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Well, I decided to start seriously designing and building my quad-copter frame.  I recently build a CNC laser and can use that to cut all the parts I need for the frame.  I plan to use polycarbonate because it is strong, tough, and fairly light.  I've been exploring ultra light design principles.  The idea is that you can use very thin materials to make truss structures that may be quite flexible out of the truss plane but you add enough trusses in the out of plane direction to stiffen up the entire structure so that it is strong enough to resist bending moments in any direction.  I call this "box trussing".  It is very similar to a torsion box that is commonly used to make very strong, light weight work tables in wood working.

The copter arms are truss structures with spacers that are also truss structures.  The arm sides are made from 0.04" thick polycarbonate and the spacers are made from 0.03" thick material.  The entire structure weighs just over 18.6 grams for one arm and will be more than strong and stiff enough to handle the forces generated during normal flight activities.  The following shows a side view with the hidden edges showing.

All modes of shear and torsion are isolated with various truss members.  The polycarbonate is fairly stiff, but flexible in an out of plane bending mode, but is very strong in tension and decently strong in compression.  Using an X shape in the truss is very important because in a shear moment, one member will be under tension.  The proof will be in building the unit.  Also, one of the benefits of using polycarbonate is that it can be solvent welded with methylene-chloride which essentially makes the joints as strong as the plastic itself.

There is only a few dollars of poly carbonate in the frame so if I crash it, it's no big deal.  I can make another one in an hour or so.