Sport Aviation 11/86
By Tony Bingelis
The final and permanent installation of the various baffle sections can best be done with the engine installed in the aircraft - before you begin to install and hook up any of the wiring or plumbing. This will afford you the advantage of having good access to all parts of the engine while you work on the baffles.
If it is more convenient you could, almost as well, fit your metal baffles to the uninstalled engine while it is still in your workshop.
NOTE: A mobile engine stand similar to the one described in the November 1978 issue of SPORT AVIATION (page 40) would make an ideal support base for the completion of the baffle installation.
Now would be a good time to make those small final adjustments to the fit of each individual baffle.
Does each baffle fit snugly against the crankcase and against its adjacent baffle? Be prepared to remake any piece that doesn't fit closely. By closely, I mean like a zero tolerance against the crankcase . . . well, maybe a 1/16 inch gap can be accepted for the moment. You should realize, though, that allowing a 1/8 inch gap here and another there can soon add up to several square inches of air leaks.
It is essential that every bit of the air entering the engine compartment be used effectively for cooling and not be allowed to escape without doing its part. Any air that blows past the baffles without being forced down through the cylinder fins equates to so much wasted engine power. Not only that, you may find yourself with a serious engine cooling problem on your first flight. Well, so much for the halftime pep talk.
Baffle Assembly and Installation Tips
Lycoming engines have a generous number of holes tapped into the engine at strategic locations strictly for the purpose of attaching baffles. These baffle attachment holes are approximately 1/2 inch deep and are tapped to accept .250-20NC-3 x 1 /2 inch cap screw fasteners. Even so, it might take a bit of ingenuity to design the type of bracket you need to take advantage of the nearest baffle attachment hole(s).
Strive for a rigid installation. The more rigid your baffle installation, the less likely that you will ever experience baffle cracking problems due to vibration induced fatigue in the thin metal.
Along this line, Lycoming has a good method for immobilizing the curved baffle ends that curve around the bottom of the cylinders. These curved baffle ends are joined together by a baffle tie rod which holds the baffle snugly against the cylinder fins (see Figure 1). You may find it necessary to make and rivet small stand-off brackets to the baffle ends as shown to be assured that the baffle tie rod clears the oil return line from the rocker box.
Holes and Openings?
1. The two openings through which the ignition wires must pass. These openings must be large enough to allow the connector fittings to pass through. The holes are usually sealed with standard plastic diamond shaped grommets screwed to the baffle.
2. The two holes for the magneto cooling blast tubes. Without this cooling air for the magnetos they could suffer failure due to extremely high magneto temperatures. Bendix magnetos, for example, cannot tolerate temperatures much over 350 degrees F before suffering deterioration and possible failure (see Figure 2).
3. The two large holes for the engine shock mounts. These holes, fortunately, are almost completely taken up by the shock mount assemblies.
4. The large opening for the oil cooler, when located in a back or front baffle.
5. The hole for the cooling air blast tube to the generator or alternator.
6. The large 3 inch to 3-1/2 inch opening in the rear baffle for ducting air to the carburetor or fuel injector (sometimes).
7. The hole for the ducting of cooling air to a shrouded engine fuel pump (optional).
8. An opening for ducting cooling air to the cabin (optional).
That is a surprising number of holes, isn't it? Some baffle installations have them all. Fortunately, a number of them are not essential and your installation could probably do without them by making alternative provisions. Anyway, you should seriously question the need for some of them before punching your baffles full of holes.
Oil Cooler Location and Installation
The simplest and lightest installations are those with the oil cooler mounted in the left side rear baffle. This is because the connecting lines to the accessory section are the shortest and most direct from this kind of installation. Another advantage this location offers is that the oil cooler is easier to mount securely. Incidentally, if you use this location, modify your aft left side baffle so that it will space the oil cooler back about one inch (or slightly more) from the number 4 cylinder fins.
Mounting the oil cooler in the front left baffle has the disadvantage of being more difficult to position effectively. I've seen some installation where the oil cooler was partially blocking the inlet air. In addition, it also dumped the hot air from the oil cooler into the upper engine compartment . . . but I don't imagine that you would do anything like that. Of course, the up-front location will require the installation of much longer oil hoses. When the up-front oil cooler is mounted flush with the inlet deck baffle, it may not get sufficient air flow through its radiator. I guess some builders have had this problem because you are likely to see this type of installation modified with the addition of a fence-like deflector riveted on the downwind side of the oil cooler radiator inlet. This I suppose is to help cram more air into the oil cooler.
Mounting the oil cooler remoted to the firewall is the fussiest of the three arrangements and is the most difficult to make. It will also result in a heavier installation because special brackets, ducts and shrouds will probably have to be fabricated to direct the air into the oil cooler. The air outlet side of the cooler must be open (never flush against the firewall, for example) so that the air can freely pass through.
I have been told that the firewall mounting of an oil cooler has another unfavorable characteristic. It apparently can cause increased noise in the cockpit due, I guess, to a resonance generated that apparently transmits engine noises to the firewall. This may or may not be true for all such installations but it is worth checking into.
No matter where you mount your oil cooler in the baffles, that area should be reinforced with doublers and/or aluminum angles.
Sealing the Baffles
Standard baffle sealing practice calls for a generous space between the metal baffles and the cowling. This space is then closed by affixing a flexible baffle strip along the upper edges of the baffling. A special baffle seal strip material is used for this purpose as can be seen in the accompanying photos. Although some builders use a wide felt strip, the material most frequently installed is some variation of asbestos with Nigen Hi-Lo temp cord impregnated with Neoprene rubber. The material is listed in homebuilder catalogs as "Baffle Material" or perhaps as "Cowl Gasket Material". It is most commonly sold as a strip 3 inches wide, 1/16 inches thick and approximately 9 feet long. A total length of about 9 to 11 feet will be required for a typical Lycoming installation. A lot depends, of course, on the baffle design and the width of the baffle seal strip that must be installed. The full 3 inch width of the baffle seal strip is seldom necessary but the extra width is handy when cutting out curved strips. An installed baffle strip about 2 inches wide would be sufficient to seal most baffle-to-cowl gaps.
Attach the baffle seal strips in any manner you prefer. That is, you can lace them to the baffles with safety wire, staple them on or rivet them on. Actually a riveted seal strip installation is the best looking and is the easiest to install. You can use a thin metal backing trim strip over the baffle seal fabric if you like but this is not necessary . . . nor are washers under the rivet heads.
Here is something else to think about. You may, someday, have to remove your baffles. By planning ahead you can make that job easier. Install your baffle seal strips with overlapping joints every place the baffles would have to be separated for removal. To do otherwise might force you to cut the baffle seal in order to remove a particular baffle. This could ruin the effectiveness of the baffle seal strip on reassembly.
Install the flexible seal strips so that they are curved inward toward the center of the engine compartment (see the photos for typical installations). This arrangement will insure that the inlet air pressure will cause the seal strips to press tightly against the cowling making the chamber virtually free of air leaks along the cowling . . . even when the cowling bulges slightly due to the inside air pressure.
Well, that takes care of sealing the space between the baffle and cowling, but how can you seal gaps between the bottoms of the baffles and the crankcase surfaces? Many builders used to seal these areas by laying a bead of clear RTV Silicone Adhesive around all baffle edges and into gaping corners.
A newer sealant material is now replacing that clear stuff. The newer sealant is still a Silicone based product, but it can tolerate higher temperatures. One such product is Dow Corning's RTV Silicone Rubber Adhesive Sealant, Silastic 732 RTV. Another is Permatex's high temperature Silicone "Form-A-Gasket" Sealant. It, too, is a RTV (red) easy-flow substance that will skin in 15 minutes and cure in about an hour. It can tolerate temperatures up to 650 degrees F. The fully cured Silastic 732 RTV adhesive/sealant is good for temperatures up to 450 degrees F for extended periods of time.
These new products are good news for the homebuilder as he can now use them in a number of places in the engine compartment with greater confidence. (They are even being used to seal around the holes in firewalls.) One other benefit gained from using these sealants is that they tend to eliminate the tendency for the thin metal baffle material to vibrate. How's that for getting your money's worth?
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