Since this is my first plane, I was more than a bit nervous about drilling holes in those big wing skins and the ribs. I reviewed everything I could find on the subject. First the BearTracks newsletter and the eGroup archive. Next Russ Erb’s CD and then Eric’s Wing manual. A quick review: The BearTracks outlines a method of strapping the skins to the wing skeleton, carefully transferring measurements from the skeleton to the surface of the skin, drawing lines, marking the rivet locations and then drilling the skins and ribs simultaneously. I think this was Russ’s approach and it is not without potential problems. Russ confessed to some gross buffoonery in transferring some measurements and the resulting miss-drilled holes relegated the skin back to the smaller parts bin. Hmmm, this is not building my confidence. Eric used a method where he strapped the skin to the skeleton and then marked the position of the ribs and spars on the inside of the skins with a Sharpie marker. He then removed the skins and used a framing square to mark the rivet line upon which he measured and marked the rivet locations. I liked this approach better since transfer errors were avoided. I still had questions about the accuracy in this method however. I thought I would give it a try with some heavy weight roll paper before committing a skin to the process. While looking around the shop for the roll paper, I came across a roll of heavy weight clear mylar. I thought this would be perfect for my test but then realized the potential for a pattern. This page explains the method I stumbled upon. Having now used it for marking the skins of my first wing, I can attest to the accuracy attainable with this method. Same kind of amazement as when you realize that dead reckoning really does work.
I made patterns for the top skin first but did not take pictures so I’ll demonstrate making a pattern for the bottom skins here.
Here is the sheet of mylar clamped to the skeleton. Pull it tightly and smoothly to the skeleton and secure with spring clamps. It was sized a little larger than a bottom skin panel but covers only two bays/three ribs.
It really is on there! You can see the bottom of the sheet hanging below the rear spar.
Marking up the mylar. The mylar is clear so you can see what you are doing. To make the pattern use an Ultra-fine Sharpie and as accurately as possible mark the following:
1. bottom edge of the Main Spar bottom flange (shown here) 2. bottom edge of the Rear Spar bottom flange 3. all rib flute locations 4. both sides of all ribs near the spars where they are fixed (indicate the rib flange edge with an identifying mark)
You do not need to mark the web of the spar. They are hard to mark accurately with the radiused edge and capstrips and this is not used in the layout.
Mark the rib flanges with a thin marker line as accurately as possible.
Mark the rib web with a thick marker line. This just helps you keep up with where the rib flange is in the whole scheme of things, so accuracy is not essential.
Mark the position of the flutes in the rib flange on the mylar too. This will help you to avoid mistakes in placing the rivet locations.
Here is the clear mylar sheet after marking on the wing skeleton, removed from the wing and laid on my building table.
OK. See the red line at the bottom of the picture? That is the mark denoting the bottom of a spar flange. I did not mark it all the way across the mylar when it was on the wing. I just marked the two sides as accurately as I could manage.
I then used a straightedge to mark the bottom of both spar flanges by making two parallel lines (blue, horizontal) to either side of the original (red) mark.
Above that you see the marks for the rib flange, rib web and a rib flute.
Here is the key for accuracy: The line of rivet dots are placed the same distance from the rib flange edge on the pattern as the line made on the ribs to indicate the flange centerline. I used .3 of an inch because it was easy to see and mark with the scale I use.
At the bottom (shown) and the top rib flange markings, mark points .3” (or whatever you choose) from the rib flange edge mark (the thin blue vertical line).
Now connect those marks with a straightedge for the full length of the rib. That is the very thin black line here..
Now you must decide where to begin the rivet line. I determined that on my wing spars that the first rivet above the rear spar should be 1.1” above the rear spar flange edge (as denoted by the double blue horizontal lines we made earlier).
I’ve measured 1.1” and placed a green dot on the black centerline to denote where the rivet line begins. .
Now determine the rivet spacing you need to use (approximately 1.5” as per plans) to space the rivets for the full length of the rib and end up at the center of the main spar and miss all of the flutes.
I used a rivet fan to help visualize the layout and check for problems. Remember the green dot is your anchor for spacing the rivets.
Use a rivet fan or just measure and make a dot on the centerline for every rivet location. I marked the final rivet locations with blue dots.
Here is the completed pattern held up against the wing skeleton. The rivet dots lay on top of the white rivet line when the blue dashed line is aligned with the rib flange edge. When you use the pattern to mark and drill the skins you will see the white line through the rivet holes in the skin when the rib is straight.
That’s all there is to it. I guess you could make a pattern for a single rib just as well and it would work fine. You could probably make it out of another material such as aluminum or plexiglass as well. I just used what I had available and it works great.
On the next page I’ll demonstrate how I use the pattern to mark a skin.