I Just Gotta Vent

One of the earliest decisions I made on mods was to replace the standard rear seat vents with a ducted system supplying air to an overhead console. While quite a bit more complicated, it seemed like a more refined and upscale solution. Many other builders have gone down this road, which includes installing NACA vents behind the baggage compartment and running SCAT (or similar ducting) to a remote-control valve to control the airflow. This then feeds air into the console cavity where passengers use additional vents to direct air onto them as desired, similar to an airliner. In theory, you could dispense with the master valve, but I’ve read that the pressurized air in the overhead console can sometimes leak out, which is less than ideal with sub-zero OATs. Since I have been working on installing items in the tail cone before it gets rivetted to the fuselage, now was the perfect time to attend to this task.

I had long ago ordered the NACA vents from Van’s and, in a fit of muddle-mindedness a couple months back, had confused them with those that come in the finish kit and get installed in the forward fuselage, behind the instrument panel. Having committed to my confusion, I believed I thus possessed two sets of vents and match drilled them for riveting to the forward fuselage and set them aside. (Note that the plans call for securing the vents with ProSeal only but I, along with a few other builders, don’t trust the smelly goop and have elected to add some additional reinforcement.) When it came time to work on the rear vents, I spent more than a reasonable amount of time looking for the “other” set of vents before believing them lost and deciding to use the “forward” set. It wasn’t until later that I realized I had dreamt up the second set all along (the forward vents are not even the same parts). The challenges with getting old, I guess. Anyway, what was I talking about?

Let’s Cut Some Holes!

Ah, yes–installing the rear vents. The big decision with installing the vents is placement. Most other examples I’ve seen feature the vents installed centered vertically in the section of fuselage skin just aft of the baggage bulkhead, below the fuselage longeron. This is roughly on the same through line as the forward fuselage vent. Variations arise based on how far aft to go. Photos taken during my “research trip” to Oshkosh in 2021 provided numerous reference examples:

I decided to follow other builder’s examples and install the vent more towards the baggage bulkhead. This also avoids a sharper bend in the ducting, assuming you want it to stay in front of the next aft bulkhead.

To locate the cutout, I first created a template using scrap aluminum (cut from the forward fuselage tunnel when installing access covers) and tracing the cutout in one of the forward fuselage skins. I then drew centerlines on both the template and the empennage section and transferred the pattern to the skin, covering the skin with masking tape to protect against boo-boos:

As you can see, I located the forward edge of the cutout six inches aft of the empennage skin edge.

Next came the delicate part–cutting the holes. One reason I’d waited to perform this mod is the intimidation factor of cutting into perfectly good aircraft skins that are already part of the finished structure. If I’d been the original builder of the empennage, I would have done this prior to riveting the skin but, since I “inherited” the component, I had to execute the cuts in situ. Fortunately, cutting the template gave me a chance to practice on a (albeit thicker) piece of aluminum without consequence.

I cut the hole by starting with a Dremel and fiberglass cutoff wheel and finishing with a fine coping saw blade, a sanding wheel on the Dremel, and some files. I was careful to trim up to the line in increments. I then test fit the vent and marked any areas with too much overlap. The final hole closely matched the contours of the composite vent. From there I match-drilled the skin using the holes I previously drilled in the vents and gave the setup a test fit.

Some days later, (after finally deciding to prime the inside of the tail cone), I dimpled the skins and attached the vents using ProSeal and AN426 rivets. I followed Rodrigo Damazio Bovendorp’s lead and made some aluminum backing plates to take some of the compression from the rivets (though I elected to make these in two pieces, instead of one.

More Holes! More Holes!

The next step in the process was to cut holes in the baggage are bulkhead to enable air to flow into the overhead console. This was a bit tricky because I don’t yet have a console and thus needed to estimate the hole position. (The size is set by the vent flange from Van’s). By examining numerous photos and estimating the outline of the console’s rear profile relative to the bulkhead, I was able to choose two locations I believe should work. My fixation with symmetry placed one hole close to the row of rivets at the top of the bulkhead, with the opposing hole offset to be equidistant around the centerline. Was this critical? Of course not, given no one will see these holes in the finished product, but it made my overactive synapses very, very happy.

Because the tail cone had not yet been riveted to the fuselage, I was able to remove it and use a fly cutter mounted in my drill press to perform the cuts. I have read that some builders consider fly cutters to be possibly the most dangerous shop tool out there, with a razor-sharp blade that’s nearly invisible when spinning and just waiting to slice through a wayward appendage. However, I found it quite simple to use, even in a cordless drill, and the quality and precision of the cuts to be outstanding. You just need to go slowly until the blade just barely cuts through the material. The resulting hole is perfect and smooth (though the resulting scrap does tend to have a very sharp edge).

I also performed the addition step of flanging the holes to give them a bit more strcngth, given the resulting thinness of the bulkhead in that area.

Taking (Vent) Control

The final step (for now) was to figure out a way to mount the Aerosport Products vent valve. However, let me begin by saying I didn’t really want to use the valve, or at least pay for it. No offense to the good folks at Aerosport, but $300 seemed like a lot to pay for some plastic and metal bits, an R/C servo, and some electronics. I have played around with servos and controllers and was confident I could manage to come up with some DIY electronics. The question was the housing. I went so far as to design my own version from scratch in SOLIDWORKS and 3D printed some test parts. I even came up with what I thought were enhancements, including incorporating miniature bearings for the vent flapper rods.

I learned two things from my efforts:

  1. It was a very effective and satisfying way to teach myself 3D modeling in SOLIDWORKS.
  2. I have neither the time nor patience to fine tune a design for a one-off component I can just go and buy.

Well played, Aerosport, well played…

So, with the sound of a far-off cash drawer ringing in my ears, I started envisioning a mount for the (nicely done, BTW) Aerosport unit. As with other builders, I intended to mount it to the upper centerline j-stiffener, adjacent to the bulkhead flanged ducts. The challenge is that there is not much metal to grab onto. Some builders have repurposed other parts, like fuel pump brackets, but I wanted something more purpose built. I also wanted it to be easy to remove the valve assembly if needed down the road. I have also learned (the hard way) that it makes sense to plan out any complicated part(s) that I intend to fabricate.

That sent me down another SOLIDWORKS rabbit hole learning how to use its sheet metal functionality. I started by modeling the plate that would sit on top (or bottom, depending on which side you think is up) of the valve assembly and provide lateral support via two 90-degree bends. This required positioning two holes for the flapper rod caps. This would connect to two L-shaped vertical brackets that, in turn, would bolt to a third bracket riveted to the j-stiffener. I modelled each of these as a part in SOLIDWORKS and then pulled them into a final assembly (including a section of j-stiffener):

The virtual version looks so refined.

Note that I chose to model bolt and rivet holes but would locate and drill these manually during final assembly.

One handy feature of designing sheet metal components in SOLIDWORKS is that it can show a “flattened” view, making it easy to get dimensions of raw materials. This came in extremely handy on this project and I was able to get all the aluminum pieces (I used .040″ stock) roughed out, drilled, countersunk (for flush rivets), and bent in no time. (You can see in one of the images below that I used a shim to set the spacing between the two L-shaped brackets.)

I used the existing rivet holes in the empennage j-stiffener to match drill into the upper mount and drilled holes for additional AN470AD-4 rivets as well as screw holes in the L-brackets and valve assembly (no returning it now!).

After the primed parts were dry, assembly went pretty quickly. I riveted the L-backets together (using the spacer again) and screwed them and the bottom bracket to the vent valve assembly using AN515-8R8 screws and matching washers and locknuts. I riveted the top bracket to the j-stiffener using AN470AD-4 rivets (the top rivets will get set when the top skin goes on) and attached the top and bottom bracket assemblies, also using AN515-8R8 hardware.

Overall, I was pretty pleased. The mount looks well designed and has decent rigidity that should improve once the top skin is riveted in place, and the assembly is easily removable by undoing two screws.

Build Date(s): 22-Jan-23
Build Time: 7 hours

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