Matt's RV-7 Project

Another of those "did a lot of work, made little observable forward progress" updates…

When I started my panel design, Garmin made two audio panels, the GMA 340 and the GMA 347. These had different capabilities – the 340 is all analog and the 347 features digital processing – as well as completely different mounting trays and connector pinouts. I went with the 347 on the basis that it must have 7 more "somethings" than the 340.

Fast forward to now… in the intervening years, they've come out with the GMA 240, an excellent low-cost audio panel targeted at us homebuilders, and the GMA 350, a high-end all-digital number. Both of these new audio panels use the same wiring as the old GMA 340, which makes my 347 sort of an odd man out. Given that the 240/340/350 pinout seems to be the favored arrangement for audio panel connectors these days, as well as the fact that the GMA 240 is also lighter and cheaper than the 347, I decided to remove my existing audio panel and convert to a GMA 240 tray. Might as well do it now while I've still got access to the wiring, as it would be almost impossible to do later.

Old audio panel tray removed:

Redoing wiring in accordance with the new schematic I drew. Fortunately it was mostly a matter of de-pinning wires from the old connectors and re-pinning them in the new ones, with only a few new wires or splices required.

Connectors tested, strain-relieved, and sealed up:

I enlisted Mary's help to reinstall the tray in the radio stack, as there was one stubborn fastener which was impossible for me to reach from both sides simultaneously.

New audio panel tray bolted in place with connectors installed. I'm glad I did this now, because the only way to access these connectors once the top skin is riveted on will be with a crash axe or a stick of dynamite. Maybe both.

Testing… everything works!

So I spent a bunch of hours removing a perfectly good audio panel and replacing it with another one. But at least the new one is lighter by a pound and a half, and its seemingly more common pin arrangement might save me some trouble down the road.

One of the unexpected consequences of being a very slow builder – although I prefer the term methodical, thanks – is that whole new technologies that barely existed when you started can advance to the point of being practically de rigueur before you're finish. One example is ADS-B, which everybody is now required by law to have by 2020… at least if you plan to fly anywhere useful. I'm already ADS-B Out equipped by virtue of having the proper kind of 1090ES transponder and a pair of compliant TSO C146 WAAS GPS navigators, but to benefit from the free traffic and weather data my tax dollars are already providing I need to install a separate ADS-B In receiver. Of course the obvious choice for my G3X system is the GDL 39R:

I'm seriously running out room to install stuff in the airplane, especially something the size of the thousand-page biography of Winston Churchill I've been meaning to read all year. I picked out a place on the F-7107R subpanel rib, where there was still some mounting structure left over from when I was plannig to put a Lightspeed ignition module there.

I used a piece of 0.050" aluminum to make a mounting plate, and bent flanges on two sides to give it some stiffness:

GDL mounted and wired. It's quite simple, just power, data, and an antenna. It will be possible to remove this unit for servicing later, but only if I press forward with my plan to cut access holes in the forward fuselage skin.

Close-up detail of antenna coax routing – an adel clamp keeps the cable from chafing on the sharp edge of the mounting plate:

For the antenna itself, I bought a 978 MHz blade antenna from Delta Pop. Relatively inexpensive for an airplane component, and gets good reviews.

According to the boffins at work whom I leaned on for specific installation guidance, the ADS-B antenna should be mounted at least six feet or so from the transponder antenna, and at least three feet from the comm antennas. It probably also wouldn't like being right next to the hot exhaust pipes. The only places on the bottom of the airplane that satisfy all these criteria are the very tip of the tail, and a spot in the middle of the forward fuselage floor about midway between the spar and firewall. I chose the forward location to making the antenna routing easier.

Of course the fuel pump and its associate plumbing are already located there, so things are a bit tight. I temporarily installed the pump assembly so I could mark the location for the antenna. The coax connector sticks up a bit, so it needs to be far enough aft to be underneath the fuel pump cover, but far enough forward that it doesn't run into the bottom of the fuel pump mounting plate.

Luckily there is a loop of fuel line in a convenient spot to run the coax straight up and through, shown marked by a dot here prior to drilling:

To mount the antenna to the floor I made a small doubler out of a piece of scrap 0.063. I forgot to take pictures of the process, but you should know what one looks like by now because my airplane is positively bristling with antennas.

For corrosion resistance and maximum electrical conductivity I treated the doubler with alodine:

As well as the floor, after removing an area of paint:

I enlisted the help of the lovely Dr. Mary to rivet the doubler to the floor. She did a great job even though I haven't given her much rivet practice for quite some time.

Antenna doubler installed, complete with antenna:

The coax runs along the floor and then does a loop to duck down through the fuel plumbing and connect to the antenna:

Side view:

View of the underside of the airplane sporting a brand-new antenna:

I'm now equipped to both transmit and receive ADS-B. Here's hoping I get this thing finished before 2020 rolls around.

I decided to sell my panel-mounted transponder and replace it with a remote-mounted unit instead. This is the GTX 23ES:

The unit itself is a faceless silver box – instead of pushing buttons on the transponder's front panel, you control it through the screen of your G3X system. It mounts in this sheet-metal tray, which I now have to find a place for.

The only practical place left in the airplane where I can mount something this size is the area under the baggage compartment floor on the passenger side of the fuselage. Needless to say, I'm glad I made my baggage floors removable.

The piece of aluminum angle shown here is riveted to the inboard floor rib with countersunk rivets, and it will act as a little shelf for the transponder tray to sit on. Some of the open holes visible in the rib are for tie wrap anchors that I temporarily removed to give myself space to work, but some are from little brackets and other things that I've since removed. This particular area of the airplane has undergone more changes than almost any other part, as I've installed various pieces of equipment and then later changed my plan and removed them in favor of something else.

Four pieces of angle plus some rivets became a pair of T-brackets, seen here riveted to the outboard floor rib. Keep reading to see what these are for.

I fabricated these two identical mounting braces out of some more aluminum angle. The mill was handy for removing excess material where it wasn't needed.

The braces attach to the transponder tray like so:

Closeup detail of how the braces attach to the tray, using #6 screws in the countersunk holes that are thoughtfully provided:

Here's how it all goes together. The transponder is oriented with the connectors facing towards the nose of the aircraft. The braces stay permanently attached to the tray, the tray sits on the longitudinal angle (not visible) and the braces are fastened in four places to the T-brackets and the inboard floor rib. It's kind of an odd setup, but it fits in the available space and is quite sturdy. Just as important, it is all fairly easy to remove if required.

Wiring… power and ground, RS-232 to the GSU 73, ARINC 429 to the two 430W's for TIS-A traffic display, and RS-232 for GPS position data from 430W #1 so I will be ADS-B Out compliant.

Wires routed and secured, all neat and tidy:

The transponder antenna coax carries high-power RF and shouldn't be routed with any other cable bundles (so says the install manual, anyway). I ran it inboard to the center tunnel, aft, and back outboard again to miss the elevator bellcrank on its way to the antenna. It's tie-wrapped to the floor rib with the usual plastic cable anchors.

You may have wondered earlier why I removed material from the middle of the mounting braces. If you look at this photo you can see that It's simply to prevent creasing the baggage compartment floor if I put something heavy on it (such as my knee!) that causes it to flex downward between the floor ribs. Well, it probably saves a few grams of weight too.

The overhead view shows why I had to make the tray removable, instead of permanently mounting the tray and just sliding the transponder in and out – no room! The under-floor avionics bay is physically not long enough to allow the transponder to be removed from the tray, even if I'd scrunched up the wiring and mounted it as far forward as humanly possible. So, to remove the transponder I'll have to undo four fasteners, lift the tray out of the floor, and then remove the transponder from the tray. I shouldn't have to do this too often, so hopefully it won't be too inconvenient.

Everything worked on the first try – after a few keystrokes for configuration, the transponder controls popped up on the PFD.

The only thing left to do on the transponder is to finish securing the antenna coax and connect it to the antenna on the belly. I'll tackle that eventually when I get around to venturing into the tailcone to finish up all the wiring back there. Right now that area is in a state of… let's go with "disarray".

Oh, you may be wondering why I decided to go to all this trouble in the first place. Why remove a perfectly good transponder from the panel just to put in a different one that's mounted somewhere else? Naturally, it was to make room in the panel for another toy… just what kind, I'm not telling yet.