wingtips are folded. This ensures that the 777X will be able to fit on every taxiway and into every gate that the current 777 can, while gaining the benefits of an increased wingspan in flight with the wingtips folded down.

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    $\begingroup$ Exactly. The folding wingtips allows the 777X to use stands and taxiways designed for ADG V aircraft, rather than needing ADG VI stands and taxiways like the A380 and 747-8. Even airports designed to accommodate A380s and 747-8s still typically have far more ADG V stands than ADG VI stands. ADG VI is basically just the 747-8 and A380, while ADG V is all 777s, 787s, A330s, A350s, and 747s prior to the -8. Bumping the 777X into ADG VI would have been a massive cost increase to operators, making the aircraft much less competitive. $\endgroup$ – reirab Dec 17 '18 at 22:08
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    $\begingroup$ But will the folded wingtips unfold in the air during flight or do they stay folded? $\endgroup$ – undefined Dec 18 '18 at 10:52
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    $\begingroup$ They would unfold on the ground when it's safe during taxi operations but before entering the runway. This ensures that the crew can confirm they are locked in place without running the risk of interfering in airport operations by taking up an active runway. $\endgroup$ – NetworkLlama Dec 18 '18 at 14:38
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Folding them up reduces wing tip vortex. So, take of and landing planes can be stacked closer together without encountering wake turbulence. Then fold them out in the cruise to benefit from the bigger wing area.

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    $\begingroup$ Folding them up reduces wing tip vortex this is the opposite of true. (nevermind that the wingtips don't fold or unfold in flight) $\endgroup$ – 0xdd Dec 18 '18 at 15:09

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