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Plywood ply direction question

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Malcolm C

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May 18, 2020
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My plans call for the box spars to have 45 degree plywood either side of the caps, what would be the result of using 90 degree plywood instead? It is half the cost of 45 degree, ( asking for a friend...)
 

rv7charlie

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If you're a conspiracy theorist, then the designer owned a plywood plant. If you're not, then it's needed for proper strength profiles (load paths) in the spar.

Your choice... ;-)
 

rotax618

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Generally shear webs don’t have to be contiguous, you can cut 45deg across a normal sheet and butt joint the sections, if you are worried that the ply lends some tensile and compressive resistance to the top and bottom chords then scarf the ends of the sections.
 

mcrae0104

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...what would be the result of using 90 degree plywood instead?
Hard to say what the result would be without talking with the designer and examining the assumptions that went into his design. Your airplane may not ever be stressed to the point it will matter, as the designer may have 1) selected the thickness based on availability of given thicknesses of plywood, or 2) included an adequate factor of safety that would eclipse the difference.

The difference from an engineering perspective is that the principal forces in the shear web run on the diagonal. If you want to learn more about this, look up "stress element" and "principal stresses." By orienting the plywood on the 45, you're aligning the grain of the veneers in the direction of the stresses.

In the diagram below, imagine we've taken a little tiny square out of your shear web. The black arrows represent the vertical and transverse shear forces (more on transverse shear in the video below). When you add them together, the resulting stresses are shown by the red arrows (SW-NE is in tension, NW-SE in compression). If your plywood is oriented diagonally, all of the layers are being acted upon in line with their grain so you get maximum strength per weight. If your plywood is oriented "straight," the layers are only developing about 70% of the strength because the principal stresses are misaligned with the grain.

1605564927000.png

Here are a couple of videos on sorta related topics...


 
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rv7charlie

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Most plywood is laid up with 90* angles; 45* would have 45* & 135* included angles between the plies. Did the designer mean to cut stock 90* ply plywood on a 45* bias, or did he mean to use actual 45* ply plywood, which would be a different animal?
Plywood - Wikipedia
(because I'm too lazy to look up a/c ply specs)
 

Pops

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The box spar in my Falconar F-12 had plywood at 45 deg angle on the box spars. The 45 deg was opposite of the front and rear and top and bottom of the box spar. It was stressed for +9 and -6.5 G's. I would build per plans. Your peace of mind is worth it.
 

mcrae0104

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Did the designer mean to cut stock 90* ply plywood on a 45* bias
Yes, plywood "on the bias" is commonly referred to as 45-degree plywood. Typically it's oriented as shown here so that one more veneer is carrying the compression (where the wood is weaker).
1605565433604.png
 
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