An air filter is a device used to filter pollutants, dust and dirt from the air that enters an internal combustion engine during normal operation.
Most air filters used in radio control have a filter element made of porous foam with a small amount of sticky filter oil worked throughout the element. The oil, a necessary part of the filter, is used to suspend particles of dirt within the filter element. The oil is soluble in nitro-based fuel and can be easily washed out with fuel as a part of routine maintenance and just as easily replaced. Some filters are of the two-stage variety, with a dry foam outer element and oil-soaked inner element.
No engine which requires the use of an air filter should ever be run without one. Doing so will allow damaging dust and dirt to enter the engine. In addition, the fuel mixture may be dangerously lean without a filter, leading to damage via overheating and lack of lubrication.