# What causes air flow towards fuselage near the winglet?

I am trying to understand how winglets are beneficial in reducing induced drag and I came across this article;

https://www.boldmethod.com/learn-to-fly/aerodynamics/how-winglets-work-to-reduce-drag-and-how-wingtip-vortices-form/

In the context of the above article, The component of velocity towards should be towards the fuselage when a vortex is shed at the tip with no winglet. But by adding a winglet the vortex would be shed at the winglet's tip and the velocity component seen by the winglet should be away from the fuselage. Can some give me an insight into what really is happening?

Is the explanation that winglets actually produce negative drag true?

• The winglet is working like a sail on a boat close hauled on a tack; most of the lift is sideways, but there is a small forward thrust component that drives the boat forward, and the winglet gives enough forward thrust to exceed its drag at high AOAs. This is why they are only seen, mostly, on high altitude jets that cruise at low indicated speed. There is also the winglet's outwash, redirecting the vortice field in the opposite direction, weakening the circulation. Stand under a winglet and you might notice its chord line is toed out a bit, to optimize its AOA into the local circulation. Commented Jul 15, 2021 at 17:35
• @JohnK I have been doing some reading and there seems to be some debate regarding winglets behaving like sails. Although I haven't been able to find any satisfactory theory suggesting other methods of their working. Commented Jul 15, 2021 at 17:51
• Note the expansion of my answer. I hope this helps to support @JohnK's reasoning. Commented Jul 15, 2021 at 19:52

## 1 Answer

Suction does.

Air flow follows pressure gradients, and the lift-creating suction over the upper wing pulls in more air, not only from ahead of the airplane (which explains the induced angle of attack), but also from the side.

The explanation of induced drag on the Boldmethod page is actually quite good; only gems like the wingtip vortices curve up and around the wingtips, pushing the air flowing over the wing downward and the graph with straight arrows on a swept wing are in need of improvement.

The better wording would be: Lift is created by accelerating the air flowing over the wing downward; this in turn causes more air from the side to be accelerated towards the center of the wing.

Regarding the comparison of winglets to sails: The sideways acceleration of the air near the tip causes an inward-bent airflow above the wingtip. If you now place a sideforce-creating surface there, its lift vector will be tilted forward by this inward-bent airflow. This will produce a small thrust component together with a large side force, just like a sail does when sailing close to the wind. This effect works in the same way as the downward-bent flow over the wing which causes the wing's lift vector to be bent slightly backwards, causing induced drag. You might, therefore, call the beneficial effect of the winglet induced thrust. Just like induced drag, it is biggest at high lift coefficients.

Of course, folding this winglet flat out as a span extension will be even more beneficial.

• Thanks Mr. Kampf. Am I right in deducing that in all these cases the major part of vortex is still being shed at the wingtip and not the tip of winglet. Commented Jul 15, 2021 at 20:38
• There is a large flow field circulating around the wing tip extending out well beyond the winglet's own length, weakening as you get farther away. The winglet is just extracting energy from the strongest core section of the circulation to help weaken it and get a bit of thrust benefit from it. Whitcomb's objective with the winglet was to get the benefit of a span increase without the actual additional spanwise space taken up and without the added bending load on the spar root, since the winglet's structural impact is absorbed at the base of the winglet and doesn't add to overall wing bending. Commented Jul 15, 2021 at 21:21
• The best article on winglets I've ever seen was a Peter Garrison (who designed his own aircraft and flew around he world with it) article in FLYING magazine many years ago where he describes it perfectly in lay terms. Commented Jul 15, 2021 at 21:23
• @Mridul Vorticity is shed along the whole span and rolls up into the two vortices. What happens at the tip is only a minor part of the whole. Commented Jul 15, 2021 at 21:53
• I guess you could say the wing itself is one big vortex generator, a delta wing being the most extreme version. Commented Jul 16, 2021 at 0:27