Match drilling fuel tank ribs

Before I could start attaching my newly-straightened fuel tank ribs to the wings I needed to ensure the tank skins were fitting correctly left to right. If the rear baffle is misaligned then the skins will be too, and it's probably not good to have the tank skins not aligning properly with the leading edge skins. The plans have you put the skin on without the ribs in place for some reason, so I had to improvise to support the skin on the outboard ends (the inboard end is supported by the joint plate I match-drilled earlier).

Yep, that's the cutout from when I made the leading edge cradle. Works perfectly!

After clecoing the skin at the nutplates, I was showing almost a 1/16" gap between the leading edge and fuel tank skins. Instead of immediately freaking out I decided to see what the gap would be like once the ribs were in place too since they'll probably affect this.

The lower part of the skin is actually perfectly aligned, which means the top part can't be far off.

Since the baffle appears to be in a good position I took the fuel tank skin off and started clecoing the ribs in place, marking them with a sharpie as they went on so I could keep them in order later. One thing I made sure to do was to attach each rib only to the baffle at first since most of the center holes had to be elongated to match the brackets underneath. Clecoing the ribs to the center holes would cause every other hole in the baffle to be elongated to match, which is far from ideal.

Don't drill it like this!

With the ribs clecoed only to the baffle for now, I got the 12-inch #30 bit and got to work, drilling and clecoing the baffle to rib holes first before match drilling the rest of the holes into the rear baffle and brackets underneath. A drill stop is absolutely a must here to avoid damaging the spar below. For the inboard ribs I had to use a scrap piece of aluminum to protect the spar, which lies less than an inch under the baffle and would be very easy to damage even with a drill stop in place.

Eventually, all ribs were drilled and completely clecoed to the baffle and brackets.

Perfect alignment!

Before putting the skin on these fine ribs, I match drilled all of the stiffeners to the lower skins. I then immediately took them off, since I won't have enough clecos to leave them attached and also have the skins on the ribs. It's mind boggling that even with all of the clecos I've collected (I think I have around 600 clecos total at this point? I don't know, I've lost track) I still won't have enough. Oh well.

To fasten the skins to the ribs is once again a lesson in patience and finesse. The skin is, as I have come to learn with these prepunched kits, extremely tight to the ribs around the leading edge. A fair amount of carefully-directed force is needed to get the first few holes fastened properly, but the rest are cake once these are taken care of.

A quick teaser...

A wing!

Upon closer inspection, the gap between the fuel tank and leading edge skins did in fact get smaller. The first bit of drilling to be done is putting #19 holes into the joint plate so the tank skin can be secured with screws and nutplates here.

From there it was reaming and drilling just like on the leading edges, except there's a lot more holes this time. Once done, the tank and brackets were unbolted and unclecoed from the spar in one piece to be placed in the cradle. Progress!

Fuel tank rib work

After putting it off for as long as I could, it was time to return to the soul-sucking task of prepping the fuel tank ribs. Yay.

To be honest, finishing the fuel tank ribs wasn't actually that bad. I'm not sure if Van's cuts these on a newer machine or what, but there were almost no burrs to remove on the ribs. A couple of quick passes with a file and a light polish with a scotchbrite pad were all it took to get the edges smooth.

Unfortunately, the tank ribs were just as deformed as all of the other wing ribs.

This was one of the last ribs I finished. Obviously, this won't do.

The fuel tank ribs have a lot more holes than the leading edge ribs, so there are a lot more places to use the fluting pliers. What's nice about this is that I could use a lot of smaller flutes instead of fewer larger flutes to straighten the ribs, meaning less overall deformation of the flanges and more precise adjustment of the curvature of the rib.

These took mercifully less time to finish than the leading edge ribs, but I'm still very glad to have them out of the way. No more rib work!

That's more like it.

Fabricating replacement Z brackets, aligning the rear baffles

The new Z brackets with the larger flanges finally arrived. I had actually ordered the wrong size brackets at first. The actual brackets have a height of 1 1/8" but I mistakenly ordered brackets with a height of 1" instead, so I had to order another set with the correct dimensions. The unfortunate thing about the larger brackets is that they only come with 1 1/8" wide flanges, meaning they have to be cut down quite a bit for this application. Not a huge deal; this is a custom installation after all.

The brackets actually come as a single piece of Z channel, so to start I cut each bracket to length. Next, I set about finding the exact middle of one side of each flange and drilling a hole to match up with the hole in the fuel tank baffle. The holes in the baffle are aligned perfectly, so that was the side I clecoed first before match drilling anything to the spar.

With the brackets clecoed in place I again used the largest bit I could to match drill the angle through the nutplates already installed on the spars. Finally, I took the brackets off of the spar and updrilled the bolt holes to #12.

The new brackets have much better edge distances for all of the holes. A quick check with the bolts revealed that there is plenty of room to bolt and unbolt this bracket easily. I had seen some builders have issues with being able to fit a socket around the head of the bolts since they were so close to the angle, so thankfully I won't have this issue.

Holding the original bracket next to the new bracket shows just how much more material there is on the new one.  

I cut off the excess aluminum with the bandsaw and deburred the edges with the scotchbrite wheel. The finished flanges only ended up being slightly larger than the original brackets, but these will work way better than the old ones. Done!

Now that all of the brackets were in place I could finally start to align the rear baffle with the holes in the brackets. As expected there were quite a few variations in where the holes fell. Read, most of the holes didn't line up at all. This was mainly due to my barely-adequate centering of the holes in the brackets when drilling them. The plans say to elongate any holes that don't match up, so these are easy fixes.

This was one of the worst offenders. Most had some slight misalignment, but this one required substantial elongating of the hole in the baffle to fit. Any more misalignment than than and I would have considered making a new bracket.

A couple ended up being perfect though!

The easiest way I found to elongate the holes was to use the #30 reamer. The drill bits aren't really designed for removing material via the side of the bit, but the reamer chews through these holes quickly.

Pitot tube mount

While waiting for the new Z brackets to arrive I decided to start working on the pitot tube mount.

I went with a Gretz pitot mount, which has all of the hardware included in the package and works perfectly with the GAP 26 pitot tube I'll be installing. The kit consists of the mast itself, a backing plate to secure the mast in place with screws/nutplates, a washer plate that acts as a spacer for the dimples and allows washers to be installed to change the angle of the tube as needed, and the necessary screws, rivets, and nutplates.

The only piece I had to fabricate myself was an angle to attach the backing plate to one of the wing ribs for strength and to allow the skin to remain flush to the rib with the weight of the pitot tube pulling the skin down.

Using the suggested hole pattern in the instructions as guidance I picked out a piece of scrap aluminum to use for the angle and cut it to size. The most difficult part of the whole process was bending it to a 90-degree angle.

I clamped the piece over a thicker angle to give it a nice sharp radius as it got bent.

The backing plate is blank, save for the cutout for the pitot mast and the screws to secure it, so I needed to drill all of the holes myself. I chose to mount the tube where most builders locate it - one bay outboard of the aileron bellcrank/tiedown bracket bay, on the outboard rib. I clamped the backing plate in place on the inside of the lower spar flange making sure I had adequate edge distance for the holes, then match drilled it to the spar.

I then removed the plate and marked and drilled the rest of the holes. I slightly modified the hole pattern suggested by the instructions to accommodate the custom angle I had made. From here I only loosely followed the instructions and ended up doing things mostly in my own order.

After drilling and cleaning the parts with acetone to remove the markings, I clecoed the bracket back onto the spar. My next task was to match drill the angle I had made to the outboard rib, but I needed to make sure the backing plate would be laying flush against the skin before I could determine the proper location for the angle. I had bent the bracket to match the curve of the skin a bit more, but just to be safe I clamped a scrap angle over top of everything to act as the "skin" and keep things flush.

With the "skin" in place I clamped the angle to the backing plate and to the rib for drilling. I had pre-drilled holes in the angle earlier to use as a guide for drilling the rib.

Once the angle was securely fastened I removed the backing plate for better overall access while drilling.

To finish this part of the job, I secured the backing plate back in place and match drilled through the backing plate into the angle. I then removed the entire assembly and turned my attention to the wing skin.

The hole for the pitot tube obviously needs to be made in the skin for it to be installed, and this is definitely the most nerve-wracking part of the whole process. To start I checked the fit of the pitot mast to the backing plate and washer plate. I'm glad I did, because both of those cutouts needed to be enlarged a bit to allow the base of the mast to sit flush against the plates.

The red sharpie marks areas I needed to trim a little to allow the mount to sit flush against the plates when installed.

Once I was sure the shape in the backing plate was correct, I clecoed the plate to the skin with some shims to act as the "spar" and marked the hole for the mast.

A bit of measuring and remeasuring to make sure things were correct, and I match drilled all of the holes for the backing plate into the skin.

Now for the truly fun part - cutting the hole for the pitot mast. I removed the vinyl and retraced the cutout from the backing plate, and I even set the pitot tube itself on the skin and used a purple sharpie to trace the inside of it as best I could just to make sure things were accurate. Once I had psyched myself up I started the cutout by drilling the perimeter of the smaller traced shape.

From there I made liberal use of the dremel tool cutoff wheel to start removing more material. It's very messy at first but it's easy to smooth out later.

Little by little I made the cutout bigger, constantly laying the back plate over top of it to check the fit.

Once I got close to my traced line I clecoed the backing plate back in place to get the final shape as close to the original cutout as possible.

After some more cutting with the cutoff wheel and a quick switch to some of the sanding bits for shaping the sharp angles and hard to reach areas, I was eventually able to slip the pitot mast through the hole.

A quick shift of the skin and the mast fits flush to the inside of the skin. Perfect!

Only thing left to do to these parts is to prime and dimple as necessary and then they'll be ready for final assembly. Of course, I have an inordinate number of things I have to do before I get to that point.

More Z bracket work, solving a problem

I was almost ready to start match drilling the angles to the rear baffle. Starting with the left wing I laid the rear fuel tank baffle in place and started lining it up with the holes I had already drilled in the Z brackets.

The final brackets to go into place were the inboard ones that will be accessible from the wing root. These are slightly different from the other bracket in that they don't have any nutplates riveted to them. The nutplates used here are riveted to the spars instead, a job I already accomplished back before I riveted the skeletons together since they were way easier to access back then.

As it turns out, Van's has you wait to install these nutplates for a reason. The holes in the spar aren't full size, meaning two things: 1) I would need to enlarge the holes in the spars without damaging the nutplates behind them, and 2) there was no way I would be able to back drill the brackets to #12 with the nutplates in the way. The first issue was easily overcome with a #12 bit, a slow drill speed, and a steady hand to avoid drilling into the nutplates themselves after enlarging the holes. The second issue required a bit more thought.

Since there was no way I could use the #12 bit through the nutplate I decided to use the largest bit that would fit to keep the new holes centered, which ended up being #19. I could then drill the initial matched hole through the nutplate and into the bracket, take the brackets off of the spar, and finally updrill the holes to #12 for the bolts. This worked better than I expected, even if I was a little concerned about damaging the threads of the nutplates with the drill bit. I'm not sure I'd recommend this method over the plans method though.

It was a good thing I was able to cleco the brackets through the nutplates. This would have been way more difficult if I couldn't.

See that top part of the bracket sticking out past the baffle? That's gonna be an issue.

As I went to cleco the rear tank baffle to the brackets, I came across an issue with the inboard bracket I had just drilled. The line where the rivet holes were supposed to be on the inboard bracket wasn't even close to lining up with the holes in the baffle. Even with the rest of the brackets lining up almost perfectly with the baffle in the correct place, I couldn't get the inboard brackets to work.

I took a look at the undrilled angle for the other tank and realized that it would have the same exact issue. There was no way to have good edge distance on both the rivet holes and the bolt holes, even with the bracket positioned exactly halfway between the two sets of holes.

Reading the plans again, I realized that I had made the mistake of drilling the bolt attach holes first and then moving on to the rivet holes for the baffle. What I was supposed to do was cleco the angle to the baffle using the 1/8" hole I had drilled in the angle, then match drill the bolt holes in the angle. Doing it per the plans still wouldn't have worked though since I would have had bad edge distance on the bolt attach holes instead of the rivet holes, so the matter became moot.

As an experiment I took some duct tape and pretended I had a Z bracket with a slightly longer bottom flange for the bolts. After slipping it into place and marking the holes I discovered that this would be a perfect solution to the problem.

Now that looks a lot better.

I soon ordered a section of Z-angle with this longer flange from McMaster-Carr. The longer flange won't interfere with anything else here and will guarantee that all holes in this piece have adequate edge distance, so it's a no-brainer to me.

Fabricating fuel tank Z brackets

Leading edges done for now, it's time to move on to the last major assembly of the wings - the dreaded fuel tanks. To be honest, this part of the build has been the one to which I've been looking forward to the least. It really all boils down to sealing the tanks, which has given many a homebuilder near-crippling anxiety.

But that will come soon enough. For now, let's start with the basics.

Since the fuel tanks are removable, they're bolted to the wing spars with the Z-shaped brackets. My job is to drill the necessary holes and install nutplates for these bolts to secure the brackets and the tanks to the spars. To start, the plans say to mark and drill the EXACT center of each side of each bracket.

I painstakingly measured, remeasured, marked, punched, and drilled all of the necessary holes in each bracket. On most of the brackets I then opened up some of the 1/8" holes to #12 for the middle bolt of three that will attach the brackets to the spar.

One set done, one to go.

To match drill the brackets to the spars I clecoed each bracket on the underside of the spar. The brackets will actually be installed on top (really the forward side) of the spar, but doing it this way allows much easier access with the drill.

A simple 180-degree of rotation of each bracket to the top of the spar after drilling was all it took to get them in their proper place.

Since it would be near impossible to access a nut and a washer back here to attach/remove the tank once assembled, the brackets have nutplates installed. Drilling for nutplates was, as always, a quick and easy affair.

Countersinking and riveting the nutplates on the other hand takes a bit more time. This had to be spread out over a couple of days.

Since the cage hangs off the edge of each bracket when machine countersinking, I simply butted another bracket up against it to give the cage some support and stability.

The nutplates needed to be attached to the brackets to proceed any further with work on the tank, so I elected to go ahead and rivet them on before priming the brackets. I'll still be priming these later once I have the time to do so, but this will allow work to continue without having to set up a whole priming session.

I got into a nice groove, countersinking each bracket and immediately riveting the nutplates in place. Before too long I had almost all of the brackets ready to go for each wing. I bolted each in place with two bolts (which should be plenty strong and stable for now).

All in a row, except for the first one...