plywood grain

[Gregory Wong]

Folks,

Normally, you cut balsa fins so that the leading edge is parallel to the grain. Is this still necessary when cutting fins from basswood plywood? Is the grain of each layer set 90-degs from the previous layer?

Greg

[Michael Parker]

OK, I know a lot about this from carpentry, not rockets or airplanes, but, I kinda think that plywood is plywood? So, yes, the grain alternates direction on each layer. Therefore, in order for the face grains to be parallel, there will be an odd number of layers - always. And, therefore, there are more plies oriented in the face grain direction, so, yes, there will be more flexibility across the face grain than along it. You can easily feel the difference if you have a sheet of 3 ply, 1/8" light ply from Balsa Machining, just bend it with your hands and the difference will be obvious. Possibly more importantly, in structural materials each layer contributes strength in proportion to it's distance from the chord line - like in an I beam, the top and bottom plates are the strength, the middle plate mostly serves to hold the top and bottom away from the chord line. So, the face grains, being farther from the center of the sheet contribute more to the strength of the sheet than inner layers do.

tl;dr - plywood is much stiffer along the face grain than across it.

[Jack Hagerty]

-- 

Plywood no, basswood yes. Just ask my ARC teams. Two of them cut their fins from basswood with grain parallel to the root. Both broke fins on landing.
 

- Jack

 

Jack Hagerty, LUNAR #0002

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