Helicopter rotors are fitted with abrasion shields along their leading edges to protect the blades. These abrasion strips are often made of
titanium, stainless steel, or
nickel alloys, which are very hard, but not as hard as sand. When a helicopter flies low to the ground in sandy environments, sand can strike the metal abrasion strip and cause erosion, which produces a visible corona or halo around the rotor blades.
The effect is caused by the
pyrophoric oxidation of the ablated metal particles.
[6][7] In this way, the Kopp–Etchells effect is similar to the sparks made by a
grinder, which are also due to pyrophoricity.
[8] When a speck of metal is chipped off the rotor, it is heated by rapid oxidation. This occurs because its freshly exposed surface reacts with oxygen to produce heat. If the particle is sufficiently small, then its mass is small compared to its surface area, and so heat is generated faster than it can be dissipated. This causes the particle to become so hot that it reaches its ignition temperature. At that point, the metal continues to burn freely.
[9]
Abrasion strips made of titanium produce the brightest sparks,
[2][10] and the intensity increases with the size and concentration of sand grains in the air.
[11]
