Countersunk the Inboard Top Wing Skin Spar Flange Holes

Tonight I countersunk the holes on the wing spar flange for the inboard top wing skin. Before I did this, I called Van's builder support line to ask two questions:

1. What Pop rivets can I use to rivet the rear rib flanges of the Leading Edge assembly to the Wing Spar Web, instead of bucking the AN470AD-4 rivets called for in the plans.

2. Do I have to deepen my countersink tool a bit more that normal to set the holes properly to receive the dimpled wing skins? If so, how deep do they need to be?

The answer to question 1 was that I can use LP-4  pop rivets instead of the AN470 rivets to make it a bit easier to set the LE rib flanges against the wing spar web. these are structural rivets that hold the ribs of the leading edge firmly against the wing spar, so setting these properly is very important.

The answer to question 2 was to check the updated Section 5 of the Van's instruction Manual which is available on Vans website in the revisions section. Section 5.5 states that for dimpled skins that must fit into countersunk holes, it is best to deepen the countersink by .007 more than when the rivet just sits flush with the skin or spar flange.

To figure out how to do this, I had to do some more research about my micro-stop countersink tool.  It has a series of "teeth in a spring loaded cage that allow the depth to be changed in very small increments. In fact, each tooth space is the same as a .001 adjustment in depth, so it is very precise. So in a nutshell, that means that I need to adjust the tool and then countersink a few holes in a test piece until the rivet head sits nice and flush against the skin or flange. Then I need to deepen the countersink by 7 more clicks of the teeth in the proper direction to deepen the hole by that much per Vans recommendation. If you do this and then set the rivet in the hole you end up seeing that the head of the rivet is about 1/64th of an inch deeper than flush. Then you take the rivet out and set a test piece with the dimple inside the countersunk hole, and check to make sure that the test piece sits flush against the skin or flange.

I tried to do all this before I countersunk all the outer wing skin holes, and I though that they were sitting OK with my test piece until I clecoed the skin back onto the spar and found out that those dimples were NOT sitting in  those holes very well as my last post or two explains. Here is the triangular teeth of the countersink tool, all meshed together:

The locking ring on the top unscrews and allows you slide the spring loaded half of the tool back to clear the teeth of the other part of the cage, and then you rotate it the requisite number of teeth to achieve the desired depth. All very nice if you know what depth you are trying to achieve, which, until Vans finally updated their manual to provide this very important information, was a mystery that most builders tried to figure out by trial and error.

Here is the test piece that I used - same as before with some new holes drilled and deburred so I could test several of them to make sure I had the tool set exactly the way that it needs to be.
Note the rivet in each hole, first the one that is flush with top of the metal, and then the one that was set just a bit deeper per Vans suggestion:

You can just see how the second hole is just a little deeper than the first, and the rivet head is no longer flush with the rest of the metal. remember to double click on the pics in the blog to enlarge them a bit more to show better detail.

And here is a pic of all the tools I had to use just to setup the test piece to make this critical tool adjustment.

Once I had the tool properly setup, I removed the fule tank assembly  temporarily to expose the holes in the spar flange, and then I proceeded to countersink the holes for the inboard wing skin.Then I experienced my next error. Some of these holes coincide with a main rib flange that sits up under the wing spar flange. Each side of each main rib (top and bottom of the rib) has two of these holes. Without thinking, I just assumed that both of these holes would get the same countersunk hole, just as I had done for the outer skins.However, there is no attach point for the fuel tank skins in this area, since they are dimpled and inserted into the countersunk for the #8 screws. These holes are already drilled, and the fuel tank skin is screwed to the flange - NOT riveted, since it is designed to be removable for periodic servicing. Since there is no skin dimple being inserted into these holes, the countersink for the top most rivet hole needs to be flush with the spar flange again. I drilled two of these upper flange holes before I stopped and realized what was going on. So hopefully I will be able to set a rivet with a squeezer in those two holes and make it come out flush with the spar flange. We'll see.

Here is a pic of the countersunk holes in the spar:

You can see a couple of the ribs where there is an extra hole in between countersunk holes in the top edge of the spar flange where the screws for the fuel tank will go. Those are the rivet holes that need to be flush with the flange and not set too deeply, since no skin dimple will be inserted into these holes.

And finally, two pics that show the test piece with the dimple in it laying on top of the angle with the countersunk holes. the first is with the dimple sitting in the properly countersunk, slightly deeper hole:

And in contrast, here is the same dimpled test piece sitting in the hole where the rivet was sitting flush with the metal (not countersunk enough for the dimpled skin). You can see how it is not sitting flush on the angle. SO this tells me that the info from Vans is correct - dimples that sit in countersunk holes need to be countersunk just a bit deeper for the skins and flanges to fit together properly.

The reason for all this, of course, is that dimples are round, and countersunk holes have straight edges. SO you have to compensate for the radius of the rounded dimple sitting against the straight edge of the countersunk hole. Kind of like fitting a square peg in a round hole, or vice versa - sort of.

Next steps:

Remove the fuel tank assembly again

Remove the inner clecoes for the rib flange holes that attached the rear baffle plate to the wing spar Z brackets

Replace the tank skin assembly back on the baffle plate

Re-cleco the inner and outer tank ribs to the fuel tank skin again, remove the clecos for the inner and outer rib flanges attaching the ribs to the baffle plate and the Z brackets

Remove the entire tank assembly from the wing

 Dimple the inboard main rib rivet holes - top and bottom - with the squeezer

Cleco the top inner wing skin and wing walk doubler back on the wing frame, removing clecoes holding the outer skin in place where the two skins overlap, placing the outer skin over the top of the inner skin edge.

Remove the outer skin

Re-countersink ALL those remaining holes that need to go just a bit deeper that are currently not quite deep enough - YES, I have to go back and drill all those holes I already did once more time.

Reset the countersink for the flush setting and finish countersinking those holes

Debur the top sides of the Z brackets and the rear holes of the Tank baffle plate

Reattach the outer wing skin, and firmly remount the fuel tank by clecoing the inner and outer ribs to the Z brackets again and securing the tank skins with #30 clecoes into the screw holes and nut plates,  and check the skin fit along the spar flange.

Then I can FINALLY put the LE back on the wing and secure it so that I can mark and measure for the cut out.

And then we go on from there. Lots of little steps, all done in the proper order. At some point everything comes back off again so I can scuff, clean and prime the inside of the top wing skins. Once that is done and the leading edge assembly is done, they will be ready for permanent riveting to the wing frame.