Fabricated the first two 408 rib rear flange additions

I reviewed all my calculations this morning and headed out to the shop. Since pictures do so much better than trying to decipher my detailed dribble in my posts, I took a lot of pics. Objectives were to use some scrap .032 2024 T3 aluminum alclad sheet metal to fabricate each part. I used a couple of different methods for both trimming the part and forming the part because I wanted to see which methods I liked better. To trim the first part I measured the dimensions - 1 and 7/32nds wide by 3 and 1/16th inches long. The length was determine by starting with 2 inches as I discussed in my previous posts. 2 inches plus 7/16 inches to cover the cutout in the 408 rib to clear the spar bars, and then 5/8ths of an inch more for 90 degree flange that lays flat against the Main wing spar web.

I cut the blank using my straight tin snips. This caused the metal at some of the edges to bend just a bit, reaffiriming the reason why I don't like to use the snips when I need the metal to lay very flat. A couple squeezes in my table vise corrected the bending to my satisfaction.

The next task was to mark a line where the flange bend needed to occur. Then I needed to find a piece of metal or wood that was also 3/16th of an inch thick to match the of the rod that purchased to form the bend. After a bit of searching I discovered that my small rapid square was exactly 3/16th of an inch thick, so problem solved. I placed the square flat on the bench, the placed the 3/16 diameter metal rod in front of the square, and then used the flat metal part to align it flush against the edge of the bench. Then I taped up the part over the square and the rod,and secure the square with my clamps. Before taping the part to the square to secure it during the bending process, I placed the bend line for the flange as close as possible over the center of the rod. Then I taped it in place.

Here is the pic before bending:

As can be seen in the pic - I set this one up to bend the short end of the metal part. The was the only way I could do it using this method. To bend the flange I was able to start it by bending it over the metal rod with my hand. Then, after it had been bent over a little, I took my rubber hammer and tried to finish the bend by tapping it down further with that. Unfortunately this caused the square and the rod to start moving as my clamps were not quite strong enough to handle that, so I removed the part and finished bending it with my hand seamer tool.

Here is the finished part placed in its planned location against the rib The bend came out pretty good and seems to follow the bend pattern of the normal 408 rib rear flange.

and again with the longer portion on the other side of the rib web,which is how I intend to attach it to the rib web when the time comes:

the mark next to the 5/16 gap note represents where the 709 rib's rear flange terminates. the whole idea for this part of the mod is to try to make the 408 rib look and function as closely as possible like the 709 rib that it is replacing. SO I ma simply trying to make the rear flange length dimension match the 709 rib dimension as close as possible, and thus the reason for added rear flange parts.

Next is the pic of the 408 rib clecoed into place on the left wing spar in the proper orientation (top and bottom of rib in the correct position). Notice the cutouts on both sides of the rear of the rib web to clear the spar bars from the main wing spar. Also note that there are no spar bars to clear in this location.All 709 ribs have no cutouts, since they are not used in locations where the spar bars exist.

Hopefully this pic makes what I am trying to do very clear:

For the second part I decided to use different methods to create the part and bend the flange. This time I used my dremel tool with a cutoff wheel instead of the hand shears. I guess I should also mention that after cutting out both parts I smoothed all edges and corners on the scotch brite wheel.

Next, instead of marking only one line for the bend line of the flange, I placed two lines. The first is the 5/8s inch bend line, and the second is the 3/32 inch radius line on one side of the part:

Next was the method I used to bend the flange. This time I placed the part in the jaws of my table vise. The jaws are protected with Gorilla tape. to keep the rough edges of the jaws from marring up the surface of the part while clamped in the vise. With the part loosely placed in the vise, I tool the 3/16 inch metal rod and placed it in front of the part on the top of the forward jaw of the vise. Then I position the part so that I could just see the top reference line appear on the very top of the rod. This is how I determined that the bend line was centered on the rod. The other major difference is that this time I will be bending the large area of the part instead of the small flange. Overall I liked this method better but I was not certain if I could secure the rod on the top of the vise jaw well enough to perform the bend. With the rod in place and the part properly positioned, I clamped everything down tight, and then took one of my bucking bars and placed it on the back side of the part and started bending it around the rod as best I could.

And here is the end result of both parts:

And finally I placed the parts on their sides so I could eyeball the edge distance for each rivet that will be driven into each flange of each part. Looks like they should work as expected:

I'll probably trim the parts down to 1.25 inches of material that will be used to rivet them to the rib web, and then each flange gets one AN470 rivet through the main wing spar. Of prepping and priming will need to be done as well. Tomorrow I will trim them and layout and drill the rivet holes.

Was nice to be in the factory again making an airplane.

Prep for rib flange mod

Since I don't remember if I already documented the process that one has to go through to determine how to size, fabricate, and rivet a custom part, I figured I would do it here.

As I previously mentioned, I have to fabricate a total of 4 small flanges, 2 per 408 rib. They are only going to measure about 2.5 inches x 1 inch, with a half inch flange on the end to match the one already on the rib, so they are not very big. Regardless, you basically have to treat this like you are doing a repair, and in that case you need to refer to AC 43.13.2B, chapter 4, Sheet metal repairs, and use the techniques and procedures outlined in that reference to determine exactly how to construct and rivet the parts together. Here are the details of how I figured this out:

1. I took measurements of the area on the rib web that is available for me to apply the additional flange parts. The goal is to make the rear flange match the same dimension as the one on the 709 rib as closely as possible. On one side of the existing flange the width needs to be about 1 and 7/32 inches, and on the other it measured about 1 and 3/8 inches wide. Both parts can be about 2 inches long before I run into the lightening hole. I may not need to use the full 2 inches but I am starting there.

2. The process for this is:
 a. Determine the required sheet thickness for the part
 b. Determine the rivet size to use for the part
 c. Determine the number of rivets to use
 d. Determine the rivet spacing/layout

The thickness needs to be the same for both parts. reference page 4-26 in AC 43.13. I will be using 2024-T3 aluminum that is .032 inches thick, which is the same material used for the 408 rib. So the total thickness will be .064. Here is where I found something interesting when comparing the info provided on page 4-27 of AC 43-13 and the info provided from an EAA SportAir work shop manual for the sheet metal class that I attended some time ago. The SportAir manual says to add the thicknesses of both sheets being joined and then multiply that by 3 to determine the rivet size to use. The example given in AC 43-13 on page 4-27 clearly shows that they only use the thickness from a single sheet times 3 to determine the rivet diameter size to use. So this is wonderful, now I am unclear as to which to use, and the problem is that if I use one formula it will require me to use 5/32 rivets, whereas the other formula will require me to use 1/8 rivets. So that's just great. The sportair manual refers to all this as a "rule of thumb", so my decision is to follow the example provided in the higher power source document, which is AC 43-13. This method allows me to use 1/8 inch rivets which is what I was expecting to use.

The rivet size is supposed to be about 3 times the thickness of the sheet, rounded up to the next largest rivet size. So 3 x .032 (single sheet per example from AC 43-13 = .096. A 3/32 rivet is .094, which is under the required .096 diameter. SO the next higher standard size rivet is 1/8 inch, which is .125. the other consideration is that if the parts to be riveted are considered to be structural, then for most standard aircraft aluminum sheet thicknesses that are used, the 1/8 inch AN470AD4 rivets should be used. So I will be using 1.8 inch rivets for this mod. See page 4-27 in AC 43-13.

The number of rivets to use is based on a formula. An example is provided on page 4-27, and the table it references - table 4-10, is on page 4-38. For my situation, 4.9 rivets are required for each 1 inch of width for aluminum with thickness of .032 inches. In my case I may be using 2 inches of "width" or less for my parts. There is a note on this table, note C, the explains that 75% of the number of rivets shown can be  utilized for single lap sheet joints,which is basically what I am doing.

.75 x 4.9 = 3.675 rivets per inch of width.
3.675 x 2 inches of width = 7.35 or 8 rivets total.

If I choose to only use 1.5 inches of "width" then the formula becomes 3.675 x 1.5 = 5.51 or 6 rivets. Using only a 1 inch overlap would require 4 rivets.

Lastly, after the number of required rivets is determined, you need to determine the rivet layout. AC 43-13 provides a couple of examples in figure 4-5 on page 4-20. The examples show the difference in strength when a double or triple row of rivets is used. This is where you have to account for the size of the part you will have to make so that it fits the area you are working with, and also the number of rivets required to attach it. The primary rule is that no matter what, the area has to be large enough to provide edge distance and enough area to use the required number of rivets as determined above. Notes that I had written on my rib from last year as I began calculating all this indicated that I determined that I needed 5 rivets on one side, and 6 rivets on the other. I think I am going to use a double row pattern. So this requires 2D edge distance around all edges of the part, or .25 inches (2 x 1/8 inch rivet diameter), and 4D distance between rivets, both next to each other and in between rows, or .5 inches. So if I decide to use only a 1 inch overlap, requiring a minimum of four 1/8 inch rivets, the part will need to be at least 1 inch x 1 inch where the rivets will attach the new part to the rib web. for each side of the part, I will need .25 edge distance, plus .5 distance to the next rivet, plus another .25 inches for the edge distance on the other side. so a 1 inch x 1 inch part should be all that is required for the joining surfaces, assuming I mark and drill my holes correctly.

Disclaimer - this information is provided for my benefit,and is based solely on my interpretation of information in a number of different reference materials as I try to determine how to fabricate custom parts for my planned modifications. Anyone else choosing to follow or utilize this information does so at their own risk. Builders must assess their own situations and conduct their own research to address their specific needs,and this info may not provide the correct information for a different application.

There now, so after going through all that, are you still sure you want to modify any part of your aircraft? It takes a lot of brain power and thinking things through before you actually do something different from the plans. As I have said before in other posts, I do NOT take any of this lightly, and I realize that any of the actions I am taking for this modification may put me at an increased risk. So I strive to stringently follow all established procedures and practices to mitigate that risk as much as possible.

Tomorrow I will make final decisions about the dimensions of the part and cut the basic shape, and hopefully bend the flanges, now that I know what bend radius to apply. As a footnote, the bend radius of the W-709 rib is only .0625, or 1/16th of an inch, compared to the thicker 408 rib, which is .09375, or half of a 3/16 inch diameter. They used a smaller bend radius on the W709 rib because it is not as thick as the 408 rib. I found that to be interesting, but I am a bit anal about such things sometimes.
Remember, the devil is always in the details. More tomorrow.

Its Been a Long Long time....

.......since my last post on this blog. Greetings fellow builders and followers. Contrary to what you may have heard I am not dead, nor have I sold my RV-8 project or anything silly like that. I'll just sum it up as I have done many many times in the past - life just got in the way. Kids, school, weather, work, etc.

I have spent the past few weeks cleaning up the shop after a long winter. I did my best to organize everything so I could continue where I left off last August. Specifically I needed to fabricate two small flanges for each side of the modified 408 rib that I was working on. One of the issues I had when I stopped last August was that I needed to determine the bend radius that Vans used to bend the rear rib flanges of the 408 rib. I needed to make sure that the new fabricated flanges are bent to the same radius as the existing rear rib flange so that any stresses on this part of the rib are consistently handled by the entire part.

Turns out that trying to determine the bend radius of an already-bent part is not an easy thing to do. All I knew for sure is that the radius was greater than 1/8 of an inch but less than 1/4 of an inch. I determined this by taking some drill bits with long shanks of different sizes and basically set it in the radius of the rear flange and eyeballed the fit. It was easy to tell if the drill bit was too big because a gap would appear between the drill bit edge and the bend of the flange. The problem is that all drill bits with sizes that are equal to or less than the actual end radius will obviously fit next to the bend radius. SO I started with smaller bits and continued to increase the bit size until I reached the largest size bit that seemed to sit flush inside the bend radius of the rear rib flange. This occurred with a 3/16th of an inch drill bit.

SO off I went to the local HD aircraft supply store to buy a piece of smooth 3/16th inch rod that I intend to use to form flanges of the 2 parts that I need to fabricate. Hopefully once I figure out how to position the rod to make the bends to the .032 scrap aluminum I am using to make the parts, the new flanges will be completed and then I can proceed with riveting them to the rib web. More on that in tomorrow's post. AC 43.13 provided the answers as to how many rivets I will need to use. I provide more info in the next post about the formula that was used and how this was determined.

I'm back!

Drilling holes in the new joiner plate

With the new joiner plate/subskin fabricated and bent into shape to match both 408 ribs, it was time to start the process all over again to drill the rivet holes. Per the post on VAF that I referenced a while back, I followed similar procedures for this, which basically involved marking the hole locations with the drill bit instead of trying to drill through several layers of metal for different parts all stacked together in a situation where they cannot be easily clamped in just the right spot to ensure that all holes are aligned through every part. Having already been through this agony several times with the original 408 ribs and joiner plate assembly, I sure as hell did not want to screw this up again, so I took my time.

The first set of holes to drill into the new joiner plate are the same set of holes I drilled before on the most inboard 408 rib. This is the part where 11/16th of an inch of the joiner plate must extend beyond the edge of the LE skin to provide a mounting point for the fuel tank skin and associated #8 nut plates and screw holes. The only difference is that the rib has already been drilled for the skin attach holes. So I only needed to locate the holes of the joiner plate through the LE skin, and ensure that the 11/16ths of an inch was properly positioned.

I placed the new joiner plate over both inboard 408 ribs and then measured the 11/16ths just as I had done before. I clecoed the LE skin to the main spar, and the rear flanges of all the ribs to the main wing spar web. I reached inside to make sure that the second 408 rib was as straight as I could possibly make it. THen I simply took a number 40 drill bit secured in my air drill without having it connected to the air line, and I inserted the bit through the predrilled LE skin holes and then spun the drill chuck manually several times to carve out just enough metal to leave a noticeable mark on the part. Once all the holes were marked this way I removed the clecos from the LE skin, removed it as well as the new joiner plate, and placed it on my drill board so I could final drill the holes:

After all of these holes for the inboard 408 rib were drilled, I had to reinstall everything again. THis time, with the inboard holes drilled through everything, I could cleco the inboard LE skin, joiner plate, and rib flange holes together. this served as a stable method to align and position the rest of the joiner plate so that the second set of holes could be marked and then final drilled, following the same procedure I used for the most inboard holes. I did not drill through the entire joiner plate - just marked the holes, disassembled the LE, and placed the joiner plate back on the drill board again, just as before. IN the pic below you can see the first set of holes on the left, and new marked holes on the right:

With all the rib flange holes drilled in the new part for both inboard ribs, I could now mark the center line of the rib flanges on the second 408 rib, cleco down the LE skin assembly one more time, and look through the holes in the skin AND the joiner plate to locate the center line of the rib flange as shown:

Remember that all of the pics in this blog can be clicked on to enlarge the photo and see more detail. I proceeded to drill all the holes through the skin, plate, and rib flanges, and after removing the LE assembly one more time verified that all the holes appear to be in a position on the rib flange that I consider to be acceptable, which is not too close to the bend radius and not too close to the outer edge of the flange. I'll have more pics of this later. This basically means that I now have a new sub-skin/joiner plate part with all the holes drilled in just the right place, even after replacing one of the original W709 ribs with another 408 rib to allow for the thickness of the wider joiner plate that I using for this LE modification. I am relatively pleased with the way this turned out, even though it added several days and hours to the build. I still need to repeat this process for the right wing, but now I know all the steps that I need to take,and all the fixtures I needed for forming the part have also already been created. Unfortunately, since I messed up the measurements on the first skin I have to order a new one, and that will take several days to receive from Vans, so it looks like I may be heading back to some fuel tank fitting and hole drilling in the near future.

Next steps are to debur the holes in the rib, and after a visit from my technical counselor John Linz, he suggested that I should go ahead and fabricate some rear rib flange extensions so that I can use both of the outer holes that were predrilled in the main wing spar for the W709 rib for the new 408 rib. This should be an easy thing to do by using some .032 scrap to create a small flange and attach it to the rib web with a correct amount of rivets per AC 43-13. That will solve the questions and concerns I had about the difference in the size of the rear flanges for the two different LE ribs. Once that is done I can proceed with marking and drilling the fuel tank attach screw hole locations on the new joiner plate, essentially picking up where I departed from the original plans.

As for the new joiner plate/sub-skin support - next steps for that are to carefully measure and mark where the flanges for all the additional nutplates will need to be located so I can trim the part to remove all the unnecessary excess. So still lots to do - just need to remember to measure about 30 times and cut/drill only once. I don't want to screw this up now or I get to start it from scratch all over again.

New Joiner Plate Fabrication Details

OK, so its a bit later than "tomorrow." Been a horribly busy week of work and other stuff this past week, but I have also managed to keep working on the plane, slow-going as that may seem. As promised, moving on with my LE modification - it all started with the piece of 12 inch wide x 3 foot-six inch long x .032 inches thick piece of aluminum sheet. First it had to be cut to size, and then the fun part was bending it to roughly the same shape as the original joiner plate, which wraps around the contour of the W408-1 L and R ribs, and is sandwiched between the rib and the LE skin.

The width stayed the same - 12 inches. This seemed to work well because it was just wide enough to span the entire distance between the first two inboard ribs, while still allowing for just enough excess on either end to allow for the 11/16ths of an inch extension on the inboard side for the nutplates and bolt holes for the fuel tank attach points, and just enough on the other end to overlap the rib flange by a bout 1/4 inch or so. I am glad I did not have to trim the width at all.

Here is the sheet that I began with:

Then came the fun part. I actually ruined the first sheet by not carefully thinking about the cut lines, so before I knew it, I ended up with yet another piece of scrap metal. In a nutshell I had to trim the edges of the length-wise portion of the part to account for some differences in the rivet hole pattern of the LE skin between the most inboard W408 rib and the and what was originally the first inboard W709 rib, now converted to another W408 rib. It turns out that the skin-to-rib flanges are shorter on the W408 rib since it was originally designed to clear the spar bar that lays down across the main wing spar web on both sides. I also discovered that the most rearward rivet hole on the LE skin is also about 1/4 of an inch lower than the same one that attaches to the original W408 rib. This again is due to the difference in the flange lengths of the W709 and W408 ribs.

In short, about half of the width of both ends of the sheet had to have the original 36.5 inch length for the first half of the part, but the second half of this edge had to be about 1/4 inch longer, or 36.75 inches, to account for the different rear-most rivet hole locations on the two rib bays. After correcting my measurement problems from the first attempt to trim the part, here is a series of pics showing the lines, radius holes, and rough cut appearance after it was all cut to size:

I used a #30 drill bit to drill a 1/8 inch hole to create a 1/16th inch radius in the sheet where the dimensions on the ends change. Then I smoothed the edges on the scotch brite wheel.

Then I had a real dilemma to figure out - I needed to try to find a way to ensure that I could bend the sheet into exactly the same shape that the original joiner plate was bent, which follows the contour of the W408 rib. THis also brought to my attention one small detail about the aluminum sheet that ordered from Vans, and that is that I was not sure how the grain of the metal should be oriented. After looking at the original joiner plate strips that I fabricated, I determined that the grain of these strips ran along the long side of the strip (parallel). Of the two sheets that Vans sent tome, I determined that one of them had the grain running across the long edge of the sheet. So I had two sheets, each cut with the grain running in a different direction. My problem was that the sheet that I had mismeasured and cut was the one with the grain running in the same direction as the original part, so I was concerned about this, but I decided to proceed with the other sheet anyway. I figured if I really need to make another one then I will know how to do it quickly. SO I considered this to be a true "experimental" exercise in how to fabricate a part of my own design. IT was more important to get the process down than to worry about which way the grain is running at this point.

This led me back to the original problem - how to form a curve in a piece of aluminum sheet that was originally only 1.5 inches wide, but is now 12 inches wide? The original skinny part was hard enough to form to the correct shape, so I was not looking forward to this wider one.  My idea was to create a series of form blocks out of wood that matched the shape of the rib, and stack them together to make a form block that was almost as wide as the 12 inch wide sheet. I already had some cut outs that matched the general curve of the nose of the LE ribs from the parts I cut to make the LE cradle. I decided to head over to HD aircraft supply and pick up a 2x10x 8 foot long piece of wood. I then traced the rib pattern onto the wood and used my jigsaw to cut out each rib form: Here are some pics of that process: 

Even though I drew the pattern from the same rib, the nose of each piece of wood did not come out as uniform as I had expected. Cutting with the jigsaw is not always very exact. So I took each one and ran the noses against my rotary sanding wheel to even them up:

With the nose sections of the wood forms mostly uniform, I now had to decide if I wanted to bolt them together to keep them tightly held together, or if could just clamp them into place while forming the aluminum sheet around the part. I decided to try to clamp them together first and see if this would work. I used several bar clamps as shown in the following pics to clamp the forms near the edge of my work table, and then slipped one side of the sheet in between the forms and the par clamps. The only other thing I needed to figure out was where to place the end of the sheet against the rear part of the forms to make sure that the ends of the part would end up in the right position. Once I determined the correct location of the end of the sheet, I clamped it down against the form, and then I started to bend the sheet around the LE of the forms by hand.

As shown from the above, this was good enough to form the initial bend, but due to the spring back it was not quite good enough to form the sheet as far as it needed to be.So I resorted to another method tht could apply a little more force than my feeble hands. I used my ratchet straps to cinch down the skin around the forms and this helped a little more, but again was not quite enough:

Next I unfortunately had to remove the part and begin that wonderful exercise of finishing up the form with my hands. This took quite a while and involved some squeezing, rolling, forming, pulling pushing, swearing, cursing, bleeding, - - you know - - basically all those things involved with trying to form a complex part by hand. Here is a comparison of the "finished" part and the original joiner plate:

Not too bad if I do say so myself. From the above pic you can also see that I clamped the rib down on the table. Once the original form was complete I began using the actual rib to verify that the curvature of the part was at least close enough to form correctly around the rib. The true test, however, came when the part was placed inside the LE skin and mounted on the Main wing spar. More on that in the next post.....

Initial Fuel tank Fit and more on the revised joiner plate fabrication

Only one thing to say about a modification from the original plans - it is HARD! More on that in a bit. Here are some lingering pics I tool to show the initial fit of the left fuel tank, top wing skin, and LE. Looks pretty close even without cinching anything down with the straps:

And I marked the inboard rear tank rib flange hole locations just to see where they lined upon the flange:
NOte the severely out of alignment rib to rear baffle holes on the right - not sure why these are so off but will have to check ribs and straighten flanges again for resorting to anything else.

Then I marked the basic location of the holes for the #8 screws that will attach the tank skin to the joiner plate. I am not going to use the stock joiner plate, but wanted to see where the holes ended up being positioned out of curiosity.

Some how the spacing on the top of the joiner was reduced from the 11/16th of an inch, so the hole location for the screw and nut plate is very close to the edge. Not sure if a dimple can be applied with it that close to the edge. Enough about the fuel tanks. The rest of my final weekend while on vacation was spent working very hard on the modified joiner plate.

I began by removing the fuel tank and putting it back in the cradle. I then removed the left wing LE and placed it on my work table, after removing all the skin to rib clecoes for the first inboard W709 rib. SO the two most inboard ribs are not attached to the LE skin:

Then I took some measurements on the main wing spar to verify that the center holes of the 408 rib and the 709 rib are in deed in the same place. This was painful, as there was no easy way to find a reference on the main wing spar for the W709 rib holes. I don't even remember exactly how I did it, but I did confirm that the center holes of each rib are in fact aligned with each other, so that was good news. This means that I could use the center hole of the 709 rib and the one I drilled in the second 408 rib as a starting point for marking and drilling holes.
Here is the 709 rib on the left and the 408 rib on the right, flipped so that the tops and bottom LE skin flanges of each rib are matched up with the correct top/bottom orientation.Note the difference in the length of the rear flanges of both ribs:

As shown in this next photo, I marked the location of the pre-drilled hole in the 709 rib on either side of the center hole onto the rear flange of the 408 rib:

You can also see the two outboard holes in the 709 rib that will not be usable on the 408 rib because they are outside of the rear flange of the 408 rib. The two most outboard holes were drilled by me to match the hole pattern in the wing spar. Instructions said to drill new holes using the hole pattern in the wing spar,which is what I did. The two holes inboard of those holes are the original factory holes which Vans says to "abandon." Easier to view in the pic than explain. I did decide to go ahead and use the two marked holes on either side of the center hole for the 408 rib, but I have some questions for a tech counselor about where exactly to drill the remaining holes - I meet with him tomorrow after work to discuss.

Next is the hole spacing of the first 408 rib in the main wing spar. I needed to compare this hole spacing to that of the 709 rib to try to determine how the rivet holes would line up (or not). IF anyone i interested, the casual observer might say that these holes are spaced about 1 inch apart. My more exacting measurements revealed that these holes are actually 31/32nds of an inch apart. Consider it useless info, as is most of this post if you are building the stock wing. You can also see my intended center line and the one that I actually mistakenly marked and drilled. We shall see how this works out.

Oh yeah, NOw I remember how I checked the center of each hole pattern cuz I took a pic of it. I used a flat wood block held along the spar flange and ran my ruler up against the wood block. GENIUS!

Next I took the original 408 rib and clecoed it to the new 408 rib to mark the existing holes in that rear flange so I could compare spacing with the 709 holes I marked previously. Why is this so important? Too many holes can weaken the part, and not enough holes can created stress points in other parts of the rib during flight. This needs to be done correctly.

And here are all the holes marked on the rear flange of the new 408 rib:

And now for the infamous hole drilling exercise of the rear flange of the new 408 rib. I had to remove the LE skin to do this, and I also had to reverse the orientation of the rib on the spar so that I could afix my square to the wing spar to ensure that all holes drilled in the rear flange of the rib are straight, and therefore the skin flanges of the rib should also be straight. THe tie down bracket gets in the way for this rib so you have to squaer it from the other side. Since the center hole was drilled correctly it did not matter how you positioned the rib on the spar. I'll just run these as a series of pics since they pretty well show the procedure I use. The only trick was that I had to use a different approach to clamping the square to the wing spar, since there is no spar bar to keep the square from sliding off the rounded edge of the wing spar flange and web. I was able to figure this out but it took some serious time to get it all secured properly before I back drilled the two additional holes into the rear flange of the rib.

And two new holes successfully back-drilled:

WOW! I never even got to the part about forming the new joiner plate/subskin for the LE! I will have to get to that tomorrow as it is past my bed time. I will just say that I spent most of my day today and part of yesterday trying to form this part. It was real bitch and I used a large number of techniques that I will detail tomorrow.