The dangers of cheap plywood

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Aerowerx

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Although not aviation, here is a good example of what can happen if you use cheap plywood.

This is the top of my computer desk. Homemade about 35 years ago. It is 3/4 inch oak veneer plywood from a big box store.

Yes, it is 3/4 inch thick, but imagine this happening on your wing D-tube cover.:fear::grave:

upload_2020-2-20_9-57-58.png
 

TFF

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It happens all the time. Doesn’t take much to hurt an airplane. I have known a couple of fabric planes in common hangars that had fabric torn. One just recently. Show me a Bonanza with original wingtips. Crowd planes in and at some point they bump. Whatever did that to your desk would be at the spar on a plane. Some people don’t get to fly, you fly an eggshell. Not a bulldozer.
 

speedracer

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I've never heard of using plywood for a spar since (about) half the layers are vertical instead of horizontal.
 

Aerowerx

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My point was that you don't know if there are voids in the inner layer when you use cheap plywood. That spot in the picture is not a ding, but the outer veneer collapsed into the void.
 

lr27

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I understand that you can see voids in thin plywood if you put a bright light behind it. This doesn't tell you much about the glue, though.
 

don january

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My 2 cents worth on ply is this. I have found some really good ply on the back of old dressers and some areas of entertainment centers. Pieces seem very dry and brittle but once cut and shaped to what your building they came out very strong and no noted defects or air pockets also took to wetting and shaping by jig very well. I had some Mahogany ply for spars for the Taylor and it had nearly 15" of bad area were the outer ply started pealing from the rest and when cutting small gussets from same sheet I found numerous defects when cutting with razor knife. I think ply is much like a 1/2" x 4" piece of wood you must physically handle the wood for a bit and see what it will tell you.
 

lr27

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For spar shear webs, I imagine it's worthwhile to spring for the really good stuff! At least near the middle.
 

dog

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My 2 cents worth on ply is this. I have found some really good ply on the back of old dressers and some areas of entertainment centers. Pieces seem very dry and brittle but once cut and shaped to what your building they came out very strong and no noted defects or air pockets also took to wetting and shaping by jig very well. I had some Mahogany ply for spars for the Taylor and it had nearly 15" of bad area were the outer ply started pealing from the rest and when cutting small gussets from same sheet I found numerous defects when cutting with razor knife. I think ply is much like a 1/2" x 4" piece of wood you must physically handle the wood for a bit and see what it will tell you.
Watch for people throwing out 50'to early70's kitchen's ,all cabinet grade ply,no voids.
I have restored those kitchens,remove the over painted hardware and swish it in a can of paint stripper, cleans up nice.Sand the fronts,etc,re paint.
Looks great with a bit of nice vintage stuff,60's modern and cool old gajets.
Or just grab the unobtainium and run.
Sometime the studding is aircraft grade spruce too.
20 to 30 anual rings per inch,no knots,in the dumpster it goes.
 

cluttonfred

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Taking this thread in another direction...I have worked a little with BS-certified marine grade okoume plywood and found it to be very good quality: strong, uniform, and light. Does anyone have a plywood strength and weight chart by species that includes okoume that could be used to calculate substitution of okoume plywood for mahogany, Douglas fir, or Finnish birch plywood?
 

lr27

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Don't forget stiffness, though I suspect that with okuome, equal strength, being thicker, will be significantly stiffer in bending. You would need to go through the design carefully to make sure there was room for the thicker wood. When going through the design, you'd need to know whether buckling, tension, or compression were critical, as this would affect how much was required. I imagine in some cases okuome would make for a slightly more offset load. Also, how strong is okuome across the grain, or in rolling shear, and would you have to change some glue joints? However, I imagine in most places on a plane it's simpler than that. You just have to know which ones. It seems to me that you might end up with a lighter airplane, though.

Forest Products Labs, as I recall, has some interesting papers about the properties of plywood. But I doubt they discuss okuome.
 

pictsidhe

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On cheap ply, the biggest worry should be the glue. If that goes, the whole sheet turns to paper... Safety factors are there to take care of small flaws. Carefully destroy a sample or three in a bench plane to see what sort of void percentage it has.
If you still want to use some, check its water resistance. A lot will delaminate if it gets wet even once. Try some soak/dry cycles and see what happens.
 

pictsidhe

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Taking this thread in another direction...I have worked a little with BS-certified marine grade okoume plywood and found it to be very good quality: strong, uniform, and light. Does anyone have a plywood strength and weight chart by species that includes okoume that could be used to calculate substitution of okoume plywood for mahogany, Douglas fir, or Finnish birch plywood?
I believe that oukome is lighter than the other woods and less rot resistant. Wood strength has a very strong relationship to density. BS marine ply should be void free and use glue more than suitable for us. I'd be quite happy to use varnished marine oukume.
 
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