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Barracuda
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Date:  9-28-2007
Number of Hours:  0.50
Manual Reference:  
Brief Description:  Center section spar

Had a question answered by Mr Siers reference the wing to body attach blocks within the spar -


Todd, Yes, this piece is for the wing to body attach blocks within the spar. The 2 inch dimension becomes the 1 3/4 inch dimension and the 2 3/8 dimension becomes the 2 inch dimension (drawing sheet 201). The grain direction here is CRITICAL, and after reviewing the builders manual, it appears there is room for misinterpretation. The grain of this member should be oriented up and down, lengthways with the piece, so that the bushings are loaded against end grain.

My intent in calling out the grain to be perpendicular to the 2 3/8" dimension (builders manual page 19) was for the bushings to pass through all the wood growth rings in the member and the face grain to glue to the webs. If the block is oriented square to this so that the edge grain is glued to the webs, the strength is about the same. The imortant thing is that when looking at the ends of the block, you will see end grain.

The bearing strength of sitka spruce at 12% moisture content, which this should be, is 37,500 kilopscals parallel to the grain and 3,000 kilopascals perpendicular to the grain. This is per the Forest Products Laboratory data which is available on the web. It compares closely to other data sources. The conversion is 435 PSI square to the grain and 5,076 PSI parallel to it. The requirement for the grain in this member to be vertical is obvious.

The load imposed into these blocks by the bushings is reacted as a shear load between the blocks and the ply webs they glue to.

I am very sorry if this has caused rework to anyone. I should have been more clear on this point in the builders manual but after a full year of many late nights writing it, and many rewrites, there were bound to be some mistakes. The Barracuda structure is extremely forgiving, but not here.

-------------- Original message --------------
From: "Wilton Matrix"

Richard, I have laid out the center section s
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