I’ve been doing a lot of research and discussion with folks who have installed air conditioners, especially the Flightline setup like mine. The general consensus is that it’s marginally effective at best. My standards aren’t extremely high, as the AC in the Diamond we fly just takes the edge off but doesn’t really cool like a car would. Others with the Flightline system have added a second condenser and one or two have routed the intake from the bottom to the side skin to avoid the exhaust mixed heated air. As my original commitment to John at Flightline was to design a side intake and exhaust, I decided to dive in and redesign pretty much the whole dang thing.

First, an overview of the setup. An intake on the bottom tail skin catches air and directs it through 6″ flex duct up and 90 degrees forward where it turns 180 degrees and goes through a 12′ square condenser. A 12″ puller fan helps air flow volume so it doesn’t rely solely on ram air pressure. Then, it heads back into a 6″ flex duct, makes another sharp 90 degree corner and exits out the bottom skin. It’s a lot of direction changing and restricting of the air.

I began by taking everything apart and checking out how it was all pieced together. The first thing I noticed was the mounting plate for the fan actually covered a 1/5 or so surface area of the condenser. Additionally, the system didn’t use basic air velocity and volume principles, so it was constantly going from small to large volume spaces with many restrictions.

My goal was to facilitate air flow through the shortest path with the least resistance, thereby providing more efficient flow and better heat exchange in the condenser. I studied several factory AC setups such as the Cirrus, Bonanza, and M350 and noticed no ducting on the intake and direct exhaust from the condenser through the skin. The condensers were also significantly larger in surface area. So the new design would be to pull ambient air from the tail cone through a larger condenser via the same fan and exhaust it through the side skin within the shortest path possible. The ambient air is provided through an louvered intake near the horizontal stabilizer.

So, I bought a 16″ x 12″ condenser and set about molding a plenum from the condenser to the fan. If I had a vacuum table, this would have been perfect to keep the mold to share with others, but unfortunately I had to lay up fiberglass and the molds were trash.

The layup came out very nicely despite destroying the mold and was a perfect fit around the fan casing. A simple bead of silicone should seal it up nicely when installed.

Next was to do a mold from the fan to the skin for the exhaust. I figured up the square inches of the fan and worked within the stringers to maximize the opening area while keeping it just under the square inches of the fan. My thought on this is using the Bernoulli principle and having the smallest (relative to the entire air path) opening creating the highest velocity at the skin. This will in theory help the exhaust penetrate the boundary layer air around the fuselage, which has also been a concern from others.

I used the same fan mounting plate since it’s no longer restricting air flow. The exhaust plenum as a lip molded in to use as a mounting surface on the skin. The weight of the setup will be supported from the stock tray so the plenums aren’t load bearing.

I put it all together and hooked up the fan to check it out. There is significantly more air flow going through and I’m cautiously optimistic that this will create a much better performing system.

Next up will be mounting it all in the airframe and cutting the skin.