This weekend, Jesse and I worked on installing a replacement piece of baffling under the rear of the engine. This piece of baffling is suspended from the rear cylinder enclosure baffling. Its purpose is to fill the gap between the bottom of the engine and the rear of the lower cowling, thus sealing the area under the engine to create positive pressure.
While performing engine compartment checks for the annual, we discovered that the baffling had fractured where it was attached to the engine, which was causing it to bend backwards in flight, allowing air to pass through and out of the rear of the lower cowling. This reduced the positive pressure under the engine. On my last flight, my CHTs were 15-20 higher than normal, and the oil temperatures were 10 degrees higher than normal.
We used .025 aluminum sheet from Spruce to build the replacement baffle. Jesse created a cardboard template from the existing baffle after we removed it from the engine. The old baffle was in poor shape; it was clearly life-expired. This rear baffle does take a hammering because it has to "float" under the engine to seal against the cowling; thus it has to flex and move in flight, which eventually causes weakness due to metal fatigue.
While he optimized the new baffle template by creating a second template, I removed the silicone baffle edge sealers from the old baffling by drilling out the rivet heads. We also saved a number of small washers that are used as load spreaders for the silicone edge sealers. If you simply rivet the silicone material to the edge of the baffle without using the washers to spread the load, the rivet head will soon pull through the silicone.
We trial fitted the first and second cardboard templates under the engine several times, with and without the lower cowl being in place, to fine-tune the shape of the replacement baffling.
We then used the second template to cut a new piece of aluminum baffle using tin snips. The piece was trimmed to match the template, and then trial-fitted to the rear of the engine, both with and without the fitting of the lower cowls.
The new baffle, in addition to being riveted to the existing rear baffling around the cylinders, is also fixed in place by being attached to one of the alternator mounting bolts.
Once we were happy with the fit of the new baffling, I riveted it to the existing cylinder enclosure baffling. We also re-inforced one section that was narrow (it had to flow around the starter motor assembly) by riveting a second piece of aluminum behind it, doubling the thickness at that point.
Jesse then left to take care of other business, leaving me with the instructions on how to complete the attachment process for the silicone edging material. This took me several more hours. I was able to re-use the existing edging material.
(Hint: Use a #30 drill bit to drill the rivet holes. A 1/8 inch bit is fractionally too small, which will cause much muttering and cursing as it proves difficult to insert the rivets prior to using the rivet gun).
Once I had attached all of the edge sealing flaps, I then found that the cowling would not attach either using the camloks. The overall fit of the aluminum was too tight. I decided to complete that optimization process on Monday.
I worked for 2 hours on Monday evening on getting the lower cowl to fit properly around the new baffling.
The main challenge was on the right side, where the re-inforced section and the top right of the new baffling section were too low. This caused the cowling to be pushed to the right, which made it possible to do up a camlok on the right side, but not on the left side.
I started to Dremel aluminum off of the baffling edge, but soon realized that the tinsnips would do the trick far quicker. I ended up cutting back the descending section on the right hand side by 1/3 inch, and cutting some of the re-inforced area away, up to the attachment rivets in some cases. I also had to bend the top right hand side of the baffle forward to prevent it from colliding with the airdam lip on the inside of the lower cowling when I installed the cowling.
After 3 rounds of removing aluminum, I finally got to the point where I could do up all of the camloks and I checked that I could insert all of the cowling screws for the lower cowling.
I then removed the cowling and re-applied some RTV sealant where the baffling fits around the starter motor, and also checked and re-applied sealant to other areas that I had sealed up to stop air from escaping.
I now have a properly fitted new lower baffle. I hope that this will improve cooling effectiveness. The first serious test will be at the end of September after I have moved house.