Dirt will hurt the engine. RC cars that use glow typically will have a oil wetted sponge breather or filter. Typically, air is "clean" once you break ground. It is only the ground handling one needs to be worried about. If you fly from grass or pavement, your probably alright. Once in the air there isn't much dirt up there. If your field is very dusty... well... I guess your engine will eat some dirt. Don't worry about it... it is probably OK as long as it isn't really dusty.

You could fashion a air cleaner of sorts I suppose, but they tend to rob a lot of power. It will vary from engine to engine I'm sure. I tested one of my engines both ways. W/O air filter, I was getting 9300 RPM. With the cleaner I was getting 9000 RPM. This may not seem like a lot at first, but prop RPM is exponential as it relates to power. I was losing 0.4HP (3.6HP vs 3.2HP estimated) just from the air cleaner. Thats over 10%!