Below are some pictures of various fixed wing aircraft with winglets. It is widely accepted that these winglets will improve an airplane's performance by reducing drag and therefore improving fuel efficiency.

Here is an official video from Boeing showing the folding wing of B777x. According to Wikipedia, the wing's ratio increases from 9:1 to 10:1 which, in turn will improve efficiency up to 7%.

Here is also an official video of Boeing 737 Max that is being tested in Boeing's trans-sonic wind tunnel. In the video, it is claimed that the addition of the split winglet will reduce drag and improve fuel efficiency.

Airbus also said the claimed the same in regards to their A380. It has a claimed 4% efficiency improvement from the addition of a winglet.

But now, Boeing is developing a new airplane which with "foldable" wing, the 777x. The wing is "foldable" to accommodate taxiway width limitation of some airports. Clearly, it is not because of aerodynamics. But at the same time, Boeing claims that it will improve efficiency up to 7%.

Then my question is, if using a traditional winglet will increase the aerodynamic as well as fuel efficiency, why does Boeing not implement this for the B777x?

  • Picture 1: Boeing 777x with folded wings.
  • Picture 2: Boeing 777x with straight wing.
  • Picture 3: Boeing 737 Max with Y-shape winglet.
  • Picture 4: Boeing 747-400 with bent-up winglet.
  • Picture 5: Airbus A380 with Y-shape winglet (like Boeing 737 Max).

Boeing 777x' wings are folded Boeing 777x' wing's is expanded Boeing 737 Max' Y-shape winglet Boeing 747-400's bent-up winglet Airbus A380 Y-shape winglet

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    $\begingroup$ This question and answer from Peter Kämpf may potentially be your answer: aviation.stackexchange.com/questions/8556/… $\endgroup$ – aerobot Oct 23 '19 at 3:41
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    $\begingroup$ "Clearly, it is not because of aerodynamics" – what makes you think so? Of course the wider span improves aerodynamic efficiency. $\endgroup$ – Peter Kämpf Oct 23 '19 at 5:26
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    $\begingroup$ Can we just auto-close questions with "really" in the title? $\endgroup$ – Christian Oct 23 '19 at 6:38
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    $\begingroup$ The wing is never folded or unfolded in the air! This is only possible on the ground, before take-off or after landing. $\endgroup$ – Peter Kämpf Oct 23 '19 at 11:59
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    $\begingroup$ @AirCraftLover: It's about the taxiways and mostly the width of the terminal parking positions. On a 98 ft runway with lots of clearance left and right there is no reason not to unfold the wings. Span brings most benefits at low speed. $\endgroup$ – Peter Kämpf Oct 24 '19 at 5:43