<October 15, 2005>
Countersunk the Wing Spars
Today I tackled the job of countersinking the wing spars. I certainly didn't want to make a mistake here so took my sweet time getting everything set-up, and drilling lots of test holes in scrap alumiminum. In the end, it went pretty easy with no major problems... Thank God!
I started by covering the gap between the layers of the spar assembly with tape to keep out the aluminum shavings. Then I countersunk for the rivets that hold the nutplates to the spar. By the way, since you have your countersink set up, you might as well do both sides AND go ahead and countersink the holes for the nutplates on the bottom that hold the inspections covers in place (check the plans to see which holes). This way you don't have to go throught the trouble of setting up your countersink tool again.
Another suggestion... if you are going to use tape to cover the seam in the spar, use something that is very lightly sticky. I used blue painters tape because I thought it was gentle, but it left a lot of sticky stuff behind when I peeled it off.
The manual says to rivet the nutplates to the spar then use the nutplates to guide the pilot of the countersink for the #8 screw. Well, that wasn't good enough for me so I made this little tool to help me countersink the screw holes. I basically involved 3-3 rivets sandwiched between two pieces of aluminum as guide pins, and a #30 hole to accept the pilot of the countersink.
I just clamped the jig into place using a spring clamp and countersunk away. As you can see the short rivets acted to keep the jig in place while the countsink pilot stayed centered in the hole (in the jig). I didn't need to use anything other than a spring clamp because there was really no pressure exerted on the jig, only on the spar.
According to the first 2005 edition of the RVator, the proper width for the wing spar tank screw countersinks is .365" to .375", so obviously I set up the countersink for the middle of the range or .370".
Nice huh? I am pretty pleased with the way these came out, but... for the inboard few nutplates (the angled ones), I used the method of using the nutplate as a guide for the countersink and to be honest, I can't really tell the difference between the holes countersunk using my jig and those using the Van's method. Was it worth it... probably not, but no harm done. The only benefit to my method is that you have the ability to deburr the back side of the hole since the nutplate isn't already installed.
Looks good from this angle.
All perfect... no really, they are.
I finished the tops of both spars and called it quits for the day.