ELT antenna

August 9th, 2014

Further progress on the ELT installation today. The box comes with an antenna, which you're supposed to mount on the exterior surface of the aircraft skin. Some guys try to get away with installing their ELT antennas inside the baggage compartment or underneath the empennage fairing, I suppose to reduce drag or simplify the installation. I'm no RF engineer, but it seems obvious to me that an antenna mounted inside an aluminum box is not going to have the same performance as one placed outside with a good ground plane. Besides, the regulation (14 CFR 91.207) that requires you to have an ELT also says it has to be inspected periodically for "proper installation" – which is governed by the manufacturer's installation guidance, and that tells you to put the antenna on the outside of the airplane. Therefore you could argue, if you wanted to be picky, that your airplane is not airworthy with the ELT antenna mounted inside. So, one more external antenna to install.

Mounting an antenna on the skin requires a doubler as usual. I made one out of 0.040" alclad, and just to be different I decided to make a perfectly round doubler using the rotary table on my milling machine:

This is about two and a half inches in diameter, with rivet holes spaced exactly one inch from the center at 45-degree intervals.

I recruited Mary once again to help me dimple the holes and drive the rivets:

Crawling back into the tailcone is no fun. It's been hours since this photo was taken and I still have a neck cramp.

Nice looking rivet pattern though, eh?

The ELT antenna doubler is mounted just forward of the F-708 bulkhead. As per usual, I put some alodine on the mating surfaces for corrosion resistance.

This location is far enough back that it won't be a problem when opening the canopy:

…but it's still far enough forward that it won't bash up the vertical stabilizer when it's whipping around in the wind.

For those keeping track at home, this brings the total count of external antennas on my airplane up to eight – or nine, if you count the two antenna elements inside the GA 57X separately. I think this should be the last one, though!

Mounted ELT

July 26th, 2014

While I was contorting myself back into the baggage compartment to work on the autopilot pitch servo, I went ahead and installed the new ELT also.

For those of you keeping track at home, you might be confused because I already mounted the ELT a long time ago. In fact I did it twice. Well, I didn't like either of the two previous arrangements, and also new and improved ELT technology has come along in the meantime. I wanted the benefits of having a 406 MHz ELT, so I bought an ACK E-04. That's the least expensive one on the market, although they are still not what I would call cheap.

Anyway, does this look like a happy face or what?

You're supposed to mount the ELT in a more or less level orientation, as far back in the fuselage as possible, and attach it to something strong enough to resist deforming in a crash. I attached the mounting bracket to the F-729 rib, using structural screws for the top two holes that go through the reinforcing angle. It's plenty strong.

Here's what the ELT looks like popped into its bracket:

I still need to connect the power and GPS inputs, not to mention mount and connect the antenna, but first I have to go to Oshkosh to work…

Autopilot upgrade

July 25th, 2014

I sold my Trutrak autopilot system and changed over to the Garmin G3X autopilot instead. Fortunately the GSA 28 servos fit into the same bolt holes as the old servos, so installing the new pitch servo was straightforward. And it's a bit lighter too.

The electrical connections were surprisingly not that hard to retrofit either. Of course the wiring is very different between the two systems, but the servos use a similar number of wires, so I was able to repurpose existing wiring for new tasks. The old stepper motor drive and torque sense wires became CAN and RS-232 lines – luckily I had the foresight to use shielded wire. The only new wire I had to run was a single conductor for the autopilot disconnect line; in the old system the red button was connected to the control head in the panel, whereas in the new system it connects to both the servos. The power and ground wires stayed the same.

I'll have to come up with something to do with this oddly-shaped hole in the panel, which used to be occupied by the Trutrak autopilot control head. I'll figure it out.

I haven't installed the new roll servo yet, since I need to redo some wing wiring, but the wiring that goes out through the side of the fuselage is now all prepared for the Garmin autopilot.

Audio panel downgrade

June 29th, 2014

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.

Compressor repair

May 10th, 2014

Ever since I bought my air compressor, a factory-overhauled unit from Campbell Hausfeld, the pressure regulator has been broken in a very specific way. When set to the On position, it would run the compressor automatically as designed, topping up the air pressure whenever it dropped too low. However, it would also do this when the switch was set to the Off position, thus making a mockery of the entire concept of having an On/Off switch. I knew all along what the problem was – a little plastic bit inside the regulator housing wasn't quite long enough to interrupt a set of relay contacts – but the inertia of doing nothing about it made it pretty easy to ignore for a very long period of time.

When we went on trips I would turn it off by flipping the breaker at the fuse panel, which is a well-intentioned idea (avoid burning the house down if the compressor springs a leak) but not particularly a well-executed one (using a breaker as a switch is generally considered bad form). This generally led to simply leaving the compressor in a "live" state overnight, which sometimes caused it to cycle in the wee hours of the morning when the falling temperature caused the pressure to drop below the set point. Although you can't hear the compressor outside our house when the garage door is closed, it is unfortunately sited more or less directly below our bedroom, and is quite audible from in there. It always would wake me up, but I'm a light sleeper so everything wakes me up (here's looking at you, annoying loud neighbor). However, once or twice the sound was even of sufficient intensity to wake Mary from her customary log-like slumber, thus making her quite grumpy indeed. So that was more or less the last straw.

Anyway, I finally got around to replacing the regulator with a $37 part from Pacific Air Compressors:

I'm happy to report that it was a quick and easy job, and now the compressor works as designed in all modalities. Most importantly, I no longer wake up to discover that I'm in the doghouse before I've even found my trousers.