I spent an hour this evening chasing down this "problem". I was setting up the jointer to plane the spar roots and in the process discovered that my 1/16" ply isn't 1/16" (or 0.0625"). Instead, it is 0.075" - about 20% over the nominal thickness.
After scratching my head for a while (and taking many measurements), I consulted the Aeronca Drawings and discovered that 1/16" plywood must have been about 0.075 even back in 1946. Bizzarre.
Here's what I found...
My logic says: The spars are supposed to be 3/4" thick. At the root, that means planing 1/16" off of each side so you can add back the 1/16" doublers to net 3/4". Assuming you take 2 x 1/16" off of the original 3/4" spar, that tells you that you should plane the spar down to 5/8". Make sense? It did to me.
But the DRAWINGS say you're to plane the spar down to something between 37/64" and 39/64" (FYI, 5/8" = 40/64") before attaching the doublers. However you're still supposed to net a 3/4" thick spar with the doublers attached. So how do you net a 3/4" finished thickness if you're adding two 1/16" plates to a 37/64" spar? Easy. Even in 1946, 1/16" aircraft ply must have been about 20-25% thicker than the nominal thickness.
Amazing.
You people need to tell me these things.
