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.
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.
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!
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:
- Plumbing Install – The cylinder mounting brackets plus the distributor outlets, electronic connectors and all the tubing
- Control Head – The “brains” of the system that gets mounted in the instrument panel
- “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.
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”.)
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:
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:
I clamped all the pieces in place and set the cylinder in position to check he overall fit. It seemed stable and well aligned.
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.
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:
Some antibiotic ointment and bandage later I had all four brackets match drilled.
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