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RV-15 Flush Rivet Leading Edges

MCA

MCA

Well Known Member
Advertiser
I'm looking for some help from collective wisdom of this forum. My plan is to use flush rivets (AN426AD4) for the leading edges and would really appreciate some extra eyes to take a look at the attached files which have been marked up to show the locations for dimples and 426 rivets. (also includes some other notes picked up along the way).

I built a -7 about 20 years ago, so this doesn't seem particularly difficult. But, I'm not sure how much (if any) Van's has vetted the idea of flush solid rivets on the leading edge. When I visited the factory recently they said it was designed for this (notice the LE access holes and rib flanges match up nicely). But since I haven't seen anyone do this yet, and my kit is a few weeks out, it seemed like something to get out in front of. Any insights are appreciated.

Sections 29 and 31 (marked up) are attached.

Also, let's not go off thread about WHY this is a good or bad idea, let's focus on the HOW. Thx!
 

Attachments

Big picture I think you will need to figure one of two things. Either figure out how to reach inside the wing everywhere to buck the skin rivets - probably an inspection port in the bottom of every other rib bay, maybe every fourth rib bay if you can get a arm into an adjoining bay from a port to buck those. Or, figure out how to attach the nose ribs to the spar from behind the spar if the skin and nose ribs are riveted first.

For the second option, maybe put nut plates on the nose ribs at all the holes where they mate to the spar, and drill the spar for screws that go in from behind.

Just a quick comment on motivation - these would be the rivets that would give the most 'bang for the buck' in terms of drag reduction payoff for the effort.
 
scsmith said:
Big picture I think you will need to figure one of two things. Either figure out how to reach inside the wing everywhere to buck the skin rivets - probably an inspection port in the bottom of every other rib bay, maybe every fourth rib bay if you can get a arm into an adjoining bay from a port to buck those. Or, figure out how to attach the nose ribs to the spar from behind the spar if the skin and nose ribs are riveted first.

For the second option, maybe put nut plates on the nose ribs at all the holes where they mate to the spar, and drill the spar for screws that go in from behind.

Just a quick comment on motivation - these would be the rivets that would give the most 'bang for the buck' in terms of drag reduction payoff for the effort.
Click to expand...
Marc already mentioned in his post above, that Vans designed in access along the entire leading edge of the wing.
There are premade openings that get screwed on cover plates during wing construction.
 
The other thing to consider is that the flush skin rivets in other models are typically -3 rivets; they buck and squeeze very easily.

The -4 rivets will be a bit more difficult to set. Manageable but something to consider.
 
I'm also curious how the flush -4 rivet will sit in a fairly curved section of skin. The larger dimpling may create a substantial flattening in the curve of the skin which would then be lumpy in a tight radius area with closely spaced rivets. I would certainly test this on scrap prior to implementing the idea.
 
Ya’ll know that there are flush pulled rivets as well…right? The Sonex’s used flush pulled rivets on the leading edge, back to the main spar (dome heads behind that). Something you can think about….then fill some mandrel holes….

“Flush” and “Solid” are not synonymous …
 
Ironflight said:
Ya’ll know that there are flush pulled rivets as well…right? The Sonex’s used flush pulled rivets on the leading edge, back to the main spar (dome heads behind that). Something you can think about….then fill some mandrel holes….

“Flush” and “Solid” are not synonymous …
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Thanks Paul. Yes, considering that as well. TBD.
 
There are many variations of solid rivets as well.
The AN426B4-X is made from 5056 alloy, so they are slightly softer than an AN426AD4 rivet but still have an lbf of about 295 which is only about 35lbf lower than an AD4!rivet.
The difference isn’t huge but it is very noticeable when squeezed with a hand squeezer.
Every little bit helps.
 
rvbuilder2002 said:
There are many variations of solid rivets as well.
The AN426B4-X is made from 5056 alloy, so they are slightly softer than an AN426AD4 rivet but still have an lbf of about 295 which is only about 35lbf lower than an AD4!rivet.
The difference isn’t huge but it is very noticeable when squeezed with a hand squeezer.
Every little bit helps.
Click to expand...
True but how does that factor in to the design?

I don't think you are suggesting just using different rivets without knowing the potential result on the airframe because they are easier to set, however, it could be taken that way...
 
I also am planning to install flush rivets on the the leading edge of the wing back to the spar.

rocketman1988 said:
Out of curiosity, what is your goal in using flush rivets for the first third? Not judging, just curious...
Click to expand...
I believe Steve Smith answered this question in his previous post.
scsmith said:
. . .Just a quick comment on motivation - these would be the rivets that would give the most 'bang for the buck' in terms of drag reduction payoff for the effort.
Click to expand...
At least that is the reason I intend to do it.
 
rocketman1988 said:
True but how does that factor in to the design?

I don't think you are suggesting just using different rivets without knowing the potential result on the airframe because they are easier to set, however, it could be taken that way...
Click to expand...
The LP4-3 rivet that comes in the kit has shown through testing to be good for about 135 lbf.
The AN426B has a spec value of about 295 lbf
Even if the spec value is optimistic by 15% (or more for that matter), the 426B solid rivet should be far and away above the blind rivet strength and fatigue wise.
 
Lots of good stuff to consider so far. Thanks for the feedback!

Three reasons to do this (YMMV):
1. Maybe pick up a knot or two. A smooth surface on first ~30% of the chord makes a difference, according to a chief engineer I work with (he's designed several popular P23 aircraft). Behind that not much. The front spar is at 18% but it is something.
2. Easier to clean the bugs off
3. I feel this kit may be too easy, at least compared with the -7. It's a bit of self-inflicted torture to get my $$ worth out of the kit.
 
MCA said:
Lots of good stuff to consider so far. Thanks for the feedback!

Three reasons to do this (YMMV):
1. Maybe pick up a knot or two. A smooth surface on first ~30% of the chord makes a difference, according to a chief engineer I work with (he's designed several popular P23 aircraft). Behind that not much. The front spar is at 18% but it is something.
2. Easier to clean the bugs off
3. I feel this kit may be too easy, at least compared with the -7. It's a bit of self-inflicted torture to get my $$ worth out of the kit.
Click to expand...
4. Dimpled joints are stronger (yes I know that Van’s designed it without and therefore non-dimpled joints are strong enough).
5. Easier to paint
6. Subjectively, flush riveted surfaces look better.
7. Similar to Marc’s #3 above - the challenge of flush riveting is intriguing.
 
I anyone goes this route, that is solid flush rivets, can you let us know what length you're using. I would like to at least have them on hand. And perhaps where to source the AN426B rivets.
 
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Jetmart said:
I anyone goes this route, that is solid flush rivets, can you let us know what length you're using. I would like to at least have them on hand. And perhaps where to source the AN426B rivets.
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My opinion is that anyone considering a rivet deviation from what is called out in the plans, needs to be able to make their own size determinations based on material stack up dimensions. There are lots of resources available to gain the proper knowledge (AC43.13, Aircraft Standards Handbook, etc)
 
I spent some time in the shop this morning getting reacquainted with a 3x rivet gun, DRDT2, and some bucking bars. The goal was to figure out rivet lengths without having the kit present. So for those of you who are more talented than I am with rivet lengths, please check this over. It's combination of shop measurements and tests on scrap, along with looking at the KAI.

Wing skin thickness:
Inboard skin: 0.025
Middle skin: 0.032
Outboard skin: 0.020

Van’s section 5 and 43.13 specify L = T +1.5D for flat stock, but I could not find a formula for joined dimple surfaces. So I used L = T + 1.5D + H to match the drawing in section 5.
L=rivet length (in)
T=material thickness
D=dia of rivet (0.125). 1.5D  = 0.188
H=height of dimple (0.040, from shop measurement on 0.025 sheet)

-4 rivet length: 0.250
-5 rivet length: 0.3125

All possible LE skin joints:

LE skin (0.032) to nose rib (0.025): thickness = 0.057
0.057 + 0.188 + 0.040  = 0.285 (-5 rivet)
LE skin (0.032) to J-channel (0.025): thickness = 0.057
0.057 + 0.188 + 0.040  = 0.285 (-5 rivet)
LE skin overlap (0.032 x 2): thickness = 0.064
0.064 + 0.188 + 0.040  = 0.292   (-5 rivet)
Two LE skin overlap (0.025 + 0.032) to nose rib (0.025): thickness = 0.082
0.082 + 0.188 + 0.040   = 0.310 (-5 rivet)

LE skin (0.032) to pitot mount (0.063, countersunk): thickness = 0.095
0.095 + 0.188  = 0.283  (-5 rivet)

It looks like -5 rivets will work across all the countersunk holes. Anyone come up with something different?
 
I'm seeing 3 difference thicknesses for leading edge skins. .020, .025, and .032.
 
Marc, that looks good to me.

Just curious (since I’m weighing the pros and cons myself), are you going to buck those leading edge rivets or pull some flush blind rivets? Seems like pulling blinds and then filling the heads with a dab of epoxy might be easier, have the same aesthetic and aerodynamic benefit, and in my case be less prone to screw ups.

See you at Buckeye Air Fair in a couple weeks.
 
MCA said:
I spent some time in the shop this morning getting reacquainted with a 3x rivet gun, DRDT2, and some bucking bars. The goal was to figure out rivet lengths without having the kit present. So for those of you who are more talented than I am with rivet lengths, please check this over. It's combination of shop measurements and tests on scrap, along with looking at the KAI.

Wing skin thickness:
Inboard skin: 0.025
Middle skin: 0.032
Outboard skin: 0.020

Van’s section 5 and 43.13 specify L = T +1.5D for flat stock, but I could not find a formula for joined dimple surfaces. So I used L = T + 1.5D + H to match the drawing in section 5.
L=rivet length (in)
T=material thickness
D=dia of rivet (0.125). 1.5D  = 0.188
H=height of dimple (0.040, from shop measurement on 0.025 sheet)

-4 rivet length: 0.250
-5 rivet length: 0.3125

All possible LE skin joints:

LE skin (0.032) to nose rib (0.025): thickness = 0.057
0.057 + 0.188 + 0.040  = 0.285 (-5 rivet)
LE skin (0.032) to J-channel (0.025): thickness = 0.057
0.057 + 0.188 + 0.040  = 0.285 (-5 rivet)
LE skin overlap (0.032 x 2): thickness = 0.064
0.064 + 0.188 + 0.040  = 0.292   (-5 rivet)
Two LE skin overlap (0.025 + 0.032) to nose rib (0.025): thickness = 0.082
0.082 + 0.188 + 0.040   = 0.310 (-5 rivet)

LE skin (0.032) to pitot mount (0.063, countersunk): thickness = 0.095
0.095 + 0.188  = 0.283  (-5 rivet)

It looks like -5 rivets will work across all the countersunk holes. Anyone come up with something different?
Click to expand...
The height of a dimple should not be considered when making decisions on rivet length.
In the interest of learning, what size is to use, I suggest dimpling some scrap material in the same thickness as you are joining, and then set some rivets and find shortest length that will result in a proper shop head.
Keep in mind that when measuring the height of shop heads on dimpled rivet joints, you are only measuring the thickness of the rivet head. Not the resultant height of the rivet head above the skin surface because of the dimple.
 
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I will be interested to see what the actual performance difference will be. I would be surprised if it was very noticeable.
 
rocketman1988 said:
I will be interested to see what the actual performance difference will be. I would be surprised if it was very noticeable.
Click to expand...
Striving to improve performance is only one of a number of different reasons for flush riveting, so I suggest we abide by the OP’s request of not turning this thread into a debate of whether someone should do it or not.
 
I would be surprised if it was very noticeable.
Click to expand...
I shan't... just looking at 'em clean leading edges will make 'em look at least 15 kts faster.

PS
This of course taking into account the sums of any of all eventual fudge factors.
 
rvbuilder2002 said:
Striving to improve performance is only one of a number of different reasons for flush riveting, so I suggest we abide by the OP’s request of not turning this thread into a debate of whether someone should do it or not.
Click to expand...

Never said to do it or not and I stated in a previous post that I was not asking to judge but only out of curiosity.

am still at the point in my 15 build where I could go either way so I think it is perfectly fine to ask as to the WHY of a change like this

I followed up with a statement that it will be interesting to see the actual difference, if any. Again, not pushing either way, just a statement.

There isn’t any argument or judging just trying to determine the path I am personally going to take…should we talk about primer?
 
Do you think the decision to use flush rivets on the leading edges could be made at the time the skins go on? Can all the holes in the nose ribs be reach with a hand squeezer to dimple them after they are mounted to the spar? I would like to see how difficult it will be to buck the rivets through the access holes.
 
Vans has told me the kit is designed for flush solid rivets, but they won't officially stand behind that and I don't believe they have actually built a wing with flush solid rivets yet. So we're on our own for now. I did email Vans about the rivets, and they demurred saying they can't comment yet. If you look at the plans you can see access holes in the right places, and the rib flanges are bent inward towards those access holes.

You'll have to make the decision before putting the front nose ribs on the main spar, as those need dimpling as well as the skin. It's unlikely you'll be able to use a hand squeezer for any of this, other than some initial prep on the ribs. My current plan is to dimple all the holes forward of the main spar (as shown in attached files above), and use regular LP4 rivets on the spar attachment, per plans. But it'll be a few months before that actually happens (kit just arrived this week).
 
MCA said:
Vans has told me the kit is designed for flush solid rivets, but they won't officially stand behind that and I don't believe they have actually built a wing with flush solid rivets yet. So we're on our own for now. I did email Vans about the rivets, and they demurred saying they can't comment yet. If you look at the plans you can see access holes in the right places, and the rib flanges are bent inward towards those access holes.

You'll have to make the decision before putting the front nose ribs on the main spar, as those need dimpling as well as the skin. It's unlikely you'll be able to use a hand squeezer for any of this, other than some initial prep on the ribs. My current plan is to dimple all the holes forward of the main spar (as shown in attached files above), and use regular LP4 rivets on the spar attachment, per plans. But it'll be a few months before that actually happens (kit just arrived this week).
Click to expand...

Thanks, reading your thread made me think that I can cleco the leading edge ribs on and then the skin to see if I can reach all the rivets with a small tungsten bucking bar.
I had a brain fart when I asked my initial question. I don't want to prep for flush rivets and then fine out I can't get to them easily or without making a mess of them.
 
Yes, you could cleco the leading edge ribs on and the skins just to double check. Sounds like a good idea to double check before committing. Who will be first? 🤷🏻
 
MCA said:
Yes, you could cleco the leading edge ribs on and the skins just to double check. Sounds like a good idea to double check before committing. Who will be first? 🤷🏻
Click to expand...

I would have a lot less concern if they were - 3 rivets but being 1/8" ads some anx.
 
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MCA said:
Yes, you could cleco the leading edge ribs on and the skins just to double check. Sounds like a good idea to double check before committing. Who will be first? 🤷🏻
Click to expand...

I was kinda hoping @rvbuilder2002 would have gotten one of the first kits and then provided the intel for the rest of us on how to do it. ;)
 
MCA said:
You'll have to make the decision before putting the front nose ribs on the main spar, as those need dimpling as well as the skin. It's unlikely you'll be able to use a hand squeezer for any of this,
Click to expand...
Not correct

Every RV-3, 4, and 6 built had large portions of the airplanes sub structure riveted together before the skin attachment rivet holes had even been drilled.
The dimpling can be done on ribs with them attached to the spar, though I agree it would be a little easier if they weren’t.
 
I have my kit. I have built multiple RVs. I have plans to flush rivet the leading edge rivets. I have not made it to the point where I can evaluate the process but will post what I find out when I get there.
 
N10KE said:
So one of Van’s most experienced former employees with years of problem solving offers to build the first flush riveted RV-15 (and verify what Van’s has indicated will be possible) - and they ignore you.

Sad commentary on the “new” Van’s in my opinion.
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No.
I only offered to be a beta builder. Nothing was specifically stated about deviating from the basic KAI build process.
 
rvbuilder2002 said:
No.
I only offered to be a beta builder. Nothing was specifically stated about deviating from the basic KAI build process.
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My mistake. I thought you intended to flush rivet your RV-15.

Regardless, I can’t think of anyone better qualified than you to work through the initial bugs and issues of a new kit - especially since you were so involved with the prototype.
 
N10KE said:
My mistake. I thought you intended to flush rivet your RV-15.

Regardless, I can’t think of anyone better qualified than you to work through the initial bugs and issues of a new kit - especially since you were so involved with the prototype.
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I do, but that wasn’t mentioned when I made the offer (The rivet type used to attach skins wouldn’t have impacted my ability to test the build process)
 
In the seemingly never ending quest to source flush headed rivets that would be acceptable alternatives to an LP4-3, I have discovered how insane the whole Cherry rivet debacle has become.

Back story
For anyone wishing to flush rivet the wing at least as far aft as the main wing spar (lots of closely spaced rivets running full span on the wing, so likely the most beneficial), a suitable flush blind rivet will be needed because of zero access on the interior of the wing in the vicinity of the fuel tank.

While looking for alternatives to tha Cherrymax CR3242 series I have looked into other manufacturers that make an equivalent (they usually even have a similar part #).

I just got a quote from a supplier that has inventory of the Huck brand version in Part # HR3242-4-2.
Since each wing will require about 125 I requested a quote for 250 (seems like a reasonable quantity for good pricing🤔).

Quote came back at $55.50……. Each 😳

The search continues…..
 
rvbuilder2002 said:
In the seemingly never ending quest to source flush headed rivets that would be acceptable alternatives to an LP4-3, I have discovered how insane the whole Cherry rivet debacle has become.

Back story
For anyone wishing to flush rivet the wing at least as far aft as the main wing spar (lots of closely spaced rivets running full span on the wing, so likely the most beneficial), a suitable flush blind rivet will be needed because of zero access on the interior of the wing in the vicinity of the fuel tank.

While looking for alternatives to tha Cherrymax CR3242 series I have looked into other manufacturers that make an equivalent (they usually even have a similar part #).

I just got a quote from a supplier that has inventory of the Huck brand version in Part # HR3242-4-2.
Since each wing will require about 125 I requested a quote for 250 (seems like a reasonable quantity for good pricing🤔).

Quote came back at $55.50……. Each 😳

The search continues…..
Click to expand...

I was just out in the shop and counted rivets on the LE ribs, I count 17 on each rib x 15 ribs -> 255 rivets per rib. That's just for the skin to LE rib, not the rivets that go into the spar. [As I type this I realized maybe you are just counting the area where access is limited?]

I'm not interested in bucking -4 flush rivets but might consider flush pulled rivets if there was a suitable alternative (and at a decent price!).

$55 each is NUTS!!
 
rvbuilder2002 said:
In the seemingly never ending quest to source flush headed rivets that would be acceptable alternatives to an LP4-3, I have discovered how insane the whole Cherry rivet debacle has become.

Back story
For anyone wishing to flush rivet the wing at least as far aft as the main wing spar (lots of closely spaced rivets running full span on the wing, so likely the most beneficial), a suitable flush blind rivet will be needed because of zero access on the interior of the wing in the vicinity of the fuel tank.

While looking for alternatives to tha Cherrymax CR3242 series I have looked into other manufacturers that make an equivalent (they usually even have a similar part #).

I just got a quote from a supplier that has inventory of the Huck brand version in Part # HR3242-4-2.
Since each wing will require about 125 I requested a quote for 250 (seems like a reasonable quantity for good pricing🤔).

Quote came back at $55.50……. Each 😳

The search continues…..
Click to expand...
Scott, are you proposing flush rivets on the main spar? If so, wouldn't that require countersinking the spar itself (not something I'd want to do, even though that is SOP for other Vans aircraft). The spar seems too thick to dimple. Looking for your guidance...
 
Those dome-head rivets are looking better and better, aren’t they? 😉

On the “other brand” kits (Sonex…Sling….) that use flush pulled rivets on the leading edge, they are just using regular Cherry N’s (not Max’s) which are way, way cheaper. And you can actually BUY them…. I would want Vans to weigh in, however, on their suitability. I have done calculations on dome-head Cherry N’s and they are very close in strength to solids - but that’s just napkin engineering, so I would certainly not promote the idea without design engineering concurrence.
 
Ironflight said:
Those dome-head rivets are looking better and better, aren’t they? 😉
Click to expand...
I know there is a good solution....I just haven't found it yet.

From what I have seen so far, other than where already specified in the KAI (and I think with some good planning a lot of those can be avoided) the holes along the main spar flange directly ahead of the fuel tanks is the only locations that will require blind rivets for flush riveting the skins.
 
jacoby said:
Did you look at the Allfast Allmax rivets? ACS has the 3242 in the -3 length for $2/ea. Looks like the -2 are produced but a quick search doesn't turn up any suppliers that carry them.
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Thanks
I have looked at them and I am pretty sure the min. grip of .126" for the -3 size is too large for rivets used for attaching skins to the wing spar flanges (but I don't have a kit yet so have not confirmed that).

Doesn't matter though, because if you try and buy any it shows "No Stock" in your cart like it does for all of the other relevant sizes of Cherrymax.
 
rvbuilder2002 said:
Thanks
I have looked at them and I am pretty sure the min. grip of .126" for the -3 size is too large for rivets used for attaching skins to the wing spar flanges (but I don't have a kit yet so have not confirmed that).

Doesn't matter though, because if you try and buy any it shows "No Stock" in your cart like it does for all of the other relevant sizes of Cherrymax.
Click to expand...

Ahh too bad. I thought since they recently appeared in ACS as "new" they'd have some in stock! I suspect all the alternate suppliers are running full speed to fill the gap left from the Cherrymax fire and are prioritizing large orders first.
 
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