Today, I am building a 3.5-inch FPV drone with DJI O3 Air Unit. While 5-inch drones offer power and excitement, they can be quite heavy and a bit intimidating to fly in smaller parks. That’s why a 3.5inch quad has the advantage over 5-inch in these situations. Our goal is to build a lightweight freestyle micro quad that can comfortably carry the DJI O3, yet without compromising on performance. Let’s get started.
Frame: FlyfishRC Volador VX3.5
- AliExpress: https://s.click.aliexpress.com/e/_Dkub2KZ
- Amazon: https://amzn.to/3D1zA9V
- RDQ: https://oscarliang.com/product-p1nh
FC/ESC Stack: HGLRC Zeus F7 Mini + 45A 4in1 ESC
- AliExpress: https://s.click.aliexpress.com/e/_DF0ePt7
- Amazon: https://amzn.to/3JMcXKu
- GetFPV: https://oscarliang.com/product-bbjt
Motors: Flash 1804 2450KV
With 3.5inch propellers, these can be used with 6S, for 4S setups, consider the 3500KV version.
FPV Setup: DJI O3 Air Unit
See my review of the O3 here: https://oscarliang.com/dji-o3-air-unit-fpv-goggles-2/
- RDQ: https://oscarliang.com/product-sadl
- GetFPV: https://oscarliang.com/product-4blv
- AliExpress: https://s.click.aliexpress.com/e/_DdQYArv
- Amazon: https://amzn.to/3RtWfm1
VTX Antenna: FlyfishRC Dual-Band antenna
These aftermarket antennas are slightly lighter and smaller than the original O3 antenna, but the weight saving is marginal (less than 1 gram). Plus, 3.5″ quads aren’t designed for long-range flying, so the original antenna is perfectly adequate to be honest. Anyway if you value 1-gram weight saving, get them here:
Radio Receiver: Ghost Atto
Flashed with ExpressLRS. I’m using this RX because I happen to have spare, you can use any ExpressLRS receiver, see my recommendations here: https://oscarliang.com/setup-expresslrs-2-4ghz/#Receivers
For around $30, the FlyfishRC VX3.5 frame is an excellent value for money. With its top-notch design and build quality, it comes with all the necessary accessories and TPU 3D printed parts.
The screws and accessories are neatly sorted in labeled bags. With the help of the assembly guide, available on Flyfish’s website, you can see where each piece fits into the puzzle, simplifying assembly.
The design follows the “split deck” bottom plates concept, just like the Source One V5.
It’s a spacious frame, compatible with both DJI O3 and Caddx Vista Air Units. The camera holder accommodates both 20x20mm and 19x19mm cameras, with different spacers provided. The camera is soft-mounted to minimize jello, especially needed for the O3 camera. Handy TPU parts are available for antenna housing, accommodating either the Vista or O3.
The arms are constructed from 3.5mm thick, 9.5mm wide carbon fiber, enhancing durability. The top and bottom plates measure 2mm in thickness. The camera mount is aluminum.
One feature I appreciate is the TPU holder for the radio receiver located at the bottom. Pressed nuts are provided for mounting, creating a clean and effective solution.
However, it shares issues I identified in the larger 5-inch Volador frames I recently reviewed. First of all, they are using a bit too many screws, and there are so many different sizes, which complicate both initial assembly and subsequent repairs.
Its generous space for components and robust arms contribute to its weight, making it heavier than I’d prefer. When building this quad, I tried leaving out as many parts as possible to reduce weight. I even removed the two standoffs behind the camera, as I believe the frame is sturdy enough without them, and replaced the original battery pad with lighter, stickier Ummagrip pads. Still, the final weight of the frame came to 63g, which is lighter than the original, but still on the heavy side for a 3.5inch frame.
The holes for the hex standoffs were slightly too small, requiring me to enlarge them with a small file, which took around 15 minutes.
Below is the assembled frame.
We’ll begin by installing all the motors and the DJI O3 unit.
FlyfishRC thoughtfully includes spacers and M1.6 screws for mounting the O3.
Next, solder the XT30 pigtail to the 4in1 ESC. I used the HGLRC RC 2 soldering iron for the job, and it worked beautifully.
Given the space constraints within the frame, I soldered the low ESR capacitor to the XT30 connector. Although not as effective as soldering it directly to the ESC power pads, it’s better than nothing. This is a typical workaround in micro quads with limited space. The capacitor I used is a 35V 470uF, compatible with 6S LiPo.
Proceed by soldering the motors to the ESC.
Here’s a top view of the ESC and the soldering work.
Next, solder the radio receiver to the FC.
Afterward, connect the ESC and DJI O3 AU to the FC and secure them with lock nuts.
The RX can then be mounted inside the holder at the bottom.
And with that, the assembly is complete.
With a 6S 550mah LiPo included, the weight rounds up to about 300g. While it’s not sub-250, we’ve used bigger and more powerful 1804 motors instead of the typical small 1404 or 1604 ones, coupled with the tanky frame, the heavy weight was to be expected. However, the powerful motors will balance that out.
How to Setup
Update DJI O3 AU firmware as explained here.
Update BLHeli_32 to latest version. Apply the recommended settings as explained here. I use 48KHz fixed PWM frequency for smooth motors.
Update Betaflight to latest version (4.4.2 at the time of writing) as explained here.
Update ExpressLRS firmware on your receiver with your binding phrase as explained here.
To Configure Betaflight, you can follow my beginner’s guide here which also explains the reasons for the settings. In the following I will summarize the changes I made to this specific quad:
- Configuration Tab:
- Board and sensor alighnment, First Gyro, set it to default
- Set PID to 8K,
- set Arming angle to 180
- Under Features, enable OSD and Air Mode
- Enable DShot Beacon, RX_Set
- In Setup tab, make sure the 3D model is responding to the quad’s movement.
- In Preset tab, load “ExpressLRS 500Hz” and “OSD for FPV.WTF, DJI O3”
- In OSD tab, Video Format, select HD, and enable your preferred elements
- Ports tab:
- For UART1, enabled Serial RX for the receiver to work
- For UART 4, select VTX (MSP + Displayport) for DJI O3 OSD to work
- Now go to Receiver tab to make sure receiver is working (enabled Telemetry), power on the quad and goggles and make sure OSD is showing
- Motor Tab: Select DShot600, check motor direction and order, enable bi-directional dshot
- Setup Modes tab, assign switches for Arming, Angle mode, Turtle mode etc.
Finally, give your setup a test flight.
While this 3.5″ build is on the heavier side, it packs a punch, offering closer performance to a 5″ model. This is mainly due to the powerful motors and a sturdy and rigid frame. The added weight actually provides more ‘fling-ability’ and makes the drone behave more like a 5″ model. If you’re into sbang-style flying, this build could be just the ticket.
However, if your aim is to build a lighter 3.5″ model under 250g, here are some changes I would suggest:
- Choose a lighter frame, ideally around 45g. Just bear in mind that this might compromise on durability and rigidity.
- Opt for smaller motors, 1606 motors would be a good choice.
- Use a lighter ESC. The one used in this build is rated for 45A, which is more than adequate for even 5″ quads. For a lighter 3.5″ quad, a 30A or even 25A would suffice.
- A previous 3.5″ build I assembled: https://oscarliang.com/roninuav-sohei-mk2/
- Other 3.5″ BNF models I’ve reviewed:
Regardless of your chosen path, the joy of building and flying your own FPV drone is the journey itself. Enjoy the build process and happy flying!