To: before landing check list
Lu, then why is it the sparks are still visible (but less due to less sand being blown about) when the helo is at flat pitch and on the ground?
As it says above sand particles come in many sizes. Consider the desert environment. There is sand everywhere and a great deal of it is entrained in the air we breathe and the air we fly in.
Because of the small size of the entrained particles while at flat pitch and the density of the sand particles in the air the energy exchange (see above) is to a lesser degree and therefor the sparking is less.
Depending on the localized airstream, in which the helicopter is flying in, static electricity, can build up on the blades even if there is no dust entrained in the air. If this charge can not drain down to the fuselage the charge will build up similar to that of a capacitor. When the “capacitor" builds to a specific point at which it can’t store any more static energy it will bleed off the blade tips as visible light.
On those helicopters where the charge can get down to the airframe there are special precautions that must be employed when loading or unloading cargo on a sling. The charge must be grounded before the ground crew can touch the hook. The same is true if picking up someone from the water using the hoist. The sling and hook must be dropped in the water prior to the downed person touching the sling or hoist cable.
It seems you edited your post while I was composing mine.
I am not familiar with the rubber strips you mentioned. If there is minimal sparking or no sparking then there is no energy exchange because of the surface hardness with the rubber strips as compared to a metal blade or, metal erosion strip. If it is in effect sparking at the tips then consider the surface hardness of the tip as compared to the rubber strip(s).
What you see under NVG conditions could be the "sparking" or it could be corona discharge which is caused by static electricity being discharged from the blades. Just imagine an enemy equipped with NVG or IR sights on his gun.
