Air filtration

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Markproa

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Bellingen, NSW. Australia.
I'm interested to know what you guys think about air filters on aircraft engines. The designer of my aircraft tells me to not use a filter on my French turbo diesel. He reckons there is not enough dust in the air to warrant one. I'm not convinced that is true in a dry place like Australia though it may be true in France.
Do all aircraft run air filters?
 

TFF

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Memphis, TN
It’s about conditions. Clean runways, clean air. Some aerobatics people are looking for the most power. That are not landing on dirt roads, sand bars, or water. Overall regular airplanes it probably should. Tropical Spitfires had extra filtering and desert helicopters have special filters. It’s about where you are.
 

Marc Bourget

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Stockton, California
John Thorp was Lockheed's "engine liason" with all the manufactures from 1938 through WW-2. He continued to consult on engines even after his retirement.

John felt a properly filtered/maintained aircraft engine would never "wear out."

pebbles are bad, but the really small stuff is like valve grinding paste and it's every where, just less in some areas.

Go with filtration, just design it properly.
 

Dan Thomas

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Many years ago I was the shop foreman in a machine shop that overhauled compressors and other air brake components. We rebuilt 17,000 compressors in my time there. There were always compressors that came back for warranty claims for some reason, maybe 1 or 2 percent of the production, and most of them were failed because of dust. Silica is hard stuff, and it's the primary ingredient in glass. Most dust is silica or a silicate.

We bought piston rings from a large US manufacturer. Those guys told us that their lab tests found that one teaspoonful of dust fed slowly into the intake of an automotive V-8 running under load was enough to trash the engine.

You don't want to run without filtration. You never know when you'll encounter a cloud of dust that takes a bunch of hours out of the engine. More than that: a single leaf sucked into the carb can foul the secondary venturi and cause fuel starvation.

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The fuel nozzle is in that secondary venturi. It's rather tight. Anything that gets in there destroys the suction that moves the fuel. I've seen it.

Carb heat is unfiltered. Not necessarily because it needs to be, but because the engine needs an alternate source of air if the filter should become clogged by snow or ice or dead grasshoppers, and the carb heat is a simple way to do that. Otherwise, the air is filtered. Some aircraft airboxes aren't the best that way, since their flapper valves can have a sloppy fit and let some unfiltered air from the carb heat side in. Aircraft quality. Huh.
 
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Martin W

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May 14, 2021
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My farm neighbor bought a brand new combine harvester with the Chrysler industrial engine which used an oil-bath type of air filter .... for some reason they forgot to put the oil into the filter ..... that engine was completely worn out in 2 weeks .

Our local veterinarian drove a Volkswagen diesel pickup truck .... the bolts on the intake manifold had vibrated loose so the engine was breathing unfiltered air .... engine was worn out within a few weeks and had to be rebuilt .

Highway Semi Tractors draw (reasonably clean) air from high above the cab and then through large paper-filters ... service centers clean those filters by a special impulse-air system that reverses flows air to blow the dust out of the filter ..... I have watched them do it and witnessed the huge clouds of fine dust that was expelled.

Airline Passenger Jets that fly through fine volcanic dust must replace the turbine engines .

.
 

TFF

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If you can see stuff blowing in the air like grass or dirt like running around on a farm, duh. To be completely opposite, I drove my Triumph Spitfire for ten years every day without a air filter and my Alfa Spider about the same. Only stopped driving them for more sane day to day. Kid seats in Triumphs not really the way to go. I don’t live in the desert southwest, or play in gravel pits. If you don’t know, put the filter on. A nicely designed partial separator where the air can turn and the junk can’t and gets ejected would be the best of both worlds, which is what turboprops and turbine helicopters have.
 

rv7charlie

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Pocahontas MS
The current versions of the RV-x carb airboxes also filter carb heat air. The only unfiltered air on current RVs would be from the *alternate air* door on the injected engines, and that should only open when there's something like an in-flight bird strike that plugs the normal air inlet.

Having said that, the kit-supplied 'air box' on one of the early RV4s (completed ~1990) that I owned was a simple 'D' shaped tube, with the hole for the carb inlet on back end of the flat and the carb heat flap about mid-way forward on the flat. Air from the chin inlet went straight back and then up into the carb; no filter.

Filtering is obviously is better, but unless you fly in really awful conditions all the time (STOL competitions out west on dry lakes, etc), the motor won't fall off if there's no filter.
 

speedracer

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Feb 4, 2020
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Klaus Savier doesn't run an air filter on his (world's fastest) LongEZ. I asked him if he worries about sucking a bird into the throttle body. He said "If I see a bird coming I lower the speed (belly) brake. " BTW, a few years ago he won the Bronze Sport Class race at Reno. Second place was a 540, supercharged F1 Rocket.
 

Markproa

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Bellingen, NSW. Australia.
Thanks guys, I'd certainly prefer to run a filter. My problem is space in the tight cowl so the only place for the filter is directly under the turbo next to the exhaust. 'Cowl on' test runs show a significant drop in power than with the cowl off. I'll just have to spend more time making partitions and a new cowl inlet, something I was trying to avoid now it's all nicely painted.
 

blane.c

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capital district NY
Thanks guys, I'd certainly prefer to run a filter. My problem is space in the tight cowl so the only place for the filter is directly under the turbo next to the exhaust. 'Cowl on' test runs show a significant drop in power than with the cowl off. I'll just have to spend more time making partitions and a new cowl inlet, something I was trying to avoid now it's all nicely painted.
The Super Cub just adds it to the bottom of the cowling.

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