What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Countersink or Dimple .032

husabeast

Member
Patron
I'm sure this has been asked in the past, but I can't find a definitive answer anywhere. The RV-8 left elevator instructions say to countersink many holes on the various spars made from .032. Section 5.5 COUNTERSINKING AND DIMPLING states that for AD3 rivets, .032 thickness or less must be dimpled and .032-.040 should be dimpled but can be countersinked if necessary.
I am leaning toward dimpling since it does not require the removal of material. It is also easier and less likely to ruin a part. Any thoughts would be appreciated.
 

Attachments

  • 2022-07-12 20-21.pdf
    193.9 KB · Views: 173
Sometimes dimpling will cause the piece to curve, this is particularly if the part is the flange of the piece.
While I am not familiar with the part that you are talking about but being a spar, I imagine it is the flange and in that case the countersinking might be the reason it says to do.
 
Sometimes dimpling will cause the piece to curve, this is particularly if the part is the flange of the piece.
While I am not familiar with the part that you are talking about but being a spar, I imagine it is the flange and in that case the countersinking might be the reason it says to do.

When in doubt, dimple. Countersinking .032 for 3/32 rivets is marginal and not as strong as dimpling.

MHO!
 
Dimple — all the way up to and including .040”

+1, I even dimpled the 0.040 forward floor and countersunk the angle. A dimple is almost always better than a countersink. On 0.032 I would only countersink if there were no way to dimple.
 
The head height on a 3/32" rivet is .036", that's slightly deeper than .032" sheet. It's apparently acceptable, but definitely not preferred. There's a chart somewhere, but can't seem to recall where.
 
Has anyone tried to dimple a piano hinge to see what happens to it?

As a general rule, yes dimple is prefered over countersink but there are applications or types that VANS has you countersink despite the skin not being very thick. RV14 has multiple examples of that.
 
Has anyone tried to dimple a piano hinge to see what happens to it?

As a general rule, yes dimple is prefered over countersink but there are applications or types that VANS has you countersink despite the skin not being very thick. RV14 has multiple examples of that.

Yes, I dimpled the MS20001 hinge that I used for the trim tab. It was hard, the material curved, etc. etc.

But, at the end of day, it worked out ok...
 
Last edited:
Hinge

Has anyone tried to dimple a piano hinge to see what happens to it?

As a general rule, yes dimple is prefered over countersink but there are applications or types that VANS has you countersink despite the skin not being very thick. RV14 has multiple examples of that.

Yes, not on the airplane. It will bow.
I think most hinge we use is thicker than .032. Seems like it's .040. Easy to countersink.
 
Piano hinge is usually the bottom layer too, meaning it recieves the dimple from other layers. Doesn't much matter if it's a dimple in a dimple, or a dimple in a countersink in the last layer, the additional strength is still there. The shop head either sits on the dimple bottom, or the smooth back side of the material (hinge). I suppose it's lighter to countersink the last layer as the rivet will be shorter and removing material. :D

There's a couple spots in the RV8 where a countersink is specifically directed in the plans where it's way too deep and knife edges. #6 c/s screws in .040 sheet. 1/8" rivets in the same .040 sheet also knife edges slightly. Even the spar flanges where the fuel tanks attach goes way deeper than the thickness of the flange material. I called Van's support about it once and told to basically "follow the plans, it's fine, I'm over-thinking things". I suppose they're right. The designer decided this was the best way to do it. Of the thousands of RVs flying, they aren't falling out of the sky because people followed the plans.
 
Piano hinge is usually the bottom layer too, meaning it recieves the dimple from other layers. Doesn't much matter if it's a dimple in a dimple, or a dimple in a countersink in the last layer, the additional strength is still there. The shop head either sits on the dimple bottom, or the smooth back side of the material (hinge). I suppose it's lighter to countersink the last layer as the rivet will be shorter and removing material. :D

There's a couple spots in the RV8 where a countersink is specifically directed in the plans where it's way too deep and knife edges. #6 c/s screws in .040 sheet. 1/8" rivets in the same .040 sheet also knife edges slightly. Even the spar flanges where the fuel tanks attach goes way deeper than the thickness of the flange material. I called Van's support about it once and told to basically "follow the plans, it's fine, I'm over-thinking things". I suppose they're right. The designer decided this was the best way to do it. Of the thousands of RVs flying, they aren't falling out of the sky because people followed the plans.

Been there, done that on my -14 trim tab. I fretted about the call to C/S in 0.032 thick material because it goes against every fiber in my structural analyst being. I stopped and pondered for a while which included doing some Solidworks models of a 0.040 in. rear spar being custom made by a local metal bending shop. Talk about over-thinking it. :rolleyes:

In the end, I carefully did what the plans said and everything went together beautifully. I first had to think through all of the variables that go into something like that before I convinced myself I won't fall out of the sky.

Vans knows what they're doing.
 
I just checked MIL-STD 403C which states the minimum sheet thickness for countersinking .125 rivets is .051"

In a different reference standard, the text explained that a sharp edge to the c'sunk sheet acts like a knife in shear. A certain amount of "cylinder" has to be maintained in the c'sunk sheet to provide enough surface area to react against to meet the shear requirements.
 
Another data point

just another piece of info,
in the airfoil practice kit for the spar section the instructions say to dimple and countersink the spar material for a good fit to the dimpled skin.

maybe doesn't apply to what you're asking for my tech counselor sure was surprised by that
 
Nuances . . .

Yes, when in doubt dimple, but if it does not fit, touch up with a countersink for a better match.

Beware - there is more at play here than the thickness - if you are doing bent parts, then they are likely 6061, if so be careful if using a pneumatic squeezer, it will stretch the material(edit) if set too tightly. If the parts are 2023, then stretching is very unlikely.

I used two flat face sets on a pneumatic squeezer as a stretcher to un-bow my tip-up canopy side rails. This will happen with hinges as well if they are heavily compressed dimples.

Just in case you weren't aware. :D
 
Last edited:
I just checked MIL-STD 403C which states the minimum sheet thickness for countersinking .125 rivets is .051"

In a different reference standard, the text explained that a sharp edge to the c'sunk sheet acts like a knife in shear. A certain amount of "cylinder" has to be maintained in the c'sunk sheet to provide enough surface area to react against to meet the shear requirements.

To expand on this a bit, you need that bearing surface on a countersink hole for pressurized aircraft as they see pressure stresses every flight cycle. The load spectrum is is pretty brutal. On non-pressurized structure with a light load spectrum you can accept a knife edge on the first layer so long as the countersink doesn’t extend into the next layer.
 
Interesting.......

Beware - there is more at play here than the thickness - if you are doing bent parts, then they are likely 6061, if so be careful if using a pneumatic squeezer, it will stretch the material. If the parts are 2023, then stretching is very unlikely.
Just in case you weren't aware. :D

Can you elaborate? I've built 4 aircraft that use 6061 exclusively and never experienced this problem. All were flush riveted and many of them squeezed.

If you are talking about using the squeezer to dimple, that may be a different matter. I only use the squeezer for riveting. For dimpling, I use a pop-rivet dimpler or vice-grip dimpler.
 
Last edited:
Good point MEL.

Can you elaborate? I've built 4 aircraft that use 6061 exclusively and never experienced this problem. All were flush riveted and many of them squeezed.

If you are talking about using the squeezer to dimple, that may be a different matter. I only use the squeezer for riveting. For dimpling, I use a pop-rivet dimpler or vice-grip dimpler.

MEL, you are correct, in that simply dimpling is not the issue. It is the lower yield strength of the 6061 combined with greater potential force from the pneumatic tool. If the tool is set too tight then it can apply enough force beyond the dimple to deform/extrude the material around the set.
 
Back
Top