More Custom Leading Edge Rib Work

Who would have thought that modifying a rib would be such an enormous task? Truth is its not really all that bad, but there are moments.....

Anyway, wanted to post a pic of the graduation cake from the party we had back on the 23rd of May.

4 more years!!!

Now back to the latest on the build. With both upper and lower rear rib flange extensions fabricated, now it was time to fit them to the rib and match drill them. I decided (rather stupidly as it turns out) to drill the holes with the rib still clecoed to the main wing spar and the extensions clamped into place against the rib web. One small problem that I ran into was that I needed to match drill the holes from the inboard side of the rib. I forgot that the wing tie down bracket and fuel tank z bracket would be in the way of the drill, so I had no choice but to use the 12 inch #30 drill bit extension so that the drill chuck would clear the brackets (just barely). I should have removed the rib and match drilled the holes on the bench, but I was concerned about the rear flanges of each fabricated flange extension sitting flush with the main wing spar, so that is why I wanted to drill the holes with the rib attached to the main wing spar.

Here the drilling has begun:

The trick was to get the flanges of each part flush with the main wing spar, and I also needed to provide about a 1/32 inch gap between the edge of each part the edge of the original rear flange of the rib sothey would not rub against each other, as shown in the next pic:

Also, trying to get all those clecos to fit next to each other was a chore. I needed to get creative about the order in which I drilled the holes. As shown above I tried to drill the first two holes to secure the part in position by drilling one hole on top and the opposite side hole on the bottom.

Next are the tools used:

After drilling the holes I deburred them and basically All I need to do now is prep the parts for primer and prime them. I am wiating to drill the final two holes in the new rear flange extensions until the LE is all primed, and the underskin mod is final trimmed to size. I need to mount the LE with all ribs in place and the entire assembly clecoed into position before I drill those two holes in the new rear flanges.

So that then led me to tonight's activity. With the flange extensions pretty much completed I unfortunately have one more rib modification to make. As shown in the pics below, which I switched the W709 rib out for the W408-1 LE rib, the missing rear flange extensions were one thing that I had to resolve. The other thing has to do with the last LE skin rivet hole that attaches the LE skin to the LE rib flanges on the top and bottom of the LE rib. Since the rear flanges are cut out of the W 408 rib, the top and bottom rib flanges are also shorter than the ones for the original W709 rib that I am replacing. SO, when I match drilled the LE skin to the 408 replacement rib, the rivet holes ended up at the very edge of the rib flange as shown in the next two pics:

SO I obviously have to do something about this to ensure that the last rivet hole at the base of the LE skin for this rib location has enough edge material to provide proper support for the rivet and the stresses that may occur in that area of the LE skin. My solution is to cut part of the existing flange away, and fabricate a small strip of metal almost as wide the rib flange that I can fit underneath the rib flange and create a kind of extension of the upper and lower rib flanges. its kind of weird but in order to do this I actually have to trim away part of the existing flange so I can fabricate a new flange and attach it so that it has the proper edge material for that final rivet hole.

For reference, here is what the W709 rib flange looks like for that final rivet hole. Note that there is at least an additional 1/4 inch of material beyond that last hole. This is what I have to create for the 408 rib.

I am going to accomplish this by fabricating a strip of .032 aluminum and then putting a joggle in it to serve as an extension of the existing rib flange.A joggle is simply a small rise at the end of a part that allows it to slide over the top of another part and fit seamlessly next to the other part.I'll start with what it looks like after the joggle is formed.

The joggle is formed by determining the location where the joggle is needed, drawing a reference line,and then position the part between two pieces of metal of the same thickness or the desired thickness for the joggle that is needed. One piece goes on one side of the part and the other piece goes on the other side of the part. You then tape everything together and there are a couple of different methods to create the joggle. You can either pound the part with a hammer, or you can place the taped up assembly in a bench vise and let it form the joggle as you apply extreme clamping force with the vise. I tried both methods just to see how each would work, and the vise method definitely comes out better.

I used two pieces of .032 scrap metal and taped them to the part, which is simply another strip of .032 metal about a half inch wide and several inches long. This was a practice piece so I was not too concerned about actual measurements.

Then you put in the vise so that the jaws will apply pressure to the joint created by all three pieces of metal:

ONce that's done,you remove it from the vise and fit the newly joggled part to the other part that it will be attached to. Here you can see the basic idea and how it will allow me to extend the length of the flange just enough for the final LE skin rivet hole.

Of course, as with everything else about this project, it is a bit more complicated, because I have some precise measurements to make to determine the exact amount of material to remove the from the existing flange so that I can create a part that is long enough to accept two flush rivets to secure it to the existing flange and then provide enough length to accept the hole for the last LE rivet hole. I wish I did not have to do this, and there is an alternative, which would involve drilling holes in the skin and the rib flange, which I am trying to avoid at this point. So the joggle is the next best option and that is what I am going to do.

"NUff" for now......

Graduations and Oshkosh Airventure Prep

I stalled a bit on the build as more rainy weather moved in over the past several weeks. Yesterday and to day we finally hit 80 degrees with no rain. The rain even threatened to cause problems with my youngest son's high school graduation, but it managed to stop long enough for that to happen outside as scheduled. Then came the big graduation party one week later. That definitely turned into a major weather fiasco. About 100+ people showed up at the house and had to move inside when the weather reared its ugly head multiple times. At least several people were interested in the plane so we talked in the garage for a while during the party.

In my 53 years I have never seen such BS weather in the month of May. All that said, at least I can finally put these graduations in the history books. All I have is four more years of college to finance, and then I am done with schools forever.

The USAF Thunderbirds were in town this weekend, flying over Aurora Reservoir. Of course, in true Colorado fashion, the only thing the media reported on the parking fiasco. This was the main reason I did not attend. Not sure if they will perform at Airventure again this year or not, but if they do then I will have a great seat to watch them from, without all parking madness.

With graduations out of the way, and the weather taking up  most of the month of May, I needed to start preparing the trailer for the 2,000 mile round trip journey to Oshkosh. I managed to get the water system checked out today and I only have one leak. Only problem is that it may require a new water pump to fix it. The electrical and everything else seems to be ready to go,minus one problem with the trailer light circuit from the vehicle that I have already traced part of the way.

I'll have a different tow vehicle this year, so that means that there are bound to be issues along the way, but I am ready for that. the AAA subscription will be paid up, and I'll have all my tools with me as usual. I'm just hoping that flooding and bad weather are not going to make this an even more challenging trip. About 8 more weeks is all that remains, and it will be time to travel again.

Here are some pics of the brackets in their almost finished form. I trimmed the edges and smoothed them on the scotch brite wheel. Then I fit them to the rib, clamped them into place, and drew the outline on the back side of the rib web.  Next I need to remove the rib and ensure that the brackets are clamped precisely in the correct place and then I will match drill the rivet holes for the rib attachment and rivet them in place. then the rib goes back on the spar where I will match drill the rivet hole for the flange that sits on the main spar web.

After a couple more home repairs I think I can resume the build. Will it ever end?????

Second Rib Flange Attach holes drilled

Tonight I did the layout, drilled the rivet holes, deburred them, and trimmed the second flange for the 408 rib mod. And here is living proof why you should probably not rely on any measurement details that I provide unless you read way ahead in my posts. I think I provided rivet length dimensions based on 2D of the rivet. This is wrong. The length of the rivet after it is inserted into all parts that being joined is supposed to be 1.5D and not 2D. SO the math for the rivet length is 2 x .032 (thickness of the rib web and the new flange), + 1.5D x the rivet diameter.

SO that is 2 x .032 = .064, + 1.5 x .125,or .064 +.1875 = .2515 total length. A 4-4 rivet is .25 inches long (4/16ths), so a 4-4 rivet is just under the correct size for the rivet required to attach both parts. I did a trial fit and the 4-4 rivet will work just fine. Originally I thought I needed a 4-5 rivet which is way too long.

So, make sure you read ahead because my initial math may be off just a bit. One thing about being off the project for extended periods - you have to relearn a few things that used to be common everyday knowledge. That is why I am so deep into AC 43.13 to make sure I do this right.

This flange is a bit wider than the first one, so my rivet spacing was widened accordingly by about 1/16th of an inch. Here are the pics:

Next step is to final trim, smooth all the edges, and then securely attach both parts to the rib on the spar so I can match drill all the rivet holes to the rib web. Then prime, then rivet them to the rib. I I may drill the final hole through the actual flange that will be riveted to the main wing spar, but I wont set that rivet until all the LE ribs are ready for riveting. Then I need to fabricate 2 more flanges for the other rib on the right wing. These are small gains, but considering this is exactly where I left off last August, this is still progress. Funny that it takes all this extra time to try to make one rib look and function just like another rib. Sure wish I could ahve Vans make a 709 rib with skin flanges bent .032 smaller for the joiner plate, just like the 408 rib. Sure would have saved me a lot of work.

Rivet layout of custom flanges

I managed to get out to the shop tonight since the rain was less threatening than yesterday. The goal for tonight was to get the rivet layout drawn, holes drilled, and the part trimmed to final dimensions. As I was doing the rivet layout something caused me to pause and go back to AC 43.13 to review something. The original plan was to space the AN470 rivets using a 2D edge distance and a 4D between rivet distance. This seemed a bit wide tome as I looked at that pattern drawn on the part. In AC 43.13 on page 4-26 is a section for Wing and tail surface ribs in paragraph 4-58 (c). It then refers to two figures, 4-13 and 4-14, on pages 4-29 and 4-30 respectively.

The epiphany that I had that caused me to review this info was that the other diagrams that I had been referring to for rivet layout on page 4-20.were based on a single lap joint of two sheets of aluminum,similar to a skin repair. IN my case all of the modes I am performing are to the Leading edge rib, not the wing skin. Sp after reviewing the two diagrams, figure 4-14 told me what I wanted to know. I decided to use the spacing on picture D, which is a doubler plate applied to the rib web over a lightening hole. Basically all this does is change the spacing of the rivets  between rivet from 4D to 3D. The 2D edge distance remains. SO I measured the rivet layout based on 2D edge distance and 3D between rivets, and that basically meant that instead of requiring 1 inch of metal, I really only needed about 7/8s of an inch. As it turns out, I kept the 1 inch dimension for reasons I will explain later. Here is the pic of the part with the rivet layout marked:

The line toward the top of the pic was drawn by position the part against the rib web, and using the very bottom of the cutout section of the rib to draw the line. This is very bottom of the rib web that must be reference for edge distance requirements for the rivets near this section of the part.

The two parallel lines that are very close to each other was an adjustment I made toward the flange side of the part. I decided to back off about 1/16th of an inch from the bottom of the attaching flange after recalling something that I read that 2D edge distance is good, but 2.5 D is better. SO I raised the level of that row of rivets by 1/16th of an inch. The edge distance of the part is not affected, but the edge distance of the bottom portion of the rib web will be affected, so I have to make sure I measure this right.

Next came securing the part so I could center punch all the holes (now 6 total instead of 4 as a result of going from 4D to 3D). I also trimmed the top portion of the part based on a line drawn 1 inch from the reference line drawn that represents the very bottom of the rib web cutout for the spar bars. I secured it with  bar clamp and center punched all 6 holes:

First I drilled all holes to #40, then deburred and final drilled them up to #30:

These pics turned out blurry so sorry about that. My camera is also flipping them around which I need to figure out how to correct, so sorry about that as well.

I still need to debur the edges after trimming down the top of the part. My scotch brite wheel was buried in the garage due to recent storms and the need to make fast room in the garage for both cars. So I'll finish that later. Here is the part temporarily placed into position on the rib. I will use the holes drilled in the part as a guide to match drill the holes in the rib web. I will need to secure the part into its final position as best I can by securing one flange to the wing spar and the other flange against the back of the rib web.Then I will drill each hole through the rib web and cleco each one as I go.

My rivet layout did not turn out as straight-line as I would have liked it but everything is within specs from 43.13 so tome that is all that matters. Hopefully I will be drilling remaining holes. Then I have to think about some priming before I can drive the rivets - as usual. Sure haven't done any of that in a while. I may prime this rib in 7220 acid etch rattle can instead of my 2 part primer just so I can keep moving forward on this part of the mod.

And the challenges continue....

I was not able to work on the project on Sunday since I had to become an Ax Man for day and chop down some tree branches in the back yard. I did it with the chainsaw but it still took all day to trim several branches. In the next two weeks I will be attending two graduation ceremonies, one high school and one college for each of my two sons. Then comes the obligatory grad party at the house. I had to cut several branches from the trees to prevent folks from knocking themselves out when hey walk outside in the backyard. Then we had some bad weather move in for next several days.

So the distractions and challenges continue, but at least all the school stuff is starting to come to an end. The new goal now is to complete the plane before my youngest son graduates from college 4 years from now.

Anyway, I still have to drill the rivet holes and do some final trimming of the fabricated flanges, and then I get to drag out the ole rivet gun and see if I remember how to pound some rivets.

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!