FL-706A pop rivet lacks edge distance

When match drilling the FL-706 bracket to the flap spar, I have a problem with edge distance.

Here you see FL-706A's flange flush to the edge of the spar.



I backdrilled one of the holes through the spar into the bracket. Then I test fit an LP4-3 rivet in the hole. Unfortunately the rivet won't sit flat in the hole due to the manufactured head resting partially on the radius of the flange.



The rivet hole seems too close to the flange of Fl706A.

I see a couple of options: either remake the piece and let the flange of FL-706A hang a bit off the edge of the spar so that the pop rivet line is brought inward a bit and the heads sit flat. But then it wouldn't allow FL-706B to sit flush, which I could fix with a shim underneath FL-706B.

Other option is to grind down the flange radius on FL-706A around the rivet heads so that the sit flat. That seems incorrect.

What to do? Did anyone else run into this?

I don't recall if I had this issue, but if I did I would just pull the rivet and move on. The rivet head and the bracket will both deform a bit.

fl706a

I'm at the same point as you, it looks like the rivet will sit just proud of the surface from the radius of the bracket it should squeeze down.

I've scoured the web's various build logs, nobody seems to point out this issue. One of my builder buddies also mentioned he did have it sit proud a bit and just squeezed it down.

I'm tempted to spotface around the rivet hole just a tad so that it can sit flat.

Your pictures show a thick fitting and a steel bolt. You have a sheet metal part and a pop rivet. Trying to spot face the sheet metal will probably break through, and start a crack. Do what your buddy did.

I just squeezed it and moved on, never gave it another thought until now. I think now I would still do the same.

I just ordered a 1/4" spot face tool with a replaceable guide. I got a #30 and #40 pilot insert. Yardstore. You may need it again on the fuse up under the roll bar attachment to the longeron for a tip up.

DGlaeser said:

Your pictures show a thick fitting and a steel bolt. You have a sheet metal part and a pop rivet. Trying to spot face the sheet metal will probably break through, and start a crack. Do what your buddy did.

Click to expand...

I see what you're saying, but the FL706A bracket seems thick enough at 0.125" for that not to be an issue.

That said, I do not own a spot facing tool and haven't done that before. I think I might be able to approximate it with a small scotchbrite wheel, but that seems sort of excessive.

Also not sure what the bigger issue is - the rivet heat sitting crooked? Or the head digging into the flange and potentially creating a stress riser?

This video shows the an example of spotfacing with a piloted counterbore in a drill press, on what looks like a very similar bracket.
https://www.youtube.com/watch?v=EoRyJz9zC3Q

Edit: Looked at the PDF drawing of FL-706A and took some measurements. Although dimensions are not directly specified, the holes in question are 5/16" from the outer side of the flange. The material is 1/8" thick, leaving 3/16" or 0.1875" between the hole center and the flange wall. The radius on the flange is 0.188". The rivet head diameter is 0.232", radius 0.116". So that leaves the 0.116" of rivet head over hanging a flange radius of 0.188". So the interference is there in the design.

The hole I drilled in FL-706A is exactly 5/16" from the edge, so it's per the plans.

I guess I will do my best to jam those rivets in and move on. I guess I could call Van's, but I don't expect them to say much except "you're the builder, figure it out".

Washer?

Another generic fix might be simply to make a contoured washer. File a curve on one side of a washer to conform.

Make sure the rivet has sufficient grip length or buy rivets of the next longer grip length if needed.

I just squeezed my rivets last night and they all pulled tight, so I wouldn't do anything to the bracket.

spaceflightmeow said:

...
Also not sure what the bigger issue is - the rivet heat sitting crooked? Or the head digging into the flange and potentially creating a stress riser?
...

Click to expand...

It won't do either. Putting a spotface on the flange would create more of a stress riser.
The aluminum rivet head will deform a bit to conform to the flange. The flange will likely deform a bit also - but not so much that you'd notice (due to the thickness).
Find some scrap to replicate that geometry and try it!

Well, I tried it.

Here's a test piece with a hole 5/16" from the flange per the drawing, with an LP4-3 rivet installed. I bent the rivet stem and slid on the wedge piece you see to the left so that the rivet could be pulled at an angle.


Doesn't look so good!

spaceflightmeow said:

Well, I tried it.

Here's a test piece with a hole 5/16" from the flange per the drawing, with an LP4-3 rivet installed. I bent the rivet stem and slid on the wedge piece you see to the left so that the rivet could be pulled at an angle.

Doesn't look so good!

Click to expand...

Again, simply add a contoured washer...

az_gila said:

Again, simply add a contoured washer...

Click to expand...

Read and understood.

I called Van's, the reply came back with the standard "there are probably thousands of RVs flying with the rivets like that", which implies "build on and don't worry about it".

What I did.

I modified my wedge/ spacer as shown below. Necessary? Probably not, but it looks a bit nicer!