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Back in about 1996 when my youngest daughter was in 5th grade, she did a project for the science fair to show the strength of triangles vs squares in a truss. Together we made up two identical "bridges" that were 24" long, and 4" square. They were made of 1/4" spruce capstrips and 1/16" plywood for the gussets, the same as we all use on our wing ribs. We blocked up the ends of the trusses on supports, then placed a block of 2x4 on the top center of the trusses, and placed a bathroom scale on top of that. Now came the fun part. With everything in a 30-ton electric-hydraulic press, we began to slowly add pressure. The open truss without triangles broke at 110 lbs, fracturing the lower longerons in tension. The triangulated truss broke at 360 lbs. You can see in the pics how small the glue areas are. When the triangulated truss broke it pulled one of the diagonals from the gusset, and the glue area is very tiny, as are all the contact areas between the capstrips and the gussets. Everything was glued with T-88. This reinforced to me just how strong a built-up rib is in a wing. Besides being a very unique science project, Jennifer won the "Teachers Choice Award" and $10.
Danny
Danny
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