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Dimpled vs countersunk strength

MIL-HDBK-5F FAA Design Book

According to this document, the design specs for a lapped joint in single shear of .032 aluminum sheet for flush head (AN426-) rivets is 217 lb - dimpled & 178 lb - machine countersunk.

This is a 22% difference - the "knife edge" condition may come into play here, and the Handbook also had a warning that in .032 material, this was a knife edge condition and was undesirable, and not approved.

This handbook is a wonderful reference for the design engineer looking for specs on all sorts of things related to aircraft design.

YMMV

HFS
 
Here you go

The rivets used in RV construction are 2117 Alloy "AD" rivets. See the milspec chart, which will give you some shear strength data. Its not the greatest chart, because it doesn't take into account variations of dimpled and countersunk stack-ups. In real world structural repair manuals, often the guidelines revolve more around minimum skin thickness for countersinking, which has a profound affect on joint strength, IE, you never want a knife-edge condition. Dimple joints do give some additional strength and durability because the skin bearing strength added in the "clinching" of the joint. When I built my -4, I used single piece upper skins of .032" (minimum thickness for 3/32"rivet CSK)and eliminated the mid-span joint. My skins are countersunk, as well as my vertical and horizontal skins, which are .032" The leading edges are dimpled. 11 years of harder than average flying, not one single "smoking rivet".
 

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From Section 5 of the build manual:

“ For AD3 rivets, a total material thickness between .016 [.4 mm] and .032 [.8 mm] must be dimpled. Material thickness between .032 [.8 mm] and .040 [1.0 mm], should be dimpled, but a countersink may be used if necessary. Finally, for a thickness of .050 [1.3 mm] and above the material must be countersunk.”

Now I know these are experimental aircraft and you can do what you want, but for what reason would you want to machine countersink 0.032” skins if you didn’t have to?
 
Really interesting information that someone nerded out on to achieve. I appreciate it.
 
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