I recently flew to Florida on Southwest. Both ways I was on a 737, but on the trip down the plane was newer and the last 5 feet of the wingtip was curved up 90 degrees. Many new jets seem to have this feature. What purpose does it serve, other then giving them a place to paint “Southwest.com” where folks can see it?
I think that those wingtips help prevent the formation of vortices coming off of the wingtips, which can be a source of wasteful drag. But IANA ærospace engineer; if one comes in and contradicts me, listen to em.
You can see the vortices that are formed by a non-winglet airplane by watching this (very cool) video.
All that the winglet does is provide a physical barrier against the formation of these vorticies. That reduces the size of the vortex, which in turn increases the lift and decreases the drag of the plane.
Interesting video.
I was amazed at how long it took for the vortices reach those smoke trails.
As an aside, I was a passenger in a smal plane that took off behing a Southwest 737 from Hobby airport years ago.
Those vortices are real! The Cessna I was in literaly rocked 90 degrees side to side.
What kind of an effect do winglets have on ground effect?
Here’s an interesting photo that shows a wingtip vortex.
This article is a reasonably good discussion of winglets and their design. Credit for their invention goes to NASA engineer Richard T. Whitcomb, who also developed the “area rule” for the fuselage shape of high-speed aircraft.
Would winglets have helped an old-time airplane like a DC-3?
When an airplane flies close to the ground its wingtip vortices are dispersed. The mechanism is obviously different from that of a winglet, but the effect is similar: reduction of induced drag.
Note that the plane has to be quite close to the surface: 0.5 wingspan yields about a 10% reduction in induced drag; for 50%, you need to be within about 0.1 wingspan.
Winglets themselves have little influence on ground effect. You’d expect a plane with well-designed winglets to see less benefit from ground effect than one without them - its induced drag is already somewhat lower.
Quite possibly. But it could be that the wing itself would need some redesign.
I’ve heard Mark Maughmer (professor at Penn State and an authority on winglets) say that for best effect the wing and winglet should be designed together. Fitting a winglet to an existing “untipped” wing can be a good deal less effective.
It should also be noted that adding a winglet increases the bending moment on the wing spar, in much the same way that an increase in span would do. So there can be structural issues.
When did you last talk with him? I used to be in his sailplane class.
It was in late June.
So you are a glider pilot?
Simple answer. Thanks folks.
No, I’ve been up with him before though. The class was designing and building them. The chemicals I was exposed to back then probably means I will have three headed kids. Fun times, lousy safety.
Yep, me too, as a pilot. Took off in a little C152 a couple of minutes after a 777. About 100’ off the runway, it felt like a giant had grabbed my wingtips…90 degrees one way, then slammed 180 degrees the other way, with no yoke control. Scariest moment of my life. I kissed the ground and obliterated the nervous shakes with too much Guiness after landing 
It isn’t only light planes that must watch out for wing vortices. They are a prime suspect in the crash of the XB-70. On a photo shoot near Barstow, CA an F-104 piloted by Joe Walker, a top test pilot and very experienced, collided with the right wingtip of the XB-70, rolled over inverted over the bomber, shearing off the vertical fins. Walker and one of the XB-70 pilots were killed in the accident.
It is suspected that Walker was momentarily distracted and allowed the F-104 to drift too close the the wingtip of the bomber. This put him into the vortex and caused a loss of control with tragic results.
In any case, photos show that the 104 suddendly pitched up into the wingtip, collided with it, rolled over and continued across the top of the XB-70 shearing off the vertical fins.
On the opposite side, though, it has been discovered that aircraft flying in close or (in the case of big bombers, transports, or commercial airliners) not so close formation can make a significant increase in efficency by using the vortices of the leading aircraft. There was a bit of talk of having trans-atlantic flights do this to save on fuel costs.