The holes are parallel with each of the rib holes on each side, starting at the very front of the LE and extending back about 4 or 5 more holes on the top and bottom of the skin. Their main purpose is to secure the trimmed edges of the main LE skin to the subskin, after cutting out the removable section, similar to the same way that an access panel's mounting bracket attaches to the skin.
The idea is that these rivets will secure the area of the LE skin that is near the cut out for the removable section, and keep it from flexing too much where the rivets that secure that ribs to the skin are attached, hopefully preventing the formation of any cracks or cause any structural fatigue or failures in this area, and to ensure that the main skin continues to sit flush next to the removable section to maintain a smooth flow of air over the entire assembly.
I measured and drilled 4 holes on each side of the LE for the bottom (for a total of 8 holes), and 5 holes on each side for the top (for a total of 10 holes). As I have reported before, the reason for this offset number of holes between the top and bottom of the LE is to ensure that the most stressed part of the skin around the LE is totally secured to the front of the LE. The curvature of the skin is different on the top and the bottom, so the number of rivets required to hold down either side's major stress points is also different.
The same thing is being done for the number of screw holes required for the top and bottom of the removeable section. The further back the skin goes, the less curving stress exists on the skin. This is primarily being done for safety, so that if for some reason any rivets or nut plate screws were to fail or come loose on the back side of the removable section, it would not want to pop up all the way and act like one large dive brake or spoiler that will kill all lift over that section of the wing (and possibly kill me in the process by creating a situation where the wing can no longer sustain adequate lift to stay airborne, or worse yet introduce a stall condition that I am unable to recover from).
The idea is similar to when you stick your hand out of the window into the slip stream of a fast moving car. If you hold your hand parallel to the road, there is less drag on your hand, and the air will continue to flow essentially undisturbed over your hand. However, if you then take your hand and flip it up 90 degrees, then all that air hits the palm of your hand and forces your hand and arm to move backward very rapidly. By choosing to extend the removable section back far enough from the major bend of the front of the LE, I hope to prevent the same rapid drag effect from happening as much as possible. If I lose anything on the bottom of the LE this is less of a concern, because the net affect of this will be added lift and drag to the bottom of the wing, which is a situation that I think I will be able to deal with and continue to be able to fly the plane.
This is also the reason why all of the attach screws for the removable LE section on both wings will be checked thoroughly before every flight, to make certain that the screws are tight and secure, all rivets are still in place, and that this section of the LE is still nice and smooth and is retaining its proper shape from front to back.
The reason I need 5 new rivets on each side of the top of the LE, and only 4 on each side of the bottom, is due to the camber or curvature of the wing on the top, and straighter, less curved shape on the bottom.Due to the increased curvature, there is a greater distance over the top of the LE than on the bottom to get to that point where the bending stress on the skin starts to decrease. The bottom of the LE skin is flatter than the top, so it takes fewer holes or less distance between holes to reach the same point of decreased bending stress compared to the distance required on the top.
I finished drawing intersecting lines through the previously drawn lines to locate the exact point for each new rivet hole, used my center punch to mark them, then took my #40 drill and MANUALLY turned the drill bit with my hand several times to remove just enough metal to ensure that each hole was appearing exactly where I wanted it. Then I plugged the drill into the compressor and finished drilling each of the holes on each side of the bay through the outer LE skin and the subskin.
I am now past the point of no return on the modification. If I screw up now, the effort to fix the problem will be ugly and complicated, or may force me to abandon it all together and just build the stock LE after ordering new ribs and a new LE skin. So before I drill any new holes, I double and triple check everything, and make certain that this is what I need to do, before I do it. That is why this act of marking and drilling a total of only 18 new holes took me about 1.5 hours to complete.
Here are the pics - two of the bottom and one of the top, showing the new clecoes in the new holes, right next to the clecoes securing the skin and subskin to the top and bottom rib flanges:
Basically, with the rivet holes now created along each side, I now need to make similar rivet holes for similar reasons to secure the main wing skin to the subskin along the rear-most edges of the removable section. The big question about this was how many rivets to use, and how to space them out.
For that I am relying on information in section 4 of AC 43.13B that explains that a "patch" to an existing skin should use the same rivet pitch and spacing as other nearby rivets to ensure that the loads are carried similarly. This applies to the rivets along the sides as well as with the most-rearward attach points on the main wing spar. SO the rear-most rivets will align as much as possible with the same spacing as the rivets that attach the rear of the LE skin to the wing spar. I am trying to do everything here as recommended "by the book."
The last part of the hole drilling exercise will be to locate and drill the pilot holes for each of the nut plates that I will need to install along the sides and the rear of the removable section. I think I have resolved the number and placement of the nut plates to use, but I am still "visualizing' that a bit more before I start drilling those #30 holes to mark the location for each screw. More on that in the next post or two.
KPR.......
Glad to see you're back to working on the plane. KPR
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