Build: roninUAV Sohei MK2 Frame (3-inch and 3.5-inch)

by Oscar
Roninuav Sohei Mk2 Frame

I broke an arm on my GEPRC Smart 3.5″ drone when testing the GoPro Hero10 Bone. It was impossible to get a replacement bottom plate in Europe (it’s a unibody design), the only option was to order it directly from China which costs over $25 plus one to two months of waiting. So I decided to just rebuild it with another 3.5″ frame, the Sohei Mk2 from roninUAV.

Massive thank you to Rob from roninUAV for sending me this frame to play with! You can find him on IntoFPV under the username “the.ronin”.

The GEPRC Smart 3.5″ FPV drone is a great BNF model which I recommend, see my review for more. Only complain I have is how hard it is to get replacement parts locally. That’s why it’s so appealing to get your frame from a reliable vendor that you know there will be replacement parts available when you need it.

Naked Gopro 10 Hero10 Black Bones Fpv Camera Crash Test

In feudal Japan, the sohei were Buddhist warrior monks known to reside among the mountain shrines.

Where to Buy?

Get the Sohei Mk2 frame from: https://roninuav.com/shop/sohei-mk2/

Roninuav Sohei Mk2 Frame Unbox Accessories Parts

Specifications

Roninuav Sohei Mk2 Frame 3.5inch Arms Dimensions

FC / 4in1 ESC mount support:

  • 16x16mm
  • 20x20mm
  • 25.5×25.5mm (whoop board)

FC stack clearance (stack height limit): 20mm

VTX mount support:

  • 16x16mm
  • 20x20mm

FPV Camera mount: 19mm (Micro Camera)

Motor mount support: 9x9mm to 12x12mm

Frame Dimensions:

  • Wheelbase: 164mm
  • Width: 134mm
  • Length: 96mm

Carbon fibre sheet:

  • Arm thickness: 3.5mm
  • Bottom plates thickness: 1.5mm and 3mm
  • Top plate thickness: 1.5mm
  • Camera mount plates thickness: 2mm

Frame weight: 48g

The manual that comes with the frame:

Roninuav Sohei Mk2 Frame Manual Assembly Instructions

Closer Look at the Sohei Mk2 Frame

This is not exactly a usual review, just me sharing my experience with this frame after building it.

It was originally designed for 3-inch propellers, but the frame designer Rob recently added option for 3.5″ propellers. It’s basically the exact same frame, just longer arms that support the larger propellers. This allows me to simply migrate all the electronics from my GEPRC Smart to this frame without much of a hassle.

The first thing I noticed was that the carbon fibre are not chamfered. You can file and round the edges yourself.

Roninuav Sohei Mk2 Frame Arms Carbon FibreThe bottom plates use seated clinch nuts to keep assembly super easy, and replacing the arms convenient.

Roninuav Sohei Mk2 Frame Bottom Plates Carbon FibreIt also uses countersunk bolts to keep the bottom of the frame flat and smooth.

Roninuav Sohei Mk2 Frame Bottom

The roninUAV Mk2 frame supports all the common FC / 4in1 ESC mount sizes for a 3-inch FPV drone: 16×16, 20×20 and whoop boards. In the rear of the frame there are mounting holes for your VTX too with 16×16 or 20×20 holes as well. It allows 20mm stack height, which is plenty.

Roninuav Sohei Mk2 Frame Side

The camera mount supports FPV cameras with 19mm width.

Roninuav Sohei Mk2 Frame Front

The MK2 frame with the longer 3.5″ arms weighs about 48g, which is only 2 grams heavier than the GEPRC Smart frame’s 46g (with arm braces).

Roninuav Sohei Mk2 Frame RearIt was relatively easy to move all the components over to the new frame, I only had to make the wires between the FC and Caddx Vista a bit longer so I could mount the VTX at the back (as it was originally mounted on top of the FC).

The other change was the FC, I had to rotate it 45 degrees in the Mk2. That was an easy change in Betaflight, just needed to set Board Alignment, Yaw to 45.

Roninuav Sohei Mk2 Frame Build Geprc Smart 3.5inch Fpv DroneThere’s no TPU mount included for the RX and VTX antennas, so you need to get creative with mounting them. I simply use the same 3D printed mounts from the GEPRC Smart. Obviously the standoffs have different width, I pushed the mounts into one standoff and tied the other hole to the other standoff. That seems to work fine and saves me a lot of trouble designing a whole new mount for it.

Roninuav Sohei Mk2 Frame 3d Print Antenna HolderAnd here we have it! The Smart 3.5″ brought back to live in the roninUAV Mk2 ! (don’t mind the missing props, still on the slow boat!)

All up weight is about 156g, which is slightly heavier than the Smart’s 150g.

Roninuav Sohei Mk2 Frame Build Finish

Notes

  • I love it comes with spare screws and standoffs
  • Carbon fibre not chamfered
  • There are only two front standoffs, most HD cam mounts require 4 screws, so it gets tricky how to mount your HD cam on this frame. Maybe you can get away with just two screws but might not be very secure/stable
  • On the bottom, the arms are held in place using countersunk bolts, however, the front two standoff screws are button head which kind of defeats the purpose of having a totally flat surface on the bottom
  • No 3D printed VTX / RX antennas mount included, so you need to get creative on how to mount them
  • Rear standoffs blocks DJI Vista USB port (doesn’t matter which way you mount it), not an issue if you use Analogue VTX
  • The tip of the arms could use a bit more protrusion for motor protection, or we could 3D print skids that go under the motor, which also include this feature

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