OK, vacation is over. The airplane obviously won't build itself, so it's time to get crackin, er drillin I guess is the better word. Over last weekend I managed to get the junk off of my garage shelves and into the shed. I had purchased a roll of some bubble wrap sheet for insulating my trailer windows a long time ago that I never used. It basically consists of the same bubble wrap material that is used for shipping, compressed between two sheets of reflective chrome plastic sheeting, and is a bout 1/4 inch thick.
With everything off of the garage shelves I then had an idea for using this stuff. It would be a great protector of my sub assemblies, providing a bit of shock absortion and preventing scratches on the skins, etc. It also just about fits perfectly on on my 18 inch wide shelves with no problem at all. The actual dimensions of the roll are 24 inches wide by about 30 feet long, which was more than enough to cover the 21 feet of shelving. I unrolled it onto the shelves, placing the excess width of the roll up against the wall, and lining up the other edge flush with the outer edge of my shelves. Then I trimmed off the excess at the 21 foot mark.
Next step was to take the long pushrod and longeron angle bundle that had spent a long time nestled away in the long crate of the wing kit, and I was now able to transfer that bundle off the floor and onto the back side of the shelves. I have to say I thought I would never find a suitable place to stow this bundle so that it was out of the way and not a risk of being damaged. Problem solved.
Then I took my right wing main spar and both rear spars and set them on the shelves. Worked out great for now. My plan is that once I get one of the wing spars on the stand, the other spar can come down on the bench, but for now I am still juggling where to put my elevators. All other wing parts have homes on other shelves, so I think I am finally ready to begin some actual work on the left wing.
I reviewed in detail my CD from Dan Checkoway, and also Ron Duren's blog site, to determine how best to countersink the holes. There is a point of detail missing from both sites that I will endeavor to fill on my site, and this has to do with the size of the pilot hole that you drill in the 1/8 inch aluminum angle scrap which will serve as a sort of backer piece of metal through which the countersink pilot will be able to insert into.
Before I get too far head of myself - here is the problem at hand:
Each nut plate has 3 holes, two for the rivets on either side that hold it in place, and the center hole for the bolt or screw that it will eventually hold. The prepunched or predrilled holes for each nut plate in the wing spar include the correct size holes for the two AN426AD3 rivets to attach them to the spar, but the prepunched center hole is already wider than a #30 countersink bit, which is what the plans tell you to use to countersink the hole so that it will accept the dimple in the .032 " thick tank skins. If the hole for the countersink pilot is too large, the countersink has no way to center itself properly, and this would result in oblong and off-center holes being countersunk in the spar flanges, which is not good.
Ron points out in his post that the real guide for everything is actually the two mounting holes for the rivets, which will then position the threads of the nut plate in the correct location that should match the dimpled hole in the tank skin, and he is correct about this. The problem, however, is that an oblong countersunk hole may not seat the dimple and/or the screw head correctly, and this may cause abnormal wear on the dimples of the tank skins over time. SO while the location and general shape of the countersunk hole is not too much of a concern, I feel that the accuracy and symmetry of the countersunk flanges surrounding the hole are quite important. SO you don't want to botch these up IMHO. The thought of having to reskin/build a new wing tank long before you need to sends chills up my spine.
So, what to do....
Vans says to cleco the nut plate into position, and the center hole in the nut plate will "adequately" provide the guide for the #30 countersink bit to keep the counterskins relatively uniform. Big problem with this method is that the clecoes get in the way of the counterskin tool.
Dan and Ron used a different approach that I am also going to use as follows:
Take a piece of scrap aluminum angle that is 1/8 inch thick, temporarily clamping it to the underside of the wing spar flange, marking the location of the center hole throught the hole in the spar onto the angle, drilling the #30 pilot hole centered as well as possible inside the marked hole on the aluminum angle, re-clamping the angle into position by positioning the hole in the angle under the hole in the spar flange, then drilling the #40 holes for the rivets on either side, clecoing after each hole is drilled. The clecoes are then removed after the holes in the angle are aligned, and the clamps continue to hold the angle in place while the center hole is countersunk to the proper depth for the #8 flush head screw. Then you move the assembly to the next set of nut plate holes and repeat the process - 360 times to be exact.
Now for a laundry list of other things that I need to do to prep for this task:
1. Put masking tape over the seams between all the layers of the spar, spar doublers, adn the top and bottom spar bars to prevent metal chips from falling in between the crevaces of those parts - excellent idea from Dan.
2. Start on the bottom flange first (goofs on the bottom don't get seen) Excellent tip from Ron.
3. Take one of the screws, cut the threads off so that only the head remains, then take an allen wrench and super glue the top of the screw head onto the tip of the Allen wrench and use this as your depth gage for the countersink. Great tip from Ron.
4. Take some scrap .032" aluminum and drill, debur, and dimple the skin with a #6 AND a # 8 dimple so that it can used to test fit the dimple in the countersunk hole. I prefer to make a piece of this long enough to fit at least two holes so you can verify that the skin sits flush on top of the spar flange, which means that the countersunk holes are deep enough. (Tip from Dan)
As a side note, I bought some tank dimple dies and now I have to also go back and research VAF again as there were contradictory reports of these dies worked any better or worse than the normal dimple dies. The difference is that the tank dies create a slightly deeper dimple, presumably to account for a layer of proseal that must surround the rivets used to seal the tank.
5. Clearly mark each wing spar with a sharpee so that the root, tip, front web, rear web, top flange, and bottom flange are clearly identified.
6. When countersinking, start shallow and work up to the proper depth. Set the tool so that you reach the correct depth without having to push too hard on the surface. The rhythm should simply be as follows: position, drill, remove, measure the depth, and move on to the next hole, with no repeat drilling needed.
7. Clamp the spar into position securely on the work bench to comfortably countersink all the holes, and protect it from scratches and dings
Looks like tomorrow will be the day to start. After several hours reviewing plans and builders logs, it is time to cut loose and and make lots of aluminum dust. I could not put it better than the way that Ron did in his post when he was ready to start this same process almost 2 years ago, so to sum up the mental leap that it takes to start something totally new, see his post.
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