Marking and Drilling Holes for Conduit

Tonight I set out to mark and start drilling the location of the 3/4" holes required for running the conduit that will contain the majority of my wire runs for various electrical components in the Wings. These will include things like strobe and navigation lights, landing lights, pitot tube heat, autopilot servo wires, antenna wires, etc. For most applications you will be running 18-22 ga. wire, so it is relatively small, but items such as the autopilot servos require between 5 to 7 wires just for that, so you have to ahve enough space to run the wires for each component.

I chose to use the Vans Conduit, which is a black flexible light weight conduit that can be easily mounted through a series of aligned holes in the wing ribs. I am admittedly concerned about using it after reading several posts from folks that performed fire tests with this stuff, all of whom comment about the thick toxic black smoke that exudes from it, and how easily it catches fire and how quickly it burns. Not a very comforting thought for a pilot. But I have also reasoned that I do not hear of any accidents or problems that are attributed to wing fires and such, and with 8,000 airplanes flying I think I am OK using this stuff as long as am careful about all of my electrical wiring when the time comes.

This process begins with a service bulletin that can be found Vans website. It contains instructions for drilling additional holes for wire runs in each of the ribs, and provides some general information about where this can be done.

 The big hole on the bottom of the rib in the above pic is the one I am focusing on now. As is usually the case, Vans fails to provide any specific measurements about the location of the holes, just a general picture. So step 1 was to figure out exactly where I need to locate the center of each hole in the rib, then how to create a template that would ensure that each hole in the rib is streamlined from root to tip so that the conduit, adn therefore the wires inside, run in a straight line.

At first glance this seems like a fairly simple process. As you dive into it a bit more you discover there are some things you need to take into account to ensure that the holes are aligned correctly from rib to rib. First is the realization that the ribs are not all the same size. Several of the more inboard ribs are a little shorter in length that the outboard ribs. Why? because the inboard ribs are attached to the doubler plates of the main wing spar, which is about 1/8 inch thick. So these ribs are 1/8 inch shorter. Then you have to determine how they are shorter. At the forward end or the back end. Since the rear spar runs in a straight line all of the ribs ends are aligned, so the difference in length is at the forward part of the rib. Why am I going through all this thought process? Because if you don't get the holes aligned relatively well with each rib it will make running both the conduit and the wire runs a bit more difficult due to the increased friction caused by the uneven line that the wires will need to follow when being pulled through. So the straighter the better.

I took measurements from the front flange of the ribs to the line formed by the perpendicular crease that separates the first and second lightening holes of each rib. This line turns out to be the best reference point for determing where to place the holes. As long as the holes are the same distance relative to this line then they should be relatively aligned with each other.

So now it is time to determine where exactly to mark the center for these holes. My process was to take one of the ribs, eyeball the distances from the document as best as I could, and draw some initial lines to define the center point and the end points of each hole. Then stare at it for a bit to see if ot look right.

I thought that the location in the above pic looked pretty good, but then I had another revelation. I recalled some posts I had read from builders along time ago that mentioned some difficulties with the conduit when it came time to start riveting the skins on. Then I noticed the relative position of my center line and each end point of the hole relative to the two rivet holes on the bottom flange of the rib. The hole I drew was just slightly forward of the center line between the two rivet holes for the skins, so I decided to adjust the hole back about another 1/8 of an inch. For those that may be interested, the location of the rear-most edge of the hole is about 1/4 inch forward of the forward most line of the straight line depression, and the bottom reference is about 5/8 of an inch from the bottom flange of the rib. I found these to be the best reference points to use for creating a template for the remaining ribs that will allow the transfer of the location of this first hole to the other ribs.

With the center point of the hole determined, it was time to drill. I used a center punch and then drilled a small 1/8 inch (#30) pilot hole. No turning back now. Next came a 1/4 inch hole to widen it just enough to fit my unibit with a final 3/4 inch step - you know, that crazy tool that I last used on my rudder rib that I had to replace after screwing it up. Fortunately, I did remember the lesson I learned from that experience. Very simply it is to make sure that when I use that unibit, or step drill as it is also known, use it ONLY with a variable speed electric drill and NOT the air drill. That is how I got into trouble before - just pressing a trigger and letting it wallow out the hole however it damn well pleased.  Not letting that happen this time - so I used the electric drill. Worked out fine.

I clamped the rib using some thick pieces of wood, since the unibit I have is quite long, and would need to penetrate at least 1.5 inches of material before the final 3/4 step would drill through the rib. From this next pic you can see how the hole was repositioned about 1/8 inch back from the original marks. I wanted everything to be equidistant between the two bottom flange rivet holes so that the effort of getting a bucking bar positioned for each of the holes would be as easy as possible.

and the unibit I used with the electric drill:

And then finally with one hole drilled successfully, it was time to make the template for the other ribs. My method - I used half of a cardboard paper file, aligned it with the edge of the straight line stiffener impression aft of the first lightening hole, and placed the bottom corner of the file at the bottom of the rib web. I taped it into position, and then drew the hole on the opposite side, using the edges of the hole in the rib as my guide. I then took an Xacto knife and cut out the hole in the cardboard template. Now all I need to do is tape the template in roughly the same position and draw the hole onto the rib web. I used regular cellophane tape, which seems to work great for adhering, removing, adn adhering again to the next rib. All of the template fitting was performed under several general assumptions, including:

1. The perpendicular stiffener impression is in exactly the same location and orientation on all of the ribs(I forget the technical term for these, but they significantly stiffend the web of the rib - see Hints for Homebuilders videos on the EAA site for info)
2. This rib width is exactly the same for all ribs.
3. I was not looking for perfection here. I figured as long as the hols are within about 1/16th of an inch aligned with each other, then that should be more than fine.
4. I laid the template down across the flat side of rib, where the stiffener impressions are inward or concave to the surface, so that the template would lie flat on the surface. The opposite side of ribs contains the outward or convex part of the stiffeners, and these stick up aboce the surface of the rib web, making it harder to apply a flat piece of cardboard - so use the other side when marking your holes.

Since the ribs have flanges that face in alternating directions (L and R desingations on the rib part numbers), all I had to do is flip the cardboard template over and align it exactly the same way as I did for the other ribs.  Only problem with this is that you have to remove the tape and reapply it to the other side of the template, so it is best to do all the ribs facing one direction first, and then switch the tape one time and do the other ribs, to avoid ripping the cardboard too much. You can also use the same template for all the ribs or make separate ones for the left and right-flanged ribs- the choice is yours.

Now I just have to finish transferring this hole onto all the ribs for both wings, drill, debur, and then I drill a couple of other smaller ones along the front of each rib and one in the rear for the nav antenna wire. I only need to enlargen this rear hole for one wing, but I may go ahead and drill antenna wires access for both wings just in case I decide to run more than one antenna. Antenna wires need to be about 12-16 inches away from other power source wires to prevent interference from the electromagnetic fields that are generated by the power wires. Fun stuff.

So that ends our lesson on how to locate, mark, drill, and debur holes in the ribs for running conduit.