Descending into a drab, unnamed war zone, the helicopter is familiar sight. As it kicks up dust and sand, the onlooking soldiers barely shield their eyes; even their most sensitive organs are grizzled. The moonlight is occluded by the grainy mist.
The casual cargo drop is then enchanted by a peculiar sight: a glowing halo adorning the helicopter’s blades. Cutting through the sand and dust, the blades smash into millions of tiny particles, each creating their own nano-crater, each contributing to the halo.
Photographs by Michael Yon [Sangin, Helmand Province]
I like to think that the seemingly spontaneous halo illuminates the otherwise dreary LZ. I like to think that perhaps, just for a second, a soldier forgets his or her dire mission and revels in the unexpected pleasure of curious physics. I like to think that one can find just a little beauty in battle: the Kopp-Etchells Effect.
The Science
When a helicopter descends into a sandy environment, the enormous downward thrust from the rotors inevitably kicks up a cloud of sand. Once in the air, the helicopter’s blades cut through this cloud, generating the halo.
But how? To prevent early degradation, most helicopter blades are coated with an abrasion strip. This strip, typically crafted out of a metal like titanium or nickel, prevents the leading edge of the rotor blade from being worn down too quickly by the various particulate hazards of the atmosphere.
This abrasion strip can handle a lot of wear and tear, but the desert is a harsh environment. Sand is harder than the titanium and nickel that make up the abrasion strip, so when the blades begin cutting through a cloud of sand, the particles hit the blades and send bits of metal flying into the air.
The Kopp-Etchells effect was first documented during combat operations in Afghanistan. The photographer Michael Yon named the effect after two soldiers who died there, Koop, a US Ranger, and Etchells, a British soldier. Both were killed in combat in this province of Afghanistan.
It turns out that the metals that make up the abrasion strips on helicopter blades, bracing themselves against the stinging force of the sand, can be pyrophoric. This means that the metals used are substances that can spontaneously ignite in air. But of course this doesn’t happen in normal circumstances; we don’t see bricks of titanium bursting into flames. Rather, the spinning blades of the helicopter generate a cloud of metal particles, just like the cloud of sand. Once in a powdered form, the metal particles can ignite and create the brilliant scenes above.
This may sound far-fetched, metals don’t just combust on their own? Right? Interestingly, we are all familiar with the pyrophoricity of metals, even if we don’t know it. Think of a metal that when struck creates a spark. The sparks that we see (maybe a dilapidated car is dragging a sparking muffler along the road) are in fact small metal particles being dispersed into the air by some impact and then igniting. And if you have ever started a fire with a flint, it’s just a simple MythBusters-esq ramping-up to imagine how the rotors of a helicopter spawn a shower of sparks.
I’d like to think that, with a tired face partially illuminated by the unnatural yet natural light emanating from the rotors, a soldier can for a moment escape from a solemn world to one filled with wonder. The astounded children watching the lab demonstration and the soldier are fleetingly connected.
CORRECTIONS made to this article, suggested by cited photographer Michael Yon, November 4th, 2012:
- All photos occurred at night, not during daytime, as my narrative suggested.
- Michael notes that all soldiers shield their eyes when a helicopter lands. My narrative takes liberty with this fact.
- Michael suggests that the Kopp-Etchells effect is caused by intense static fields. However, in my research, nothing indicates this. The pyrophoricity of the metals involved is a well-established phenomenon, and is in my estimation a better explanation, given that both researchers and military officials are unsure of the cause. In fact, looking at the photographs, the “sparks” created are inconsistent with a massive, simultaneous, static discharge.
A wonderful post. A wonderful tale.
Much appreciated, thank you.
I can’t imagine how reassuring that must be to look up and see the only things between you and some pretty serious gravity issues, being ground away in a shower of sparks like a knife blade on a grinding wheel!
Awesome article. I forwarded it to a researcher in the UK. Note: I have not said this is caused by static. Others often say this. I do not know the causes.
Note noted. Thanks for the awesome photos and the feedback Michael.
Thank you Kyle for posting this tribute to two very brave soldiers, from Michael Yon.
I added the following at his site:
The ‘flint’ is not a metal, but a mineral similar to quartz. You strike it on a steel (file) to generate sparks to create a flammable tinder ignition. (Old Boy Scout training)
Causation:
The Kopp-Etchells effect still appears to me to be closely aligned to the sight of a small meteorite burning up through the atmosphere. One of the first shots Michael posted in straight low-light exposure showed it well, as does the top one here with the IR included.
Pitting is occurring by the larger dust pieces abrading the blades; the illumination is very pronounced at the ends of the blades, where the tip-speed is greatest. The impingement of the smaller dust-motes into the boundary air-layer of the blade creates the extreme frictional heating of the dust – hence the “halo”.
The color of the illumination is not the whitish-blue one expects from an electrical discharge, but rather the yellowish color of something burning. I do know that the reason the helicopter produces its distinctive “whap-whap” sound is that the blade tips are exceeding the Mach 1 speed barrier, and the halo is more pronounced at those tips.
Then again, maybe I am full of hot air! ;-)
Hey Phil,
Thanks for the corrections on flint. I have made the changes.
However, I doubt your causation. Are you suggesting that there is enough heat imparted to the dust to cause the dust to glow? To make what is basically sand glow in this way would surely make the blades themselves glow. Also, if it really was a heating effect, we wouldn’t see this “shower” phenomenon. The halo would be strictly confined to a small area of the blades, but we can see in the pictures that it radiates out.
I agree that the color of the halo suggests something burning. And, as you know from your boy scout training, small metal particles can ignite (oxidize) in the presence of air. Given this, I still think that, like the flint example, metals are being dispersed from the blades by contact with the sand, and those particles are then igniting.
Thanks for the feedback!
There might actually be more than one effect going on. Again, I just don’t know. I did hear from a researcher in the UK yesterday. His paper is not yet complete. I have seen many thousands of helicopter landings in places like this. Sometimes they glow, but never as brightly as I saw in Sangin (here). Strangley, and I told the researcher this, on some times it was really bright, as you see. Other nights, exact same landing zone, with no noticeable change in weather (no rain, for instance), they would not glow even with same type of helicopter. Now, it is possible that they were different helicopters with different rotors, but it was always CH 47 helicopters and some nights there were rotor fireworks and some not. Leaves me scratching my head. But one researcher, American in that case I think, emailed that there might be at least two different causes. When the research paper is done, the researcher said he would send it and I can publish. Will let you know.
That’s fascinating Michael. Be sure to let me know what comes of the research.
I have run Kopp tests in supersonic chambers at NASA Glenn – the effect does not occur when the sheath is composed of a periodic table refractory metal with a special type of interstitial ionic boride in a FCC bravais, with a Knoop factor over 800 but not more than 1200. Since this unique bravais can conduct electricity but is resistant to oxidation, this suggests (falsifies) that the effect is not atmospheric, nor “static” halo electrical, rather, direct Kinetic Combustion of particulates released from the Nickel sheath that protects the titanium nose cone of the rotor blade. Given the dramatic erosion rates of this sheath fleetwide and that the sheath coats the final 54″ of the rotor blade, and given that this final 54 inches is where the sparks fly – this is much like the sparks from a nickel steel grinder in a lab, or a diamond cutting blade slicing nickel steel. Direct KE, and nothing else.