When Prof Ed first brought up the issue of inspection holes in 3-bead skins, I asked Mike Sellers at Univair to comment. I've edited his two e-mails on thh subject, but the bottom line is this-- when SB-31 was issued, they did not explain how to do inspection holes in a 3-bead lower wing skin simply because they were unaware of that configuration. After that, late-model Erco and Forney owners started asking for more information, and Univair started trying to track the airplanes that had the 3-bead skin, but they were unable to correlate them. If you have a 3-bead skin, I think the message is that you can cut the holes wherever you need to as long as they don't impinge on the spar.
I have examples of all three types of skin in my "collection". My airplane has a smooth left lower skin and a 2-bead right lower skin. I also have a 3-bead skin with the center section I bought from Weston Webb in Idaho. Don't know if that helps, but the subject of how lower wings skins came in so many configurations sure is interesting Ercoupe history! Maybe Bill Baine can add more information. "Service Bulletin 31 does not address a three bead skin because we have no reference in any of the original data that even suggests the possibility of a 3 bead skin. We simply were not aware of a 3 bead skin when we wrote the bulletin. It wasn't until the Service Bulletin hit the streets did we have people mentioning that they had a 3 bead skin. So to that end we don't know what serial number break there is. It has been a while since the big rush of kits went out the door. When we were in the middle of this, I believe we had started a list somewhere to see if there was a particular run of serial numbers to see if we could come up with some sort of correlation. By the time we decided to start keeping track, a number of contacts had been made inquiring about the 3 bead skin, and we weren't anticipating the issue. So, a number of folks went by us before we started collecting numbers. By the time that rush had passed, we didn't have enough data to make any conclusions. Since then we here of somebody else about twice a year. The only thing we have been able to come up with was the total numbers out there weren't great. I don't think the total number of contacts we had on the matter would have even amounted to 50. In trying to figure this out, we wondered if this was an undocumented change on the production line, if it was done somebody in the field because they felt the skin needed stiffining, if it was a screw up that got conveniently ignored or what. Mike" "Dave, Sorry for the delay I took some time off to help a family member move to another state. The original number of the skin was 13047-1 L/R (2 bead). It was replaced by 13147-1 smooth skin via an Engineering order dated 11/23/45. According to my records serial numbers 136, 137, 138 were completed on 11/23/45. S/N 139 was built on 11/24/45. While I do not know the exact serial number break, I would say that the smooth skin appeared on aircraft somewhere around that s/n 139-140. Going back and looking at margin notes in my parts book, I have noted that some later aircraft "i.e. Forneys" had the 3 bead, but we have no engineering record of it usage. Regards, Mike"
