Front Engine Oil Cooler Duct

Over the last many days, I have worked on the front engine oil cooler duct. It is a simple part, but has taken a few cure cycles to make. I broke the part down into multiple pieces to make it easier to fabricate. Just like the rear engine diffusers, I decided to make the duct in two halves, then join them. This makes the cloth placement and wrapping easier. It also makes de-molding from the plug easier. Not only is de-molding from the plug easier, but it also allows de-molding without destroying the plug in the case that you need to remake one of the halves or need to make a small change and would like use of the plug over again.

The first task was to decide where to put the oil cooler. I decided to hang it off the engine mount behind the left bank of cylinders. This is a common location, and there is an “oil cooler-sized” volume there. The cooler is suspended from the engine mount using adel clamps. The ducting will be structural in that is will also support the weight of the oil cooler. The flanges on the duct that create the seal will do double duty and support the cooler flanges. The duct in turn will be secured to the engine mount via a strut and adel clamp arrangement. This will in effect rigidly mount the cooler. The duct/strut support will turn the cooler from a cantilevered arrangement into a fully supported accessory.

The duct shape was defined by using splines to make poster board templates. One representing the opening in the back of the baffle, one representing the face of the oil cooler, and the third representing the “best” aerodynamic and expanding path between the two. The poster board was refined and then I made the three foam core profiles. These could then be hot glued together and filled with insulation foam and pour foam to allow me to carve the shape.

From there it is the same as you saw with the rear engine diffusers where I cover with vinyl tape, wax, and spray with PVA mold release. Then layup the first half. After the first half cured, I released it, put it back in place, and then prepared for the second layup. Once I had both halves, on the mold and cured, I drilled in 4 places for clecos that would allow me to locate each piece back in precisely its designed location. The two halves then get epoxied together with the clecos in place.

When the two duct halves were one, the assembly was checked for fit. Everything was well, so I applied mold release tape to the oil cooler and hot glued the duct to the cooler in 4 small places about the size of a pea. Then I was able to do a “match-Layup” to the cooler. I did all 4 sides of the cooler. Two of which are the mounting flanges, the other two are the drawn cups of the cooler. This both gives me precise mounting flanges as well as precise sealing flanges. Once cured this was again released from the cooler.

The mounting flanges get reinforced with phenolic to help with the crush loads of the mounting bolts. I have yet to drill the holes, but for the most part, this completes the oil cooler duct. I may still put one ply of carbon over the whole thing. It does seem somewhat light, being only 2 ply. It is under internal pressure, and is certainly adequate for that, but I think one more ply of carbon would give it a bit more rigidity and do a better job of transferring load to the eventual mounting strut.

I will likely also add an exit duct at some point too, directing the flow towards the cowl exit. In addition there will be some mounting details that will keep the oil cooler from fatigue cracking and provide the flexible coupling between the baffles which are mounted to the engine and the cooler/duct which is mounted to the airframe. I will cover that in another post once it’s ready to be mounted.

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