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I've been reading some articles online about flying wings aircraft design. But articles about box wing design (of similar depth to flying wing articles) seem harder to find.

For illustration, here are two example aircraft, one of each design, i.e. flying wing and box wing respectively.

Northrop YB-35 Northrop YB-35 Sunny box wing aircraft Sunny box wing aircraft

From the two images above, I can easily figure out a similarity and a difference between the two aircraft: The swept wing and the vertical stabilizer

What are the major similarities and differences between the two aircraft configurations? For example, if the sunny box wing aircraft doesn't have a swept upper wing, will it still be stable in flight, and is wing sweep necessary for flying wings and box wing aircraft?

Guy Inchbald
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dammy999
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  • How else am I supposed to present this question? I think we need a better version of StackExchange :( – dammy999 Apr 27 '20 at 11:50
  • Someone sees it as three separate questions rather than one. I will take the liberty of editing it a little and if you don't like it then one of us can undo my edit. – Guy Inchbald Apr 27 '20 at 12:36
  • Sweeping the wing gives a little pitch stability, and the rear mounted props also helped the YB-35. Lack of length makes it harder to control the wing aerodynamicly in pitch, but hang gliders do this by shifting weight. Both designs could have straight wings and tails, but the sweep also gives a bit more yaw stability. One possibility for the boxwing would be wing mounted rudders and a rather bird-like horizontal tail. But stability is key for safety, even if it does not look quite as "modern". – Robert DiGiovanni Apr 27 '20 at 23:38

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A good place to start is the biplane flying wing with endplates that box it in, so that it is both a flying wing and a box wing. The Dunne D.5 of 1910 was the first certified stable airplane ever to fly. It was a swept design, with pronounced wash-out towards the tips, effectively putting the tail at the end of the wing. But its ancestor the D.1 glider had proved stable without any endplates. Although they did improve the stability of the D.5 and D.8, their main function was to reduce tip losses. They even lacked a rudder; the planes turned OK using the ailerons alone but could not land in a crosswind.

By comparison the box wing just flips the lower wing backwards, so it acts as the horizontal stabilizer and there is no need for washout. But it also loses directional stability so a tail fin or similar is necessary. For this reason many box designs, such as the Sunny, sweep it less than the front, so that the swept fore wing and endplates are enough to stabilize it.

The monoplane swept flying wing typically has no endplates. Dunne achieved the same effect by sharply drooping the wing tips. But in most designs, the washout reduces lift at the tips enough to limit tip losses.

Sweep is not strictly necessary. If you use an airfoil with reflex camber, tilted up at the trailing edge such that it has a stationary centre of pressure as AoA changes, it will be inherently stable and no other tricks are needed. But it will be very sensitive in pitch and the controls will not be well balanced. Also, it will be unstable or minimally stable in yaw unless you play tricks similar to Dunne's, such as adding endplates and toeing them in to create differential drag.

Guy Inchbald
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  • With this answer of yours, I think I'll go with the swept wing design. But sir, why is it so difficult to ask questions appropriately on stack exchange? How else am I supposed to present this question to be worthy of been asked here? – dammy999 Apr 27 '20 at 11:47