Purpose of countersinking

[AviatorJ]

I've always thought the purpose of countersinking was purely aesthetic. The main reason being to have a smooth surface once screwed, riveted or otherwise. Is this the case or is there some added bonus of strength?

I'm asking because I did my rivets on the read spar of the VS last night. Onbe of the previous parts I had countersunk was the rear spar doubler. Here's a picture from someone elses log;



This particular piece gets riveted to the rear spar and wouldn't be visible on the final build, it also doesn't have anything directly butt up to it. Only thing I could think of is they wanted to reduce the rivet length, so they had it countersunk...

[Auburntsts]

Originally Posted by AviatorJ

This particular piece gets riveted to the rear spar and wouldn't be visible on the final build, it also doesn't have anything directly butt up to it. Only thing I could think of is they wanted to reduce the rivet length, so they had it countersunk...

Actually it butts up against the aft end of tailcone when the VS is installed. Take a look at Section 11, page 11-5, step 5 and you'll understand why the plan calls flush rivets in this area.

[Mel]

Many times flush rivets are used because another piece must be installed directly to the surface. Flush rivets are not stronger than universal rivets.

[TFeeney]

Justin -
Take a look at page 11-1 where the vertical stab gets mounted to the empennage. That piece is forward facing and mates flat against the back of the tail cone - which is why its countersunk.

Tim

[AviatorJ]

Originally Posted by Auburntsts

Actually it butts up against the aft end of tailcone when the VS is installed. Take a look at Section 11, page 11-5, step 5 and you'll understand why the plan calls flush rivets in this area.

Now it makes sense! Thanks

[rv7charlie]

Originally Posted by Mel

Many times flush rivets are used because another piece must be installed directly to the surface. Flush rivets are not stronger than universal rivets.

Technically correct, but *dimpled joints* (not countersunk) using flush rivets have been tested as stronger in shear than non-dimpled joints using the same diameter universal head rivets. Including the dimple apparently increases the effective shear area of the joint.

At least, that's what I read on the interwebs....

Charlie

[RVbySDI]

That picture brings up a question. I cannot think why one would drill and countersink holes with the blue plastic still attached to the piece. That plastic came off as soon as possible for me. Once you find your first sign of corrosion underneath some of that plastic you will understand why.

[jchang10]

I was thinking the same thing. It would interfere with deburring as well. You will have burnt plastic pieces possibly melted to your Al and your tools. Also, I especially primered all my non-Alcad pieces such as this anyway, so no worries about scratching it.

[AviatorJ]

Originally Posted by RVbySDI

That picture brings up a question. I cannot think why one would drill and countersink holes with the blue plastic still attached to the piece. That plastic came off as soon as possible for me.

Can't take credit for the picture, I just wanted to have a picture for the talking point. I personally take the plastic off the entire piece.

One thing to note though, I went outside to look at the plans to see where that piece ends up going. When running my hand over my rivets I feel a bit of an edge, some more than others... but now I'm not happy with my work.

I ordered more rivets from Vans and am going to redo about 20 of the rivets. This will remove the primer I put on the piece when I start making deeper countersinks. Should I reprime each hole after drilling and deburring? Or leave it since the new rivet will smash it down. Also is it better to have the rivet more 'sunk' than 'raised'? Fun times!

[Mel]

Originally Posted by rv7charlie

Technically correct, but *dimpled joints* (not countersunk) using flush rivets have been tested as stronger in shear than non-dimpled joints using the same diameter universal head rivets. Including the dimple apparently increases the effective shear area of the joint.
At least, that's what I read on the interwebs....
Charlie

I believe we were talking about machine countersinking here, not dimpling.