Subskin prep - Drilled and Dimpled Remaining nut plate rivet attach holes and Reassembled the LE

Today was a busy day at the factory, but for the amount of progress I made today you would probably not believe me. I finished sanding the edges of the cutout section of the LE skin smooth and straight, and used the combination of my pneumatic rivet squeezer and C-Frame tools to dimple all remaining #40 nut plate rivet attach holes on the subskin. I had to switch from my flange yoke to the standard 3 inch yoke to be able to reach all the rivet holes along each side of the subskin, and this worked out nicely. I had to use the C-Frame to reach most of the rear-most rivet holes on both sides of the subskin, and this also worked out well.

Unfortunately I had to use the #30 cleco method to secure the nut plate to the subskin so that I could align and drill the holes for each nutplate on the top side of the subskin, just as I had previously done on the bottom side. Just as before, I had trouble keeping the line of rivet holes in alignment with other straight edges, and some of them got a bit slanted. As I stated before, these holes are just for the rivets that attach each nut plate to the subskin, so the MOST important hole is actually the center hole where the screw will be inserted, and not the rivet holes.

I used the same technique to mark the alignment of the rivet holes on the top as I did  on the bottom, by placing a straight edge along the center line of each #30 screw hole and drawing short lines on either side of each hole that I used to align the rivet holes of the nutplate after it was clecoed to the subskin through the screw hole.

I drew the lines along the sides on the outer side of the skin, and drew the lines for rear nut plate holes on the inside of the skin, just as before. As shown from the pic above, I also came up with a different clamping and prep method of drilling these rivet holes. This time I really did not care so much about trying to rig up something so that I could clecoe the nutplate in plate as shown above, and then ensure that I was drilling each rivet hole into a piece of wood. I reasoned that any slag or burr on the holes would be removed by my deburring tool afterward, and so this time I chose NOT to waste a substantial amount of time coming up with ridiculous ways to ensure I could drill each hole into a piece of lousy wood. This worked out fine, except for the slight shifting of the nutplate because the clecoe just does not apply enough force to keep the drill from moving the nut plate around. SO I much prefer using the nut plate jig which prevents this from happening, but since I was not yet ready to drill the #30 hole to a #19 hole yet, which the jig uses as a pilot hole, I could not use the jig tool.

I used my 4x4 clamping blocks to provide some height to allow the cleco mounted from the underside as shown in the next photos to clear the table and saw horse ends, and the next pic shows how I clecoed the first hole after it was drilled, which keeps the second hole from moving around as well, so that both rivet holes are in alignment with each other.

As each set of rivet holes was drilled I detached the nut plate and moved it to the next hole and re-clecoed it in place, and repositioned and reclamped the subskin as necessary to retain some rigitidy in the subskin near the nut plate locations so the drill would not slip or cause the skin to warp and the hole to become mis-aligned. Here is the series of pics as I continued to drill each set of nut plate holes along the sides:

With the sides done it was time to drill the rear nutplate hole. I had to flip the subskin over and reposition and reclamp it once again. I was able to use the edge of the table to help position the skin so that the clecoe underneath butts up against the edge, keeping the skin nice and stiff while drilling.

The last pic above actually shows the holes after I dimpled them. The mot important thing to call attention to here is that I have NOT yet final drilled the screw holes to the proper size because I need to make a template first so that I can fabricate a properly sized part that will replace the cutout section in the skin. I cannot final drill the screw holes to #19 and dimple them for a #8 screw until the template has been created and at least 1 or 2 .025 removeable LE skin sections have been fabricated and fitted in place. To do that I need the screw holes to remain at #30 for now. 

Next I had a bit more cutting, filing and sanding to do on the subskin. A long time ago I had marked the location where the hole for the tie down ring is located by tracing the hole I had already drilled in the LE skin onto the subskin, which, until today, had been covering up that hole. I used the cut off wheel o n the Dremel tool to carefully cut away the metal in this area, staying just OUTSIDE the drawn lines so that  could ensure that the subskin would have enough clearance to screw in the tie down ring. Here is the pic of that after filing, sanding, and rounding all corners was completed:

With most all of the subskin prep work done, it was time to reassemble the LE for what I hope will be the second-to-the-last time before it goes together permanently. the subskin was attached first using all the rivet holes that are not part of a rib flange hole. This gets the subskin in the proper position for the ribs. Then the two inboard-most ribs are clecoed to the subskin and outer LE skin. This process always takes some work because the ribs are a very right fit in this area due to the thickness of the subskin. Again, the goal of all this was to reassemble the LE with the newly cutout section in the skin that exposes the subskin that will be used to attach the removeable LE section.

Here is a series of pics I took to show the fit of the LE and subskin after remounting it all back on the wing spar again. Overall I was pleased, but the problem that I feared and mentioned in a previous post is exactly what I encountered. The outboard are of the cutout section of the LE skin shows a bit of a gap between the subskin and the outer LE skin, whereas the inboard section seems to be draped over the subskin fairly well. This will make fitting the removeable part a bit of  a challenge, but we'll see how that goes. I am not overly concerned about the gap because I think the rib in that area combined with the subskin provides more than enough support to keep the LE from deforming in any way as a result of this gap. In other aircraft designs, such as the Sonex for example, I know that their ribs do not even wrap around the contour of the LE skin. Instead, they stop well short of the very front of the skin so that NONE of the ribs have any rounded contour that wraps around the LE at all, leaving a void between the rib and the very front part of the LE skin.

As long as the shape of the front of the LE skin remains consistent across its entire span I do not see a problem with this. I think this will occur because the rest of the LE outer skin is stressed, so it retains most of its shape due to the tension of the bend that is force-ably wrapped around the top and bottom flanges of each LE rib.

First pic is the top side of the LE with the subskin now showing through:

Next are the inboard and outboard curves of the front of the LE skin showing the gap I mentioned between the LE skin and the subskin  on the outboard section. Both pics are takne from the top side of the LE:

 

The next set of pics is the same series taken from the bottom side of the LE

And finally 2 pics that show the clearance of the tie down ring hole through both the outer LE skin and the subsin after trimming it away from the subskin. Looks pretty good to me, but I may decide to wien both of the holes a bit to make certain that stresses placed on the ring from high winds etc. will not start contacting the edges of the LE sin in this area. The ring can be screwed in and out with ease.

So this was a day that was very long in the making. I had to wait 2 weeks for it to warm up enough in the garage to get to this point. took about 3 hours or so to do everything described in this post. Next steps are to make the file folder template and fit it to the cutout section of the LE, mark and drill the #30 screw hole locations in the paper template with the LE firmly mounted back on the wing spar, remove the template and use it to trace the new removable section of the LE onto a piece of .025 aluminum. I may decide to go buy some Home Depot grade stuff to practice on until I am certain that I have the outline of the part measured, cut, and formed correctly. My concern now will be what I need to do to make the curved front end of the removeable section match the curvature of the inboard and outboard outer LE skin. Not sure if I will need to apply some shims due to the gap, or if I will be able to pre-stress the removeable section similar to the outer LE skin so that it will retain its shape with the rest of the LE skin. Time will tell on that one.  Hopefully I will be able to get to that point this week since the weather is finally supposed to be warmer.