FPV Aircraft

Third fresstyle Build

The second freestyle drone was a monumental improvement from the first, but I wanted to improve its performance. A key factor to performance is trust to weight ratio, and so I wanted to build a lighter, more powerful drone.

I upgraded the motors to ones with a higher Kv (2700Kv), but if the voltage is constant and Kv increased, the amp draw will be increased, and so I upgraded to a 60amp ESC. I chose a 4in1 ESC for multiple reasons, firstly weight saving, secondly the arms on the frame were really thin, and thirdly I had used one on the race drone and was impressed by this form factor. The frame was lighter, utilising thinner bottom and top plates, and much thinner width arms (but thicker depth), and generally being a more compact design. The 3D printed antenna mount was also replaced with a lighter moulded one. A lighter micro eagle runcam analogue camera was chosen, and I was very impressed with this on the racer drone. A new FlightOne RevoltOSD flight controller was chosen. This frame did come with a moulded mount for a GoPro, but it was bulky, at the wrong angle, and I wanted to start using an ND filter - So this was the start of 3D printing GoPro mounts.

3D Printed GoPro mounts

Why did I need one? - Previous frames had come with mounts, but this one didn't (or it wasn't the correct angle or allow for ND filters). And so needed a mount that could be at the same angle that I liked to run my analogue camera at, and also have space to fit both the GoPro, and a glass ND filter in front of it. We were flying a GoPro session, and unlike the larger GoPros, the ND filters didn't click onto the front, and so the only option were ones that stuck, but then you could only ever use 1 strength of ND filter, or just glass ones, but they needed some way to be attached. So the objective was GoPro mount at a specific angle, designed to hold both the camera and ND filter securely.

Testing 6s batteries

At this time there was a general trend from using 4s to 6s batteries in the industry. For the main reason of battery sag. As power = Current x Voltage. Using a higher voltage battery will provide the same power at a lower current. This means the rig runs cooler and suffers less battery sag at high throttle. Battery sag is when at high current draw the internal resistance causes voltage to drop, causing a temporary loss of performance that recovers when throttle is reduced. As 6s draws less amps, less energy is wasted as heat, and as the starting voltage is higher, a temporary drop in voltage will feel less noticeable on performance. Both these mean 6s suffers less sag, and provides similar performance throughout the entire flight.

Because 6s is a higher voltage, you need to have lower Kv motors to achieve the same RPM. But to test out using 6s batteries before buying new motors, I used a setting on the flight controller that could effectively limit the Kv of the current motors. However, I hadn't accounted for the increased energy spikes, and so at low throttle the drone flew fine, but at higher throttle the voltage spikes exceeded what the capacitor was capable of absorbing, which fried the MOSFETs on the ESC. Due to the higher voltage of 6s, and the larger energy spikes and harsher electrical noise, both the voltage rating and capacitance of the capacitor needs to be increased.

From theoretical advantages, online reviews and videos, and the testing I had managed to do, I was impressed by 6s batteries. So I bought a new ESC, a higher voltage and capacitance capacitor, and a few 6s batteries. The flight controller setting to limit the motor KV was working smoothly, so I didn't need to purchase new motors.

Fourth freestyle Build (new frame)

The third freestyle build was again a large improvement in performance from the second one, with weight saving, increased power, and smoother flight. Then the upgrade to using 6s was another leap in performance, and using the TBS crossfire radio system improved range.

But although performance was improved, the build wasn't fundamentally designed around using these new components. The only components still being used from the initial design were the FPV camera and flight controller. The 6s batteries were larger and heavier than the 4s ones, the new capacitors were larger, and the new VTX was larger. This meant that the layout of the components in the rear was messy, and there were concerns about the strength of the top plate.

This highlighted a design choice that needs to be made. Do you try to optimise purely around weight saving, or also factor in things such as ease of use and durability? For example the previous frame's rear section tapered in, which saved weight, but made it really difficult to secure the VTX, RX and capacitor.

So I decided to put all of the components on a new frame, which was slightly larger. Two main parts of this were the increased space in the rear section, to more securely mount the VTX, RX and capacitor, and a larger top plate, for more secure mounting of the battery. The frame also has specific slots to mount capacitors, more durable front standoffs, a raised plate to reduce oscillations to the GoPro, and a split bottom plate which allows for easier replacement of arms.

So the design choice I made was to not purely optimise for weight, in order to have a securely mounted, durable setup, for smooth flying and video footage. There is also a case that depending on the flying style you want to have, a heavier drone will carry more momentum when off power, and so if this is advantageous there is no trade off to a heavier build. But if I also wanted a really lightweight build, the vision was to put a new set of really smooth motors on this heavier rig, and then use the previous frame and current motors to build a lightweight 6s drone.

Lessons from extensive flight testing