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I need to know more about the "splayed wingtips" design made by Airbus in their latest conceptual design "birds of prey".

How would wingtip vortices work on this plane? How are they reduced by the splayed wingtips or the feathered tips?

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    $\begingroup$ Welcome to Stack Exchange! I hadn't heard of this design before this post, so you may want to provide a link. I found this one on twitter. $\endgroup$ – yshavit Aug 30 '19 at 17:44
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    $\begingroup$ You may also provide a picture/drawing of such design. $\endgroup$ – Manu H Aug 30 '19 at 19:44
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    $\begingroup$ Do we even know they work? Airbus (and every other aircraft company) tends to show many concepts just for the PR and publication count. The designers may not yet know how useful it would actually be, or may even be convinced it's a complete nonsense, but publish it anyway. $\endgroup$ – Jan Hudec Aug 31 '19 at 20:56
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Splayed wing tips are like .... Fowler flaps. Totally useless for cruising flight unless .... you want to try the "coffin corner" at perhaps 60-70,000 feet. The best wing for cruising remains the high aspect albatross style. However, they may wish to fold them back in cruising flight. The writer had imagined this for the Etrich Taube. Unfolding them on approach would greatly reduce the probability of tip stall and make lots of drag. Strictly a low speed gimmick. (By the way, slats also do this very well, actually helping high AOA pitch control).

However, the design does hold some interesting concepts: no vertical stabilizer AND a high wing mount. Sort of a fuselage mounted on a flying wing with an Hstab for better control. These concepts, along with engine mount location, bear further study. The rest is for the birds.

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  • $\begingroup$ "How would wingtip vortices work on this plane? How are they reduced by the splayed wingtips or the feathered tips?" If you meant they work like Fowler flaps, can you also address the wingtip vortex question, which is not present on the flaps? $\endgroup$ – AEhere supports Monica Aug 31 '19 at 11:54
  • $\begingroup$ @AEhere well, if you put the Fowler flaps at the ends of the wings (along with slats) it would be awesome (but unneccesary). At higher speeds (that the albatross flies) these structures are not needed (possibly because the vortex is shed more to the rear). The Etrich Taube wing maybe a good starting place for this project. Splayed wingtips are excellent for very slow flight only (as the circulation remains near the wingtip). But I did see the Airbus reaching down with a titanium talon, gracefully snatching a whale out of the water (LOL). $\endgroup$ – Robert DiGiovanni Aug 31 '19 at 13:28
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The basic idea is to try to extract as much of the available energy in the tip circulation as possible, and it's thought that soaring birds like eagles have this bit figured out. What it is in essence is an array of winglets.

You have a spiral flow field circulating around the tip, with the most intense part of it near the center - the tip vortice - but there is a circulation flow extending well away. The flow, being a tip circulation, is not coming from the "normal" direction that the rest of the wing sees. You can exploit this by sticking airfoils in spots were the local flow allows the lift vector to be beneficially exploited because is it canted forward somewhat (lift and/or thrust), as well as creating a local "downwash" that tends to oppose the circulation, effectively weakening it.

A single vertical winglet, the original concept, does this by creating a thrust component by the lift it generates in the circulation (horizontal toward the fuselage, but also with a forward component - thrust), as well as an "outwash" that opposes the circulation. Because it's vertical, it's not making any vertical lift and doesn't add to the wing bending and doesn't add to the span dimension. It's a sail, like on a boat, making thrust as well as redirecting the circulation back in the other direction, and in fact winglets were originally called "tip sails".

It's kind of a "well, if one works, why not add as many as I can" kind if idea. I can add more winglets if I want at different angles; some will be making some vertical lift depending on how horizontal they are - more or less acting like added span, some with a more vertical orientation will be making mostly "thrust". All are trying to extract energy from this circular flow, that is normally wasted, creating extra lift and/or thrust depending on the exact location and orientation.

Like winglets, these kinds of devices generally are only worthwhile when airplanes cruise at higher lift coefficients where the circulation is strong - that is, low indicated airspeed. To get that condition, you need to be above 30000 ft, more or less. This is why winglets are rare on turboprops and piston airplanes - they don't do that much good, except maybe enhance climb performance.

None of this stuff is new (aside from things related to jet propulsion, most things in aviation had been already tried by about 1940). I'd be surprised to see this sort of thing on a turboprop because it would only really work at low indicated speeds, and turboprops cruise at high indicated speeds. But maybe Airbus is on to something all new - who knows.

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