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R716 placement/edge distance

miles_gloriosus

I'm New Here
after fabricating the 716 fairing attach strips i clamped them on the inside of the rib as far into the radius on the rib as i could, marked the holes and measured. found that the holes would be only 7/32 from the edge of the strip. any further into the rib and it starts riding up the radius.

is this strip supposed to go between the rib and skin? or is 7/32 acceptable in this case?
 
The R716 strip goes on the inside of the rib. The min edge distance is 2D from the centre of the hole. In this case is an AN426AD3 rivet so a 3/32 hole. You would need 6/32 or 3/16 edge distance. So with 7/32 you’re good to go.
 

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Guys- It seems like this topic comes up about once a month and folks are probably getting sick of me chiming in, but I hate to see people go down a path that will have them throwing away good parts or otherwise wrecking something in an attempt to solve a problem that doesn't exist.

Everybody throws around the 2d edge distance like it was handed down from the mountain, but thats just a rule of thumb that will quite frequently give you surplus over the "real" number.

MIL-R-47196A is very clear in that the standard minimum edge distance to the center of a 3/32 driven rivet in a dimpled hole is .156" i.e. the decimal equivalent of 5/32"

To be 100% clear, A design print or other manufacturers specific engineering guidance is the final authority, but if it doesn't specify something different, then the expectation is that you follow standard industry practices. That would be 2D if which will keep you out of trouble, or look up the real minimum if you need to eat into that rule of thumb.

OP- The Mil-Spec I'm referring to has been around since WW2 in one form or another. You can find the most current version (revised in 1974!) linked to the tech support section of Van's website. you really owe it to yourself to go take a look at it.
 
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I get your point but how does MIL-R-47196A stack up against AC 43.13-1B? That document states 2D for edge distance.
 
The R716 strip goes on the inside of the rib. The min edge distance is 2D from the centre of the hole. In this case is an AN426AD3 rivet so a 3/32 hole. You would need 6/32 or 3/16 edge distance. So with 7/32 you’re good to go.

wouldnt this be 2.5 as it should be dimpled all the way through? the skin isnt thick enough for countersinking.
 
I get your point but how does MIL-R-47196A stack up against AC 43.13-1B? That document states 2D for edge distance.

Thats certainly a valid question, but if you look at the intent, AC 43.13-1B is geared toward repair and the guidance is more of a one size fits all generic approach.

Quote:

"This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions."

The section of I believe you're referencing is section 4-57 If so, it says in part;

"Unless structural deficiencies are suspected, the rivet spacing and edge distance should duplicate those of the original aircraft structure. If structural deficiencies are suspected, the fol*lowing may be used in determining minimum edge distance and rivet spacing.
(1) For single row rivets, the edge dis*tance should not be less than 2 times the di* ameter of the rivet and spacing should not be less than 3 times the diameter of the rivet."

So the question in play is simply how do you determine if structural deficiencies are suspected?

If Vans spells out a dimension, it's pretty cut and dried that they can specify bigger, smaller, whatever. They have done the engineering to back it up, and you as the manufacturer/assembler also have the engineering to back it up in the form of the assembly print.

If there's no dimension, we're now expected to use industry accepted design practices, right? Everybody points to the 2d callout in AC43.13. There's certainly nothing wrong with that, but what I'm saying is that in addition to AC43.13, MIL-R-47196A is approved data that it isn't specifically geared toward repairs or structural deficiencies.

It's also referenced by Vans in section 5 of their build manual as justification for why other areas of AC43.13 aren't written in stone. Specifically when they talk about the size of driven shop heads.

I've spent quite a bit of time on this reply because I'm genuinely trying to help, but my fear is that I've made it as clear as mud, so I guess I'll just end my manifesto by saying if you run into a situation where you've got less than 2d and if you want to know FOR SURE what you're dealing with, call Builder Support :)
 
Might regret what I did here

I did a mod here - I put a row of K1000-08 plate nuts on this aluminum strip. The reason being that in my rudder I will have a TailbeaconX and faired in a Whelen flashing red beacon on top. Removable cover on the bottom allows me to have the rudder's electrical connection in here along with some service access, etc.

However, it turns out the fibreglass fairing may be too thin for my preferred fastener choice (AN507 countersunk screw) to have a nice flush fit. In fact the AN525 washer head screw may be as smooth :eek: Anyone aware of another 8-32 threaded fastener to consider here?
 
I've spent quite a bit of time on this reply because I'm genuinely trying to help, but my fear is that I've made it as clear as mud, so I guess I'll just end my manifesto by saying if you run into a situation where you've got less than 2d and if you want to know FOR SURE what you're dealing with, call Builder Support :)

No, you have made it easier to understand. And would fully agree with the final statement.
At the end of the day part of the reason for experimental aircraft is education. And I’m certainly getting that!

For the OP:
I’ve not seen anywhere where 2.5D is stated for dimples. I’m working to 2D most of the time, unless Vans states otherwise. And as above, you can go less if required. But if you’re not sure a quick email to Support would answer your question.
 
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