Wings 70, Total 533 - Fluting and straightening rib flanges

Rib work continues. Here is a shot of one of the many main ribs already fluted, with fluting pliers to one side. I managed to finish deburring all the main ribs - well, mostly anyway. Small cutouts for joggled flanges will need to be sanded down by hand, but the main parts of each flange are done. I ran each of them twice across the scotch brite wheel on the grinder, angled from the outside and again from the inside. I have found that this seems to adequately debur both sides of the edge of the flange, whereas if you only run the edge across the wheel from one side, it still leaves some roughness on the other side of the edge. I also deburred each of the corner cutouts on the wheel, but more about that later.

All ribs come out of the forming blocks with a pronounced curve or warp across the web. It's a metal thing, and it's just the way that it is. Every rib has to undergo the process of fluting and straightening of the flanges to ensure that the prepunched holes will line up with the same holes in the skins.

To do this somewhat methodically, I did the following;

1. Must have a flat surface on the bench large enough for the rib
2. Lay the rib on the bench with the curve facing up. (flanges will also be facing up)
3. Look at the rib from the side, and note the hight point of the warp on one side
4. Take the fluting pliers and flute the high point until the rib flange sits flat on the bench. A good review of the hints for home builders video on the EAA web site will help you understand what needs to be done, but the methods in that video and the ones I am using are a tad different (mine are easier).  I think it is the Making Ribs video part 4 or 5 in the sheet metal category). The interesting part of this video is a question of what you should do first - straighten the flanges or flute the ribs.

In the end you may need to keep doing a little of both until they are just right. I chose to begin by fluting the ribs to align the rivet holes on the flanges as much as possible, and then straighten the flanges using the homemade flange bending tool that I made. Take the time and make the tool - you will be glad you did. Details are on VAF and also in my empennage posts - search the blog under Building Tips and Tricks. Attending the EAA Sport Air Sheet Metal class will also give you experience with this.

Anyway, keep fluting the high points of the warp until it seems that the flange and web are sitting square on the bench. Then do the other side of the rib the same way. The ends of each rib are straight bends, and are not curved that the top and bottoms of the rib, so these flanges will NOT require any fluting - only some straightening when the time comes.

5. Now set the rib on one of the flanges, with the other facing upward. Take a straight edge (my 24 inch thin metal ruler from Cleaveland tools works great for this) and align one end woth the edge of the outermost rivet hole on one end of the rib, and then align the other end of the straight edge with the hole on the opposite end of the flange. Now look at the alignment of the holes along the rest of the flange. Chances are that not all of the holes will align along the straight edge exactly the same as the two outermost rivet holes. This means that a bit more fluting is necessary.

6. When finished fluting, use the flange straightening tool to straighten each of the rib flanges, then recheck the holes again to ensure they are still aligned. If they are not, then repeat the fluting process, adding new ones if necessary, or deepening or reducing existing flutes as needed.

Time for some more pics to illustrate:
The warp:

They can also be a bit twisted from front to back as shown in the next pic:

Next is fluting the highpoints of the warp. DO NOT fall under the spell that you must flute the gap between every single rivet hole on the flange. Most of the time you only need to create a few flutes to completely straighten the the web of each rib. So far I seem to be averaging about 4 or 5 flutes per flange on the Wing ribs. Remember to place the pliers half way between the rivet holes, and NOT insert the fluting pliers until they bottom out at the bend of the flange. Keep the pliers about half way out from the bend to maximize the effect of the flute. Fluting is skill that must be practiced. The more you do it, the better you get. There are tricks to knowing just how much pressure to apply to each flute, and where to place them - and you will learn these tricks by practicing the art.

And after the flute is applied. The shadow makes it seem that the bow is still there, and it is, but not as much as it was before the flute was applied.

Think you are done with fluting?- think again. Looks pretty good though.... Notice that the flanges are still not 90 degrees to the rib web (boughed outward just a bit.)

After you think you have the flanges all straight, check the alignment of the rivet holes:

Now for the fun part. Some of the holes may dissappear under the straight edge, while others may have a noticeable gap between the straight edge and the rivet hole, as shown in the above two pics. An easy way to figure out how to resolve these alignment problems is to remember the following:

Holes that vanish under the straight edge require more fluting, while holes that are gapped require less fluting. Of course this all depends on which side of the hole you are holding the straight edge. What becomes difficult is determining the exact amount of pressure to apply to each flute so that the flanges come out perfectly straight. Another problem that I tend to have is that after I have verified that the hole alignment is as perfect as it can be, I experience an optical illusion that tells my brain that the flanges are over bent, but the hole alignment does not lie, as long as your straight edge is really straight of course, and that is what you need to focus on.

Once the flanges are straight, the rib should also sit flat on the bench. There may be a small amount of offset, but that is usually due to twist in the rib, which you can correct by applying torque in the opposite direction of the twist. They don't have to be absolutely perfect, but the more aligned each rib is, the easier it will attach to the wing skins.

Now on to the deburring pics. First is an example of running the flange edges across the scotch brite wheel:

Next is deburring the notch on the trailing edge corners of each rib:

And next is the cutouts on the leading edges of each rib:

I only wish that I had a grinder that had an extended shaft to provide extra clearance from the grinder motor, but you learn to make due with the tools that you have, so you adapt and overcome.

Then lastly is the flange straightening. Keep in mind that I posted some pics a bit out of process order. May fix that later but not right now. Just remember to debur, flute, and straighten - that seems to be the order that works best for me.

If you take a straight edge (angle edge or triangle is best for this step) and check each rib flange, you will most likely find out that they are not set to 90 degree angles to the rib flanges. For the RV-8, all rib flanges on the wing ribs are supposed to be 90 degrees to the rib web, and will need to be straightened. Notice the gap in the next pic:

In comes the straightening tool

One of the challenges of using the tool on the wing ribs is the slight interference of each of the stiffening holes. I decided to basically ignore them and applied pressure to the flange as necessary. There was no damage to any of the lightening hole flanges/stiffeners - they just spring right back into action as if nothing ever happened.

And the rib flange is now square after being straightened with the tool - works like a charm. Remember, the bending brake angle is 11 degrees over 90. This seems to provide the right amount of bend with the amount of spring back due to the thickness and tension of the metal kept in mind. Alot of this also has to do with where you hold the handle as you apply the pressure to the flange, and how much pressure to apply. It is possible to over bend the metal slightly if you apply too much pressure. Again, practice makes perfect here.

I do try to avoid the areas where I have added flutes to each flange. Use the straightening tool just on one side of the flute or the other, slide the flange down, and apply pressure again. you can make a bigger one with a 2x4 but I have found that I like the control and the ability to straighten smaller areas using the 2x2 on my tool as shown:

Still a bunch of ribs to do - this will probably take me at least the rest of this week in my spare time to get all the ribs done. Then I get to scuff and clean and prime each one..... If I am lucky I will be riveting the ribs to the main spar just before I leave for Oshkosh 2012.