Part 3 -- Singlecopter

 

Section 1: The Concept

Introduction

While a quadcopter is both statically and dynamically unstable, a more traditional approach similar to a helicopter is still statically stable. Helicopters are, however, mechanically complex

– they require swashplates. An alternative to helicopters is to revert back to a model similar to the initial design, a bicopter, but this design also requires two motors and two servos, which is considered to be expensive, and requires mechanical complexity to some degree. However, the Bicopter’s control scheme is that, to pitch, it shifts its CG a bit to tilt itself to the direction it wants to go. This specific technique can be utilised in roll directions too, and so the CG-Shift Controlled Singlecopter is Developed.

Early Design & Development

Developed in parallel with the quadcopter right after the first crash. he initial design of this UAV is solely based on simplicity. It requires minimum manufacturing of specific components in which it is made out of purely commercial-off-the-shelf components. The initial design consists of a simple balsa cube, with all the components attached to the surface. The battery was initially designed to be suspended under the cube to provide a “pendulum effect” in which under disturbance, it will reorient the UAV without needing any electronics. On the sides, there are four limbs on each side, in which, each of them is attached to a single servo. This servo limbs will act as the ones that shifts the CG away from the thrust vector in order to create pitching or rolling moments.

(Conceptual Design)

Components & Systems Concept

Powered by a single 3S 1500mAh battery, it is projected that the UAV should be able to fly up to 90 minutes under no payload, and up to 50 minutes with payloads. Equipped with a single 2.4GHz ELRS receiver, the projected range of this UAV was up to 2km. The telemetry system is a little bit of a bottleneck however, where since it uses ESP32 hardware under WiFi link for auxiliary functions like payload control and camera stream, the range is limited to only up to 500m.

The UAV uses MG90S servos, which shares a similar specification to its plastic counterpart but being more reliable due to its metallic gears. Each of these servos would be attached to a limb which has a weight on the lower tip. When these limbs are rotated, the CG of the UAV will shift, causing a theoretically significant change in the moment vector.

(connections diagram)

(logic diagram)

(wiring diagram)

Section 2: Prototyping

Attempting to prototype the concept turns out to give a much safer and simpler design alternative which doesn’t externalise the components. Using a small water bottle (300ml), the first prototype of this UAV uses an avionics bay for ease of the avionics & control system’s iterative design. The prototype also utilised only three limbs rather than the initial design’s four limbs, this not only greatly reduced the projected weight by up to 20g, but also eliminated the need for a BEC in the UAV.

The battery, which was supposed to be suspended, now has a small housing enough to keep it in position, this makes it much safer compared the the original concept.

The prototype features an “Avionics bay”, which is just a simple plywood plate where all the components are attached to. It consists of the main flight controller using STM32F103C8T6, a single MPU-6050 IMU packed as the GY-521 module, and the ELRS Receiver. 

Around 7 Flight Tests were recorded, which only were recorded.

On the first flight test, on the initial lift off, the UAV seem to be initially stable on the pitch and roll axis, which this might be due to the UAV spinning uncontrollably in the yaw axis due to the lack of proper yaw control. However, after gaining some altitude the UAV immediately "tumbled" and had a headfirst dive into a mattress The conclusion was to add small vanes to aerodynamically counter the torque created by the main propeller.

The second test flight was not recorded, however it was on this test flight that the UAV managed to fly slightly higher before getting another head-first dive and breaking the propeller.

The third to fifth test flight bears similar results, however on the fifth test flight, I decided to recheck whether the feedback works as intended or not, and whether it has enough latency to actually respond to changed in attitude.

The sixth test flight ended up in a new yet different disaster, where the UAV seem to be experiencing an error similar to a voltage sag, where the throttle got stuck in 100% whereas the rest of the electronics seem to experience a drop in power. To counter this, I verified the arm/disarm switch and directly went off to the last test flight which again, ends in a head-first dive.

Section 5: Downfall & Closure

Test flights results show that using CG-shifting mechanisms to control the pitch and roll of this simple aircraft is not possible. The limbs are simply not fast enough, and even if they are fast enough, the CG still doesn’t shift too much to actually create a pitching or rolling moment, not to mention, the yaw control which were supposed to be controlled by small vanes in the limbs, had proven to be useless. While this project should have work in theory, manufacturing is limited by the available hardware, which there happens to be no 3D Printers or Laser-cutters around. Not to mention the computer used to design the system was not adequate to run proper 3D modelling software as it even lags to design the early design of this aircraft. Simulation is another case, where due to price constraints not all hardware can be precisely modelled in Simulink/MATLAB. Another vital constraint is that the microcontroller unit seems to be slightly under the actual requirements for such a complex system, where its 20kB RAM might be overloaded in flight.

On the bright side, the control logic was validated despite wasn't properly tuned. The 1 Prime Mover + 3 Servo is the baseline of fixed-wing UAV configuration, meaning that the software, while still buggy, will most likely be able to work on other fixed-wing platforms.

Last Updated 2025.08.30 21:03 (final)

Main Documentation:

https://docs.google.com/document/d/1xhnqCpLLievuv9DtGMCRpozWEj4NXXEhfYtOLel3koM/edit?tab=t.0

GitHub (Archived):

https://github.com/AltmzTrn/IrengUAV/tree/main/Archive/2025/09.09