Drilled Tank Drains and Fuel Cap Flanges

I positioned the fuel drains and match drilled them to the skins.

For the fuel cap flanges, I drew reference lines at the top and bottom.

I then positioned the flanges in the precut holes in the tank skins so that the hole is centered and the lines I drew are visible in the top and bottom rivet holes.

Before drilling, I confirmed that the fuel caps are centered in the hole in the skin.

Both fuel caps are now match drilled to the skins.

Fit Capacitive Fuel Senders

There are two options for fuel quantity senders.  The first is to use a float that reads the fuel level with a hollow plastic float on the end of a wire that sits on the top of the fuel in the tank.  The position is read through a resistive mechanism.  This is a well-proven design but has a few drawbacks.  The other option is a capacitive fuel sender.  This has no moving parts and can measure from the first few ounces in the tank to the last few.  The only disadvantage that I’m aware of is that it must be re-calibrated when you change fuel types.  Since I don’t plan on doing that much (if ever), this is a non-issue for me.

The capacitive senders require mounting two electrically isolated plates in the fuel tanks.  The capacitance between these plates and the tank is measured by a small AC signal.  The fuel acts as the dielectric and changes the capacitance.  Once calibrated, this system is easily accurate to a tenth of a gallon.

The first thing is to cut some plastic tubing to length.

These slip over the screws (after putting on a large countersunk washer)…

…and through a UHMW washer…

…and three more UHMW washers on the other side.  As you can see, the screw can’t possibly come in contact with the rib.  This is what keeps the plates isolated from the tank.

The screws thread into nutplates that are mounted to the capacitive plates.

Here is one of the plates after mounting it to the rib.  The two outboard plates have to be modified at the bottom to clear the stiffeners.

Drilled Tanks

I match drilled all of the holes in the tank skins, ribs, and baffles.

The skins must be countersunk where they attach to the baffles because the baffles have to be slipped in after the ribs are riveted to the skins.  I only got one side of one skin done because I’m exhausted.

Fit Tanks to Spars

I used the Checkoway method to fit the tanks to the wings.  Since his site has been taken down, I’m going to document this rather carefully so that other builders can refer to it if necessary.

First up is to assemble the tank.  I’ve heard of many other builders struggling with getting the ribs to seat fully in the leading edge using the instructions Van’s provides.  I’m using an alternate technique that works very well and let me assemble each of the leading edge section in about 5 minutes with virtually no struggle.  The first step is to cleco the first few holes on the top side of the skin near the leading edge.

Next, pad the leading edge with something that is firm but will conform to the leading edge somewhat.  I used a dense foam mat that’s folded over a few times.  Stand the section up on the leading edge with the rib you’re working with directly over the pad.  Push down firmly with one hand while the other hand puts a cleco in the topmost hole on the bottom of the skin.  By pushing down firmly, you can easily get the hole in the rib close enough that the cleco can be pushed through and the cleco can be used like a lever to pry the holes into alignment (make sure you’ve pushed all three pieces of metal at the tip of the cleco through both holes before doing this or you’ll damage the cleco).

Next, remove the plastic from both sides of the tank baffle and cleco it in place (yes I know I hadn’t removed the plastic yet here).

Next, drill a 3/16″ (or #11) hole in each of the z-brackets.  The root most bracket should have its hole shifted about 1/16″ away from the web (relative to the center) to allow a wrench to be placed over these bolts.

The rest of the brackets should have the hole shifted about 1/16″ toward the web.  This will give a little more room on the other flange for the pop rivet tool to seat the pop rivets that hold these brackets onto the tank.

Using this center hole and some scrap bolts, tighten these brackets firmly to the spar (making sure to get the flange orientation correct).

After all brackets have been bolted to the spar, back drill them through the holes already in the spar.

Here is a closeup of what you should end up with.

Next, remove all of the brackets from the spar and attach platenuts to all but the inboard z-bracket.  For the inboard bracket, the platenuts are attached to the rear of the spar.  Go ahead and attach those platenuts as well now too.  After that, bolt all of the z-brackets to the spar.  I used all three bolts here because I found there was some minor hole adjustment that had to be done for the bolts to fit through cleanly and I wanted to make sure that that was done before the z-brackets were match drilled to the tank baffle.

Put the tank on the wing and push it firmly against the outboard leading edge.  Use some straps to pull the tank tight against the wing so that the skins are touching.  Make sure the joints are tight all the way around before proceeding.

Here’s a closeup of the joint between the inner and outer leading edges.  You can see that the leading edges are perfectly flush and the joint between the skins has virtually no gap.

I’m doing both wings at the same time.  Here’s a shot of both tanks in place.

While the tank is in place, drill the tank to joint plate holes out to #19.

Now, drill the inboard z-bracket to the tank baffle and inboard rib.  This is the only part of the baffle that can be reached right now.

Take off the outboard leading edge.

This gives you access to the outboard z-bracket.  Drill these holes out to full size.

Now, remove all of the clecos holding the tank skin to the spar, baffle, and end ribs.

After removing the skin and interior ribs, you are left with the baffle in the perfect position to ensure tank alignment.

If you zoom in on this picture (click any picture on this site to zoom in), you can see that the centerlines that were drawn on the flanges show through the holes.

Now, drill the remaining holes in the baffles to the z-brackets.

This took me an entire day, but both tank baffles have been drilled to the spars.

Drilled Outboard Leading Edge Skins

I match drilled the outboard leading edge skins to the ribs and spars.  I didn’t take any pictures since it would look just like the picture from yesterday.  I also drilled out the holes for the tie down ring.  Here is one of the tie down rings partially screwed into the tie down bracket through the new hole.

Finished Drilling Main Skins and Fit Outboard Leading Edge Skins

My buddy Andre stopped by tonight and drilled all of the bottom main skins while I worked on getting the leading edge skins ready to install.  I started with fabricating a simple cradle to hold the skins while fitting the ribs.  This is just a couple of pieces of particle board roughly cut to the shape of the ribs and lined with some old carpet padding I had laying around.  The shape really isn’t that critical since it just needs to help hold the skins closed while you get the clecos in place.

Update: Particleboard is really the wrong material for this as this fixture will take a fair amount of abuse during the build.  Both of these U shaped pieces broke near the bottom of the U while working on my outboard leading edges.  I didn’t need them anymore, so I didn’t fix them, but save yourself the hassle and use plywood.

Andre and I clecoed the leading edge ribs into place and fit the leading edge of the left wing in place.

The innermost rib needs to be fitted with a joint plate where nutplates will mount to help hold the fuel tank.  This was a real bitch since there are no holes in either the joint plate or the rib and the fit is really tight.  After futzing with it for about 10 minutes, I finally got everything in place and drilled a few holes to hold everything in alignment.  The only issue I ran into was one of the holes that attach the rib to the main spar ended up with too little edge clearance.  I doubt it will be an issue, but I’ll check with Van’s just to be sure.

After Andre left, I knocked out the other leading edge in an hour or so.  These still need to be match drilled, but that can wait until tomorrow.

Drilled Top Skins and Fit Bottom Skins

I got started tonight by fine-tuning the lower wing clamps to the point that there is no measurable twist.  I then drilled out all of the skin holes on the top skins (~1000 holes) and this also entailed moving every cleco.

Afterward, I pulled out and attached the lower wing skins.  Here you can see the three access panels that are on the bottom of the wings to make inspecting the wing, aileron control linkage, pitot/AOA, autopilot roll servo, etc. possible.