[DaleB]

How many NAS1097 rivets are "safe"?

Today I started riveting the bottom outboard skin on my right wing. I hadn't planned on doing it until later on, but have found I regret putting things off when it comes time to pay the piper later. So, since I had it clecoed in place I just decided to go ahead and rivet.

But... since I'd planned on finishing the skin later, on the bench, I hadn't drilled the flap hinge holes where the outboard skin is, and the skin is un-dimpled and the flap brace not countersunk. If I'd had it on the workbench I would have noticed this but it's on the wing stand, and I didn't notice/remember until it was too late. Bloody heck. With the skin riveted as it is, I can't get in there with a squeezer or countersink cage on any but maybe the last couple of holes. Not good.

So I think I can drill those holes to #30, lightly countersink the skin and use "oops" rivets (NAS1097AD4). They will look fine, but will they give that area enough strength? Is there a limit to the number of "oops" rivets you can use before compromising strength because of the smaller manufactured heads?

 

[DaAV8R]

Bottom skin

If I understand your question correctly, I would be inclined to dimple the bottom skin and flap brace in assembly and then countersink the hinge for these holes only. You probably won't get quite as crisp of a dimple in the flap brace using this technique but that means you won't need to countersink the hinge very deep, which I see as a good thing.

I wouldn't use 1097's as they don't have much of a head on them. The flap hinge and flap brace look like a high load area to me and I wouldn't trust a bunch of ganged up 1097's. Probably be good to check with Van's on this one.

 

[rzbill]

All of us forget stuff. Don't feel like the lone ranger. I am convinced this is where the rubber meets the road as far as the "education" part of experimental aviation comes in. My most painful one was on the rear spar through the cabin. I had to completely disassemble due to missing 2 rivets.

I want to clarify something that was inferred but not stated in your post.

In order for the flap brace to be riveted to the inboard skin, the flap hinge is also riveted in place, correct? And the outer section of flap hinge( 9 rivet holes) may not be through drilled? If not drilled, go ahead and do so.

OOPS
In regards to using 1/8" 1097s, I'd probably call Vans to get their nod of approval. I think the 1/8 shank busts the edge distance on the hinge and of course the head grip is smaller than standard.

DIMPLE
Assuming the flap hinge is in place, that is a lot of material to try to dimple all at once. I'd probably simulate it with some scrap before trying it on the wing. Decide on a path after the test.

PER PLANS
An alternate method that would allow completion per plans would be to drill out enough of the inboard hinge rivets until you can flex the brace for c'sinking and dimpling of the skin. If you have a set of pop rivet gun dimplers, and are willing to use the c'sink tool in your deburr handle, you don't have to flex the brace too far to do both operations.

At the times I wanted to be really careful drilling AD3s, I would start with a 1/16 drill bit before moving to #40 and then breaking the head off with the shank of a #40 bit.

Good luck

 

[Mel]

These rivets were designed to be used occasionally for cosmetic reasons. They should not be used continuously in a structural area.

Flap loads are some of the highest structural loads of all control surfaces.
Flaps are the only control surfaces that are deployed onto the wind stream and stay there.

 

[DaleB]

In order for the flap brace to be riveted to the inboard skin, the flap hinge is also riveted in place, correct? And the outer section of flap hinge( 9 rivet holes) may not be through drilled? If not drilled, go ahead and do so.

Correct. Only the holes corresponding to the outboard skin are not already drilled, countersunk and riveted.

If I understand your question correctly, I would be inclined to dimple the bottom skin and flap brace in assembly and then countersink the hinge for these holes only. You probably won't get quite as crisp of a dimple in the flap brace using this technique but that means you won't need to countersink the hinge very deep, which I see as a good thing.

Assuming the flap hinge is in place, that is a lot of material to try to dimple all at once.

I'm certainly not opposed to dimpling the two pieces in assembly and countersinking the hinge, but in order to do that I'll have to drill out quite a few of the inboard hinge rivets. I'm really not seeing much of a way of avoiding that. I think today I'll get the wing up on the workbench again and try removing rivets until I can get stuff far enough of the way to do that. I may be able to countersink at least some of the flap brace holes, even if I can't get to all of them. For the ones I can't I'll try to countersink the hinge. I've already emailed Van's to see what they think. I'm hesitant to drill out a bunch of rivets of the potential for deforming the flap hinge and/or damaging the inboard skin.

 

[Walt]

NAS1097 rivets are not an exceptable substitute for standard AN rivets. It plugs the hole and in a shear application it may be acceptable from an engineering standpoint but should not be used as a "substitute".

In many of the specs I used to work with you were allowed a 1 in 10 defect, so in a line of 10 rivets you were allowed 1 that did not meet standard criteria. I would consider this a good general rule for the use of "oops" rivets.

Standard practice is to step the hole up to the next "standard" rivet size.

 

[bill v]

check with vans but 20 years ago vans had you dimple the hinge

 

[DaleB]

Well, today I got a call from Van's. I'd never gotten an answer to my original email, maybe it got lost, but I re-sent it last night.

The recommended fixes were to either use NAS1097 rivets but add one in between each of the 9 original holes, or use 3/32" universal head rivets in those 9 holes. It's the bottom of the wing where they won't show up as badly as they would elsewhere, and I could set them with a flush set instead of a cup set and the heads would be even less obtrusive.

He recommended against either dimpling or countersinking the hinge. Countersinking will weaken the hinge, and dimpling -- well, I already know that's a losing proposition, too much distortion of the hinge.

So tonight or tomorrow, whenever i get out to the shop again, I'll try some AN470 rivets set with a flush set on some scrap material to see how that works. Depending on how that goes I'll decide how I want to proceed.

 

[rzbill]

or use 3/32" universal head rivets in those 9 holes.

DUH. Of course! That is a great solution. AN470AD3s have a pretty small head. I don't know if I would bother mashing them with a flat set. Make them look like you meant to put them there.

And FYI, don't spend a lot of time looking for some in the kit. There aren't any. You will need to scrounge a handful. There are other places where they are useful too.