# Opinion | Halos in a time of war: The Kopp-Etchells Effect

• The rings of sparks one sees above a helicopter in action are caused by tiny shards of pyrophoric metal, igniting spontaneously as they fly through air
• The Kopp-Etchells Effect is a reminder that in the wrenching tragedy of war, sometimes you stumble on a touch of beauty

For an election campaign and season dominated by talk of military operations, what might an appropriate topic be for a mathematics column like this one? I’ve been pondering on the question. For, I don’t want to subject you to yet another election-related screed, even if it turns out to be mathematical. But I do want to remind us all of the reality of war: It is too conveniently used for political gain, but it kills people.

Yet, sometimes, it also produces great beauty. Some of all that in this column, via a phenomenon I first read about several years ago.

Now I’m sure you know something about helicopters. At least this much: They have rotors on top and, during wars, they are used to ferry troops and supplies. Probably pretty routine stuff. If you were a photographer in a war zone, I doubt you’d be going out of your way to photograph helicopters. And yet, there’s at least one war photographer I know of, Michael Yon, who has taken plenty of images of military helicopters in action. Why?

To answer that, think first of a sight I’m sure you’ve seen often: Sparks flying when a piece of metal drags along the ground, or rubs against another piece of metal. Maybe it’s an old car with some piece of its chassis loose and scraping the road surface as the car speeds past. Maybe it’s the wheels of a train against the rails it rides on, as it picks up speed.

Why do we see those sparks? You think “friction" and you’re right, but that’s only a partial explanation. That is, it’s not just friction that’s the cause. For example, you wouldn’t see such sparks if the car dragged a rod made of plastic or wood along the ground. There is certainly friction, and it might wear away the rod, but you don’t see any sparks. Try a palm frond, or a tablecloth, or a newspaper, or the pretty stilettos Poorna aunty gave you. No sparks from any of those. A glass bottle? It will simply shatter, and right away.

So, what is it about metal and, in fact, only some metals?

For sparks to appear, the substance that’s dragged along the ground needs to be “pyrophoric", a word whose Greek root means “fire-bearing". A pyrophoric material is prone to spontaneous ignition under certain conditions, without necessarily being heated to high temperatures. Many relatively obscure substances have this property and must therefore be handled with some care. But some familiar substances are also pyrophoric: Aluminium and iron, for example.

I’m guessing you read that last sentence and your brow wrinkled. You are wondering why your aluminium thali, or your house keys and the always rusting keyrings, have never spontaneously caught fire. The point here is my mention above of “under certain conditions". Those conditions are met when objects made of these metals are subjected to friction, like with the surface of a road. What happens then is that the friction chips away tiny pieces of the metal, and these pieces spontaneously ignite as they fly through the air, and voilà: The shower of sparks that you’ve seen plenty of times.

This is the mechanism at work in more common cigarette lighters, or in now obsolete guns known as “flintlocks". In the guns, for example, a small and sharp piece of hard stone called flint is held cocked in position. When the trigger is pulled, it rapidly rotates forward and strikes a piece of steel. The friction between flint and steel shears off small shards from the steel. They ignite, and that shower of sparks is directed into a little pan filled with gunpowder. In turn, the gunpowder ignites and…well, you get the picture: The gun fires. With the cigarette lighter, it’s much the same. When your thumb turns that minuscule wheel, it rubs against a piece of metal and scrapes off tiny pieces that catch fire. At the same time, you’re releasing some of the fuel from inside the lighter and…well, you get the picture again: You have a flame. (Not that I’m suggesting that you smoke.)

So, if you now have an idea of how those sparks you see so often are formed, what is the connection to helicopters, and to military ones at that?

Well, Michael Yon took a number of photographs of helicopters in battle zones. He found that when landing or taking off, they often had what looked like gleaming halos above them, sometimes even two or three halos. With these rings of light glowing in the dust-laden dusk, complemented by landing lights in assorted other colours, the photographs are striking and strangely beautiful. Now “beautiful" is not a word I thought I’d ever apply to helicopters, which are not quite the most graceful of aircraft. But take a look at several of Yon’s images on his blog post.

I also find those photos strangely moving, especially after reading Yon’s text that accompanies the images. At one point, a soldier, Captain Mark Hale, looked over Yon’s shoulder as he sifted through his helicopter halo shots to put together that post. “Mark had particularly liked the next three images," writes Yon, in which “the halos appear like distant galaxies (or) like the rings of Saturn". Those are apt descriptions, as you’ll see on that page. And soon after, Captain Hale, as you’ll read on that page, was “killed while aiding a wounded soldier".

Because the halos match the motion of the rotors—some helicopters have two or three rotors, so two or three halos —clearly the rotors produce the halos. But how? Yon asked the helicopter pilots, and their Command Sergeant Major Jeff Mellinger. This was their explanation:

“It is a result of static electricity created by friction as materials of dissimilar material strike against each other. In this case titanium/nickel blades moving through the air and dust...The most common time is when fuel is being pumped. When large tankers are being fuelled they must be grounded to prevent static electricity from discharging and creating explosions."

Sounds reasonable enough. The dust does indeed have something to do with the phenomenon. Yon’s photographs are from his time with British and American troops in Helmand, Afghanistan, in 2009. When the helicopters land or take off, especially in the high-altitude deserts of that country, their rotors kick up a small storm of dust. There’s friction as the blades rotate through the dust.

But in an article in Nautilus in 2013 (Mystery in Motion, Beauty in Battle, 24 July 2013), the science writer Kyle Hill explained that this theory has problems, because “a static discharge looks nothing like a shower of sparks".

So what does look like a shower of sparks? Here’s Hill again: “When a helicopter descends into a sandy environment, the enormous downward thrust from the blades inevitably kicks up a cloud of sand. Cutting through the sand and dust, the blades smash into millions of these tiny particles, each sandblasting metal from the blade…Every so often, the metal blasted from the blades produces a miniature meteor shower…When a helicopter’s blades begin cutting through a cloud of sand, the particles hit the blades and send bits of metal flying into the air."

That’s right: The halos are caused by tiny shards of pyrophoric metal, igniting spontaneously as they fly through the air. The same phenomenon, that is, that’s at work in cigarette lighters and flintlock rifles and bits of metal that decrepit old cars drag behind them.

There in war-torn Afghanistan, Michael Yon captured a gorgeous visual treat that was, for him, simultaneously a reminder of the beauty in science and the despair of war. Yet while the pilots and soldiers around him were familiar with it, they had no name for it. On his blog, he asked: “How can the helicopter halos, so majestic and indeed dangerous at times, be devoid of a fitting name?"

So he came up with his own name. At about the time he was photographing these silky helicopter halos, two young soldiers died in combat in Helmand: An American, Benjamin Kopp, and an Englishman, Joseph Etchells. For Yon, the halos were almost like a tribute to them and all the deaths in this war he had witnessed so closely. He writes: “And so a fitting name had arrived to describe the halo glow we sometimes see in Helmand Province."

The Kopp-Etchells Effect, then. A reminder that in the wrenching tragedy of war, sometimes you stumble on a touch of beauty.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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