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The Boeing 777 has an optional folding wing mechanism to fit into tighter spaces, but it has never been ordered by airlines, according to Wikipedia. Why is this option so unpopular?

Now with the Boeing 777X, Boeing is again offering a smaller, less complex wing folding option. Has this been ordered, and are other airliner models planned to incorporate folding wingtips?

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    $\begingroup$ Ethan, please consider passing by in chat before asking questions, your posts keep being flagged by the system as short and incomplete. You seem to have the right amount of curiosity, but if you don't learn how to ask questions it will be difficult for others to give you answers. $\endgroup$ – Federico Aug 25 '15 at 7:54
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    $\begingroup$ @Federico I usually dont ask question Iin the chat thats just me. My questions may have negative results but I still get the answers I need. I personally think the chatroom is more for asking for definitions. And didn't you ask a question in chat didnt get the answer and had to ask the question on the main site. $\endgroup$ – Ethan Aug 25 '15 at 11:48
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    $\begingroup$ Ethan, I am suggesting you ask help in chat on how to properly ask the questions on the site, I am not suggesting to ask the questions only in chat. $\endgroup$ – Federico Aug 25 '15 at 11:51
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    $\begingroup$ It's a nice question thought from Ethan, but I feel like my upvote rep should go to Peter for putting in the effort here. $\endgroup$ – digitgopher Aug 31 '15 at 4:48
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The folding wingtips are found mostly in naval aviation. The reason is the space constraint inside the hangar of the aircraft carrier.

USS Ronald Reagan

"US Navy 070128-N-9712C-003 Aircraft fill the hanger bays on board USS Ronald Reagan (CVN 76) after recently embarking Carrier Air Wing One Four (CVW- 14)" by U.S. Navy Photo by Mass Communication Specialist 2nd Class John P. Curtis.

As can be seen from the photo, folding the wings offers the only practical way of fitting fighter aircraft inside the cramped hangar space. For example, the F/A-18C/D had a wing span of 11.43m; this was reduced to 8.38m with the wings folded, a 26% reduction.

However, the price to pay is an increase in complexity, weight, price and increased maintenance. The figure shows the wing folding mechanism of a F 18.

F 18 Wing fold Source:polepositionimagery.com.au

The major drawbacks of folding wings are,

  1. The increased weight and complexity due to the folding mechanism. The folding mechanism takes time to operate and there are safety concerns if they get stuck in a position.
  2. The folding of wings limits the fuel capacity and as a result, the range of the aircraft, as fuel is usually not carried on the folding part of the wings.

F-18 Overview

Source: zarco-macross.wikidot.com

As can be seen, the F/A doesn't carry fuel in the folding part of the wing. In fact, after the airlines didn't take up its folding concept in 777, Boeing added fuel tanks to that area and increased range of the 777-200/300.

There has previously been no reason for airliners to fold their wings, as airports offered enough space for operations and ground movement. However, as the size of aircraft and their aspect ratios grow (to reduce drag), designers are finding it more and more difficult to keep the wingspan below the 80m 'box' which airlines believe is the maximum they can cope up economically.

The first major program to be affected by this limitation was the A380, which limited the wingspan of the aircraft to less than 80m and reduced its aspect ratio.

Comparison of aircraft

"Giant planes comparison" by Clem Tillier (clem AT tillier.net) - Original Work. Licensed under CC BY-SA 2.5 via Commons.

Basically, Boeing is facing the same problem. The proposed 777x is the largest-wingspan aircraft Boeing has ever produced. However, in the case of the A380, Airbus managed to reduce induced drag through the use of wingtip fences, among other design decisions. The 777x is expected to have a raked wingtip, which is not as space effective as the wingtip fence.

777x Wingspan

Source: Flightglobal.com

As a result, Boeing is considering folding wingtips. However, there are important differences between the original plan in 777 and the proposed plan in 777x.

In the original plan, Boeing proposed to fold 21' of the wingtip. However,the new plans call for folding only 10'.

777 Wing Folding

Original Wing folding plan in Boeing 777. Source:www.seattletimes.com

777x folding tips.

Image:Boeing

As can be seen from the figure, the folding plan is much more modest. This is not expected to have the drawbacks associated with the the original plan, as,

  • The end of the wing will not house controls.
  • As the size of the wing to be folded is smaller, the folding mechanism will not be as heavy and complicated.
  • That part probably will not carry fuel.
  • Advances in actuation systems since the original 777 will reduce the weight penalty.

To put things into perspective, the 747-400's winglets were 6' feet tall.

Airlines have already placed orders for 777x and it is their operational record that will determine if other aviation manufacturers (Airbus, that is.) will follow suit.

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why cant this apply to all airliners?

It can

It can apply to any new or redesigned airliner. Its probably not something you'd need, want or be economically feasible to retrofit to existing in-service airliners.

Disadvantages

Whether the manufacturer or operators want the associated disadvantages is another matter.

  • additional expense,
  • complexity,
  • weight and
  • safety/maintenance burden

Reliability is critical. A 777x may be unable to vacate a runway if a wingtip fails to fold, it has to do this in a short time (of the order of 20 seconds).

Should the wingtips not fold up and lock into place correctly after the aircraft lands, it could effectively paralyze an airport by delaying both departures and arrivals

- Liebherr-Aerospace senior executive Heiko Lütjens

It's just the tips

Note that only the tips fold, otherwise there would be many more problems in catering for hydraulic, electrical, structural and fuel systems.

Competition

The 777x has a wingspan of 71m. Only when folded are it's wings about the same width, 64-65m as it's competitor, the A350-1000.

Airport gate width

Wingspan matters when designing an aircraft to operate into current sized terminal gates at the specific airports used by the makers' customers.

Boing want their 777x to be able to fit where the old 777 and it's newer competitors can fit.

aircraft    wingspan   passengers
747-400     60m        345†
777-300     61m        299†
A320-1000   65m        369
777x        71m        350‡
A380-800    80m        469†

† Most of the passenger numbers are from BA for 4-class operation. They don't represent anywhere near max capacity but are probably closer to typical?

‡ Typical 3-class operation.

Airbus had to put serious effort in getting airports to accommodate the A380. I guess Boing really don't want the 777x to be competing with the higher-capacity superheavies for space at the bigger gates.

if the tech already exists

The notion of folding wings has been around a long time. Since before 1913

enter image description here enter image description here enter image description here

It has mostly been used on small aircraft that operate from facilities where space is severely limited.


Boeing's ideas in this area have been known since at least 2006

enter image description here

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    $\begingroup$ For completeness I would add a small description of the drawbacks (weight, complexity, fuel storage/pipes) to explain why it is not that common. $\endgroup$ – Federico Aug 25 '15 at 8:36
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    $\begingroup$ @Federico: Thanks, I've made that more explicit. $\endgroup$ – RedGrittyBrick Aug 25 '15 at 9:04
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    $\begingroup$ All of Otto Lilienthal's gliders had folding wings (backwards, not up) so they could fit through the standard railway wagon door. $\endgroup$ – Peter Kämpf Aug 25 '15 at 13:44
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    $\begingroup$ Note that the part about 'effectively paralyzing an airport' would only apply if the taxiways were not approved for ADG VI aircraft. Any airport that can handle a 748 or A380 would not be affected. They just wouldn't be able to park the plane in an ADG V stand. For airports that have not been designed to support ADG VI aircraft, they'd most likely bring out ops vehicles to help ensure wingtip clearance during the taxi. They might also need to not use an adjacent runway or taxiway during the taxi. It's unlikely that it would actually be forced to just sit on the runway for long, though. $\endgroup$ – reirab Feb 7 '18 at 8:33
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The B777x share most of the characteristic of B777. However, while B777 is Code E(ICAO)/Group V(FAA), due to b777x wingspan, it falls into Code F(ICAO)/Group VI(FAA) catalog.

Currently only A380 and b748 is in Code F/Group VI catalog, which the fleet size is limited (317+123), only a few airliners has such planes and not many airport can parking place and other infrastructures for those plan.

When B777x provides wing folding option, it can feet the code E/Group V parking place. The B777x can be served for most of the airport and reduce the cost.


Wing fold is not new idea.It appears after WWI, when aircraft carrier has been developed, because of the limited hangar size, most of the naval aircraft are designed as wing folded.

E-2 Hawkeye with wings folded: enter image description here

F-18C with wings folded: enter image description here

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