Time to make an engineering change. The oil is cooled with two oil coolers. The primary oil cooler is in the back with the engine. There’s a secondary oil cooler in the nose that’s used to cool engine oil and provide cabin heat. A factory supplied NACA duct is installed in the side of the nose, the oil cooler is mounted and another duct exits the bottom. Two 1 1/2″ tubes are inserted into the exit duct that supply warm(er) air to the cabin area. A small flap/diverter is installed in after the tubes that would block the air from exiting outside and instead directs it to the cabin. Here’s the problem: At cruise speed, the air is forced through the oil cooler at an incredible rate. And in the winter, the air at cruise altitude can 20 – 30 below zero. Given that, I think the oil cooler would increase the temperature of the outside air maybe 50 degrees. Which means that the cabin would be “heated” with 30 – 40 degree air.

So here’s the plan: I’ll put an additional flap between the NACA duct and the oil cooler to block the outside air. Then run a return line from the cabin to the oil cooler. That way I’ll be heating inside air that already warm.

This plan requires fabricating almost all the parts.

Here’s the plan view. On paper.

I’ll make a form out of foam and cover it with fiberglass. I used a spreadsheet based the formula for the NACA design developed by the NASA predecessor in the 1950’s.

Here’s the form made from glued blocks of foam.

This is the result. Needs a little cleanup but not bad.

This is the duct for the air after it goes through the oil cooler and then out the bottom of the fuselage. I had to make a flange where the oil cooler mounts to it. Because the geometry of the design is different, I had to make a slight modification to the opening so it would cover the entire oil cooler.

Duct cleaned up with the flap that blocks outside air.

Flap open. You can see the inside return air port.

The flap mounts to the bolt that is filed down, drilled and tapped.

Now it’s time cut the opening for the NACA duct. This part always drives me nuts! I’m cutting a hole in the airplane! What if I cut it wrong? Oh well.

Here’s the mark for the opening

Inside view where the duct will go.

After the cut. Hope it’s in the right place.

Duct installed.

Inside view of the installed duct.

The whole assembly installed with oil cooler.

View from the outside with the door open and closed. You can see the oil cooler inside on the left picture.

Partially disassembled. This is the lower portion of the whole assembly (where the heated air exits the bottom of the fuselage. With this lower door closed, the heated air will be diverted into the cabin through two two tubes which will have flexible tubing that supply warm air to the pilot and co-pilot side of the cabin.

Same view with the flap open that blocks the cabin supply lines and exhausts the air outside.

Later, I’ll attach control arms to the pivots for the two doors. Then cables can be attached which will allow opening and closing the doors from inside the cabin