AC4 – a bit different quad


I have been watching a new DJI Mini 2 quad. Especially its motors and props. Thinking that DJI can optimise motor and prop combination for efficiency. Mini 2 specs say 30min flight time and people got 25min. Still good result for 250g drone with 2S battery. I got curious if I could build a lighter drone with the same motors, could I get an even longer flight time?
I thought, that my AC3 would be the last project, but couldn’t resist sketching several versions and ended up with this design.

This quad is intended to be sub 250g, with 4k HD on board and with long range capability.

See more info on the build and part list on

Update 7.3.2021: Arms cause motor vibration around their twist axis. So they need to be redesigned. See video below. Developing continues.

Part 1 video of the developing this quad

If you are interested in this frame, don’t hesitate to contact me. I’ll consider to put it on sale at shop.


Top plate

  • 2mm
  • Two 20×20 slots in the front and the rear
  • Twist-lock v2 hole for HD camera mounts
  • Battery strap slots for 15mm wide strap
  • Big hole at the front part for the power lead

Bottom plate

  • 2mm
  • 25.5×25.5 holes for AIO Toothpick FC
  • 20×20 slot ends for centre FC+ESC stack
  • One 20×20 slot in the rear
  • Twist-lock v2 hole for something


  • 3mm
  • 2mm at the tips for motors
  • Front arm is straght
  • Rear arm is 90° boomerang
  • 3mm holes for mounting to the bottom plate
    Attention! Arms are under redesigning. So the new versions are coming.


  • 4x 4x20mm standoffs
  • 8x M2 6mm hex screws
  • 4x M3 10mm hex screws
  • 4x M3 lock nuts

3D printed parts

  • Nano camera mount
  • TBS Immortal or FrSky R9 T-antenna front mount
  • Combined mount for TBS Unify pro32 nano, antenna and Beitian BN-220 gps module

3 thoughts on “AC4 – a bit different quad

  1. So how does it fly? Any dvr or at least flight times? Seems like a good use case for inav or ardupilot. Great work.

  2. Thanks, JuHu!
    I did last Sunday a time testing flight. The quad was shaking during all flight. I still managed to get 22min of the flight time. I wasn’t satisfied with the result. After that flight, I tested quad’s motors on a bench and found out that they were shaking at some frequencies. Arms are stiff against the bending, but not twisting and this allowed motors to vibrate. I’ve attached some spars between the side arms with electric tape and this improved a lot! The spars added some weight but I’m waiting for my next spare time to make a new flight time test. It should be longer as no energy is wasted in a fight against the vibration.
    I also drew a new arm design with integrated spar to order new cut and wait a few weeks for them to arrive from China to Finland. Frustratingly slow progress 🙁

  3. Anything ever arrive in Finland? Or was the frame so good the People’s Party confiscated it and kept it for themselves? 🙂

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