AN426AD3-4 rivets specified
Running into this issue today, I thought that given the difference in thickness of a dimpled v's countersunk 3 layer sandwich, that ultimately the optimum method would be defined by whatever rivet Van's specifies for this application. According to the plans, it is an AN426AD3-4 rivet. I created a test sample out of two layers of 0.040" sheet to represent the spar and doubler webs, and a top layer of 0.025" to represent the skin. I dimpled both holes on the skin layer, then on the middle layer (the spar) I countersunk one and the other I dimpled. On the bottom later (the doubler) I dimpled one and left the other untouched. Note that I used a small diameter 3/32 female dimple and did not get any spar/stiffener deformation. Clamped tightly together and making sure I had countersunk far enough that the sandwich had no air gaps, I then measured the rivet length exposed, and from the base of the dimpled setup, there was 0.115" exposed. For the countersunk setup, there was 0.141" exposed. Given that a 3/32 rivet has a diameter of 0.093", this means that the optimum rivet length exposed is 1.395 (1.5x the diameter). Thus the -4 rivet length provided almost perfectly matches the countersunk setup. I squeezed both rivets until they met the 1.5x diameter required head diameter. For the countersunk rivet, it looks like perfect depth using the rivet gauge, and for the dimpled setup, it is just shy of being ideal according to the gauge, as one would expect. That said, it still looks like a well formed head, and given that I loath countersinking and that Van's has specified that either method is applicable, I think I will proceed with the dimpled head. I guess I could always use a -4.5 rivet in that location. Just as a final note, if you are going to be dimpling, be careful that the yoke doesn't scratch the spar or the doublers when you squeeze the dimple, because of the acute angle of the web in relation to the spar cap. I left a light scratch on the doubler because of this, so I buffed it out, then ground a small shoulder on the top of the yoke and shielded it from the surface it was rubbing against with some duct tape. Just keep an eye on it as the tape wears. Anyway, given that I've blown way too much time on this exercise (I guess we all suffer a little from OCD when it's your butt in that aircraft), I need to get back to driving rivets. I hope this better spells things out for fellow builders who way over think things.
Tom.
