No. 2775:
Wingtips
Audio

Today, airplanes, birds, and their wingtips. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

I'm fascinated by wingtips: The black wingtips of a white ibis or the white wingtips of a black vulture — the blunt wingtip of a P-51 Mustang or the Spitfire's wings curving almost to a point. How does a wing end, how should it end, and why?

My wingtip fascination began a few years ago when I first saw those funny little upturned winglets at the end of jet-liner wings. I'm supposed to know a thing or two about fluid mechanics, but I didn't see how those worked. Once I found out, I was a lot less embarrassed by my ignorance. For they are not simple.

The crucial thing about a wingtip is this: The air under the wing is at a higher pressure than the air on top, so it tries to flow out from below, around the wingtip, on to the top. But that moving air gets caught by the stationary air outside and whipped away in a corkscrew motion.

The pressure and temperature of air inside that trailing vortex are both low. And, if conditions are right, the low temperature causes water to condense into thin white droplet trails. That usually happens only at low altitude, and if the humidity is high.

(We mustn't confuse those trails with the white trails we see behind high-altitude jet-liners. Those double, or quadruple, trails, extending miles across the sky, are called contrails. They're made of the water formed when aviation fuel burns. It condenses, or freezes, into the particles that make up the contrail.)

But back to wings: We can't often see their tip vortices, but it takes a lot of energy to create them — we wish we could be rid of them. Winglets divert air coming from the bottom of the wing so it rides up the winglet adding a bit of lift. Then it spins off its top in a much weaker vortex. Winglets save significant energy.

And they aren't really new. The British had a patent for a wing endplate in 1897 — six years before the Wright brothers flew. Variations have popped up on occasional airplanes since 1910. NASA and some business planes have used then since the '70s. Boeing finally put them on a new version of the 747 in 1985. Since then they've appeared everywhere — on gliders, even on propeller and windmill blades. And they appear in all kinds of shapes.

So what about birds? If winglets are so useful, why don't birds use them? Well, the big soaring birds all do. They have a kind of double variation on the idea. They splay four or five wingtip feathers as they soar. That breaks up the air-flow from below. And they also curve those feathers upward to create arrays of small winglets.

If that's tricky to visualize, check out the Engines web site. I'll post some of my photos of birds doing exactly what airplanes have finally managed to do as well. So look at airplanes and look at birds. Each displays its own unique wingtip — one suited to just what it needs in its own magical act of flying.

I'm John Lienhard, at the University of Houston, where we are interested in the way inventive minds work.

(Theme music)

Notes and References:

Wingtip vortices, on contrails, and on wingtip devices. The Wikipedia article on bird flight only briefly mentions the birds' use of splaying feathers to alleviate tip vortices and makes no mention of their use of upturned tip feathers.

All photos by John Lienhard.

This episode was first aired on March 2, 2012