The Sohei Mk2 is the 150 class iteration of the flagship Ronin series. However, the Sohei keeps the same carbon fiber thickness as its bigger brother making it far more resilient than other frames in its class. This build is based around a 4S 700mAh lipo and 1404 motors spinning 3 inch props. The result is a very nimble and lightweight (224g AUW) ripper that can take a beating for a solid 4+ minutes of flight time.
As I’ve been doing with recent builds, this one is also equipped with the HDZero digital FPV system. However, since this is meant to be a basher build, I’ve kitted it with the inferior Foxeer Digisight v2 as larger sensor, micro cameras (Runcam HDZero, Foxeer Digisight v3) are still in early release as of writing. Still, with enough tweaks to the settings, you can get a fairly decent image from the Digisight v2.
In addition to the build itself, we’ll be looking at the tuning and filtering strategy as well as a weight analysis so you can have a better idea of how to approach your own sub-250g build.
The Sohei is assembled identically to the Ronin. The arm pairs are secured between the bottom plate and brace plate.
Note the proper orientation of the brace plate and bottom plate.
You guessed it, I’m once again going with a Flywoo Goku GN745 F7 AIO flight controller with 40A BLHeli_32 ESCs rated for 2-6S.
Widely used by FPV hobbyists, MG Chemicals (422C) silicone conformal coating also comes in a UV sensitive version which will illuminate under UV-A light (black light) to easily identify areas which are not coated.
A 35V 330uf capacitor at the lipo has proven sufficient to mitigate noise for 4S builds. I recommend low-ESR capacitors from Nichicon or Rubycon. I am using 16AWG silicone wires for the lipo harness measuring approximately 2-1/2 inches.
It is also highly recommended to use a capacitor with the HDZero digital FPV system as it can be susceptible to voltage spikes.
I’m using a TBS Crossfire Nano RX strapped to a 20×20 stack plate and a full-size TBS Immortal T antenna tail mounted with a TPU mount. A Mini Immortal T antenna can also be used saving upwards of 3-5g.
Click here to download STL files for 3D printed parts designed for the Sohei Mk2.
The HDZero TX5S.1 (whoop style) VTX provides both the FPV feed as well as the HD flight footage. As mentioned, I am using the inferior Digisight v2 since I expect to crash this build often.
I am using a Xilo AXII antenna for the VTX. As it is a direct U.fl connection to minimize signal loss as well as weight, I’ve secured it with a liberal dab of hot glue. I’m so far pretty impressed with this budget antenna.
The new “kuwagata” lipo pad is custom cut for the Mk2 line.
The Xing 2 1404s are spinning Gemfan Windancer 3028 tri-blades.
With that much left in AUW, I could mount an Insta360 Go or Naked Gopro and remain sub250g. Despite being already quite the pocket rocket, I plan to swap out the 1404s with some even higher KV 1407s as well as higher pitch props. Cos why the heck not.
The build is very capable of what some might consider a fairly aggressive filtering strategy relying primarily on RPM and Dynamic Filtering. If you have a clean noise profile, your build should be able to handle this filtering without issue. As a reference for comparison, the following are the gyro_scaled for pitch and roll for this build.
After filters, the noise profile for the build is very clean.
This clean noise profile was achieved using minimal filtering. As mentioned, this strategy relies primarily on RPM and Dynamic Filtering. If your ESCs are BLHeli_S, you can use third party firmware to allow for RPM Filtering or compensate with additional notch filtering. Ultimately, you can enable sliders and incrementally reduce filtering to its optimal level (be sure to check motor operating temperatures often).
Based on the filtering, the following PIDs and Feed Forward settings were used.
Weight Analysis
The following highlights the top five components that make up the 224g AUW. As is the case with most of these builds, the lipo and the motors make up half the weight. For this build, exactly 50%. The frame makes up 19% of the AUW and is the second heaviest component. The flight controller is a very distant third heaviest at only 4% of the AUW. The take-away here is that battery and motor size greatly informs the AUW. (You can hover over the chart components for details.)
The following chart sorts all of the components by weight.
Of note, wiring and solder as well as 3D printed parts each accounted for 4% (9-10g) of AUW.
The Sohei Mk2 is a particularly robust frame leveraging the smaller 150 class to provide a versatile sub250g platform. The purpose of this log is to provide ideas and inspiration for you to come up with your own perfect setup.
73
All products mentioned and used in this build log were purchased at retail. All links to product websites are non-affiliate / non-commission and no solicitations were made by companies to the author to promote these products.
Sohei Mk2 Build Log
Updated on February 15, 2022
The Sohei Mk2 is the 150 class iteration of the flagship Ronin series. However, the Sohei keeps the same carbon fiber thickness as its bigger brother making it far more resilient than other frames in its class. This build is based around a 4S 700mAh lipo and 1404 motors spinning 3 inch props. The result is a very nimble and lightweight (224g AUW) ripper that can take a beating for a solid 4+ minutes of flight time.
As I’ve been doing with recent builds, this one is also equipped with the HDZero digital FPV system. However, since this is meant to be a basher build, I’ve kitted it with the inferior Foxeer Digisight v2 as larger sensor, micro cameras (Runcam HDZero, Foxeer Digisight v3) are still in early release as of writing. Still, with enough tweaks to the settings, you can get a fairly decent image from the Digisight v2.
In addition to the build itself, we’ll be looking at the tuning and filtering strategy as well as a weight analysis so you can have a better idea of how to approach your own sub-250g build.
Build Log
The Sohei is assembled identically to the Ronin. The arm pairs are secured between the bottom plate and brace plate.
Note the proper orientation of the brace plate and bottom plate.
You guessed it, I’m once again going with a Flywoo Goku GN745 F7 AIO flight controller with 40A BLHeli_32 ESCs rated for 2-6S.
A 35V 330uf capacitor at the lipo has proven sufficient to mitigate noise for 4S builds. I recommend low-ESR capacitors from Nichicon or Rubycon. I am using 16AWG silicone wires for the lipo harness measuring approximately 2-1/2 inches.
It is also highly recommended to use a capacitor with the HDZero digital FPV system as it can be susceptible to voltage spikes.
For this build, I’m using iFlight Xing 2 1404 3800KV motors spinning 3 inch Gemfan Windancer 3028 tri-blades.
I’m using a TBS Crossfire Nano RX strapped to a 20×20 stack plate and a full-size TBS Immortal T antenna tail mounted with a TPU mount. A Mini Immortal T antenna can also be used saving upwards of 3-5g.
The HDZero TX5S.1 (whoop style) VTX provides both the FPV feed as well as the HD flight footage. As mentioned, I am using the inferior Digisight v2 since I expect to crash this build often.
I am using a Xilo AXII antenna for the VTX. As it is a direct U.fl connection to minimize signal loss as well as weight, I’ve secured it with a liberal dab of hot glue. I’m so far pretty impressed with this budget antenna.
The new “kuwagata” lipo pad is custom cut for the Mk2 line.
The Xing 2 1404s are spinning Gemfan Windancer 3028 tri-blades.
And it’s never complete without the Tiny’s Femto RGB LEDs.
AUW with a Pyrodrone 4S 75C 700mAh lipo with an XT30 connector is 224g.
With that much left in AUW, I could mount an Insta360 Go or Naked Gopro and remain sub250g. Despite being already quite the pocket rocket, I plan to swap out the 1404s with some even higher KV 1407s as well as higher pitch props. Cos why the heck not.
Tuning & Filter Strategy
The build is very capable of what some might consider a fairly aggressive filtering strategy relying primarily on RPM and Dynamic Filtering. If you have a clean noise profile, your build should be able to handle this filtering without issue. As a reference for comparison, the following are the gyro_scaled for pitch and roll for this build.
After filters, the noise profile for the build is very clean.
This clean noise profile was achieved using minimal filtering. As mentioned, this strategy relies primarily on RPM and Dynamic Filtering. If your ESCs are BLHeli_S, you can use third party firmware to allow for RPM Filtering or compensate with additional notch filtering. Ultimately, you can enable sliders and incrementally reduce filtering to its optimal level (be sure to check motor operating temperatures often).
Based on the filtering, the following PIDs and Feed Forward settings were used.
Weight Analysis
The following highlights the top five components that make up the 224g AUW. As is the case with most of these builds, the lipo and the motors make up half the weight. For this build, exactly 50%. The frame makes up 19% of the AUW and is the second heaviest component. The flight controller is a very distant third heaviest at only 4% of the AUW. The take-away here is that battery and motor size greatly informs the AUW. (You can hover over the chart components for details.)
The following chart sorts all of the components by weight.
Of note, wiring and solder as well as 3D printed parts each accounted for 4% (9-10g) of AUW.
The Sohei Mk2 is a particularly robust frame leveraging the smaller 150 class to provide a versatile sub250g platform. The purpose of this log is to provide ideas and inspiration for you to come up with your own perfect setup.
73