Air Mixer

At the point of getting ready to close up the front of the plane, I ran into the issue of installing the heater ducts and vents. Originally, I had envisioned running the ducts along the center cover up to the fuel valve, where I would attach a panel to hold the vents.

That was a good idea until I sat in the plane and found there was simply no room to do that. In fact, it was turning out to be a bit tighter than I would have imagined.

The idea that immediately sprang to mind is to utilize the existing ducts and vents as provided by Van’s, and install a “mixer” to select either outside air or heated air, just like a car.

It seems that there aren’t too many off the shelf solutions that will fit two inch duct work. At least, none that are reasonably priced. The obvious answer was, if I can’t (afford to) buy it, then I will build it instead.

After browsing around the local Home Depot for about an hour, I found a number of things that looked like they might be useful. There were a few duct related items for household use that looked interesting, but most of the exhaust fans and dryer exhaust stuff is three or four inches in diameter and therefore not useful.

I knew the plumbing department would have PVC and ABS plastic “Y” and “T” connectors that might be useful. The problem here turns out to be one of finding the right size. Two inch pipe is not the same as two inch duct as far as connecting things together goes. Pipe is generally measured by inside diameter. Then, there are different kinds of pipe that have different thickness walls. The closest thing I found that might work is a 1 1/2″ ABS (black plastic) “Y” pipe fitting.

I enthusiastically bought one and went home to the garage.


It turns out that the angle of the “Y” portion is almost exactly the same as the NACA duct outlet, which means the “through” portion of the fitting runs straight in line with the fuse. The outlet side is just under the subpanel, which makes for a good support point.

Since this was looking good, I decided to go ahead and carry the idea forward. I figured that simply mixing the outside air and the heat all the time was no good, especially if the OAT is really low. So I need a way to block off the outside air and only allow heat in. A butterfly valve seemed like the easiest solution here.

I started off by cutting out a 1 5/8″ disc of aluminum for the valve.

Browsing through the junk hardware box, I got a three inch machine screw, some washers and some hex nuts to use as the valve stem. I marked the pipe in the center near the “Y” which would attach to the NACA inlet. Using a piece of tape helped to see the mark – the black plastic kind of absorbs the Sharpie marks.

To attach a cable to the swivel and allow control of the valve, I cut a short rectangular piece of aluminum to be used as a control arm. This piece gets sandwiched between a couple of hex nuts.

Using a piece of tape, the disc is held in place behind the stem. A couple of small straps are fashioned to connect the disc and stem, and everything is JB-Welded together. Just be sure to NOT get any JB-Weld on the pipe fitting, and don’t permanently attach the disc to the fitting!

Wait 24 hours for this mess to dry. Then, try swiveling the valve open and closed to be sure it is not binding. Make any adjustments as required. You may want to create a seat for the valve to seal off the NACA vent even better; use the same method as described by Van’s to seal their cheapy plastic eyeball vents.

As for installation, there are a couple of options. First, if you sand off the flange on the NACA duct with a Dremel tool or sandpaper, the expanded opening of the pipe fitting will slide over the NACA duct outlet, albeit very tightly. Another three inch machine screw through the fitting and the NACA duct will solidly hold these two in place, OR a solid attachment to the subpanel. I wouldn’t suggest gluing the two together as the valve may need servicing someday.

The duct hoses are also a very tight fit as the expanded ends of the fitting are just larger than two inches. However, you CAN get the hoses to slide on. Attach them there with some hose clamps.
A more elegant solution is to spend another $4 on a ten foot length of 1 1/2″ black ABS pipe and some ABS cement. This pipe is almost exactly 2″ outer diameter, so the duct hoses are a nice slip fit. In fact, I took this a small step further and bought a 90 degree street elbow. Note in the picture that the “street elbow” has an expanded end to act as the female fitting, and an end that is turned down to the O.D. of the pipe.

I used about a 5 1/2″ length of pipe and the elbow to bring the flex duct run a bit more direct to the heater box. On the other end of the “Y” fitting, I put in a 2″ piece of pipe to be used as the flange for attaching the duct to the eyeball vent. Here is a pic of the assembly before being installed in the plane.

And the final installation:

The least expensive locking control cable from AS&S is about $18, which is the most expensive part of the project. All in all, about $50 and a couple of days to put everything together makes for a nice clean installation.

Enjoy!