Composite matererials are materials made from multiple other materials which are bound together to take advantage of their different material properties. For example large concrete structures have rebar inside of them for additional tensile strength while the concrete provides the compressive strength. Another thing to remember goes back to Intro to Mechanics and beam bending, where the the second moment of inertia of a beam scales by the beams height cubed times the width. This shows the material height plays a larger roll in strengthening the part than the middle material, or the material farther away from the neutral axis is more imortant for stiffness. In composites this is an important concept because the core material does not need to be structural excapt in compression, it can then be optimized to be light weight, like foam, and the surrounding materials can provide the strength. This is why composites are used in airplanes.
Quad-Copter Body
I decided to make a body for a quad-copter because I needed it to be light weight but very strong to withstand a lot of crashes. I'd acquired most of the parts for a quad-copter, but up until now the only body I'd tried was some coat hangers and duct taped I'd hacked together. From this I learned the necessity for the drones body to be very rigid, otherwise it is impossible to control, again, ideal for composites.
I started by laser cutting out a cardboard outline of the bodies shape to use as the core. I then cut out linen strips to use as the fiber and mixed up the epoxy as per the instructions on the box to use as the matrix. I layed out everything I would need, including the peel ply, so the epoxied part wouldn't stick, and some batting to absorb extra epoxy (bleeder material).
I then dunked the linen strips into the epoxy and wrapped the cardboard outline like it was a mummy. It was surprisingly difficult to keep the strips from falling off the piece. I noticed this on some other peoples projects too, the epoxy is not sticky at all when it isn't cured, so it's like trying to mold wet cloth.
I layed the mummy like soaked linen and cardboard mess on the peel ply, wrapped in batting and then clamped it down between two sheets of wood. You could also use a vacuum bag or hydro static loading. The purpose is to apply pressure so the air is pushed out and the fibers and the matrix are compressed tightly together, which fills in the gaps in the fiber and gives the piece its strength.
The next day when I came back everything had cured very nicely. The part peeled off the peel ply with a little bit of pulling. The body was a bit more flexible than I had hoped, and also a bit heavier, but it smoothed out well and all the strips were solidly connected. I was able to sand down the edges with a dremel and file, and it looks pretty nice.
Now time to make it a quad-copter. I had to re-drill the holes to attach the motors with a drill press, which went right through the part like wood. I cut some holes to feed the motor cables through to keep all the electronics on the bottom away from the propellers, something I'd learned from my first quad-body because the propellers can cut through the wires like air.
For the time being I zip tied all my electronics together underneath the body. I will soon get screws and tiny washers to attach them properly. I also need a LiPo and will make a second deck below to protect the electronics when the copter lands/crashes.
