Breathing is Good!

Installing Oxygen Cylinder Brackets

Call me old fashioned, but when it comes to operating an aircraft, I have always maintained that it is important to remain conscious. A key requirement for most carbon-based lifeforms that respire is oxygen. Most people take it for granted but if you fly in a non-pressurized airplane, it’s something that requires special thought. Furthermore, if you’re building such a plane you need to decide whether to furnish oxygen for you and your passengers or go all ‘BYOO’.

For some aircraft, especially those that fly “low and slow”, supplemental oxygen may not be a top concern. It wasn’t something I thought much about when I owned my Maule–until it was. During a long cross-country flight, returning to the Seattle area from the Midwest, I needed to climb to around 12,000 feet to clear a broken cloud layer that was “just chillin'” up against the west side of the Cascades. By regulation, I didn’t “need” supplemental oxygen (though I would have if flying under part 121 or 135), but it was high enough to feel the effects. As I was contemplating the intricacies of FAA oxygen regulations, I also found myself noticing how pretty the puffy clouds were and how everything was just so lovely and…in other words, the first signs of hypoxia.

Clouds and pointy rocks–what could be better?

When planning my RV-10 build I considered whether to install a built-in oxygen system, as some builders have, or to rely on a portable solution whenever I planned to fly at high altitude. My initial decision was to opt for the latter, partly due to being unsure how often I would really need oxygen and partly because of the cost of a permanent system. Things began to change, though, as I got further into the build. First, after pulling the trigger for pricy upgrades like after-market rudder pedals and fuel and brake systems, I became less resistant to spending large sums of money. I also thought more about the prospects of cross-country travel from our current home base in the Denver area. Facing east, the landscape appears as a flat table top for as far as the eye can see. Turn around and it’s a different story. It’s impossible to avoid the big pointy rocks if you want to head west and safe margin of error above terrain puts you squarely in the “oxygen required or highly recommended” zone. In the end, it was my wife who pushed me over the edge, reasoning (correctly) that, not only would oxygen come in handy, but a built-in system made sense for the trim level I was building towards. So, as always, when the spouse gives the go ahead to spend money, I didn’t let the moment pass.

Extensive Research?

The process of building a sophisticated airplane is a funny thing. Some decisions you obsess over, compiling graduate student levels of research materials. Others you make quickly, even if there are cost and complexity implications. When it came to the oxygen system I went with the latter and chose one from Mountain High Equipment and Supply Company. The company has been around for almost 40 years and sells all types of built-in and portable oxygen systems aimed at the Experimental/Amateur-Built market. I went with their top-of-the-line EDS-4ip system, which is a “pulse demand” system that includes all the components for a permanent install in a 4-place aircraft like the RV-10. According to the, a pulse demand system (as opposed to constant flow, like you find in hospitals) is the more efficient because it dispenses oxygen only when it detects a user’s inhalation, leading to dramatically higher durations of a given supply. Plus, as a builder who likes to build, it comes with lots of stuff to install!

New kit always makes me happy.

I’m sure an exhaustive search would have turned up additional options, but I decided to go with this setup primarily because other builders have done the same and I knew I could learn from their experience installing and using the system. Also, Mountain High makes it easy for builders like me who are working through the build at a more relaxed pace. They sell the system in three “kits” that let you spread out the cost and work:

  1. Plumbing Install – The cylinder mounting brackets plus the distributor outlets, electronic connectors and all the tubing
  2. Control Head – The “brains” of the system that gets mounted in the instrument panel
  3. “Fly Away” – Oxygen cylinder and regulator, cannulas and mask

After a quick call to Eric at Mountain High, the Plumbing Install kit was on its way to GAF (Gilbert Aircraft Factory, a.k.a., my garage).

Planning the Cylinder Bracket Install

After unpacking the kit and expressing the requisite “Ooh!”, “Ahh!”, and “Cool!”, I set about planning the cylinder bracket install. However, without a cylinder as a reference, I was unsure exactly how much space it would take up. Fortunately, Rodrigo Damazio Bovendorp clued me in to Mountain High’s loaner program. For a small deposit they will ship you a decommissioned cylinder to use during the installation. I didn’t know about this when I placed the original order, and so had to wait a few more days for it to arrive, but I’m glad I had one in hand as it made visualizing the outcome I wanted a lot easier.

It’s big, green, and MEAN! Okay, maybe not mean. In fact, it seemed quite friendly.

The first step in installing the cylinder is choosing a location. I found builders electing to go with numerous variations, ranging from inside the baggage compartment to behind the baggage bulkhead, either mounted horizontally on the floor or diagonally on the side. I’ve also seen at least one builder mount it from the underside of a shelf that spans the area behind the bulkhead. In the end, I chose to mount mine on the bottom of the empennage, port side, behind the baggage bulkhead. I determined this would make it easier to remove than if it were mounted vertically or upside down. Plus, I planned to rivet part of the bracket structure to the skin and j-stiffeners using existing rivet holes. I felt that letting it “rest” on the bottom would be a bit more secure than “hanging” it off the side stiffeners, especially in turbulence. But I’m not a structural engineer so maybe it makes no difference.

I began by placing the cylinder and bracket assembly into the tail cone and positioning it nearly up against the rear bulkhead. I wanted to leave enough room for the regulator (which I estimated to be about 4 inches deep) behind the baggage bulkhead panel. (Note that Mountain High does offer a remote regulator option, which some builders have gone with. I’ve not yet decided one way or the other.) I aligned the cylinder to be roughly parallel to the two j-stiffeners and marked the position with masking tape. (I say “roughly” because the stiffeners themselves are not parallel and so I just “eyeballed it”.)

Plenty of room to get the fit I wanted.

Designing and Fabricating Brackets

I considered a few ways to fabricate the mounting brackets that the supplied brackets would bolt to. I thought about building a full shelf, but that seemed like overkill and a waste of material and weight. In the end, I decided to go with two longitudinal brackets riveted to the skin tied together using transverse crossmembers. Each bracket would include reinforcing bends for rigidity. Additionally, the supplied bracket dictated locations for nut plates that would receive AN3 bolts, so I had to decide exactly where these would land laterally. Because I also wanted to secure the fabricated brackets to the sides of the j-stiffeners (and thus needed to get a rivet gun or puller into some cramped spaces), I decided to position the port-side holes to the outboard side of the outboard stiffener and the starboard-side holes to the inboard side of the inboard stiffener. A final consideration was ensuring that I could get a bucking bar underneath the inboard bracket for the skin rivets. The additional clearance required also enabled me to level the mount laterally by varying the height of inboard and outboard brackets. With me so far?

Here’s a rendering of what I had in mind:

Note that the brackets aren’t perfectly aligned due to the angle of the j-stiffeners.

These design requirements led to me cutting out the following pieces of .040″ aluminum stock:

  • (2) 9″ x 2 3/4″ for the crossmembers
  • (2) 5 1/4″ x 3 7/8″ for the outboard brackets
  • (2) 5 1/4″ x 3 3/16″ for the inboard brackets

I choose 5 1/4″ for the length of the brackets because it enabled me to “capture” 5 rivets in the underlying j-stiffener.

I added 90-degree bends to the crossmembers to match the width of the supplied brackets and trimmed some of the material away in the center to provide clearance for the cylinder:

For the outboard brackets, I created bends that resulted in a 3/4″ vertical section flanked by a 5/8″ flange to match the j-stiffener on one side and a (roughly) 1 3/8″ flange for the crossmember attachment on the other. The inboard brackets have a 1 3/8″ vertical section in the middle, to account for the height difference imparted by the skin curvature, surrounded by 5/8″ and 1 7/8″ flanges for the j-stiffener and crossmember attachments, respectively:

Test fitting the brackets.

I clamped all the pieces in place and set the cylinder in position to check he overall fit. It seemed stable and well aligned.

Another test fit.

The next step was to attach the brackets. I started by flipping the tail cone on its side and drilling out the necessary j-stiffener rivets.

I’m the sure the original builder was very proud of these rivets. Too bad.

You can see from the photo that on the outboard one I miscounted and drilled out an extra rivet. This would come back to haunt me later when match drilling the brackets. I thought I was holding the bracket in the right spot, with my fingers out of the way, but ended up drilling through the bracket and into my index finger. Argh! Oh, well, time to update the board again:

I’m thinking about just using permanent marker.

Some antibiotic ointment and bandage later I had all four brackets match drilled.

Progress paid for in blood.

For the rivets tying the brackets into the sides of the j-stiffeners, I used a rivet spacer to evenly locate five holes using a #40 drill bit and then went back and enlarged them to 1/8″. I then clecoed the brackets in place and match drilled the crossmembers to them using a #30 bit. At this point, because I had a bit over five inches of bracket to work with, I elected to drill an additional set of bolt holes to provide for future flexibility if I didn’t like the position of the cylinder. Extra attach points would let me customize the position of the mounting straps against the cylinder body.

Happy with the fit, I drilled 3/32″ holes for the nut plates and enlarged the bolt holes to accommodate an AN3 bolt. I countersunk the nut plate attach holes for AN426-3 rivets then primed the parts and let them cure overnight.

The following day I assembled all the parts, starting with riveting K1000-08 nut plates to the brackets. I then riveted the brackets to the j-stiffeners and tail cone skin using AN426-3 and LP4-4 rivets for the bottoms and sides, respectively. The outboard brackets went on with little difficulty due to easy access to the rivet locations. The inboard brackets took much longer. For the blind rivets, I used sections of trailing edge wedge material to create a shim that provided clearance between the head of the rivet puller and the curved edge of the stiffener. It also turned out the space I allocated under the large bracket flange was a bit tight for me to get a firm grasp of the bucking bar. I resorted to using a wooden block to position the bar in place so I could then apply pressure with my fingers. In the end I was able to get the brackets attached, though I chose to drill out and redo a few rivets that I wasn’t pleased with.

Once the brackets were in place, attaching the crossmembers and cylinder hardware was straightforward. My hole locations turned out not to be exactly perfect but the flex in the bracket flanges helped to get everything aligned and bolted down.

Next up will be locating the fill port and starting the plumbing and wiring.

Build Date(s): 28-Jan-23 to 29-Jan-23
Build Time: 11 hours

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