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I've read here on Aviation SE that the aerodynamics of winglets on an aircraft are easily described as "extending the airfoil". Thus, as I understand it, a winglet is like stretching the airfoil a few feet and "bending" it upward.

Now, this answer mentions the various types of winglets used in contemporary aircraft, but I'm not sure the B737-800s I've seen (such as the one in the below image) with double winglets perfectly matches any of those categories1.

enter image description here source

Thus, my question is: How can one best describe the aerodynamics (and benefits) of the second, downward-pointing winglet in the context of airfoil extension?

1 I claim that the 737-MAX's "split scimitar" as mentioned in fooot's answer is different than the aftermarket double winglet seen in the United (Continental) B737-824 in the image.

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Winglets don't have some mysterious magical wing tip extending property. They don't add to the total lift of the wing (except to the extent they are canted).

The best way to conceptualize them is they extract energy from the tip flow and convert it to THRUST which can be deducted from the induced drag. They are little wings making lift (wing..let, see?), except the lift isn't up (unless they are canted, where there is a small vertical lift component). They work the way boat sails propel a boat forward and were originally called "tip sails".

The flow striking the winget on top is curling inboard toward the fuselage so that there is an outboard angle of attack at the winglet's leading edge that creates a lift vector (being 90 degrees to the local AOA) inboard and slightly forward.

The split scimitar doohicky just flips the effect over to take advantage of unexploited flow below and behind. The flow striking the winglet on the bottom is angling outboard, causing a lift vector to be applied outboard and slightly forward (again at 90 deg to the local AOA).

If you examine most winglets you will see that they aren't just fins that "block" the flow. They have an airfoil cross section because, as I said, they are wings making lift... just not up.

The benefit of the lift being created at the winglet is only sufficient to exceed the thrust and weight penalty of the winglet itself at lower indicated airspeeds where the vortices are stronger. For this reason you (almost) only see them on high altitude aircraft that cruise at low indicated airspeeds.

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    $\begingroup$ … but they do add a little lift, because they raise the pressure difference at the (horizontal) tip from zero to some small value. And the flow hitting a properly designed winglet is pushed outward (like the downwash behind a wing). $\endgroup$ – Peter Kämpf Jun 11 '18 at 4:03
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    $\begingroup$ Good point. Actually I was thinking about downwash thing and was going to add something about the "outwash" generated by the winglet itself (of the upper one; "inwash" of the lower one) counteracting the circulation and that could be thought of equivalent of adding span, but I think the energy recovery of the lift generated by the winglet itself is the biggest factor and is the best thing to emphasize when describing them. One of the few articles I ever saw that properly described winglets was a Peter Garrison article in FLYING mag in the 80s that used the boat sail analogy to great effect. $\endgroup$ – John K Jun 11 '18 at 12:46
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    $\begingroup$ The boat sail analogy, the reduced lift vector tilt, the reduced downwash and the "outwash" explanations are only different ways to describe the same phenomenon. $\endgroup$ – Peter Kämpf Jun 11 '18 at 17:10
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A reasonably complex question. What you see is call a Split Scimitar Winglet. Lots of references on google. Simplest explanation "their intended effect is always to reduce an aircraft's drag by partial recovery of the tip vortex energy" (look up Wingtip device on google).

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$ – Carlo Felicione Jun 10 '18 at 7:12
  • $\begingroup$ I respect your opinion, though a more correct answer could be just "google it". Explaining how wingtip devices work on this forum seems a waste. The question is hard to answer as each time I read it I think of some other tack. $\endgroup$ – Robert Clark Jun 10 '18 at 18:38
  • $\begingroup$ Also these scimitar wingtips have been around on B737-800's since 2013. Lots of data out there. $\endgroup$ – Robert Clark Jun 10 '18 at 19:10
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A better answer maybe that United believes that a cruise performance boost of 30-40% is worth the effort.

Thus, my question is: How can one best describe the aerodynamics (and benefits) of the second, downward-pointing winglet in the context of airfoil extension?

You can extend the wings many feet which reduces drag and also makes for a very ramp unfriendly airplane. Winglets make wings behave as if they where longer. Wingtip vortices are reduced with the one upward winglet the lower or downward scimitar tip reduces vortex loss even more.

Hope this helps.

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    $\begingroup$ 30-40%? Where did you get that figure? It's usually more in the range of 2-3% $\endgroup$ – TomMcW Jun 10 '18 at 20:51
  • $\begingroup$ I should have sited the source, sorry. I doubt that it is correct or at least far from context. https://gizmodo.com/how-these-simple-scimitar-winglets-make-the-737-a-whole-837341873 $\endgroup$ – Robert Clark Jun 12 '18 at 6:11
  • $\begingroup$ 2 - 3% seems low also. The cost of modifying a fleet of 373-800s must have been high. If all you get back is 2 percent, even if after the investment, hardly seems worth the trouble. A few hundred million here, a few hundred million there, what do I know? $\endgroup$ – Robert Clark Jun 12 '18 at 6:24

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