I first looked at retrofitting my quad to run 4S batteries, but I needed new motor controllers. The bigger battery would also shift the center of mass, making it unbalanced. Given the cost and crappy performance I figured I might as well dump some more money in and build my dream machine.
The first thing I wanted to do was correct my complaints with the typical frame designs. Mainly I wanted the battery protected from crashes. During the races a number of people would smash their batteries in a crash. LiPos are pretty volatile and I wanted to mitigate the risk of a battery fire. In addition to the battery armor I wanted the arms to be replaceable. It is much cheaper and faster to replace a single arm instead of the entire base frame plate. My quad has a cracked arm, and I didn't want to spend the money on a new frame plate (I fly more gently). Finally, I wanted the frame to be unique.
The rest of the hexrotor design focuses around protecting the battery. Most frames have the battery held (exposed) on the top or bottom of the quad with a few velcro straps. I worked to design the hex frame so it wraps around the battery. I split the hex into two portions, a base plate that handles power distribution (battery connector, motor controllers, and all of the wires associated), and a top deck that holds all of the electronics (flight controller, video camera, video transmitter, RC signal receiver, on screen display, and signal LEDs). The battery sits between the electronics deck and the frame. This ensures the battery is safe from damage... or at the very least I'll have already broken all of my electronics before the battery gets damaged. The battery placement also provides a very even weight distribution. The overall center of mass ends up perfectly in the propeller plane, meaning the hex is very well balanced.
I wanted the hex to be similar in size to a standard quadrotor, so I had to pull the props in closer to each other. This led to a problem... the FPV camera would see the props. Thankfully the top deck design eliminated this problem. The deck sits high enough that a highly tilted camera (necessary for these faster 4S racing quads) wont see the props.
The max center to center distance on the hex is 270mm, which is only slightly larger than my quadrotor frame which is 250mm.
Overall the hex has been a fantastic build for me. It met all three of my design goals. The arms are replaceable, but I have yet to break or even damage one. I think the sandwich design has enough flex to reduce the stress during an impact. It tends to cartwheel in crashes, which also reduces the forces on the frame. The battery armor design has worked FANTASTICALLY! I have flown into so much stuff, and the battery has been fine every time. As for my uniqueness goal... It really hits the nail on the head. The look and sound really get heads turning. The robustness and mass have also made it a bit terrifying in races. I have had a few mid-air head to head collisions, leaving the other quad needing repairs.
I will probably build an upgraded frame in the future as new electronics come out (maybe I'll make my own from scratch), or as the racing trends shift. For now, I am happy with the hex and It will be quite a while before I am once again machine limited and require a faster - higher performing machine.