Warbird Information Exchange

This comes from the topic "Mustang Cowl Question". It concerns the air filters which have been sometimes been removed from the Merlin engines on Mustangs. J
John Beyl, and many others, don't feel filters are needed on 51s since they are "not operated in dirty air" now. And John asks about how a Spitfire is set up.
!st. Unless you are flying indoors, or maybe in Florida last week, the air is dirty, it is full of dust in most places. At Oskosh the 51s park on the dirt(grass). When you start and taxi out you are sucking in lots of dirt, especially if there is another plane ahead of you. Many other places are also dusty, Midland, Chino, Leeward, etc. Obviously most Mustangs are not operated in North Africa, or even Arizona, but most places are dusty. If you don't think so try this: after attending any airshow all day long, go back home and get a white towel, wet it and wipe your face, neck and arms. The towel will be brown with dirt. And an engine pulls a thousand times more air than just your body encounters. Paper air filters came into use in the 50's when stock cars were racing on dirt tracks. It was found that a 300 hp Hemi could lose 100 hp with no filtersjust over the course of a race due to loss of compression from dirt intake and piston ring wear. Obviously a severe case, but I sure wouldn't want to pump even a small amount of dirt into my Merlin, if I had a Mustang.
2nd. A Spitfire has the intake underneath the prop at the front of the cowling through the scoop. There is a flap on the scoop controlled by the pilot with a lever in the cockpit. The Pilot Notes say to keep the flap closed, thus using non ram and filtered air for takeoff, climb to 1000 ft, and for landing. The closed position, non ram, makes the engine draw intake air through a large air filter element mounted under the engine. You still have plenty of power with the filter in use, normal takeoff is 44 inches, more if needed. Once you are at 1000 feet, normally you open the flap and the ram air bypasses the filter and goes straight back to the carburetor. The normal air filter is sort of pleated mesh, and is about 3 feet long by a foot wide. A cheap, easy modern upgrade is to get a sheet of oil wetted foam from Bracket Filters in Arizona which drops right in place and filters much better than the original.
3rd. A Spitfire air intake is right in front so if you were to taxi or run up the engine in dusty air without the filter in use, the dirt would go right into the carb and the engine. The Mustang may have a slightly different intake. I think in a 51 if you just removed the filters, left everything else stock and ran it with ram air closed on the ground you might not get as much dirt because the intakes are higher up. As for power, I am not at all sure, but I'd bet the Mustang, just like the Spitfire can develop adequate takeoff and climb power with the filters in use, even if not full power. I have done a loop in my plane with the door closed, filter in use, and not lacked power. I used about 50 inches, 2850 rpm with an entry speed of 260 knots.

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Bill Greenwood
Spitfire N308WK

John, when you say most warbird were designed without air filters, I don't see which you mean. One of the most prolific trainers, the T-6 certainly has air filters, as did Spitfires, 23,000 of them. I think Me109 s had filters, am not sure,but that's 30,000 or so. Even Piper Cubs, Champs, Stearmans, most everything that I know of has filters, if it is made after about the 30's. I don't think North American went to the trouble and expense to put a system on a p-51 that added weight, cost and complexity unless it was needed. How about Allisons in P-40s, or 38s, do they have filters?

I'd be willing to bet that 99% of warbird or Mustang owners who say you don't need air filters on their airplane engine worth $100-200K, have got an air filter on the car they drove to the airport, even if it is a old pickup truck or in my case a 76 BMW worth maybe $1 K.

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Bill Greenwood
Spitfire N308WK

You win Bill

Man I hate to start. Bill, many warbirds do not have airfilters. P40 is like the P51, most are deleted. F4U, TBM, B25, etc. have a selection for "alternate air" but no filters. The wildcat has filters. The big difference is that your car is on the road 100% of the time. It's tires, and other car's tires kick up alot of dirt, all the time. Your car is more sensitive to dirt than the bigger engines. That's not to say that filtering isn't the best for the engine. It's just not always the most practical. We do oil analysis on every oil change, on every airplane. The silicone (dirt) is never at a level that is anything but normal. One time, in our Spitfire, after Oshkosh, it was higher than normal. Go figure.

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Blue Skies,
Doug

www.cavanaughflightmuseum.com

Doug, Many planes may be flying without filters. But I do doubt that many if most mainstream warbirds or even civilian planes were designed without air filters after the early 30s as John has said. I don't have your experience with all the other types, so I could be mistaken, but I think a plane without filters would be the exception in design.Hey, if they are good enough for a DC3, they are good enough for me. Some pilots have said they don't need filters since they now operate off hard surface. But few airplanes today are designed for dirt runways, but virtually all have filters, whether it is a Cessna or a Cirrus or a piston King Air. So far the last 60 years or so, designers of many types of piston planes have thought that the extra weight and expense of a fliter system is worth it. And next time you go to a show, Corsicana, Ellington, etc. try what I wrote with the wet towel at the end of the day. Heck, some places like Reno or Midland everything is coated in dirt.

You make a good point about the oil samples. Obviously one can operate a 51 without filters and get by, especially with frequent oil and filter changes. But even a normal oil sample does not say "NO DIRT", it just says not an excessive amount. Have you compared oil samples from the same plane operated with and without filters? I suspect the one sample you got too high in the Spitfire is because the intake is so direct and if you taxi or takeoff with the door open and the filter bypassed you give the dirt a direct path. Also some new Spitfire pilots think opening the ram air door is going to cool the engine more on the ground. It won't, the air door has nothing to do with the radiator cooling. Maybe Addison is so surrounded by tall buildings the dust doesn't get through, but there is certainly dirt at Osh or Leeward Air Ranch.

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Bill Greenwood
Spitfire N308WK

I know that Joe has the perforated plate, but does not have a filter behind it when we work on it, but not being any kind of an expert I don't know for sure what it does to engine operation or longevity. It is possible that his is not in the air long enough at any given time.

But Bill is right about dust and dirt everywhere.

As a photographer, I can see the build up on my camera's and lens's at "any" event, and we are not just talking about a light dusting either. A lot of this dust and dirt has silica in it, and I can imagine that it could play hell on an engine as it could on the glass of my lenses (same hardness factor since they are both essentially glass) if not cleaned properly.

What I have discovered is that Joes plane makes a very unique "sound" with these perf plates.....with the air moving past the openings, it has a very different sound in high speed turns.

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Supporter of AirpowerWest
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There's no doubt North American wanted the filters used when they designed the air induction system for the 51.
I can't speak to the Supermarine issue for the Spit but Bill I'm sure has opinions on that installation.
I can speak for the Mustang, or at least my own experience with it.
We used the solid plates and never had a single issue with crap being pulled through the engine. Oil inspections were ok and so were the annuals.

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Dudley Henriques
International Fighter Pilots Fellowship

There's lots of pics of 8th AF Mustangs with the solid plates in use over the filtered air intakes. IIRC, I read that was an icing concern, with the intake freezing over and then the mechanism for switching back to ram air being frozen as well, but whatever the reason the guys who flew and worked on them every day had no qualms about unfiltered air from nearly everyday ground ops on up to 30,000'. 'course, with Merlins and Merlin mechs/parts, etc., being readily available, why worry?

Wade

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If I recall, the only restrictions were potential issues above 12K in UFA with the hot air selected. It screwed with the high altitude mixture compensator

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Dudley Henriques
International Fighter Pilots Fellowship

Who's "Joe"???

Glen, Joe Thibodeau. Re the icing over the intake grilles, I had never read about that, and don't quite understand how that works on a Mustang. The air filters would only be in use for taxi, takeoff to 1000 feet, landing and taxi in at most, unless you happen to be flying in really dusty air like N. Africa. For all other normal operations the control would be set in ram air so the intake air would be coming directly from the front scoop and bypassing the filters. It wouldn't seem to make any difference if the side scoops were iced over or not. In cruise at altitude you would not be drawing air through the side grills into the carb anyway. My Mustang manual shows the plates to cover the side intakes "for cold weather operation" , but doesn't give much of an explanation. It seems a lot more parts and complicaton than a Spit. I have also not read of any problem with intake icing on Spitfires, and I don't know of any "hot air" control on them. Again, just like a 51 the air filter system is to be used on or near the ground( where the dirt is) and not in cruise or at altitude. There"s no hot air control on a Spit, though I guess when the filter is in operation you are drawing air from inside the cowling and its hotter than the outside ram air, maybe that is the key. Or else the N. American guys wanted a few more gadgets.

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Bill Greenwood
Spitfire N308WK