I don’t think there is any doubt that every ‘terrestrial’ vehicle needs an air filter.  They run on dust laden surfaces, and, at times, on unpaved surfaces which themselves are dust; resulting in the ingestion of a lot of abrasive material.

The rational (at least mine) for not using a filter on an aircraft is that, a) it spends little of its time on a surface, and then on one that is paved a periodically blown off by other aircraft, and b) the amount of particulates above a few hundred feet (and certainly at a few thousand feet) is very low; probably similar to what goes through a filter in dusty conditions.

If my flights averaged something like a couple of hours each, there is about 15 minutes on a relatively clean surface for every 2 hours up in clean air.  If the rpm in the air is 5500 and that on the surface is 1800, then the fraction of air volume used where there may be some fine particulates of concern is about 4%.

So I’m willing to accept that in exchange for keeping the ram air manifold pressure increase of about 1.5” Hg that I can get when cruising close to 200 mph for the 2 hours that I’m not on the surface.  A compact filter is going to eat most if not all that ram pressure.  If you aren’t concerned about the pressure loss, or you can include a sufficiently large filter enclosure, and not lose too much of the ram pressure in expansion, contraction and friction losses; then it makes sense to include an air filter.

I am definitely not advocating not using a filter.  Just giving you my rational, however faulted, for going without one.  (BTW, I do have a screen at the largest diameter in the intake duct to prevent anything bigger than about 1/32” from getting through).

FWIW,

Al

With all due respect; I don’t disagree with what you’re saying here, but I do take offense at you calling Tracy, myself, and others “idiots” because we have chosen to run without an air filter.  My intake is in front of the prop and above the wing strake.  Anyway, why climb on that issue?  There has not been a single aircraft incident that I am aware of that has resulted from not using an air filter.  Let’s pick an issue which has caused a failure.

I won’t argue that running without a filter may reduce the engine life; and I may consider one later.  And yes, the engine would eventually fail; every engine will – filter or no filter - if you run it long enough.

Al (I guess it’s past my bedtime)

Rusty, and Group,

 I understand your comments Rusty, and think you are both right and wrong. I believe it is easier, for most people, to be successful with a standard aircraft engine. I DO NOT believe this is because the Lyc, Conti, or whatever is inherently more reliable. I do believe that the ancillaries are better developed for "conventional" aircraft engines.

Because we [FlyRotary or ACRE] are rotary enthusiasts we hear about EVERY rotary problem. If we heard about every Lyc problem caused on a daily basis we might never fly spam again! I am not trying to offer so anecdote to soothe the rotary faithful, as all failures are just that, failures. I would remind everyone that ALL Lyc installs in certified aircraft now come with AIR FILTERS!

I'LL SAY THIS FLAT OUT, IF YOU DON'T RUN A FILTER YOU WILL HAVE A FAILURE! I don't care if your running a conventional aircraft engine or conversion. Running unfiltered is simple idiocy.

It should also be said that for Lycoming to be having crankshaft failures, and a significant number of them to initiate an AD is not only sad but smacks of reckless disregard. So saying that the aircooled aircraft engine is super reliable is simply setting yourself up for a fall. If you look at the statistics most of the engine failures are "certified" engines simply because there are more of them!

ANY ENGINE properly maintained and not operated outside it's normal limits will work just fine thank you, provided it is not a deficient design to begin with. The rotary requires a GOOD water and oil cooling system. If you aren't willing to work on that, stop now and don't hurt the reputation of the rest of us and go buy a Lycoming. The rewards for a GOOD installation are many but won't "just happen." As Tracy said you must go in with the knowledge of the needed systems or you will fail. If you do slipshod work, Rotary or Lycoming, YOU WILL FAIL. Someone put the old saying on the site, "How do you eat an elephant?" "One bite at a time!" This is the proper way of looking at the jobs needed to be successful. You MUST do all the jobs. (eat every bite) And most importantly you must do all the jobs well. Safety wire everything even though it's a pain in the ass. If you can't do all the jobs yourself it's no crime. Get HELP!  Buy the redrive from Tracy. Or buy from Mistral, or buy them from Marcotte. Build your system up and test it on the ground. If you overheat your system on the ground don't assume it will be better in the air! (It might be, but also might not be!) Plan for an adequate sized radiator. That would be a MINIMUM of 2 cubic inches pre HP, and that only if you have PERFECT ducting. (3 c.i. per HP would be a safer bet) Don't expect you are the miracle guy that can change physics for your own project.  Rusty has brought up some good points in pointing out that if you want to fly soonest buy conventional. Bernie, you can probably rebuild your system from all new parts for less than the cost of two Lyc cylinders. As an older guy facing family pressures I can understand how you might not want to though. This is no crime. Your needs are your own, though I would prefer to see you go back to the rotary and succeed. We all need to go in with our eyes open. When setting up race cars and motorcycles I would look at every part from the stand point what will happen if this part breaks? If the result would be a crash I would redesign the part until I was SURE it wouldn't break. If you don't think a part is adequate DON'T USE IT! Your responsibility is to find a part that IS good enough. Don't just hope that everything will be OK. Work it out. Run your system enough to be sure of it in the air. I hope everyone takes this in the spirit intended which is to have us all come home safe.

Bill Jepson