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What is the Kopp-Etchells effect?

A unusual phenomenon occurred when American photographer and war correspondent Michael Yon accompanied American soldiers to Afghanistan in 2009 and began photographing troops and their equipment.

This strange phenomenon occurred every time a helicopter took off or descended and kicked up a large amount of sand into the air. Then, the rotor blades were beginning to shine like “distant galaxies”.

Kopp-Etchells effect observed on a US Marine Corps MV-22 at an undisclosed location in Southwest Asia. Image credits: Wikipedia.

“The halos sizzled and shimmered and really crackled, but in still photos they looked like more intricate orbital bandsMichael Yon wrote in a post on his personal blog.

Yon asked the pilots about the phenomenon, but nobody seemed to know exactly why it happened. However, they had their own theories. Yon describes talking to a pilot who said the halos were “the result of static electricity created by friction when dissimilar materials collide with each other«.

But this theory has a problem. A static discharge does not produce a shower of sparks as seen in these images.

the real explanation

Kopp Etchells
Kopp Etchells effect. Image credits: Michael Yon.

When a helicopter is very close to the ground, the rotor blades kick up a huge amount of dust and sand into the air. To protect the rotor blades from abrasion by this dust and other dangerous airborne particles, the leading edges of the blades are equipped with strips of resistant materials such as titanium or nickel alloys.

These materials work very well under normal battlefield conditions, but the desert is an environment that adds even more harshness. The grit is harder than the titanium and nickel that make up the abrasion strip.

Then, when the blades pass through a cloud of sand, the sand particles hit the blades and open small grooves in the surface of the blades themselves. this sends small pieces of metal flying through the air. this fine metal powder spontaneously “ignites” when it comes into contact with atmospheric oxygen, a phenomenon known as pyrophoricity.

Is the same phenomenon that causes sparks to fly when you sharpen a knife or when you hit two stones (or metal parts) each.

Not all metals exhibit pyrophoricity, but some do.such as iron, aluminum, magnesium, calcium, uranium, titanium, tungsten, bismuth, hafnium, thorium, osmium, neodymium, lithium, sodium, potassium, rubidium, cesium, and some others.

Michael Yon helicopter
Image: Michael Yon.

When Michael Yon discovered the phenomenon, he was surprised that he didn’t have a name yet.

“How is it possible that the halos of the helicopters, so majestic and sometimes dangerous, don’t have a proper name?”, he asked himself.

Kopp Etchells effect
Kopp-Etchells effect. Image: Michael Yon.

Yon decided to name this phenomenon as Kopp–Etchells effect, in memory of two soldiers who died in combat in Sangin, Afghanistanin July 2009: benjamin koppa United States Army Ranger, and Joseph Etchellsa British soldier.

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