RV-3 Fuse Skins - to Dimple or Countersink?

[Ironflight]

I did something unusual the other day - I actually read a few pages in the RV-3 construction manual. I know, I know...what was I thinking? The manual is fairly sketchy, and foe someone who has built before, most of the details are already in our heads, but sometimes, you actually find out something interesting. Like (in this case) a lien that says "you can dimple or countersink the .025 skins before flush riveting". Well, since we are getting close to permanently attaching skin, I made up a little test piece of .025 riveted to a scrap of stringer using AD3 rivets - I did some with dimples in the skin and some with countersunk holes. In terms of final finish, there was no comparison - the countersunk rivets were perfect, whereas you could detect the curved edges of the dimples very easily. the countersunk rivets were perfectly flush on the flat skin - beautiful!

So I am curious how all the "old school" folks have done their fuselages, and entertain comments and thoughts.

Paul

 

[sandifer]

Dumb question since I'm not a builder...does countersinking .025 material leave enough contact area on the skin for the rivet to firmly hold on to? Seems like you'd have to be extremely careful with your CSK depth.

RV-3's can't be beat.

 

[rvaitor87]

to dimple or not to

that is the question, now for the answer. When my dad and I built ours, we dimpled the skin, we tried it both ways on a scrap piece and there was some concern as to the countersink depth, and the amount of contact area, so we dimpled, after 20 years of flying aerobatics included, we still are holding together, and still looks good.

Hope this helps

 

[WAM120RV]

Dimple

Hi

I was suprised by your question as I always understood that Vans said do not machine countersink a skin of less than 32 thou. So, I checked in my RV 4build book....... my memory has not failed me!!!!

Machine countersinking 25 thou will resuly in too large a hole and loss of strength. Even with 32 thou the recommend its done with the skin clecoed to the ribs to prevent countersink wander, and still warn that it can result in an oversize hole and week joint if any deburring is excessive.

Another point to consider is that with dimpling the skin dimple sits in the rib dimple which in itself adds strength to the joint.

I was also taught that where there is a risk of a gap between the skins and the rib when you start rivetting, to hit the rivet a couple of times to swell the rivet, then put a small nut between the bucking bar and the rib and give the rivit a light tap with the gun to draw the skins together, then remove the nut and a couple of more taps should see the rivit set.

It ensures the skin is flush to the rib and that the two are locked together. I am not saying do this on all of them just those where the skin is not drawn up to the rib. It is in effect what the back rivetting tool does.

I hope this helps
WAM

 

[Brian Vickers]

Machine countersink .025?

I am an old school builder of an RV-4. My construction manual plainly states that .032 is the thinnest sheet material that can (should) be machine CS. Even with .032" it is real easy to create oversize or oblong holes. Only the top skins (turtle deck) on the RV-4 are .025. The sides are .032. I have found that back riveting does a pretty good job of leveling a dimple CS rivet. I have a big round mushroom shaped bucking bar that works very well for this. (purchased from Avery) The dimple CS rivets on my .025 turtle deck are very flush and level with the surrounding surface. Test your back riveting skills and tell us what you think.

 

[Mel]

Standard procedure for machine countersink vs. dimple countersink calls for material thickness less than .032" to be dimple countersunk for 3/32" rivets. Machine countersinking material less than .032" thick does not leave enough material for a good structural joint.

BTW, I machine CS the rivets on my horizontal and vertical stabs on my -6 and would not do it again. It worked fine and still looks good, but it was a lot more trouble to get them consistent. I also had to "shave" a few of them. Dimples are much more consistent.

 

[Kyle Boatright]

BTW, I machine CS the rivets on my horizontal and vertical stabs on my -6 and would not do it again. It worked fine and still looks good, but it was a lot more trouble to get them consistent. I also had to "shave" a few of them. Dimples are much more consistent.

I (and several others I know) did exactly the same thing and wouldn't do it again. Dimpling is quick and certain. Countersinking is a bit fiddly, especially working around curves and such. Should I build another RV, everything possible gets dimpled as opposed to machine countersunk.

Of course, dimpling does create an opportunity for figure 8 holes and misplaced holes and dimples...

 

[Ironflight]

Ok folks, this is embarassing, but you know how it is when you get old....I got home and re-read the quote from the manual, and it says "You can dimple OR Countersink the .032 fuselage skins"....which, by the way, is all of them......

So the CORRECTED question is...what are folks thoughts on countersinking instead of dimpling those skins? Sure makes a nice finish!

 

[szicree]

"fiddly" is a good word

I found that countersunk rivets have the potential to look superior, but in practice it's difficult to maintain the exact depth required. Doing a good job of it will take far longer than dimpling and it's guaranteed some will need shaving or will simply be too deep (not good). My policy if building again would be dimple any place I can.

 

[n5lp]

A

B

I'm no engineer and I'm sure both are plenty strong, but I have seen a lot of smoking rivets. My eyeballs make me prefer "B". It just looks like the skin is much less likely to work because it is more locked in place by the dimples.

By the way these images are from a Disney WWII film that gives a good overview of flush riveting.
http://www.youtube.com/watch?v=IagmAgHvcWQ

 

[Mel]

My Opinion:

Always dimple CS as default!
It will almost always be more consistent, and in my opinion, slightly stronger.

 

[Ironflight]

Lots of great responses guys - thanks! Good capture for the archives as well - I don't rememebr seeing this topic addressed in detail before.

 

[proppershope]

Tank Die

You might want to try your fuel tank dies from Avery. They leave the rivet just a little below the surface and you may need the next size longer but they leave the surface nice.

 

[apkp777]

You can always do both. Soft dimple then finish up with the machine counter sink.

 

[Danny7]

I watched that old walt disney movie on riveting, they mention using different angles for the different pieces- 100 degrees for the rivet head, which they say would translate to 110 on the bottom of the first piece of metal, so the underlying structure would get a 110 degree dimple, which they said would produce 120 degrees on the bottom of the second piece of Alu. i've read here that some people were recommending ribs being dimpled with a tank die to let the skins sit "more flush". I suppose if i can see a difference maybe i'll try the tank dimple die for the underlying structure and see if it makes it nicer, honestly my dimples i've thought looked pretty flush so i didn't see it being a problem, but in the C- frame i whack em pretty good.

 

[FredMagare]

So the CORRECTED question is...what are folks thoughts on countersinking instead of dimpling those skins? Sure makes a nice finish!

It takes a bit longer, but I've noticed that if one dimples a skin then lightly turns a deburring bit in the dimple, the rivets sit straight and flush. It might be worth a try to see what kind of results you get.

Good luck!

 

[rv4dude]

DIMPLE, DIMPLE, DIMPLE

22 years of sheet metal work does provide a tiny secret, just as said in the above post. Be very careful...but here it is. Dimple your hole just like any other. Now, use a hand held Dogleg Deburr Tool with the proper sized piloted countersink bit...VERY gently touch countersink the hole...3 LIGHT turns works well. This will remove about .002" or less of material from the hole and the rivet "thinks" it has a nice machined nest to sit in, but you have the strength of the dimpled hole and it looks a little nicer. DO NOT try to "re-machine" the dimpled hole to look like a normal countersink in a .040" or thicker material. Too much and you will reduce the thickness of the dimpled material and reduced tension strength of your sheet metal. "What about the outer alclad protection you just removed from the dimpled hole?" For most of us, the airplane skin will outlive us builders. For those near salty areas, you can use an alodine touch pen on each hole. I believe the act of the process will make some feel better in the long run. Try it with a piece of .025" and squeeze a few rivets. Now try some .032", hint hint...you can be a bit more liberal with .032 and achieve and nicer hole and a better finished outcome. Use a flat edge and a flashlight to compare your art work.

For the newer builder...Please stick with dimpling, we love our RV friends and other airplane friends. Seriously, please stay with dimpling with anything .032 and under. Yes, some say .032" can be machined, but it is too easy to over machine and knife edge the bottom of the hole. The materials we work with is too thin to "fix" with the next size larger diameter of rivet. In most cases, the strength of a dimpled hole is much stronger.

Copied from another source:
"This is a design guidebook of material strengths and fastener strengths that is approved by the Army, Navy, Air Force and the FAA.

Shear strength of 3/32 MS20426AD flush rivets (values are ultimate strength in lbs.) Sheet thickness
Dimpled Countersunk
0.032 217 178
0.040 217 193
0.063 217 216
This shows a 18% loss of strength for every countersunk rivet in 0.032, and an 11% loss in 0.040 material. The Handbook also had a warning that in 0.032 material, this was a "knife edge condition" and was undesirable, and not approved."

 

[rv72004]

It takes a bit longer, but I've noticed that if one dimples a skin then lightly turns a deburring bit in the dimple, the rivets sit straight and flush. It might be worth a try to see what kind of results you get.

Good luck!

You are right about it sitting nicer. My only concern, and yep I know its exagerated, a machine countersink/ cleanup would remove the pure aluminium surface from the alclad which would could allow corrosion to result more easily under the rivet head.
As I am told by friends, "you arent building the shuttle", couldnt resist.

 

[apkp777]

You are right about it sitting nicer. My only concern, and yep I know its exaggerated, a machine countersink/ cleanup would remove the pure aluminum surface from the alclad which would could allow corrosion to result more easily under the rivet head.
As I am told by friends, "you arent building the shuttle", couldn't resist.

As soon as you drill (or punch) a hole in the aluminum you exposed the non-alclad interior. There really is, as you indicated an "exaggerated" believe that airplanes corrode at contact with the atmosphere. The whole of commercial large aircraft use ONLY machine countersunk holes with no hole prep prior to riveting. In all my years with Boeing and UAL, I actually never dimpled a single hole. till I started my RV. Dimpling eliminates the knife-edge condition of thin gauge aluminum, but isn't design as a corrosion prevention method. The combination of dimpled followed up with light machining does help to make the rivets sit better. Although, once you start pounding it with your 3x rivet gun anything can happen.

A good solution to the original post may also be to polish the rivet heads/skin. That has a very nice "burnishing" effect and takes away the tooling marks left from riveting to boot.

 

[JonJay]

Does Boeing glue the skins to ribs?

As soon as you drill (or punch) a hole in the aluminum you exposed the non-alclad interior. There really is, as you indicated an "exaggerated" believe that airplanes corrode at contact with the atmosphere. The whole of commercial large aircraft use ONLY machine countersunk holes with no hole prep prior to riveting. In all my years with Boeing and UAL, I actually never dimpled a single hole. till I started my RV. Dimpling eliminates the knife-edge condition of thin gauge aluminum, but isn't design as a corrosion prevention method. The combination of dimpled followed up with light machining does help to make the rivets sit better. Although, once you start pounding it with your 3x rivet gun anything can happen.

A good solution to the original post may also be to polish the rivet heads/skin. That has a very nice "burnishing" effect and takes away the tooling marks left from riveting to boot.

I was told they bond the skin to the ribs. I know of one RV builder doing this and countersinking everything, inlcuding .025. He said he was using the same glue as Boeing.??

 

[apkp777]

Don't want to go down a rabbit trail here, but when I was at Boeing we didn't bond anything. There were some skins that were bonded (tear staps). Of course, I was in Everett and all that we did was final assembly so the fuselage was mostly put together. We just shot the joints. All that may have changed. I was there 20 years ago. At UAL more recently and we used a ton of ProSeal to put pressurized skins together. No "glue" was used on interior structure.

Back to Paul's original thread. I think the issue is that the -3 has thicker skin than the new RV's and gives you more options to work the holes. Machine countersinking works great when you have the thicker skin.

One thing you could consider Paul is to use a "reduced head" rivet. I believe they are NAS1097's. Wicks has them in -3's. That would work just fine I bet!