Thought I would add to the knowledge base.

I spent several years in North Africa flying piston powered helicopters and
airplanes. When I arrived, the TBO on the Hiller 12 with a dry filter was 25
hours. We installed a filter based on a Caterpillar design that had a mesh
filter with oil and a 180deg turn for the air above the oil. TBO's went from
25 to 125 hours. Had a Sikorsky H-19, where the crew chief had a metal box
filed with gasoline. At ever stop, he would take out the air filter and
replace it with one that had been washed in the gasoline and then oiled.
With that procedure, we got about 100 hours out of the engine.

On a Dehavaland Otter, the air intake was on the lower cowl. On take-off,
you could see a whirlwind of dust come right up the back of the propeller
and into the intake. Needless to say, 125-150 hours to TBO. We also had the
Beaver, the intake was on top of the cowl. Don't remember what we were
getting for TBO, but it did not have engine problems operating off of
unimproved desert landscape.

The helicopter would make their own dust storm, so every take-off and
landing was in a big cloud of dust. On the airplanes, it was obvious that
the location of the intake made a significant difference in how much debris
was sucked into the engine.

Giff Marr
LIVP/20B 31%