3D Print the Drone Air Frame
The DS3 variant of our Magpie air frame is a simple 220 mm Quad. The prototype drone airframe was the Martian II as described in Part 1 of this series. The Martian II is constructed of carbon fibre, which is strong, less flexible but heavier than the PLA that we are using to 3D print our design. For example the arms, bottom deck and power distribution board mount weighs 81g in carbon fibre compared with the PLA equivalents which are 42g. Our components are a similar thickness to the Martian II bits except the flight deck, which is 1mm thicker.
Apart from the cost, availability and difficulty of 3D printing carbon fibre there is also a safety issue with this material. If a carbon fibre air frame crashes and breaks, you need a special clean up kit. Breathing in carbon fibre is not good for you. It is classified as an acute toxic substance and its effects can be similar to glass fibre or asbestos. It can also be very sharp and small particles will irritate the skin.
The design was put together in SketchUp, exported as an STL file and then turned into GCode instructions (which the 3D printer understands) using FlashPrint. FlashPrint actually converts the STL file to a GX file. A .gx file is a GCode file with an additional binary header that contains a thumbnail preview picture. FlashPrint is the slicing software used for the FlashForge Guider II 3D Printer. Whatever slicing software that you use should work with the STL files that you can download from Thingiverse.
The complete Bill of Materials (BOM) for the Magpie DS3 Drone is provided in Figure 2.
Start off by 3D printing the 4 arms (note that the arms come in left and right orientations and you need two of each) and the bottom deck. We have used black and white PLA filament in keeping with our Magpie theme. As shown in Figure 1, the arms attach to the deck using eight M3 nuts and bolts.
Attaching the Quad ESC
Once you have the arms attached to the bottom frame, you can add the quad 20A Electronic Speed Controller (ESC). The RacerStar ESC that I bought has mounting holes which line up nicely with the Nano PDB (not a fluke). The quad ESC sits on four M3 x 6.3mm nylon tapped hex spacers.
Another set of four M3 x 6.3mm nylon tapped hex spacers go on top of the ESC (Figure 5). The Power Distribution Board sits on top of these.
Mounting the Motors
We need to mount the four brushless motors to work out the correct wire length to the ESC. My RacerStar motors came with two sets of M3 screws, 8mm and 6mm. The thickness of the frame arms is 4mm, so use the 6mm screws to ensure they don’t short out the motor coils. For now you can just do them up finger tight but eventually you will want to apply loctite to ensure that the motors don’t fall off mid flight (bad).
The RacerStar ESC can be programmed using the BLHeliSuite software on PC. With that we can reverse the motor direction but you need compatible Flight Controller software. For our purposes, it is probably simpler to just swap two wires if the motor is going in the wrong direction.
Our motors indicate their default rotational direction (clockwise or anticlockwise) by arrows on the top of the motor and different coloured propeller nuts. The black nuts rotate clockwise and the red nuts anticlockwise. To minimise the amount of work, you should mount the motors in the configuration shown in Figure 4.
You will need to thread the motors through the prop guards (Figure 5) before you can screw them onto the mounting arms. The prop guards sit between the motors and the arms and are held in place by the motor screws.
Once you have mounted the motors you can then wire them to the Quad ESC (Figure 5). While you are doing this, also solder the ESC battery connection to the appropriate pads (+ and -) on the Power Distribution Board and a connector to allow you to plug power into the flight controller. Make sure that you put the heat-shrink on before you solder the wires to the ESC and leave enough slack in the wires so that you can remove the board if required. If you want to be able to disconnect the battery power from the ESC then use some sort of connector (e.g. bullet plugs) instead of soldering. This is what we did for our prototype (Figure 6).
If you have a servo or ESC tester then now is a good time to test that everything is working and the motors are spinning in the desired direction. Next up we will look at mounting and testing the flight controller.
Mounting the Power Distribution Board (PDB) & Flight Controller
Before placing the PDB on the four M3 x 30–35mm bolts, connect the ESC power and control cables. This is easier to do before placing it on the stack.
The PDB should have M3 anti-vibration mounts in the mounting holes. This will reduce the amount of motor vibration picked up by the IMU (Figure 7). Secure the flight stack with 4 x M3 nylon lock nuts.
Four M3 x 35mm spacers are used to connect the top and bottom decks of the drone. To minimise the transfer of vibration, the top deck is not connected to the flight stack (Figure 8). The SBUS connection cable to the X8R receiver goes through the hole below the Nano 33 BLE. The X8R is mounted on the top deck with the battery.
Mounting the X8R Receiver and Battery
In the Magpie DS2 version we didn’t have prop guards and the X8R could be mounted on the bottom deck. In a subsequent version (DS4?) we will put it back on the bottom deck but this will require a redesign of the rear prop guards. For now, we will mount the X8R on the top deck.
3D Print the X8R mount and covers (Figure 9). This is then secured with cable ties or double sided tape. Similarly, the LiPo is cable tied to the top deck. Make sure that the propellers wont hit the cable ties.
The completed drone is shown in Figure 10. In the next article we will load the flight controller software into the Arduino Nano 33, calibrate the Inertial Measurement Unit (IMU) and test the motors.