Since my quad decided to run away from me I have to build a new one. I spent more than one year designing the previous one and it went through 4 major iterations which ended up with a light, strong and easy to build quad. But not a perfect one though.
There are 2 issues that bother me with my quad and all quads in general:
- They are hard to carry around. Their propellers are sticking out, their arms are large and they are sensitive.
- The propellers are exposed when flying which is dangerous for everyone (including for the the propellers themselves). I could build prop guards but they are heavy and make the quad even bigger
Lately I’ve been thinking how to improve on this for the new quad. The first issue can be improved by making a smaller or a foldable one quad. I tried in the past to make a foldable tricopter out of balsa (before I had a 3d printer) but it didn’t work out that well. It was either too flimsy or too heavy. A smaller quad is attractive but it reduces flight time significantly since not all components can be scaled linearly (such as the rpi).
While looking for alternative designs I came across this – a single ducted prop UAV. It’s an interesting idea to have all axes controlled with the servo elerons but I think it’s a bit too radical for me. I need something more practical – and I think I got it:
A ducted coaxial propeller UAV, with control surfaces for pitch/roll. Yaw is done using differential thrust. I’ll call it CUAV – Coaxial UAV
So, some specs:
- 2x 10×4.5 inches CF propeller
- 2x 2209 980kv motors. They max out at ~900 grams of thrust for a total of 1800.
- 2x 20 amp ESC. Not decided which one, not important for now
- 2x digital 9g servos with metal gears for the control surfaces
- 6mm 3k CF tubes for the structure with ABS 3D printed parts
- Raspberry PI 3 brain, wifi video link, RFM22B telemetry and control link
I made a schematic as well:
The motors are arranged with the props in the center, pointing towards one another to protect them. The ‘chassis’ is made from the motor mounts – 2 identical pieces (one for each motor) with 4 CF arms, connected on the outside.
The weight should be around 640 grams, with a diameter of ~30cm and a height of ~14cm.
The advantage of this design is that it’s pretty easy to carry – the sensitive parts are protected by the duct, and the props should not hit anything even in the case of a crash. Also I think it will be more efficient that the last quad due to having only 2 motors and way bigger props (10 vs 7 inches)
The challenges will be:
- To design the chassis to be stiff and light.
- To arrange the components – especially the RPI – so that it doesn’t block the airflow. I think the battery and GPS will be on top and the RPI + camera on one side of the duct.
- To write a new motor mixer that controls the servos depending on the desired torque. The trick here is that the torque depends on both the angle of the control surface but also the motor thrust (how much air flows on top of it)
Since I first have to make a new GS, this will have to wait a while. Hopefully by the time I start putting it together the RPI Foundation will have released a new camera module and a RPI 3 Model A…