Messed up the holes to attach the floor stiffeners to the F-704

[idubrov]

So I messed up the two holes that attach the two central floor stiffeners to the F-704 bulkhead: I clecoed the bottom skin to the aft part of the F-704 only so the front part of the F-704 bulkhead was not positioned properly.

I upsized the holes to #30, which fully cleaned it.

Now the question is, what do I do next? These two holes are special in that I am supposed to countersink the F-776 central skin and the F-704 flange, dimple the F-772 bottom skin and leave the floor stiffener "flat".

However, with the AN426AD4 rivet, I am concerned that the thickness of the F-776 plus the F-704 won't be enough for it. Seems risky to me.

So, that leaves me the following options:

1. I can use "oops" NAS1097AD3 rivet. It's head is about the same as the AD3 rivet, so I can follow the original plan of dimpling the outer skin and countersinking the other skin and the flange.
2. I can dimple the both skins and the flange, then countersink the stiffener, hoping that somehow I will be able to fit it, contrary to what instructions say. The angle, however, doesn't have much thickness for the AD4-sized countersink either (but it is probably easier to fix the angle -- do a fabricated joggle, for example).
3. I can put a universal rivet there. Leave these joints alone, don't dimple nor countersink anything, just put a domed head. Will probably loose one or two knots of the cruise speed

I am leaning towards #1 or #3, with a slight preference towards #1. What do you think?

[wirejock]

Originally Posted by idubrov

So I messed up the two holes that attach the two central floor stiffeners to the F-704 bulkhead: I clecoed the bottom skin to the aft part of the F-704 only so the front part of the F-704 bulkhead was not positioned properly.

I upsized the holes to #30, which fully cleaned it.

Now the question is, what do I do next? These two holes are special in that I am supposed to countersink the F-776 central skin and the F-704 flange, dimple the F-772 bottom skin and leave the floor stiffener "flat".

However, with the AN426AD4 rivet, I am concerned that the thickness of the F-776 plus the F-704 won't be enough for it. Seems risky to me.

So, that leaves me the following options:

1. I can use "oops" NAS1097AD3 rivet. It's head is about the same as the AD3 rivet, so I can follow the original plan of dimpling the outer skin and countersinking the other skin and the flange.
2. I can dimple the both skins and the flange, then countersink the stiffener, hoping that somehow I will be able to fit it, contrary to what instructions say. The angle, however, doesn't have much thickness for the AD4-sized countersink either (but it is probably easier to fix the angle -- do a fabricated joggle, for example).
3. I can put a universal rivet there. Leave these joints alone, don't dimple nor countersink anything, just put a domed head. Will probably loose one or two knots of the cruise speed

I am leaning towards #1 or #3, with a slight preference towards #1. What do you think?

How about a #4 option?
If memory serves, the rib sits inside the 704 flange. How about fabricating a doubler with the correct diameter hole.
If that's not an option, I would go with Option #1.

[nohoflyer]

I would use a normal an426 4-6 rivet and countersink as appropriate. You won’t notice one ot two larger rivets at the end of the day.


I just did this area but a pic of what you’re talking about would make this easier to imagine.

[idubrov]

This is what it looks from the outside:



The universal rivet head is the hole in question.

This is what it looks like inside, with the floor stiffener:



And this is without the floor stiffener:



That golden flange of the F-704 bulkhead and a 0.025" skin above it is what gets counter-sinked, to the "substructure" depth (flush + 0.007", as per section 5).

The "blue" skin is the one that gets dimpled. So, the sandwhich is:

1. Blue skin -- dimpled
2. Another skin you can barely see the edge on the photo -- countersunk.
3. F-704 golden flange -- partially countersunk, to the depth necessary
4. Stiffener angle -- nothing.

My concern about the rivet head size is not about its visual appeal (who cares, this is on the bottom), but that #30 countersunk is larger than #40 and there might not be enough thickness of 2 + 3. Which is why I am thinking about using NAS1097 rivet that has a smaller head.

[sahrens]

Just to make sure I understand you drilled the two holes that attached F-772, F-776, F-704 and the floor stiffeners misaligned and then drilled them to size 30 to correct?

There are no edge distance issues with F-772. There might be with F-776 and F-704 but they are sandwiched in between F-772 and the stiffener. Having said that there is still a potential for cracking in F-776 and F-704 at that location. What is the remaining edge distance of the floor stiffener? Is there sufficient edge distance for a -4 rivet? If so you could use a AN426-4-X for a rivet. If not I recommend replacing the floor stiffener assuming there is sufficient room to position a new stiffener up to the web of the F-704 to have proper edge distance.

In any event contact Vans so they can advise on the structural issues (if any) of insufficient edge distance for a -4 in F-776 and F-704. I don't think that's an issue but it will only take a few minutes to get their advice.

I believe that NAS1097AD3 rivets are not considered structural and would not be appropriate for this application.

Good luck, hopefully the AN426-4 rivet will workout for you.

[sahrens]

I posted and you posted photos at the same time. So let me take another stab at this. I would replace the stiffener.

Based on your photo there appears to be insufficient edge distance for a -4 rivet in the stiffener. Again based on your photos I would say you might be okay on edge distance on the F-704 and F-776.

I believe you will be okay to counter sink but again to be sure, talk to Vans.

[idubrov]

Originally Posted by sahrens

I posted and you posted photos at the same time. So let me take another stab at this. I would replace the stiffener.

I measured the edge distance on the stiffener as ~0.240", which is more than the minimum ~0.219" (as per the MIL Spec).

[idubrov]

Hmm, based on my calculations (and also a quick caliper check), the countersink depth for the AN426AD4 rivet is ~0.039", which is the thickness of the forward bottom skin. So another option is to simply countersink the forward bottom skin only (instead of dimpling it and then counter-sinking the understructure, as per the instructions).

I think, I like this option the most as it keeps the danger away from the most precious part (F-704 flange). It would leave a knife edge on the skin, but seems like it should be acceptable ("minimum acceptable thickness" as per Section 5). I can also maybe make countersink a bit less deep, as it is allowed to shave up to 0.007" from the rivet.

Will see what Van's support would suggest...

[PaulvS]

It will be interesting to see what Vans Support says however I'd be inclined to dimple the outer skin rather than machine countersink because dimpling is stronger and a knife edge in that location could result in a smoking rivet since the skin is subject to vibration from the exhaust and prop. The F704 should be thick enough to take the slight countersink that remains after the cutter has gone through the F776 skin. It would be easy enough to do a test on some scrap.
BTW that row of rivets is all AN426AD4-6 on a RV-6, for interest. I machine countersunk per the plans F604 (.063 thick), which is the equivalent of F704. See att pic for that detail.

[sahrens]

Originally Posted by idubrov

I measured the edge distance on the stiffener as ~0.240", which is more than the minimum ~0.219" (as per the MIL Spec).

You are correct that is the mil-spec. Interestingly AC 43-13 states the minimum edge distance is 2D or in this case 0.250. It will be interesting to see which standard Vans recommends