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Baffling Your Lycoming Part 1
By Tony Bingelis (originally published in EAA Sport Aviation, October 1986)
Some 13 years ago, I wrote an article entitled "Engine Baffles For The C-85 And The 0-200 Continentals". It appeared in the October 1973 issue of Sport Aviation. So, it seems only fair (and about time) that I accorded the popular 4-cylinder Lycoming engines the same, if somewhat belated, attention.
Actually, broadly speaking, there is very little difference in the way 4-cylinder Continental and Lycoming engines are baffled. The Lycomings, because of their large starter ring gear, require a slightly different baffling up front. Then, too, while the Continental side baffles are usually made in one piece, the Lycoming practice is to make two separate baffles for each side. There are other subtle differences - like points of attachment (Lycoming has more of them), and the design of the inter-cylinder baffles.
I will confine my baffling expose to conventional tractor installations. By that I mean those installations that have the propeller up front where you can see it if it stops.
As you know, in a conventional 4-cylinder air cooled engine installation, the cooling air is allowed to enter the cowling just behind the propeller through inlets, and into the chamber-like space above the engine. However, we know there is more to this business of cooling an aircooled engine than merely causing air to flow into the engine compartment and past the engine.
The amount of cooling the engine would get from such a haphazard flow of cooling air would be inadequate as the incoming air would be spewed all over the engine compartment, and only a small portion of it would make its way down through the cylinders' cooling fins. For this reason, it is necessary to install barriers (baffles) inside the engine compartment, so designed as to force the cooling air down through the cylinder fins. In effect, this baffling limits the amount of air entering the engine compartment to that which will effectively cool the engine. That, in turn, reduces cooling drag and the wasteful use of engine power for ineffective cooling.
Your basic objective in baffling the engine, therefore, is to provide a pressurized "leak-proof" chamber in which the cooling air has no alternative but to exit down through the hot cylinder fins, picking up engine heat as it scoots on out through the air outlets provided. In doing so, the engine will benefit from the maximum potential cooling for a given atmospheric temperature and airspeed. Although this is a very simplified, very broad generalization of the concept, I think it conveys the basic idea of what this baffling effort is all about.
The New Engine
Take a good overall look at that engine of yours. A new engine, or one that has been freshly overhauled by an engine shop, will be as naked as a new born babe. It will, undoubtedly, be a basic engine unit (I hate to use the term "stripped") uncluttered by the non-presence of a lot of the goodies you will need. It most probably will not have any baffles or attachment brackets - with one exception. Your engine should already have the inner-cylinder baffles installed. These are specially designed and fabricated baffles which are installed at the Lycoming factory before the engine leaves the premises. These baffles (between the cylinders) always stay with the engine. However, to be sure you have yours, check your engine to see if they are installed, and are in a serviceable condition. Your engine must have these baffles installed or it won't cool properly no matter how effective your other baffling might be.
The Used Engine
A used engine can be just as skimpily equipped and as barren as a new engine. On the other hand, you might be fortunate in having an engine that is already equipped with baffles. Chances are though that they may not be in very good condition. Some of the baffles may fit poorly and you may even find a crack here and there. Also, there is the likelihood that the installed baffles are too short in some places and too tall in others. Still, a set of baffles - good or bad - can be a valuable resource for you.
A serviceable set of baffles that happens to be too tall can always be trimmed down with a pair of aviation tin snips. On the other hand, baffles too short for your cowling may have to be replaced. If you don't want to go to all that work, you could modify them by riveting extension pieces to bridge the gap. It won't look as nice as a single piece baffle but most likely nobody will notice it or comment on it.
At any rate, don't throw away that old set of baffles even if they are not airworthy. At the very least, you could clean and disassemble them and use them for patterns.
When you do make your new set of baffles, don't forget to allow extra material along the top edges where it may be needed. While you are at it, take the opportunity to improve the fit of each individual baffle. Just because the baffles were factory made is no assurance that they were originally fitted to as close a tolerance as many an enlightened homebuilder sets for himself.
On Buying A Set of Baffles
Some kit producers and independent entrepreneurs provide baffle kits for a particular engine and, perhaps, a specific aircraft design. Of those that I have seen, a number of them are rather crude and relatively unfinished. They appear to have been roughly cut and have edges and corners that have not been deburred. You probably would be better off to make your own rather than order such a set. Other baffles I have seen are well finished pieces exhibiting good workmanship - and quite complete - with predrilled or marked attachment holes. Baffles like that would probably be worth the extra price charged. In other words, I would infer that it might be a good idea to check with other builders to find out just what you would be getting. Avoid buying a pig in a poke (you wouldn't want it on board anyway, would you?).
Thoughts About Baffle Design
There is very little variation in the design of the engine baffles you see installed in the general "Wichita Fleet". The differences, for the most part, seem to be up front where the cooling air enters the cowling. Some installations employ a level or a gradually sloped, ramp-like, baffle which causes the air to flow smoothly up to the cylinders and thence into the chamber above the engine. Further down the flight line you are sure to see an installation where the front of the engine is blocked by a perpendicular wall of baffles which appear to be capable of stopping the cooling air in its tracks. To me, it would seem that such an installation would be more appropriate for a bull dozer rather than a fast airplane. Still, these manufacturers are an enterprising lot and I am sure they have their reasons for that sort of turbulent entry for the cooling air.
Lycoming engines have a peculiar design feature - a huge starter ring gear impaled on the crankshaft smack-dab behind the propeller. It effectively interferes with streamlining efforts and complicates cowling design because it gets in the way of the cooling air. However, thanks to large spinners and prop extensions, this blunting effect can be greatly minimized. It does, nevertheless, introduce a baffling challenge in that it is very difficult to seal the area behind the starter ring gear and the crankcase sufficiently enough that the cooling air doesn't take a shortcut and bypass the cylinder fins.
Accurate Baffle Templates
It is difficult to make a complete set of baffles without the benefit of full sized patterns or templates. Unless you do have access to a set of templates, I'm afraid you will have to make your own.
Baffle templates are important if you want to save time and avoid wasting expensive aluminum. Without the use of templates, you may find, to your disgust, that you have to remake several baffles before you obtain an acceptable fit. The most aggravating difficulty will stem from your efforts to locate the baffle attachment holes. If you guess wrong, you will have a baffle with a badly elongated hole or two. Elongated holes are an embarrassment to the viewer and to the owner alike. In a baffle it is not a critical structural matter usually . . . just embarrassing.
You can make a full sized set of templates by starting with the baffle patterns illustrated in Figure 1. Simply draw a bunch of one inch squares across a manila folder and trace the lines as shown, from one square to another. All of the baffles can be laid out in this manner quite quickly.
Use manila folders as your template material unless you intend for them to be used many times over. In that case, a set of aluminum templates would be better. The light cardboard quality of a manila folder makes it easy to cut with scissors.
After cutting out a rough pattern, place it on the engine to check its fit.
Snip away any tight areas with scissors until the template fits perfectly. If parts of the template have been cut in too deeply, don't worry, the fix is easy. Just bridge those gaps with small pieces of masking tape. Continue this cut and patch process until you are satisfied with the fit.
Next, tape that corrected template to the engine with pieces of masking tape in preparation for locating the baffle attachment holes. How would you do it? You can't see through a manila folder, can you? I have tried probing around for the underlying hole with an ice pick or a metal center punch with some success. After locating the right spot, I would poke the ice pick through the template and into the hole. This really marks the hole's location all right. But, somehow, I have never been able to drill all the baffle holes accurately from this template. One problem, I suppose, is due to the ragged edges around the punched hole which makes it difficult to transfer the exact center of the hole with a punch to the metal baffle. As a result, after the holes are drilled, one or two may be off enough that they must be elongated slightly before the fasteners will go in.
Here is a more accurate way to locate your baffle attachment holes. Round the end of a short piece of dowel and use it to rub over the cardboard template in the area where the hole is known to be. You will feel the thin cardboard give way slightly when you pass over the underlying hole. Stop right there and twirl the dowel between your fingers over that hole location. As you do, the sharp edges of the screw hole will cause an embossed image of the hole to appear on the template as a beautiful round circle.
Making the Baffles
Remove the template from the engine and lay it over a piece of aluminum. Most builders use either .025" or .032" 2024 T3 aluminum for their baffles. Some prefer the softer 6061 T4 or T6 because it is more readily available locally. Then, too, they feel the softer material is less prone to crack.
Tape the baffle template to the aluminum blank with a few scraps of masking tape to insure that the template will not slide while you are tracing it. Trace the template's outline onto the aluminum with an indelible ink marking pen. (A SHARPIE pen is perfect -most stationery counters have them.) After the tracing is completed, carefully make your center punchmarks through the template every place you have an embossed hole location. Drill the baffle attachment holes with a 1/4" bit.
The easiest way to cut out baffles is on a bandsaw fitted with a 1/4" metal cutting blade. You can cut aluminum with the blade running at regular speed.
Baffles can also be cut out with aviation tin snips. The thin metal will be easy to cut but the process will take longer. This is because tin snips leave little crimp marks along the line of cut. These must be removed by filing as they might become starting places for cracks. Leave a good 1/16" extra margin when using tin snips.
All edges should be deburred or filed smooth with smooth cut files. You will probably use a flat file, a half round file and a small round file in finishing the baffles. If a lot of material must be trimmed away, a rotary file chucked in a drill press can do the job quickly - but be careful.
I would suggest you temporarily install each baffle as you complete it to simplify the fitting of the adjacent baffle.
When making some baffles you may find it necessary to cut a few long slits to separate the portions of the baffle that must be bent to curve around the bottoms of the cylinders. These slits, or cuts, can be made with a hacksaw fitted with a fine tooth blade. Cut the metal baffle with the blade tilted to a shallow angle in order to obtain the straightest cut.
When a portion of a baffle must be bent to form a corner or a similarly difficult bend, it is necessary to drill a relief hole at the junction of such bends. Use a 3/16" drill. Chamer each drilled relief hole with a larger diameter drill bit held between your fingers and twirled lightly over the hole to remove the sharp edges and chips. Do this faithfully for each hole you drill in the baffles.
Radius of Bend Reminder
Any bends to be made in the baffles should have a rather large radius to avoid overstressing the stiff, somewhat brittle 2024 T3 aluminum. Those using the softer 6061 T6 can make their bends somewhat sharper. This is really no great problem when working with thin material (.025" or .032"). However, the bend radius should be no less than 3 times the thickness of the baffle material. That would be, say, .075" radius. This is about the same radius you would get if you were to bend the metal baffle around a 5/32" drill bit or a rod for a 90 degree bend.