After the rigamarole surrounding oil filters, there’s far less drama with aviation air filters. That’s because aircraft spend most of their time in relatively clean air well above the ground.
But aircraft have to land eventually, and near the ground, there’s always some engine-wearing grit floating about. This grit wears the cylinders and piston rings, and gets into the oil, causing various abrasive effects—hence the need to filter out dirt and bugs.
Paper-element filters common in cars are excellent at straining earth-bound malefactors, but are also restrictive to airflow. Thus, most aircraft use oil-impregnated foam filters, which are less restrictive yet still strain out the boulders.
Washing and re-oiling foam filters is a staple on dirt bikes and hot-rod cars, but for aircraft, replacement is the standard practice. The cost is surprisingly low, and miscues such as over-oiling the foam are avoided. Brackett Aero is the main player here, and it offers a large catalog of pre-cut filters. Some Experimental/Amateur-Built aircraft use custom-shaped filters, and these are easily cut to suit with scissors or a razor blade from either bulk Brackett material or some other Brackett application.
One thing about foam filters is that they age out as the oil dries. Two years in service is a good replacement interval. Installation is screwdriver-simple, but it is bound to leave your fingers as sticky as flypaper.
There are paper-element filters along with reusable, washable air filters from Tempest, mainly for the certified market. Washable, reusable, oil-coated fabric (not foam) filters from K&N are common on ultralight and Rotax engines.
One aircraft-specific air filter thought is keeping bugs out of the fuel servo on Bendix-style fuel injection systems. These systems use four tiny ram tubes jutting into the airstream to meter the fuel. With no air filter, small insects can get stuck in one of the ram tubes, making the engine run excessively rich (too much gasoline). Engine power drops dramatically, pilot heart rates rise just as dramatically for good reason, and it can all be avoided by fitting an air filter.
Another aircraft oddity is that a few models allow bypassing the filter at altitude. This increases engine efficiency when the risk of dirt and debris is limited.
The takeaway for pilot-owners is that an air filter is a good thing, and the common foam-and-oil type is an easy replacement item.
Has any one done a Bracket Air filter comparison to a K&N or similar oiled filter? I’m planning my cowl now, finding a good stock air filter to design around would be ideal
Without digging into the internet I don’t know of a comprehensive test of any of the popular air filters. In a former life I was around filter tests at K&N and I suspect a foam filter such as a Brackett performs roughly the same as a fabric filter. My opinion is a paper element captures finer grit than foam or fabric filters, but is also more restrictive to air flow. Considering the mainly clean environment aircraft operate in foam/fabric filters seem fine. Backing that up is aircraft engine shops don’t note cylinder wear due to lack of air filtration.
Do an AI search on flow and filtration comparisons between synthetic pleated filters (Tempest & Donaldson), Challenger (K&N type) and Brackett (oiled foam). The foam filters have the highest flow restriction of all three by a large margin, and only a ‘good’ rating on filtration efficiency. The synthetic pleated filters are rated excellent in both categories.
27.7 inches of water pressure equals 1 psi manifold pressure, The 2022 tests shown from Donaldson says a maximum difference of 4 inches wp or.15psi manifold pressure is needed to make 1 hp difference in a 172. At max flow the Donaldson was 1.8 ” wp better than Brackett..
The question I have now is this: does a normal setup using probably a Marvel-Schebler carburator work better with a free flow of air versus the slight restriction of a clean air filter? 2 cycle engines are notorious for requiring slight back pressure (in the exhaust) to develop full rated power. I realize we are talking air filters here, not exhaust systems. What’s the score with flat 4 and 6 engines regarding air filters? Who else might know? Then again there’s the Powerflow Exhaust folks who may have something to add here. Give us a follow-up when time and information becomes available please.