goatflieg

goatflieg

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Yesterday I tackled the infamous rudder leading edge. It was obvious that the main issue was going to be the lower sections, because I needed to create the curve more at the base of the skins than at the edge, and using traditional rolling methods puts the curve in the wrong place for proper fit later. But I went ahead, using various methods I've learned from other builders, and ended up carefully massaging the pieces into shapes that were close enough to get clecos in place without creating a crease at the spar. The end result was that I ended up with a fit that looks good on the outside; a nice, tight, smooth fit... but it's ugly on the inside. I tried to make sure that the skins were in good contact at the rivet points, going to great length to push the inner skin back towards the outer skin. I was mostly successful; all the heads are tight against the exterior skin, but I think there are one or two rivets in the center of the lower run that set with a very small gap between the skins at the rivet point.
So I have two questions. Since all but a couple rivets are pulled tight, and all the heads and exterior fit is good, do I sweat this? I also was wondering about the shear forces involved in getting these parts into alignment. Since I didn't want to create the dreaded spar creases, I massaged/held/taped the edges into alignment in order to set the clecos, and then match-reamed the holes and set the blind rivets one by one, working from the center of each section outward. Since they're all riveted now, will the aluminum eventually relax into the held shape, or do those shear forces remain strong for the life of the part, and might that become an issue in the long run?
Just curious about all this; I'm sure it's all familiar territory. I'll be posting a project update soon, and will include photos. I tried to get a picture of the inside of the curve... but it's a difficult space to photograph with any kind of clarity.
 
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goatflieg said:
So I have two questions. Since all but a couple rivets are pulled tight, and all the heads and exterior fit is good, do I sweat this? I also was wondering about the shear forces involved in getting these parts into alignment. Since I didn't want to create the dreaded spar creases, I massaged/held/taped the edges into alignment in order to set the clecos, and then match-reamed the holes and set the blind rivets one by one, working from the center of each section outward. Since they're all riveted now, will the aluminum eventually relax into the held shape, or do those shear forces remain strong for the life of the part, and might that become an issue in the long run?
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In general, a gap between the two riveted panels is undesirable. This particular joint is (I think) fundamentally non-structural, so it probably won't hurt anything here. It's hard to judge severity without photos.

A few words on general principles may be useful, as a typical blind rivet and a solid driven rivet are quite different. The typical blind rivet shank does not expand inside the hole, so an oversize hole remains unfilled. A driven rivet does expand its shank diameter when compressed, thus oversize holes (within reason) are not a problem. Second, a blind rivet tends to pull parts together, a function of the mandrel bulb being pulled toward the tool. The effect is slight or nonexistent with a solid rivet.

With that in mind, an observer might conclude that the edges may have needed a little more bending to lay down flat on each other...as the blind rivet was not able to pull them together before the mandrel broke.

A reamer or clearance drill should be used with caution when used to correct hole alignment. If the resulting holes are oval shaped, (1) the blind rivet cannot prevent the parts from moving in relation to each other if subjected to alternating loads, and (2) an oval hole will more easily allow the bulb end to pull through if subjected to a tension load. All single shear rivet lines are in tension when near failure.

The spring tension in the aluminum wrap does not relax, at least not within a time frame relevant to our purpose. That's ok, as it does not matter.
 
I put clecos in every set of holes before I resized them so they stayed aligned well; there was no shear forces during reaming so the holes weren't oversized. I used a reamer instead of a drill bit because it leaves less of a burr on the inaccessible edges. I also used clamps wherever I could reach to hold the parts together tightly. It was just two of the center rivets of the lower section that seemed to pop early. Although the outer edge was in good contact with the underlying piece, looking inside it was hard to tell if the two pieces of metal were firmly nested. On all the rest I made sure they mated up well. Thanks for the info. I'd like to come up with a reliable way to roll the inner part of those lower flanges and leave the outer edges more straight... and if I do come up with a way, I'll share it here.
 
goatflieg said:
I'd like to come up with a reliable way to roll the inner part of those lower flanges and leave the outer edges more straight... and if I do come up with a way, I'll share it here.
Click to expand...

Yes. As you've discovered, this is a process that involves a lot of "English".
I'm not sure what rolling method you used, but I have settled on using a piece of EMT conduit cut to length for each of the three sections of the LE, a lot of Gorilla tape(!), and vice grips for handles (actually, I think I drilled them and used a screwdriver). If you remove all the rivets in the section you need to tweak, you can tape the EMT to the edge of the skin and kind-of push while you un-roll it, (to keep the skin near the spar from developing a reverse crease). Hard to explain, but just try to localize the roll forming to the area of interest.
Be advised that removing the pop rivets is not easy because the skin is not supported well when you are driving out the mandrels, especially in the lower section. Be careful! Support the area around the back of the rivet if possible.
 
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