Mid-fuselage bottom skins ready to rivet

Finally coming back to this section and making progress!

I took a step from section 44, and drilled the mid fuselage skin nutplate holes to size - the goal is to attach the two forward-most nutplates before riveting the skin on, since they're in somewhat tight spots after the skin is installed. The others should be easy to squeeze rivets for later:

We finally primed the bottom skins:

then proceeded to remove the vynil from both for riveting:

We then dimpled the skins, and next will start riveting them on.

Unfortunately in that batch of primer I forgot to include the TAS antenna doubler, which is way easier to rivet on before the skin is in place (because the heat tee goes over it), so I'll do that batch before continuing with this section.

Also related to the TAS antenna, I was thinking of cable paths for it, and wondering if I could put the TAS in the tailcone and run the 4 cables straight back (so exiting the tunnel to the right, then going through the rear spar bulkhead. The main question I had was about drilling a big (1/2" or so) hole through the rear spar bulkhead, so I contacted Van's support and send them this annotated picture, and they confirmed it's OK to do so:

Rear spar bulkhead hole location for running cables to the back

Time lapse:

Total mid-fuselage ribs and bottom skins rivets: 412
Total mid-fuselage ribs and bottom skins time: 84.2h

Forward fuselage skins and ribs deburring, fuel return path and bottom TAS antenna location

I finished deburring the remaining parts for the forward fuselage, and dimpled the ones that were already primed (the forward fuselage ribs):

Most forward fuselage ribs deburred parts

Deburring and trimming mid seat rail supports

Dimpled forward fuselage ribs

I also did some work in planning for the fuel system. Since I plan to use the EFII System32, I followed the steps from others who have run hoses for it, and may end up getting the hose kit from Aircraft Specialty, so I followed their instructions, starting with enlarging the systems brackets holes - the original one is the right size for a 1/2" snap bushing (plus another which is meant for brake lines and thus much smaller), but I'm planning to use 3/8" fuel hoses with grommets, so I enlarged it to 5/8" for the grommet). I did this by transferring the holes to a backing piece, enlarging them to the right direction on the backing piece (away from the screw hole and away from the top bracket edge), then transferring back with a unibit:

Transferred existing holes to a backing piece

Enlarged holes in backing piece, to transfer back to brackets

Enlarged systems bracket holes

Systems bracket with grommets

Of course I did screw up and had to re-make two of those top brackets :)

I also enlarged the forward fuselage rib holes to 9/16" for the fuel return lines:

Enlarged center hole for fuel return line (before deburring)

I added a hole for those lines through the side skins and center section side plates:

Hole position on the side skin for fuel return line 

Enlarged fuel return side skin/side plate hole

Per the post on antenna positions, I made an antenna doubler for each tunnel section:

Antenna on aft tunnel doubler

Forward tunnel doubler for TAS antenna

I'm not going to install the forward doubler just yet - the aft tunnel is harder to rivet in (with the heat tee over the spot), so I'll just add that one for now).

Next up, a bit more deburring and then primer and riveting to complete this section!

Time lapse:

Total forward fuselage ribs and bottom skins rivets: 0
Total forward fuselage ribs and bottom skins time: 61.3h

More on antenna positions

For antenna positioning, I tried to put the forward tunnel parts around the place they'd go, and reailze it gets really crammed in there (plus, forward of the fuel valve goes the EFII pumps, filter, etc.):

The forward tunnel gets pretty crowded

Photos of the fully-assembled tunnel from others also seem to confirm that.

so I'm now reconsidering the TAS antenna position - putting it in the aft tunnel may be well worth the trouble of running the cables to it, and on the upside that'd bring it close to being aligned with the top antenna - so I checked whether there's clearance for the cables below the pushrod, and there seems to be enough:

Potential position for the TAS antenna in the aft tunnel

Just to give me some margin for decision, I'm going to add doublers to both places, and decide later.

Furthermore, I looked at the G5 installation manual and realized that it has an internal antenna - unclear if that'll be enough, but I'll be holding off on adding a third GPS antenna for the time being, and if I do I may add it to the top of the dashboard, just for some installation diversity.

Updated antenna positions

I had previously planned the antenna positions both disconsidering convenience of mounting the antennas, and assuming I'd have the MotoPOD on the bottom. Given this, I took some time to re-plan their placement taking both into consideration (click on any of these for a larger version - original file here). In these drawings, the solid circles are the ideal ground planes, and other circles are additional restrictions.

Top view - ELT, Stormscope, COM, GPS and TAS antennas

I no longer need separate UAT and transponder antennas. Given that Garmin requires a minimum of 3' of cable from the transponder to the antenna, I moved that antenna to the rear part of the tailcone:

Side view with all antennas

Bottom view - transponder, COM, NAV and TAS antennas

I added a third GPS antenna - one for the GTN, one GPS+XM for the G3X (PFD only), and one for the G5.

TAS antenna details - ideal and minimum ground plane plus distance to other antennas

I moved the bottom TAS antenna to inside the forward tunnel, since the aft part of the tunnel has no easy way to run cables to it, and very little clearance near the skin, with the elevator pushrod going through it. The distance between the top and bottom antennas is about 21", which should add a bearing error of about 0.2˚ for a target at 1 mile (perfectly acceptable, I doubt the TAS has that level of precision anyway):

Distance between the two TAS antennas - this introduces ~0.2˚ bearing error for a 9 or 3 o'clock target at 1sm

With the transponder antenna moved aft, I'll try using the original XPDR antenna hole for my bottom COM antenna. It's unclear what kind of performance I can get from the ground plane at that location, but since the ground plane area is only about 20% smaller than the top antenna's, I'll just plot the VSWR after the tailcone is attached and see (and if that doesn't work, I'll add another hole in the baggage area and plug this one).

COM ground plane area is ~1371sqin (ideal is ~1662sqin, ~20% more)

I was also briefly worried that the tailcone antennas might be too close to the ground in a maximum takeoff rotation position, but a quick measurement showed that's not the case (i.e. the tail would hit the ground first):

Tailcone antenna ground clearance at maximum rotation attitude

Given that none of these require any structural changes for now (only the top TAS antenna will give me trouble), I'm deferring any installation steps to later when I actually have the avionics - who knows what else may change until then.

Mid-fuselage ribs and bottom skin deburring and dimpling

The outboard rear seat ribs weren't aligning with the skin holes, so I had to flute them, which was quite annoying when already attached (I even got a smaller fluting plier just for this):

Fluted outboard rear seat rib

I match-drilled them properly and removed the skin, then started deburring and dimpling all the ribs and bulkheads for

Deburred and dimpled ribs

All center section bulkhead holes and the holes with tabs on other bulkheads were machine-countersunk:

Machine-countersunk holes on aft center section bulkhead

Dimpled and machine-countersunk holes on rear spar bulkhead

Dimpled and machine-countersunk holes on fuselage bulkhead

Next I need to deburr and prime the skins so I can start riveting them on.

Time lapse:

Total mid-fuselage ribs and bottom skins rivets: 412
Total mid-fuselage ribs and bottom skins time: 75.0h

Forward-fuselage bulkheads and bottom skin progress

Forward + aft portions of the fuselage attached together for drilling

I measured the wing spar at exactly 1.7" thick. When I first attached the center spars with spacers, I instead measured it at close to 1.8":

Spacers measured around 1.8" (should be 1.7")

Turns out that the thickness of the blue vynil adds up quickly, and 28 layers of it per set of spacers wasn't helping. I removed it and deburred the parts for some extra smooth coupling:

Spacers with all the blue vinyl removed

That got me almost all the way there (about .02" to go):

Spacers much closer to 1.7"

Since the spacers were slightly curved, I just applied force to make them flat. I could have torqued the nuts, but I don't have a torque wrench that can do the required 1100in-lbs (will order one for when I need to attach these semi-permanently until the wings are actually attached), so I instead just squeezed them with clamps - first with my usual plastic clamps, which did pretty much nothing:

First attempt at compressing spacers, using regular clamps

Then I used the heavy-duty clamps with some felt padding (idea credit goes to the wife):

Successfully compressed spacers with heavy-duty clamps

and with that, holes were magically aligned :) so I final-drilled all of those, and started deburring some of them.

Time lapse:

Total forward-fuselage bulkheads, ribs and bottom skins rivets: 0
Total forward-fuselage bulkheads, ribs and bottom skins time: 44.4h

Firewall riveting mostly complete

I modified my #30 dimple die using the grinder, to dimple the bottom-most holes in the firewall bulkhead:

Modified dimple die for dimpling the bottom holes of the firewall bulkhead

then continued riveting:

Fully-riveted firewall forward side

Fully-riveted firewall aft side

With this, the only parts missing on the firewall are sealant (which I plan to apply later) and the rudder pedal brace (which I didn't want to install until I figure out how it'll work with the ControlApproach pedals).

Time lapse:

Total firewall rivets: 596
Total firewall time: 58.4h

Mid-fuselage ribs riveting done, skins started

We finished riveting fuselage bulkhead and baggage ribs:

All mid-fuselage ribs and bulkhead riveted in place

I clecoed the bottom skins on and final-drilled all the holes:

Mid-fuselage section with skins on

The floor stiffeners required some fluting to line up properly, then they were final-drilled as well:

Floor stiffeners, fluted and final-drilled in place

Mid-fuselage section with skins and floor stiffeners only (it barely clears the ceiling!)

I found that the holes from the skin to the outboard seat ribs don't align :( so I'll need to flute that part, which will be especially tricky with it already riveted in place:

Since I only have a vise-grip-style fluting plier, which would be even harder to use in this space, I've ordered a regular plier from Spruce and will give that a try when it gets here - I have plenty of other holes and edges to deburr in the meantime.

Time lapse:

Total mid-fuselage ribs and bottom skins rivets: 412
Total mid-fuselage ribs and bottom skins time: 67.5h

Mid-fuselage ribs riveting, part 2

With the parts all primed, I started by riveting various small parts to the ribs:

Step mounting brackets and seat rib subassemblies riveted together

Of course, while doing this, I realized that two little parts had been forgotten when doing primer :( so one more primer session had to be done:

Two seat belt attach lugs I forgot to prime :(

I got lazy this time and decided to do it with rattle-can primer. The color difference doesn't really matter since any apparent parts of the interior will be painted anyway:

Rattle-can-primed seat belt attach lugs, riveted to the rib

I also inserted the heat tee, which allows me to continue riveting the ribs:

Rear tunnel with heat tee installed

I riveted some of those ribs so far:

Most rear seat ribs have been riveted in place

Time lapse:

Total mid-fuselage ribs and bottom skins rivets: 326
Total mid-fuselage ribs and bottom skins time: 55.2h