Hello,
New EAA member and first time builder here. I have just purchased the first kit for my future aircraft, and it is on it's way!
The kit is designed around universal headed blind rivets. I would like to convert this to flush rivets for the skin, but am having some trouble engineering the right solution ...
The big issue is that the kit comes pre-punched with holes to final size. This means that dimpling stretches the hole, too large for the rivets normally sized for that hole at its original pre-dimpling size.
So a 1/8th hole gets stretched to .148 to .150 for example.
I've thought up a few theoretical solutions:
- Upsize the holes to an undersized hole for the next size up rivets, and dimple to obtain a final sized hole of the right size. For example, drill the 1/8th hole to something just under 5/32nds, and dimple to 5/32nds final size. Problem is this would require special dimple dies that do not exist (5/32nd dimple die with undersized pilot), whether in the 120 degree or 100 degree head world. Also, not knowing what every hole in the plane looks like, I could run into issues with the dimple being to close to an edge, edge distances, and so on. The A/C appears to also use 5/32nd rivets, which means going up one size from that, which gets quite big!
- Dimple as one normally would, and use oversized cherry max rivets. Turns out that in some circumstances at least, the post-dimpling hole ends up too big for even such a rivet. With 0.016 skins, the hole ends up about .004 too large. On .024 skins however, the hole is just right and could be reamed to #27 (the right size for the oversize CherryMAX -4). That being said, the tensile and shear requirements are far below what the CM provides, so the slightly larger hole may yet still provide the required strengths, even if reamed to #25 or 24. I am not exactly equipped for such testing however! I've sent an e-mail to Cherry Aerospace, but I'm not holding my breath ... This would by far be my preferred solution however.
Has anyone ever undertaken such a project, or know of someone who has? Any sheet metal experts with some ideas on how one might approach this?
Any help or insight would be much appreciated!
Thanks,
J.F.