4" Long range sub 250g quadcopter with 1404 3800 KV motors.
This is version 2.0 of my first build.

My goal with the the frame was to make it as light as possible but also to improve the resonance issues I had with my V1 frame by adding some triangulation on the rear arms with the hope of making them stiffer. I also made some larger cutouts under the AOI to try and keep the weight down. Did it work? No. LOL. This frame still has some resonance peaks at 70 Hz on roll and 50 Hz on pitch, which made it really difficult to tune.
The bottom plate is 2.5mm thick and top plate is 1.5mm thick. With 20mm standoffs and screws it weighs in at 19.6g.

After experimenting with 2S and 3S on my V1 design, I realized that 3S is the way to go for longrange. So I designed for 3S 18650 Li-Ion battery specifically.
I also wanted to try INAV instead of Betaflight this time. I wanted to try the INAV waypoints mission feature so that I could program the quad to fly consistent laps around a park for battery testing. Unfortunately, this didn't go too well, because the frame osciallations were throwing the quad off course when navigating turns. But, I absolutely love INAV's true Return to Home function and Position Hold feature. The only catch is, you need to make sure you use a compass and you select a FC that is supported by INAV. Of course you also need the usual components for long distance missions, so it has a lost model buzzer/light, GPS & barometer.
To save additional weight I switched from the IRC Ghost Atto Duo RX to a regular Atto RX with reduced the number of antenna's.
I also switched from the IRC Tramp Nano VTX to a TBS Unify 5G8 V3 VTX which up'd my power output from 500mW to 800mW. TBH, that Tramp Nano is a really good VTX and it easily got me out to 11.1 km with my previous build.

I also used titanium screws throughout the frame for weight savings.

I made my own 3S pack with a small spot welder and nickel strips.
Credit goes to Roteron on the IntoFPV forum for the awesome idea to eliminate the balance lead to save weight and instead use a magnetically attached balance lead during charging.

I designed the frame using Fusion 360 and had it cut by CNC Madness.
Full build log here on intoFPV forum: https://intofpv.com/t-mayfly-2-0-sub-250g-4-3s-rebuild-13-km-goal

Here's a video from my longest ever flight (maybe a sub250g quadcopter record??). 15.1 km out (30+ km total distance) and 34 min flight time.