Archive for the ‘Fuselage’ Category

ELT antenna

Saturday, 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

Saturday, 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…

Fastener madness!

Sunday, November 28th, 2010

I finally got sick of being embarrassed about a few of the rivets in the F-705 bulkhead – other builders will know exactly which ones I'm talking about – so I drilled them out and replaced the ones that are impossible to properly buck with AN525 structural screws.

View from the aft side. I also drilled out and replaced the rivets whose shop heads are hidden under the canopy decks, because I didn't have a tungsten bucking bar last time I tried to set these and they were kind of ugly. This time they were relatively easy to redo using two pounds of element 74 and a double-offset rivet set.

A quick spritz of paint and you can't even see the screw head unless you know right where to look.

Then it was back to working on the baffles… except wait, it looks like one of the screw holes in the engine case isn't tapped deep enough. Maybe I can just force the screw in there?

Nope! Big mistake. Twisted the head right off, and now there's a broken screw stuck in the engine case. In the very expensive engine case.

What to do? Over the course of half an hour I went through all five stages of grief:

  1. Denial – "That hole can't be very important, and I can still make the baffles work without it, right?" (no)
  2. Anger – "Stupid screw, you suck for not being stronger! Stupid hole, you suck for not having enough threads!"
  3. Barganing – "Maybe I can build some kind of brace to transfer the load to a different hole?" (no)
  4. Depression – "I should just chop this thing into bits and push them out to the curb and let them be hauled to the dump."
  5. Acceptance, starting by very carefully drillling through the center of the bolt:

…then going to the store to buy a bolt extractor:

The screw broke in half and I had to re-drill it and restart the extractor, but I got it all out!

Whew, what a relief:

Then I did what I should have done in the first place, which was to tap it about 1/8" inch deeper so the screw can be fully inserted:

Now it can be threaded in far enough to hold this bracket in place. No lockwasher yet since this screw will be removed and replaced many times before the baffles are done.

Very expensive engine not ruined after all. This whole episode really took the wind out of my sails, but I sure earned this:

I also spent many hours this weekend researching stuff about the baffles, and I think I almost know what to do next. Now that the Great Screw Disaster is resolved, baffle work will resume next time…

Fuel plumbing rework

Sunday, May 2nd, 2010

Lately, most of my free time has been taken up by house chores and other pursuits, leaving little time for the airplane. Today I did manage to redo the fuel plumbing that I'd previously discovered was interfering with the center tunnel runs.

I replaced the straight bulkhead fittings with 45-degree ones:

A view of the new plumbing run on the left side:

Right side:

There's sufficient clearance between the wiring and plumbing now:

Here's what it looks like from the aft side, once the fuel pump/valve/filter assembly is removed from the airplane. Lots of intertwined pipes here and not much room to fit them.

After changing the angle of the banjo fittings on the fuel selector valve, I tightened and safety-wired the nuts:

This is a common sight when you're working with rigid tubing. Cut a line too short, or overbent it, or kinked it, or scratched it? Toss it and start over. My recycle bin has plenty of 3003 aluminum in it this week.


VOR antenna

Sunday, November 1st, 2009

A forum thread convinced me to buy and install a traditional cat whisker VOR/ILS antenna on the bottom of the fuselage, rather than the hidden wingtip type often seen on these aircraft. I'm happy to give up half a knot for reliable navigation performance.

I decided to mount the VOR antenna just aft of the F-710 bulkhead, which is just barely accessible when the empennage is attached. A doubler ties into the bulkhead and the F-779 bottom tail skin.

The doubler is made out of 0.063" alclad. Here it's drying after having alodine applied, although in retrospect I'm not exactly sure why I bothered to do this – the "puck" part of the antenna is plastic, and the mounting fasteners don't make electrical contact with anything in there. Oh well, at least it won't corrode.

Here it is riveted in place with the antenna attached via nutplates. The brown stain is alodine that ran downhill while it was drying.

It would be pretty easy to use driven rivets here during the initial build of the tailcone, but on a nearly finished fuselage with the empennage installed it's essentially impossible. Cherry Max rivets to the rescue.

Test-fitting the VOR antenna using some random bolts… when I go to install it for good I'll use AN525 screws instead:

When the rudder and elevators are installed, you won't be able to step on the VOR antenna, so no danger of tripping over it while walking around the back of the airplane.

Yeah, it's hanging out in the breeze, but the airflow down there will be pretty disturbed anyway, so it shouldn't cause too much drag (he said, despite having only a journeyman's understanding of aerodynamics). At least the nav radio reception should be pretty good!

Now that all seven of the antennas on the fuselage are mechanically installed, it's time to move on to wiring them… but that will have to wait for a future work session.