Step 6.1 Fuel tank stiffeners part 2

It seems somewhat ironic that the first step in making a leakless fuel tank is drilling hundreds of holes in it.  The necessary second step is to fill those holes with rivets and Pro Seal.  Pro Seal is a 2 part epoxy based sealant especially formulated to be sticky, gooey, messy, difficult, and uncompromisingly unpleasant.  It is also impervious to gasoline which makes it ideal for sealing up the aluminum fuel tanks.  

The process of sealing the tank is fairly straight forward:  Put Pro Seal in between every surface to be joined.  This means coating the dimple for every rivet, the skin and ribs and rear baffle where they meet, and every fitting entering tank.  

In the photo below I have already cleaned the skins and stiffeners a Scotch Brite pad and then with MEK.  Then I placed a dab of Pro Seal in each dimple on the outside of the tank skin along the row of stiffeners. I set the rivets in place and tape over them, holding them temporarily.  The skin can now be flipped over to coat the skin and stiffener with a thin coat of Pro Seal.  Then the stiffeners are set down over the line of rivets protruding through the skin.  At this point, the row is ready to be back riveted.

After the rivets are set, the formed head also known as the shop head, is coated with a dab of Pro Seal to discourage fuel from using that rivet as a pathway out of the tank.  The last step is to inspect each rivet and edge fillet with a magnifying glass.  It doesn't have to be pretty, just fuel tight.  

Not wanting to appear stingy with the whole Pro Seal experience, I allowed Carol to help out.  Here she applies the black death to the shop heads around the fuel filler fitting.

Here are some little trap doors that are fabricated to prevent fuel from running out of the inboard bay and away from the pickup tube.  The doors cover the lowest hole in the rib separating the 1st and 2nd bays.  In the photo below the hinge pins are not yet cut, but you get the idea.

In the next post I begin the fuel tank assembly using the Rick Galati fay sealing method.  Stay tuned to get the exciting details on this one.

Step 6.0 Fuel tank attachement flanges

The fuel tanks attach to the main spar, but they also attach to the fuselage at the leading edge with a big bracket made from 3/16" aluminum angle.  Did I mention that these things are beefy? Well they are.  The first thing to do is to cut the angle to length which in this case is 3-3/4".  

There are also doubler plates that fit inside the inboard and outboard ribs of the fuel tanks.  In the case of the inboard rib, the doubler plate is opposite of the attachment bracket and must be match drilled with it and inboard rib.  The doubler plates shown below are precut by Van's to the leading edge shape, so they only need to smoothed down with a file and then a little bit on the Scotch-Brite wheel to finish.

With the attachment angle cut to length it was time to trace the leading edge shape onto one side and then follow the dimensions from the plans on the other.  The plans leave out some of the necessary dimensional information, but it doesn't seem to be too critical so I'll just keep going.  The missing dimensions are the radius at the top of the flange which is labeled R1 on the plan, the width of the base of the flange, and the lower radius where the flange meets the base.  I think Van intended that we just freehand a pleasing shape here.

Shown above are the right and left attachment flanges as cut (on right) and after smoothing and polishing (on left).  And below is the finished product after laying out and drilling the attachment holes. In the left tank I'm using a flop tube fuel pickup for aerobatic operation.  With a flop tube, the fuel exits the tank near the leading edge, hence the big hole in the left bracket. 

The fuel tank access hole is cut from the inboard rib. The center of the access hole is found using a  geometric construction from your high school geometry class.  Oh, your dog ate that home work?  Ok, draw a vertical line through the circle to be removed about midway between where you think the vertex might be and the left or right edge of the circle.  Now bisect that line and extend it across the circle.  The bisected line will pass through the vertex. Repeat this process starting with a horizontal line through the circle and you will have found the center of the access plate where the bisect lines cross.

Because I'm a just a crazy daredevil with life insurance, I've elected to apply a fly cutter to this task.  I adjusted the drill press to its slowest speed, 250 RPM, so as not to miss any of the carnage.  I crossed my fingers and let it rip.  Fortunately, no one was injured during this process, but it is a bit scary.  It may be too intense for small children.

And finally, the access hole doublers.  The nutplates are attached to the inside of the doubler and flush riveted to the rib.  The access plate will then be able to seal against the rib's smooth outside surface.  The doubler is used as a drill guide to drill through the rib for the attachment screw holes.

  

Carol came down to help out with match drilling the leading edge doubler plates and attachment flanges.

Meanwhile I'm dimpling the #8 fuel tank attachment screw holes.

And a little deburring...

Then match drilling the flanges of the fuel tank fillers and drains.

And finally, dimpling the skins and stiffener plates.  The tank is now ready for assembly!

Step 5.9 Fuel tank stiffeners

After excitement of match drilling the Z brackets, this post may seem a little tame.  That was quite an ordeal that will not soon be forgot. We laughed, we cried, and after just the right amount of pain and agony was experienced, we eventually we got it done!  Now it's time to push on to the fuel tank stiffeners.

The stiffeners are short pieces of aluminum angle that are riveted span-wise along the bottom of the tank for additional support against the weight of the fuel.  There are two rows of stiffeners in between each rib.

As was the case with the empennage, the stiffeners arrive in the kit in long pieces that are cut to length and then the edges are dressed with the Scotch-brite wheel.

There are four different lengths of stiffener and the angle stock is not marked for which is which.  In addition, three of the four types have the same number of holes.  This would seem to make identifying  the B, C, or D types rather difficult.  The faithful reader of this blog may be thinking that this sounds suspiciously like a diabolical plan to encourage the builder to make grievous fabrication errors that would naturally lead to the purchase of additional parts from the man himself, Mr. Van. I'm sure that the omission of any identifying information is innocent enough [wink], and in any case, (after some pondering and sufficient time had elapsed) I discovered that Van has actually slipped up this time -- It seems that the pre-punched holes have different spacing and can only fit in certain bays within the tank.  OK, I get it.  Never mind.

  

So here they are.  24 shiny stiffeners ready for match drilling to the tank.  Not quite ready, first they and the matching tank skin need to be uniquely marked so that after drilling and deburring they can be reassembled into the same location.  Now they're ready.

Drill, drill, drill, drill, drill, drill, drill, drill, drill...














Next on the list is machine counter sinking the row of rivet holes that attach the rear tank baffle.  Shown below is the micro-stop counter sink cage with a #40 piloted counter sink bit.  I put the skin on some scrap plywood and drilled through the skin in to the plywood.  The holes in the plywood guide the counter sink pilot so that the bit doesn't get to wobble about chattering and making oblong holes (this is the secret desire of a counter sink bit).  So after some preparation, very nice, round counter sinks result.

Following all of the counter sinking of the tank skins, I drilled screw holes around the periphery that will attach the tank to the spar and to the W-423 splice plate.  And finally, the screw holes are dimpled for a #8 counter sunk screw.  At this point, the tank skins are nearly ready for final assembly with ribs and the black death also known as Pro-Seal (epoxy sealer).  There are a number of other things that go into the tank before we're ready to assemble, so the skins will be set aside for now.  

The final step for this posting is to attach the nutplates to the W-423 splice plate.  The splice plate is shown above sticking out from the leading edge section.  In the photo, I have the nut plates clecoed to the outside where I am using them as a drill guide for their attachment rivets.  Each nut plate is fastened with two flush rivets, so after the holes are drilled they must be counter sunk.  The screw holes are then enlarged to #19 and dimpled.  Finally, the nut plate is riveted to the inside of the splice plate completing the assembly.

Step 5.8 Fuel tank Z brackets

We'll begin this post with my three legged dog Roxy.  No particular reason other than she has been present for much of the work on this airplane and deserves some recognition.  [applause]  Just kidding about the three legs.  

The first step in assembling the fuel tanks is to prepare the Z-brackets.  It seems that there are two camps when it comes to this step:  the Van's way, or the Dan Checkoway method.  Everyone seems to agree that Dan's method is superior, but there are plenty of sage advisers online to offer the tried and true: "follow the manual and you won't be sorry"  or the somewhat less helpful "I did it the Van's way and it worked out fine."

So what is a Z bracket?  It's an aluminum extrusion whose cross section is in the shape of a "Z."  These extrusions are about 4.5" in length and are fastened vertically along the front face of the main spar and form an interface between the fuel tank and the spar.  The Z bracket will be riveted to the tank on one side and have nut plates to capture bolts coming through from the other side of the spar.  Removal of the fuel tanks for inspection or repair is facilitated via these bolts. 

I decided to investigate Dan's method but I ran into a road block right away -- his website is no longer available.  To learn about this now mysterious method, I would have to gather the information second hand from those who claim to have followed it.  Right away I find that the method is not really documented  as thoroughly as I would like.  More like just a mention here or there and some pictures.

Here is I summary of what I believe the much bally-hooed Dan Checkoway method to be:

1.  For six of the seven Z brackets, mark a line lengthwise along one flange 1/16" offset from the center,  away from the web.  The offset should be such that additional clearance to the web for a socket or wrench is achieved.

2.  Find the center of the Z bracket along the line and drill a #12 hole.

3.  Temporarily install on the spar and tighten down with AN3-4A bolt.  Make sure the bracket is square to the spar.  Check the line through the other two holes in the spar and clamp the bracket securely.

4.  Match drill the other two holes through the spar and bracket.

5.  Install the nut plates on the brackets.

6.  Install the brackets on the spar.

7.  Assemble the fuel tank with clecos and install on the spar.

8.  Position the 7th Z bracket optimally in the inboard location under the inboard fuel tank rib.  This will be the bracket that did not get the nut plates.  Clamp the bracket securely and then match drill the screw holes through the spar.

9.  From either end of the tank, match drill the tank's rear baffle to the Z bracket and rib.  Then secure with clecos.

10. Remove the fuel tank skin and the tank's inside ribs.

11.  Match drill the inside Z brackets through the rear baffle.

Lastly, and this is very important, Sit down and have a beer!

Armed with a semblance of the Dan Checkoway Z bracket alignment method, I went to work on marking, drilling, and nut plating the Z brackets for both wings.

After the brackets are drilled for the bolts they are then prepared for nut plates.  Using the nut plate itself, the ears are drilled through.  The last step is to counter sink the other side so the rivets holding the nut plates on will sit flush.  This allows the Z bracket to sit flat against the spar.

With the nut plates complete, the next step is to mount them on the spar.  Since they will get mounted and removed several times before the fuel tanks are complete, I'm only putting two of the three bolts on for now.

The Z brackets are on the spar and I've clecoed the left fuel tank together.  

I placed the tank on the left wing and match drilled the W-423 splice plate the attaches the outboard end of the fuel tank to the matching leading edge section.  Note the strap to hold the tank down tight before drilling.

The final step for the Z brackets is to match drill through the back of the fuel tank and in to the forward flange on the bracket.  The tank skin must first come off to permit access.  The inboard end rib is match drilled through to the Z bracket.  Here it is shown after I removed the rib and was in the process of removing the tank skin.

At this point I finally had to face the consequences of not following the build manual's order of construction regarding the leading edge section and fuel tanks.  After I had finished the leading edge sections I, impulsively it seems, decided to rivet them on to the spar.

Unfortunately, with the leading edge section in place, it is not possible to match drill the  outboard end rib through to the Z bracket below because the end rib's flange faces outboard leaving no room between it and the leading edge section to access the rib flange from that side.  Crap!  While it certainly would be preferred to lock down the far end of the tank's rear baffle before match drilling the interior rib-Z bracket connections, there is another way for us scofflaws. (People who don't follow the manual.)

Step one on getting myself out of this predicament was to release one side of the tank skin leaving the other firmly attached to the spar.  Then reach up inside and match drill the next rib in from the outboard end to locate the tank's rear baffle to the Z bracket.  With that rib's Z bracket clecoed down, the rear baffle is now set firmly from end to almost end.  OK, so far, but what about the inaccessible outboard Z bracket?

Well, I can see it up in there just laughing at me, but what to do about it?  Here is a closer view.  The bracket is just barely visible in the middle at right, between the rear baffle (above)  and the spar (gold).

My solution was to spray some primer through the gap at the Z bracket and baffle.  This will mark the bracket's exact location relative to the baffle. When I remove the tank I can reposition the bracket up to the paint line then do the proper match drilling.  Whew!  Disaster averted.  Now on to the next calamity.

The final step on the Z bracket odyssey is to complete the tear down of the fuel tank, removing the skin and ribs to expose the rear baffle.  Now the interior Z brackets can be match drilled.

With the rear baffle exposed, the remaining Z brackets are easily match drilled.  So now I remove the baffle to reveal my primer paint trick.  Fortunately, the line is very sharp which will facilitate the re-positioning nicely. 

And so the last Z bracket is clamped to the baffle and match drilled.  Now that I been through this once, I should be able to complete the right wing Z brackets with a minimum of drama.