wallofdis asked: Ha, no, I don't think anything is superfluous. I was just wondering about the (obviously hella obvious) differences in the design of commercial and military planes, as one is wont to do, and then about how some commercial jets have little fins in places that others don't (like the vertical ones on the edge of the wings, which airlines conveniently put their logo on so you can see it from inside the plane).

Ahhhhh those are winglets! They actually serve a very useful aerodynamic function.

If you think about the way they taught you airplanes fly in like 4th grade (Bernouli’s principle), you know that supposedly the shape of the wing creates high pressure on the bottom and low pressure on top. That’s kind of true but it’s not the whole story. Anyway, the point being is that the two masses of air meet at the wing tip. Obviously since high pressure seeks low pressure, the air tends to sort of curl up over the edge of the wing creating what we call “wing tip vorticies.” They’re the reason sometimes you see pictures like this.

Anyway, these vorticies not only create a lot of additional drag but they can be dangerous to airplanes flying in trail. It’s what we call “wake turbulence” and it’s kind of like being the wake of a big ship in that it’ll knock you around pretty good. In fact it was a major contributing factor in the most recent major airline crash (American 587 in 2001). So by putting these winglets on the end of the wing, it cuts down on the ferocity of these vorticies which does two things: 1) Reduces wake turbulence, and 2) Decreases drag which of course improves fuel effeciency. Some have them, some don’t, but doesn’t affect the airplane’s ability to fly.

The fact that most airlines have turned them into billboards IS in fact purely corporate cosmetic, but the devices themselves serve a pretty important purpose. For more on winglets, see this question Paul Brady asked me about them awhile ago.