I’m hoping to dive into my first self-build freestyle quad soon (not before some essential soldering practice!).
I’ve been looking at doing a 3.5” build with the aim of keeping it under 250g, if possible, with a 4S 850mAh battery (as I have a few and want to avoid buying more).
After some research and compiling the shopping list below, it seems this might not be entirely possible, but I’d love to hear any thoughts, advice, or tips:
Frame: AOS 3.5 V5 (68g - not sure if this includes the TPU parts) Flight Controller+ESC: Either iFlight Beast F7 V2 55A AIO (9.2g) or GEPRC F722 AIO V2 (8.8g) Receiver: Radiomaster RP1 ELRS Rx (2.2g including antenna) Motors: iFlight Xing2 Unibell 1404 either 3000kV or 3800kV (total 36.4g including wire) Props: EMAX AVAN Scimitar 3.5x2.8x3 (total 7.2g) FPV System: DJI O3 Air Unit (40g including air unit, camera module, antenna, and coax cable) Buzzer: ViFly Finder Mini Buzzer (2.7g)
The dry weight sums up to around 166g. My existing Tattu R-line 4S 850mAh batteries weigh 97g (officially 105g), bringing the theoretical take-off weight to around 263g. I need to shave off 14g!
I have so many questions but I guess my main ones are:
Has anyone managed a sub-250g 3.5” FPV quad with an O3 AU system and 4S 850mAh batteries, or is it wishful thinking without compromising battery capacity?
Any thoughts on my choice of parts?
Choosing between 3800kV and 3000kV motors - any advice?
Capacitor: Stick with the one packaged with the AIO FC+ESC, or upgrade?
Any imparting wisdom would be greatly appreciated!
Sadly I think that the only way is to make a compromise somewhere. I just weighed my 3.5Inch at about 155g without battery. Mine uses a Caddx Vista (30g) as VTX and I use 720mah batteries that are 72g so 227g all in.
Adding a O3 at 40g and your 850mah batteries to mine for example = 262g so over again.
Sticking to O3 again plus the 720mah = 237g
Since the battery takes up the most proportion of the weight if you can consider going to 750mah maybe you might get it under. Alternately, strip the O3 casing to save 17g, but you’d reduce durability a fair bit.
Parts look ok, but maybe the ESC looks overpowered for 1404 motors. Mine is only 20A (see below) and capacitor if you go with that FC would be overpowered too so fine
Motors on mine are 3800KV and is BetaFPV X-Knight 35 (I installed a different FC, ELRS reciever and ViFly Buzzer but otherwise stock)
I don’t know DJI, but why not a Vista? It must be lighter… I use HDZero… so super-light Race and Whoop VTXes.
I doubt you’ll need as high an ESC rating as 55A with a 3.5" you may be able to shed some weight there.
Personally my 3.5" frame is a grinderino. At 65g it is just a little bit less that the AOS 3.5, maybe not enough. But my 3.5" is not below 250g. I did manage sub 250 with an Armattan Tadpole HD 3". that flew nice, before I bashed it to pieces…Oh and that ran on 3S.
Only way I converted my 3.5 to digital was hdzero to keep under 250g with an 800 or 850mah battery. The vista was too much. Tbh not sure which is heavier but I’d assume the O3 weights more than a vista, though you won’t get the onboard recording with a vista.i wouldn’t worry too much about slightly over 250g the rules going to change so that anything with a camera be classed the same (even a 30g AUW tiny whoop). Get A16 and you’ll be fine.
The DJI O3 Air Unit is more compact and lighter than the original DJI FPV Air Unit (39.5g vs 53.4g). However, it’s larger and heavier than the the Runcam Link / Caddx Vista (33g) and the Walksnail Avatar VTX/Cam combo (28g).
You make a fair point. I do have a GEPRC Mark 5 already. Despite the luxury of flying under A16, I still struggle to find enough open space to fly it comfortably. I figure a sub-250 could help increase my flying opportunities, and it might be a fun alternative to the big, noisy 5"!
I’ve got 2 toothpicks and a whoop. Although I don’t fly them much any more because they are analogue.
I think a 3.5" is gunna be a struggle to keep below 250g not impossible but like stated above you’ll sacrifice durability and flight time to achieve it.
Thanks for the useful tips. You’re certainly right about the compromise somewhere and probably the battery mAh is the safest and most meaningful way to do this.
I’ll check out some lower rated ESCs as there does appear to be around a 2g weight saving here. If I can find lighter options for other parts as well, even a gram each then I might just be able to chip away at it and hit the <250g target.
For what it’s worth, I also have a 5" and can fly under A16 - but as a suburbia-dweller, sub-250g is basically what makes it practical for me to stay in the hobby. If I’d relied on flying full-fat 5" to learn to fly I’d still be on the ground!
So if you feel like being tied to 5" means you’re struggling to fly as often as you want, I’d say sub-250 is absolutely worth a go.
At the moment I turn up to my local park with a sub-250 5", a “traditional” 3", a 3S toothpick, and an Odonata. And I fly whatever seems considerate to the other park users on that particular day!
Am I correct in saying with either of the following AIO FCs I would need to solder the input of the DJI O3 Air Unit directly to a Vbat pad or (if there isn’t one) to the (+) XT30/XT60 pad on the board to power it?
Flywoo GOKU GN 405S 40A AIO (MPU6000)
Flywoo GOKU GN 745 40A AIO BL_32 V1.2 (MPU6000)
Whereas on the following boards, the 6 pin connector for plugging in the Air Unit would pass through Vbat and should provide enough power without soldering a wire elsewhere?
Thanks for the reminder!
