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Proper Strength of Plywood

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TerryM76

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On the Cygnet I am building Sisler calls for 1/4" marine plywood, no face designations, but no core voids larger than 1/8" and no patches within some distance (don't recollect the number at the moment) of the main spars. I purchased Roseburg marine ply from Menards in Owensboro, KY. I've cut all my ribs, haven't cut spar doubler plates yet. Bought 2 4x8 sheets. There were just a couple of voids in the two sheets. One less than 1/8" and one slightly more. I had 1 rib that ended up with that larger (3/16") void and plan to do what was said above and slide in a sliver with T88 epoxy around it.

In addition to the marine ply Sisler also indicated that clear spruce or pine was acceptable for the caps on the false front spar and the rear spar. The front caps are 1/2"x1/2"x156" and the rear caps are 1/2"x5/8"x156" on 3/32" mahogany aircraft ply webs. The lattice on the wings is also called out as clear spruce or pine and it is 5/32"x3/4" material cut to length (about 72"). Lots of room for material interpretation/substitution here. The main spars are called out for 3/4"x5 7/8"x156" spruce. Mine are ash of the same dimensions. The wings are over designed/overbuilt for the airplane if anything and very interesting.
Any particular advantages for choosing marine plywood other than cost?
 

tdm3

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I assume Sisler called that out because it was cheaper and more than adequate within his design. I'm using it because he particularly called for it, it's cheaper and it's more readily available.

David
 

Dan Thomas

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You must be younger than I am. Or British 4x2s shrank faster.
The 2x4 or 2x6 or whatever was the dimension of the board when it was cut from the log. Rough boards. When they were run through the planer they were reduced to 1-5/8" x 3-5/8" up until around the late 1960s or so, and then they were reduced to the dimensions we see now. Rough-cut wood now is nowhere near the old 2x4 etc. dimensions, since it wastes too much wood. Good trees are disappearing, so the processors have to minimize the waste. That's why we see rough saw marks on some areas of planed boards, and one or two corners with bark still on them.

My Dad and Grandpa were lumber-mill guys. Grandpa was a lumber grader. I used to watch him grading lumber as it went by on the green chain, and the stuff he marked "cull" (junk) is now what I see being sold as construction #2 or better. There were a lot of "clear" boards when I was young: no knots at all, nice straight grain. Lots of big trees. I miss that stuff.
 

TerryM76

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I assume Sisler called that out because it was cheaper and more than adequate within his design. I'm using it because he particularly called for it, it's cheaper and it's more readily available.

David
Thanks David. Do you know if there is a significant weight difference between the two?
 

Little Scrapper

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I bought a full sheet of Meranti and tested it. It's brittle. It snaps very easily. I would avoid it for anything in a aircraft where structural integrity matters.
 

tdm3

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Terry, my understanding is that both mahogany and birch would be heavier in the same thicknesses but I have no idea how much difference.
David
 
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