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The Boeing 777x has huge wings to generate lift even to the point of having to have a hinge and being able to bend them.

Would it not be advantageous to have 2 wings like a biplane or one behind another? And what disadvantages would there be?

A biplane wing of a given span and chord has twice the area of a monoplane the same size and so can fly more slowly, or for a given flight speed can lift more weight. Alternatively, a biplane wing of the same area as a monoplane has lower span and chord, reducing the structural forces and allowing it to be lighter.

This is from Wikipedia so possibly not reliable.

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  • $\begingroup$ I hope the wikipedia poster realises that biplanes are out of fashion for a reason. $\endgroup$ – Koyovis May 19 '17 at 8:08
  • $\begingroup$ I'm not sure if this is an exact dupe, but these two questions probably cover the essential information: here, here. $\endgroup$ – Pondlife May 19 '17 at 12:47
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    $\begingroup$ Why not combining stacked and successive, and use 3 instead of only 2, like this... $\endgroup$ – mins May 19 '17 at 17:55
  • $\begingroup$ Not really what I had in mind. I was thinking a normal biplane (wing over wing) or wing and a second wing further back on the plane. @mins $\endgroup$ – SRawes May 19 '17 at 17:57
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A major disadvantage to that design would be the aft wing had a disrupted airflow over it as it travels through the fwd wing's wake, so lift is greatly diminished.

As to doing a stacked biplane design typical of small sport biplanes in use over the years, this is probably frowned upon for two reasons:

  • High Drag - more structure in the relative wind always means more drag anemic this was one of the major advantages to the cantilevered wing developed in the 1930s.

  • Heavier - more wings means more structure both in terms of the second wing itself plus the mounting structure to mate the wing to the fuselage.

A single cantilevered monoplane is by far the most efficient solution save only a flying wing, but these are not commonly used in commercial travel which has been discussed in previous threads.

Now that's not to say a biplane may not be useful for commercial aircraft. Researchers at MIT proposed a supersonic biplane design which usea the configuration to dramatically reduce its sonic boom.

enter image description here

Whether this idea survives the CGI proposal and becomes actual hardware is another story; this business is full of concept aircraft and precious little flyable hardware.

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  • $\begingroup$ For the biplane wouldn't it be equal drag as having 2 wings could be half the length of would they need to be more than half? Same for weight wouldn't you need half the length which would mean force isn't leveraged as shorter wing so lighter structure? $\endgroup$ – SRawes May 19 '17 at 17:10
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    $\begingroup$ You also have to take into account, the mounting structure, increased flat plate area, form drag, etc from such a design. It's not quite that simple. $\endgroup$ – Carlo Felicione May 19 '17 at 17:21
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    $\begingroup$ @SRawes, the main reason why biplane would have more drag is the shorter span. The induced drag decreases with span and there is no way around that. $\endgroup$ – Jan Hudec May 20 '17 at 22:01
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The Boeing 777x has huge wings to generate lift

No, Boeing 777x has long wings to be efficient.

Generating lift using a finite wing comes at a cost of induced drag and this decreases with wing span. While parasite drag increases with frontal area. The induced drag also decreases with speed, while parasite drag increases. Optimal performance is achieved at the point where induced and parasite drag are equal.

As the designers improve the aerodynamics to reduce the parasite drag, the optimal point¹ shifts to longer wing span as that decreases the induced drag, and increases parasite drag slightly so they are equal again.

So the B777x has the wing span that gives it optimal efficiency. No other wing shape would be able to provide that efficiency.


¹ Assuming the cruise speed should remain the same, M0.85 somewhere around FL360–FL400. Optimal wing span decreases with increasing cruise speed.

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