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.

Filing the Edges and Corners

Spent about an hour or so tonight filing the edges and the corners of the cutout area of the LE. It was finally warm enough in the garage again to work on the plane. This was basically a repeat of the same effort I used to fabricate the new inspection panel cutout from long ago. The only difference is that the edges are longer and are on a curved surface instead of a straight one. I used the same tools that I used for the access panel job to file down the metal edges of this cutout. I used a standard flat file, a curved rasp, a round rasp, a small jewelers curved file, my small metal ruler to check for edge straightness, and the SafeAir inspection plate that already has the 1/2 inch-radiused corners so that I can use it to test the shape of the corners and adjoining straight edges on either side for a true square fit. Here is the pic of all the tools as I started to file the edges on the top of the LE skin:

I had to once again reposition and re-clamp everything several times so that each edge was supported properly as I started filing. I had to position the edges of the cut section just off the end of the table allow enough clearance for the files moving back and forth, and allow enough room to reposition them as needed. Every time I work with the files I am amazed at how precise you can be with them. The key is to have patience, work slowly, only remove small amounts of material at a time, and CONSTANTLY check your progress against your drawn lines to make sure you are not going to far and removing too much material, which would be devastating at this point. 

I started with the edges and corners on the top side of the LE skin. I worked on those first, so that I could get those shaped properly. Here you can see the "fit" of the inspection plate on the unfinished edges of the LE skin before I started removing material with the files. the edges and the corners all had extra material that needed to be removed. I filed a bit and then kept re-fitting the inspection plate until I was satisfied that both edges were straight, and the 1/2 inch radiused corner was also fitting correctly. Both of the following pics show that there is still some filing to do.

I used the curved rasp to remove the material in each corner by slowly removing the metal and checking the curved line drawn on the skin. The goal was to just meet the sharpee line as well as each edge on either side of the radiused corner, and repeatedly check the fit with the inspection plate until I was satisfied with the fit.

After a few minutes, this side was finally starting to look pretty good:

I have this tendency to try to make everything fit almost perfectly. Some folks would say I have OCD in that regard, but the truth is that I enjoy the craftsmanship part of this project so much that it gives me sense of accomplishment when I am able to form metal with a degree of precision that I used to think could only be accomplished with wood. The reason why it is so important for this to fit together well is that any gaps around the edges will cause the airflow to be disturbed, adding drag to the airplane, and this will just slow me down or rob me of extra performance that could otherwise save me some fuel or allow me to fly faster or to reduce my stall speed considerably. This is where airplane building separates itself from other types of building. If it ain't "perfect," it will most likely deprive you of something, and it may even put your life at risk.

One problem with a "perfect" fit is that I am not allowing much room for primer and paint to be added to the parts later on, so it is very likely that the edges on some of these removable or tight-fitting parts may have to be "reduced" just enough to allow for several mills of that stuff to be applied and still fit together nicely. I'll deal with that later.

Here is the top after most of it has been filed to my satisfaction:

Then I flipped it over and started working on the bottom half. I was able to file the edges out to the tip of the bend so that I would have a good enough reference to match it against line from the other side as much as possible.

Here is a shot of how I would apply the flat file to the edges

Getting closer to completing the bottom corners with the curved rasp

And the last two pics I took of each bottom corner after fitting the access panel to each of them and finally being satisfied with the fit.

I was a little concerned that I may have messed up one of the edges by cutting too close to the edge with the cutoff wheel, but after checking it closely I think everything worked out fine. I used the metal rulers to check the straightness of each of the edges, and even though some optical illusions started messing with my eyes to make me think that the edge was not straight, the ruler sitting flush up against it was proof enough that I should stop filing and walk away before I really messed it up.

SO overall this did not take very long to do - certainly not as long as I though I would, and I am ready for the next step which is to sand all the file marks off of the edges, especially in each of the corners. If I do not manage to remove all the burs and potential stress risers from these edges, they will most certainly start to crack at some point, so proper deburring is an absolute must.  I will sand each corner and each edge until ALL the file marks are gone and the corners are smooth. There are a lot of burs from the Dremel cuts on the inside of the skin, so I will need to make sure that those are removed as well.

So it was a good session tonight with no surprises or issues. Now I'm just working down the remaining list of To-do's I posted previously to get this finished as soon as possible. The part of this that has me nervous is when I fabricate the removable section and start drilling screw holes in that and then start attaching it to the subskin and checking to see f that part will lay completely flush with the rest of the LE skin. Personally I don't think it will for a number of reasons, but I have been pleasantly surprised by my workman before - may be I will again. I sure hope so.