Step 23: Cabin Air Vents
I thought I could short-cut the process by buying a ready-made vent system from Aircraft Spruce. Unfortunately, I couldn't find one to fit. The NACA scoops were too big, too small, or just didn't suit what I was trying to do. So I resorted to making the parts myself. Yep, these babies are beautiful and the system will work perfectly. But it took 30 hours to build everything. If you have a choice, take the short cut and buy a ready-made kit.
Cabin Air Vents
I'm using the plastic vents that came with the chapter kit. Simple, cheap, effective, lots of air flow on those warm summer days! I chose to mount my vents in the extreme upper outboard corners of the instrument panel because (1) these vents leak a little air even when closed off. I didn't want cold air blowing onto my flying hand all the time like it does in the plans location; (2) the plans location interferes with my canopy latching handle; and (3) I wasn't going to put instruments or switches there because these areas of the panel are hidden from view by the removable top deck. So installing the vents there made the most of the real estate I wouldn't have used otherwise.
The bad part about this location is that the vents are now located higher than the NACA scoops. It's no longer a straight shot out of the NACA scoops and into the air vents like it would be if I followed the plans. Even a rocket scientist knows you can't get scat hose to bend that radically. Scat hose is flexible, but not that flexible !! My solution was to fabricate some manifolds that cover the back of the vents and provide the flanges for the scat tubing the will come from the NACA scoops.
The diagrams explain how I made the manifolds. The key was to use 2.75-inch PVC plumbing couplers from Lowe's and styrofoam balls from the craft store as molds. The couplers match the diameter of the flange on the back of the vents. The styrofoam balls provide a rounded, hemispherical shape that will make it easier to locate and graft on a flange for the scat tubing the will come from the NACA scoop. It'll become obvious what I mean by that a little later on. I laid down 2 plies of BID onto a board first covered with plastic and peel ply. I wrapped the couplers in box tape (for mold release), wrapped the couplers with wet peel ply, then wrapped the couplers with 2 plies of BID. I placed the couplers onto the 2 plies of BID and joined everything using small strips of BID as structural fillets. I applied 2 more plies of BID over the board BID to help integrate the coupler BID to the board BID, and to increase the thickness. The first picture shows how I used wooden semi circles to force the structural fillets to form 90-degree corners and to force the flanges to cure flat. I used sockets to weight down the semi circles. Once cured, I removed the couplers and trimmed the manifolds to size. To enclose the manifolds completely, I stretched some saran wrap (for mold release) over the some styrofoam balls, shoved the balls up through the manifolds, and glassed 2-BID over them. Once cured, I dug out the styrofoam and smoothed the edges. The second and third pictures show how the vents and manifolds interface with the instrument panel. Yes, I had to hog out the foam on the back side of the instrument panel to get the vent to swivel properly.
The vents and the manifolds are held in place with clickbonds that are floxed onto the back side of the instrument panel. The fourth and fifth pictures show my Rube Goldberg approach to holding the vents in place while the flox on the clickbonds cured. I wrapped the clickbond threads in tape so they remained free of errant flox. I also applied electrical tape onto the vents for mold release.
I located the scoops per the plans. I didn't like the shape of the NACA scoop template provided in the plans. So I elongated mine by about 3/4-inch (I made the nose (entrance) longer) and shaped it to be more like the theoretical shape it's supposed to be. I carved two male molds from urethane foam and screwed them onto a board covered with box sealant tape (for mold release). I glassed them with 2-BID and once again used forms weighted down to hold the BID down to form the flanges I'll use later for floxing the scoops onto the side of the fuselage. After cure, I cleaned up the scoops and trimmed the flanges. I traced their footprints onto the inside of the fuselage, hogged out the foam, and checked the fit of the scoops to the fuselage cutouts. So far so good.
Getting the Air from the NACA Scoops to the Vents
Part 3 of this little endeavor is determining the tube orientation from the NACA scoops to the manifolds, then glassing the flanges onto the manifolds and scoops. I took some 2-inch plastic tubing and some 5-minute glue. The first picture shows how, by eyeball, I lined up the tubing between the manifold and the scoop, then glued the tubing to the manifold. Once the glue was set, I removed the manifolds from the plane and glassed on 2-BID flanges over the tubing. Once the flanges were cured on the manifolds, I took a longer length of this same tubing and determine exactly where and how to cut through the back of the NACA scoop. That's shown in the second picture. Once the tube angles were set, I glued the tubes onto the NACA scoops and removed them from the plane. I spread bondo onto the outside of teh scoops and tubes to form the transition areas . I shaped the transition areas, applied tape (for mold release) over the bondo, then applied 2-BID to form the transitions and flanges. The third picture shows this. Once cured, I reached inside the scoops, scraped all the bondo away, and sanded the insides nice and smooth. I filled, sanded, and primed the insides of the scoops, then micro'd the scoops permanently onto the plane. I did this with the plastic tubes and manifolds installed so that everything was held in perfect alignment while the micro cured.
Cosmetics, Cosmetics, Cosmetics
For some reason, the shape of the scoops did not match up exactly with the NACA cut-outs I made in the fuselage skin. They were wider in their mid sections. So I added a little micro inside the scoop and contoured their insides to match the NACA cutouts. I taped these metal templates onto the sides of the plane to act as edge guards. They kept me from altering the shape of the NACA cut-outs while I contoured the micro. I applied sacrificial primer to the scoops and sanded them out. Here are the results. Now they wait for final primer along with the rest of the plane! Time to move on to something else!