In the last post I went through the process of designing and realizing my first carbon fiber H-quad. While the molding and lamination process itself was successful, the design was based on an open section, a fact that made it twist excessively even though it was quite resistant to bending. I thus went on to redesign the central carbon fiber “unibody” frame to be torsion resistant.
Torsion: Open vs Closed Sections
Closed (tubular) elements exhibit orders of magnitude better torsion resistance than open, thin sections. The figure below says it all:
An interesting thing to note is that the shape of the section doesn’t play much of a role in torsion resistance. The key factor is whether the section is open or closed. With this in mind, a closed, tubular section would provide much better performance.
Designing the new carbon fiber frame
With this in mind I fired up Rhino3D once again and set out to create a new design. The final, revised design is below. You will notice that I reserved space for the newly arrived NAVIO2 flight controller, along with a Raspberry Pi 2, instead of my old CC3D controller. Using this new setup gives me infinitely more capabilities in terms of aircraft control, video streaming, and even advanced features such as computer vision using OpenCV, or interfaceing with sensors such as the VL53L0X rangefinder. It also requires additional space and provisions for connections, mounting etc. I located all components in CAD into the frame before production.
The above render also includes custom motor arms, which were supposed to be 3d printed and constructed out of tubular carbon fiber cloth. I didn’t get around to building them yet, so I’m sticking to 3k carbon fiber tubes and 3D printed motor mounts for now.
Printing and preparing for lay-up
In order to have a clear print without imprecision due to print height, I reinforced the section with inner flanges, and printed the body in four pieces. I then joined the pieces together with cyanoacrylate glue. In order to achieve better precision in joining I glued plastic guides at the inside of each section, so that when bringing them together the parts would slide in precisely. I used cut pieces of zip-tie for the guides.
Carbon fiber lay-up
I started by adding a layer of epoxy to the mold1. Then, I started to roll the cloth around the mold. When I finished the first turn I kept on adding a tiny bit of epoxy between layers of carbon fiber. I applied a bit of tension while rolling to ensure the layers stay tight together. I did about four layers of carbon fiber, which resulted in a strong structure.
After curing I had to remove the interior mold, which practically meant destroying it. The removal was actually easier than I expected, with large pieces of 3D printed PLA coming out in one chunk. Then, I went through the usual procedure of trimming out the edges of the carbon fiber cloth, and sanding them to get a nice finish.
The result, together with motor arms, is below:
The frame is really strong and light. I tested it both in bending as well as in torsion, and there is almost no displacement. There’s still a lot of work left, namely fitting of components, and (most importantly) cutting out the necessary openings for both components as well as cable harness to pass through. It is expected that after the openings are cutout the frame will lose a bit in rigidity, but still I expect that it will be quite strong.
Below you may see a snapshot of the frame after just one opening has been cut out (the one for power distribution), and hanging strips are placed on the exterior to help dry-fit the components.
And here’s the quadcopter’s maiden flight (a GPS loiter test, really):
This post presented a tubular “unibody” quadcopter frame made out of carbon fiber. Overall this experiment has been a success. The tubular construction minimizes torsion and minimally impacts weight. The 3d printed mounts for the motors used in this build are available in Thingiverse.
What are your thoughts on this design? Share your experience in the comments below!
- For more details on the preparation and lay-up of the carbon fiber, see my previous post