I’ve been fascinated by coaxial drones for some time now so I decided to design and build the simplest example of one I could. From the outset I’ve known this isn’t going to be an outstanding performer (I’m aware of some limitations and drawbacks of such a design) but decided to go ahead with it because, hey, there’s not too much else going on at the moment. I’ve also decided to make the whole project available for everyone to download, improve upon, tune better and basically just have some fun with. In the bottom of this post I’ve added a link which should give you everything you need to get to the same point I’m at now- CAD files, STL files, item lists, an assembly drawing, firmware and my initial (poor) parameter list.
Arducopter supports coaxial drones where the yaw control is provided by differential motor/propellor torque and pitch and roll is controlled by passing the downwash over control surfaces. However, I decided to go a different route and achieve pitch and roll control by independent thrust vectoring of the props. To pitch forward, the top prop is rotated with a servo about an axis above the COG of the drone, causing a forward pitching torque. Likewise, with roll, the bottom prop is rotated about an axis below the COG of the drone causing a rolling torque. All up there are just the four actuators- two motors and two servos.
The drone was designed in Solidworks and made use of some components I already had laying around. The parts were 3D printed at Shapeways out of Nylon 12 material (“Versatile Plastic”). I’m quite proud of how neatly it turned out; almost all of the wiring and components are internal, including the battery.
I decided to use the Matek F405-Wing flight controller because it’s almost perfect for this application. It has an inbuilt 5V power module for the flight controller, power and signal distribution for two motors and an in-built BEC for the servos.
Some questions that you may be wondering:
- Does it fly? Well, yes, but not that great just yet. I’m still in the tuning phase. I have one small snippet of it flying though (apologies for the atrocious audio and video): https://youtu.be/LSyZhLYHXaQ
- Will it do a flip? Probably, in fact I’ve almost flipped it accidentally.
- Is it useful? Not really. But it is fun and a good learning experience.
- Do gyroscopic forces affect the control system? Why yes they do, quite a bit. For example, when pitching the top propeller forward, gyroscopic precession imparts a rolling torque onto the airframe. To counter this, I’ve added a feed-forward term to the Coax Copter code which adds a portion of the pitch PID output to the roll output and vice versa. So when pitching the drone forward, the top prop pitches forward and the bottom prop pitches sideways to counter the torque imparted by the top prop pitching forward. This parameter is called MOT_RP_FF.
- What does it weigh? Roughly 500g with battery according to the CAD model.
- How much power does it need in hover? About 100W. So efficiency is about 5g/W.
- Can I 3D print it at home? I’m not sure- the SLS printing at Shapeways allows me to print some geometry that isn’t well suited to FDM printing. It would likely need a slight redesign for printing with FDM.
You can find all the stuff you need to build your own here: http://s000.tinyupload.com/?file_id=41679256676614028922