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How does the wraparound wing in this AOPA article reduce drag? One would think all that extra leading edge and surface area would make it worse. One more link also.

enter image description here

The Parsifal Project has produced a proposed design dubbed PrandtlPlane, an homage to Ludwig Prandtl who developed an idea for an airplane wing that boosted efficiency (and reduced drag) by wrapping around on itself to form a box around the fuselage.

This paper on Conceptual Design of High Subsonic Prandtl Planes

Title: Conceptual Design of High Subsonic Prandtl Planes

Author: Zohlandt, C.N.

Contributor: La Rocca, G. (mentor)

is also proving to be most interesting reading

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It does not reduce drag, but creates more of it.

This concept is called a "box wing" and is one variety of closed wings. Another is called a "joined wing" and was popularized by Julian Wolkovitch. However, only people who don't understand how a wing produces lift can be enticed by this concept - it has too many drawbacks for the modest reduction in induced drag it offers. The general idea is to sweep the horizontal surface of a T-tail forward and stretch it out until it joins the backward-swept wing and relieves it in bending, similar to a strut.

Similar to winglets, the vertical extension of the wing structure will involve more air into the production of lift, which reduces induced drag, but this extended structure must be bought with a hefty mass increase which will reverse the gain in induced drag, because much more lift needs to be produced in order to lift the additional structure.

Note especially that the rear wing (which is supposed to act as a tail surface) will be loaded in compression in order to support the wing. Making this rear wing narrow will make it susceptible to buckling, which can only be avoided by a stiff and heavy structure, and will increase its friction drag due to its much lower Reynolds number. When a winglet-like element is added between wing and horizontal tail, this winglet is now loaded in bending and will also need to be made much stronger and heavier than a conventional winglet.

Calling this a "Prandtl plane" is to completely misunderstand the 1924 publication of Ludwig Prandtl, who very well understood how wings create lift, about the benefit of box wings.

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    $\begingroup$ I was surprised that you hadn't answered this question but, before I could say, "Gosh, Peter Kämpf hasn't posted his 'box wings don't work' answer", "1 new answer has been posted". :-) $\endgroup$ – David Richerby May 30 '18 at 20:48
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It behaves like a wing with a higher effective aspect ratio, thanks to the wingtips going full circle, while fitting into smaller overall dimensional restrictions. This wing can also be thin without suffering as much structural weight penalty as a normal wing would thanks to the bracing effect of its box shape (but there needs to be more angle between the wings than in the picture to get significant structural improvement).

The price for that is that is still adding weight and becoming a tandem wing with possible interference between upper and lower surfaces. Is it better overall? It appears that the general case is no, but it most likely has its niches where it's better.

There's been a related question: Do box-wings suffer from induced drag the same way as normal wings? - that has a lot more detailed information in the answers. Be sure to check it.

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Bottom line, it's a tandem wing with a little fin joining the tips. Basically a variation of this contraption, which does fly, more or less.

https://en.wikipedia.org/wiki/File:Henri_Mignet_HM_14.jpg

Truth is, there isn't a hell of a lot under the sun in aviation that wasn't already thought up by the 30s.

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