Monday, July 25, 2016

Hexrotor FPV Racer

It took a few months, but I got to the point of being machine limited when FPV racing. When I say machine limited, I mean I basically don't make mistakes and basically run at 100% throttle until the battery dies. The quadrotor also got to the point where it just couldn't keep up with the other racers. When I first started most quads ran with a 3S battery (3 lipo cells, or 11.1V nominal). In the past few months that trend changed to most people flying 4S batteries (14.7V nominal). The extra voltage makes the motors spin faster and provide more thrust. Most of the 4S quads have thrust to weight ratios of about 6:1, with some approaching 10:1. My 3S quadrotor... probably 3:1.

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.

This led me to designing my hexrotor. There's one or two hexrotor frames available for purchase (small compared to the hundreds of different quadrotor frames). I also made it radially symmetrical, which is not something I could purchase. The hexrotor factor took care of the unique and cool requirement.

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.

Here is a printout of my hex in a 1:1 drawing next to my quadrotor. I like to make these 1:1 drawings to get a better feel for the design and to see problems I might miss on the computer. I guess I'm kind of old school for this kind of thing.

Without shop access I sent my parts out to be machined. I found a site that did custom carbon fiber parts specifically for FPV racers. The price was better than what I could find for just the raw carbon fiber plate. I ordered enough parts to make two frames as well as spare arms for each frame. The two frames and spares came out to the same cost as the one frame for my quadrotor...

The frame is a sandwich of arms between two main base plates. Most existing frames with replaceable arms use 4 screws, some of which are close to the edge of the arm. These arms always break at the screw holes. I used two screws for each arm, directly in the center. This minimizes the loss in strength of the arm from the screw hole placement. I also made sure that the arm will always fail before the base plate. What's the point in having replaceable arms if the base plate cracks instead?

The max center to center distance on the hex is 270mm, which is only slightly larger than my quadrotor frame which is 250mm.

I 3d printed all of the electronics mounts. This time I used tough resin. The tough resin is a newer formulation from Formlabs (the brand of my 3d printer). It can take significantly more impact before shattering. It also tends to be softer and more flexible.

I temporarily put the electronics deck on top of the frame to see how it would look. The frame looks very compact until the deck is raised to make room for the battery.

I couldn't find a power distribution board (they were all built specifically for quadrotors), so I had to just make a wire bundle. It isn't the most elegant solution, but I made it pretty clean.

Dealing with all of the wires and motor controllers was a huge pain. Mounting it to the frame took a lot of care to avoid getting things tangled, or putting something in the wrong place.

My desk was a disaster during the build. I should probably get a dedicated table to work on instead of my computer desk. The good thing is it makes me motivated to finish the project faster, otherwise I basically can't use my computer.

Here is the completed hex next to my quadrotor. It isn't significantly bigger, but it definitely has the cool and unique factor going on.

The battery used is a 1800 mAh 4S (14.8V nominal) 75C LiPo. 75C means the battery can be discharged  in 1/75 of an hour. Maximum continuous current is 135 amps! (1800 mA*75).

I added some addressable LEDs to the back. Each one can be set to any color. I have it set up to act as a turn signal as well as change color depending on the throttle and state of the hex (if the flight controller is armed or not).

The hex stacks up pretty nicely against other quads. The extra thrust is more or less canceled by the extra weight. The hex has no advantage over the quads except for some redundancy (in theory I can lose a prop and still fly). The hex is more dense than the other quads, which makes it slightly less vulnerable to wind. It sounds amazing and has a pretty intimidating effect on the other racers.

I ended up buying 8 batteries. This easily keeps me flying continuously as long as i have my 4 port battery charger in the field with me.

The LEDs really saved me in this crashed landing. The grass grew pretty high in the field I race in, and the hex buried itself in. Without the LEDs I might have permanently lost it.

I also later purchased a gopro and mounted it (5-28-2017). The new mount also came with a bunch of repairs and maintenance. I mostly just had to clean the electronics and replace an antenna that had broken in a nasty crash. It has two antennas (simultaneously receiving), so it didn't cause any issues.

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.

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