I had purposely not dimpled the top row of rivet holes on the F-1270 fuselage side skins so a precise match drilling of the longerons could take place. Now that the longerons have been match drilled, the top row of holes on the F-1270 side skins need to be dimpled to accommodate the flush rivets being used on the DOG Aviation RV-12 project. Also, the newly drilled holes in the F-1255 longerons need machine countersinking to accept the dimples being placed on the side skins.
Before beginning the process of machine countersinking the F-1255 longerons for the flush rivets, a thought occurred to me. I know the CS4-4 flush rivets Van’s uses at some locations on the RV-12 have a slightly smaller head than the Gesipa flush rivets being used thus far. The smaller rivet head should require less countersinking depth thus removing the least amount of metal leaving the longerons stronger. Don’t think this is that a big of an issue here, because all of Van’s aircraft use the same longeron material. Some of them are 200 mph aircraft and can also do aerobatics, however their machine countersunk rivet holes are 3/32" at 100 degrees and not the 1/8" at 120 degree as will be necessary for the RV-12, which obviously requires removing more longeron material to begin with.
Readers who have been following the DOG Aviation Blog from the beginning may remember prior to beginning assembly, it was discovered the dimple dies originally purchased were leaving the Gesipa PolyGrip flush rivets selected for the RV-12 project a little proud. So a switch was made to dimple dies made by Bob Avery. The Gesipa rivets being used have a slightly larger diameter head thus required a slightly deeper dimple and the Avery dimple dies made for a perfect fit … all was good.
Van’s CS4-4 flush rivet on the left and on the right is the Gesipa PolyGrip rivet being used on the RV-12 project. The size difference between the two rivets can easily been seen here.
Time for a little research … the DOG Aviation R&D department used the two pieces that were cut off the aft portion of the F-1270 side skins (.025" thick) for a little test. The Avery 120 degree dimple dies were used to dimple one piece and the Cleaveland Aircraft Tool 120 degree dimple dies were used to dimple the other piece. The results: The Avery dimple dies created dimples .011" deeper than the Cleaveland dimple dies. The
Apparently not all dimple dies are created equally. The smaller head of the Van’s CS4-4 flush rivet fits nicely in the shallower dimple created by the Cleaveland dies … it is ever so slightly proud (but I’m nitpicking). If the Cleaveland dimple dies are used to dimple the top row of holes in the F-1270 side skins, the longerons will not need to be machine countersunk as deeply. … and leaving more metal equates to stronger.
Test using Avery dimple dies and Cleaveland dimple dies … the Cleveland dimple dies (pictured on the top) created dimples that measured .011" (measured front to back) shallower than the Avery dimple dies.
At the end of the day, removing less material from the RV-12’s longerons will yield a stronger longeron. So the decision has been made to use the Van’s CS4-4 flush rivets along the top row of the F-1270 fuselage side skin to F-1255 longeron attachment. The rivet holes along the top row on the side skins will be dimpled using the Cleaveland 120 degree dimple dies. That means the longerons will be machine countersunk to a depth .011" less than would be required if using the Gespia flush rivets. That does not sound like much, but one needs to take into consideration that the machine countersink bit will be creating a 120 degree conical taper in the rivet hole … as such, countersinking deeper ends up removing a lot more material because the hole also gets much wider at the top.
Hopefully, the dimpling and machine countersinking can begin during the next work session after the nutplates and air vent are added onto the left side skin.