Matt's RV-7 Project

Another item that's shown in one of the plans drawings but not really mentioned in the assembly instructions is the F-796A fuel tank attach brackets. These protrude from a spot on the forward fuselage, and get bolted to the corresponding mounting ears on the forward ends of the wing fuel tanks. I don't need these brackets yet themselves per se, but the mounting of them also involves a series of parts on the inside of the fuselage, which is a job I need to complete.

The interior parts for mounting each tank attach bracket are a piece of angle and a stackup of aluminum spacers:

I cut and shaped these to fit between the lower longerons, on the forward side of the F-902 bulkhead. This required unhooking the fuel tank vent plumbing and moving it out of the way. My recommendation to anyone reading this is to install these parts much earlier, so you don't have to rejigger your vent lines like I did.

I drilled the upper and lower bolt holes by match-drilling through existing rivet holes. Then I installed the tank attach bracket with a temporary hardware store bolt in its upper mounting hole, and match-drilled its lower bolt hole through the centerline of the mounting parts below. That step was much easier once I picked up a 12" long 3/16" drill bit. The rivets visible under the bracket in this photo are just sitting in their holes to help keep things aligned.

After drilling and deburring everything, I primed and painted the mounting angles with my interior color (Rustolem Dark Pewter textured) and painted the steel attach brackets with Dark Machine Grey enamel. While I was having a painting day, I also painted the spar gussets from the last post in the same interior color.

I had to enlist Mary's help to drive the rivets, which are in an inconvenient spot for solo riveting:

I made sure that I'll be able to bolt the tank attach brackets in place when I need to, but I'm not going to attach them just yet because they seem like a real knee-banging, pants-tearing jobsite hazard.

Back inside of the fuselage, I was able to reshape the fuel vent lines and re-clock the elbow fittings to clear the newly-installed mounting angles. Thank goodness for soft 3003 tubing.

The taildragger RV-7 has a pair of hefty aluminum gussets (F-7114) that bolt to the main spar and the lower fuselage longerons, fulfilling the same purpose as the main gear weldments which would be there in a tri-gear RV-7A. The plans don't really say when to attach these parts, other than you obviously need to have them in place before you mate the wings. I'm not to that stage yet, but I'm trying to get everything possible done inside the fuselage, so I decided I'd better check these gussets off the list.

First I made a set of drift pins by cutting down some appropriately-sized hardware store bolts and grinding a the ends to a taper point. By chucking a bolt into a hand drill and spinning it against my belt sander, I was able to make a nice taper fairly quickly, and it warmed my hands up to boot. A quick pass with some emery cloth made the ends nice and smooth.

Before doing anything else, I used the taper pins to check that the factory-drilled bolt holes in the spar were okay. There was a brief moment of panic when I could only get about half of them to go in, but with the aid of a hole gauge I figured out that the actual problem was that the F-704C/D plates were slightly misaligned with the corresponding F-704E/F bars that they were riveted to. This must have been due to the quickbuild factory failing to bolt the plates and bars together properly before match-drilling and riveting. I decided the best fix would be to carefully file the holes in the F-704C/D plates only, leaving the bores in the much more structurally significant F-704E/F bars untouched. This was fiddly, but I managed to make it work. In this photo you can see how out of alignment the parts were before I finished this repair:

I bolted the gussets to the forward face of the spar using short hardware-store bolts. You have to reshape the bent flanges on this part to make it fit properly, but it's no big deal if you go slow with the hand seamer. I drew a centerline on the outboard flange and used a paint stick to prop up the gusset at the correct angle to pick up the existing holes in the lower longeron.

From the outside of the fuselage I match-drilled into the gusset through the existing rivet holes, clecoing as I went. Here you can also see the temporary hardware-store bolts I used to affix the gussets in place while drilling:

Not too bad, plenty of edge distance:

I repeated this process for the other gusset, then drilled the five holes in each part up to #10 to accept an eventual row of bolts. Then I removed the gussets, deburred, and got them ready for painting. These won't actually be installed until the wings go on, so in the meantime I'll just shoot some paint on them the next time we get a decent day for painting.

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!

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…