My daughters and I have built several quadrotors out of aluminum and also a mini quad out of kite parts (see beatty-robotics.com for these). But for this next project, we decided to make a quadrotor out of carbon fiber. We applied everything we learned on our previous builds and also tried a few new ideas that we hadn't tried before.
The arms of this new quad are made out of (4) 8" lengths of 3/4" square carbon fiber tube. On our past builds, the motor mounts often bent (and had to be replaced) on "hard landings," so this time we dispensed with the motor mounts altogether. We drilled the mounting holes directly into the arms (as shown in the picture below). That may be worse. It might just break the arm instead of bend the motor mount (which would be a worse failure mode obviously). We'll see how it goes. I noticed recently that the 3DR kit also appears to use the same technique on their aluminum arms.
The center plates are made out of two 6" x 6" squares of carbon fiber. In previous builds, we always stacked multiple plates vertically to mount all the electronics and store all the gear. In this case we decided to "go wide" instead of "go tall." The idea was to flatten it out and make it more crash resistant, and to reduce overall weight and parts by having fewer plates, fewer fasteners, and no standoffs at all (the arms separate the plates). It also provides more of a fixture-surface for the arms and it gives us a place to store the ESCs and all the wires (We solder our own Power Distribution Unit out of Deans connectors). We'll see how that works.
Next, we wanted to try the switch idea that I had discussed and asked about in the forum. We have installed a decent-sized (but relatively light) illuminated automotive toggle switch. When the quad is stored in the house, the battery will be disconnected, but out in the field, we'll use the switch to turn the quad on and off. It's very quick, easy, obvious (red), and it can be reached without putting your hands in the path of the props. We're hoping this will be convenient and safe. In retrospect, it might have been even better (safety wise) to put the switch on the outside edge of the underside. (Note: I'm aware that 20 amp automotive switch may or may not be suitable for the ultra-high amps of a quad rotor, but refer this Forum post for our extensive discussion on that subject). We'll see how it goes.
We've had a lot of trouble with landing gear in the past. We've tried a number of approaches and none of them have been satisfactory for us. On this build, at least initially, we decided to test out the 3DR landing gear pieces available in the DIY-Drones store. They seem like they are going to work well.
I have uploaded pictures of the completed quad as well as an image of our CAD drawing and a few in-process pics. It's too wet to fly today, but hopefully we will be able to post a video soon.