Autopilot controller mounting

October 21st, 2014

I couldn't mention it at the time, but the reason I moved my transponder was to make room for a GMC 305 autopilot controller. It's an optional add-on, but I really like how it streamlines the whole operation of the autopilot and flight director, so I wanted to find a way to fit one into my radio stack. The transponder had to go in order to make room for it, but I think it was a fair trade. I already ran the wires to connect it, I just didn't take photos.

The GMC 305 is slightly taller than the GTX 330 transponder it's replacing, so I had to remove the lower filler plate from the bottom of the radio stack and shave off about 0.050" with the milling machine.

While I had the mill out, I made some 1/2" x 3/4" angle out of scrap material. Having a milling machine sure is handy.

These little angles will become the mounting brackets that will attach the GMC to the radio stack:

After a lot of fiddling I got all the various mounting holes drilled. There's not technically enough room to fit both nutplates, but the spacing between them is coincidentally perfect to allow two nutplates to share one rivet. That was kind of a lucky break.

I painted them black since the edges will be somewhat visible from the cockpit:

The brackets are attached to the radio stack mounting rails with #6 screws:

And here's the GMC 305 installed. I don't currently have the proper black cap screws that this would normally use, so I just grabbed some shiny screws from the bin for test fitting purposes.

You can see above that there's just barely room for the rudder trim and flap switches now. My panel sure is full of stuff.

Breather tube and exhaust patch

September 21st, 2014

So anyway, I decided to rethink my crankcase breather setup and remove the vacuum valve that was plumbed into the exhaust pipe. This thread on VAF has a lot of back-and-forth about the relative merits of such a setup, but it's posts like this one that really got my attention. I decided I'd made a mistake and violated my cardinal rule – learned through hard experience – of not being an early adopter for any changes to critical components of the aircraft structure, engine, or fuel system. I will keep the air/oil separator installed, since the operation of such a device is well-understood, but the vacuum valve needs to go.

I was able to pull the #4 exhaust header off the engine without dropping all four pipes, which was a real time-saver. I sent it off to the manufacturer (Vetterman Exhaust) to have a patch welded over the big hole drilled in it.

Then I went out and did what I ought to have done in the first place, which was to buy a bunch of 5/8" aluminum tube and a big old bending tool. These tools aren't cheap, but fortunately I managed to find one for about half price at an online tool vendor that was cleaning out their stock. (large tube bender shown here with my favorite Free State seasonal for scale)

Good thing I bought twelve feet of tubing, since it took me a few tries to get the shape exactly right and not kink anything:

Finally, here's the finished piece. It doesn't look like much, but it was very carefully shaped to fit in a very confined space.

Starting from the bottom of the oil separator, the new breather tube immediately jogs towards the left side of the airplane before bending downwards.

The S-bend allows it to avoid the prop governor cable, which was my main obstacle previously. A short piece of rubber hose and some clamps join the breather tube to the outlet of the oil separator.

Another view looking up from the bottom, showing how the tube hugs the firewall and avoids the prop cable:

Here's an overview of how the bottom end of the tube is arranged:

An adel clamp into a convenient nutplate secures the tube to the firewall:

At the bottom end, the breather tube is attached to the engine mount by another pair of adel clamps, and terminates about a half inch above the left exhaust pipe. In this photo and the one above, you can also see the "whistle slot" I drilled in the tube to provide an escape path for crankcase gases in case the end of the breather tube freezes up in cold weather.

Some builders argue that the bottom end of the breather tube should be angled towards the cowl airflow exit or else the engine will leak all its oil; you can find just as many who will swear up and down that it has to point the other way or else the engine will leak all its oil. I left it cut more or less parallel to the exhaust pipe – we'll see how it works.

Meanwhile, the #4 exhaust pipe came back from Vetterman good as new. I think I actually spent more on postage than I did for the repair.

A small patch now covers the previous hole. That's some nice welding:

All pipes reinstalled with new lockwashers and nuts torqued:

So long, crankcase valve. The moral of the story, once again, is that sometimes the old ways are the best.


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.