BiSolomon Frames for 3-3.5" Quads --- An Evolution of Solomon's Toothpick ---

By Martin Flite on Nov 28, 2023

3  0  0

In the previous post (, I showed my "Solomon's K3" which was a module type 3" Toothpick frame consisting only of arms, screws and nuts. I thought it was robust enough and sufficient to hold down the vibrations. However, it was not convenient for replacing a broken arm and it seemed difficult to apply directly its structure to other styles of quadcopter such as a double stack frame. So, I made BiSolomon Frames. They are not simple as Solomon's, but it is very easy to replace a broken arm and to generalize.

Design Concept

To keep robust structure of Solomon's, two independent and Left-Right symmetric Solomon's frames are combined and their connected arms are deformed for robustness and generalization. There are many possibility for this deformation. I respected its robustness too much that it became overkilling and heavy.Alt Tag

Frame Variations

Alt Tag1. BiSolomon WX3 TP (WB: 148.6mm, Carbon Weight: Approx. 20g)
2. BiSolomon WX3 Single Stack (WB: 148.6mm, Carbon Weight: Approx. 29g, Stack Height: 15mm, 20mm)
These two types are mainly for 3", but can also accommodate 3.5" props. Their arms and the lower bottom plate are common. The upper bottom plate is chosen for Toothpick or Single Stack. Frankly, I hesitate to call this TP version a 3" Toothpick frame because of its weight and excessive robustness. According to Andy RC, it might be better to call it a Twig class. However, I think "Twig" is still only used for special designs by Racer X FPV.

Alt Tag3. BiSolomon WX35 Single Stack for 14mm width Camera (WB: 174.2mm, Carbon Weight: Approx. 45g, Stack Height: 20mm)
4. BiSolomon WX35 Double Stack for 14mm, 19mm, 20mm width Cameras (WB: 174.2mm, Carbon Weight: Approx. 53g, Stack Height: 15mm)
These two types are made for 3.5" props. The single stack model has a slightly larger space between the camera and the FC. The double stack model for 19mm width camera and that for 20mm width camera (DJI FPV 1G, O3) are identical except for the camera mounting side plates and spacers. As for the double stack model for 14mm width camera model (not shown in the picture), a slightly longer upper bottom plate is used because the low camera position requiring the camera be put forward in order to obtain (almost) no props view.

Alt Tag

Build of BiSolomon WX3 Single Stack

Alt TagFor this build, I chose 23mm short motors 3450KV from FPVCycle to give them a chance to make a comeback. These motors were purchased two years ago and stored unused after rather disappointing tests with 4" props. The flight controller is the SpeedyBee F745 AIO 35A AIO, used on my 3.5" Solomon's frame with the RCinPower 2105 plus motors, resulting in very good flight performance and PID tuning convenience. The propellers are HQProp 76mm x8. FPVCycle's 23mm short motors can also handle lighter 3.5" props up to Gemfan Ducted 90mm 3 blades, but not 8 blades. The weight of this build shown in the right picture is 190g.


Fist, it is necessary to mention the motor mixer. As far as FPV flights are concerned, the flight characteristics of the wide-x frame can be quickly accustomed after a few flights and do not cause any inconvenience at all. However, in order to enjoy line-of-sight flights as well, I think it is necessary to set the motor mixer appropriately. The motor-to-motor distance of this frame is 115.5mm and 93.5mm; their ratio is 93.5/115.5 = 0.810. Therefore, the Betaflight Configurator CLI commands to properly configure the motor mixer are as follows:

mixer custom
mmix reset
mmix 0 1 -1 0.810 -1
mmix 1 1 -1 -0.810 1
mmix 2 1 1 0.810 1
mmix 3 1 1 -0.810 -1

Alt TagWith Betaflight's default pid, the aircraft hovered steadily and flew slowly indoors without any strange behavior. As usual, I ran the PIDtoolbox Basement tuning in angle mode before flying it outdoors.

Blackbox Log

Alt TagOn the test flight, I performed 0-100% slow throttle punches, some quick flips and rolls, and obtained a blackbox log file showing improved results. The rather strong resonance that remained a bit in the unfiltered data of the Solomon's K3 frame is gone. In short, this BiSolomon frame is not only useful, but also improves vibration and noise performance. I am very glad.

Flight Performance

The batteries used in the test flights were GNB 4S 530mAh 90C / 930mAh 120C, CNHL 4S 1100mAh 100C, and Tattu 4S 1300mAh 120C. All but the Tattu 1300mAh have already been in use for 6-12 months, so these tests may not be ideal.

  • Hover Throttle: 15% with GNB 530mAh (51g), 18% with GNB 930mAh (99g), 20% with CNHL 1100mAh (148g), and 20% with Tattu 1300mAh (150g).
  • Maximum Speed : 150km/h with GNB 930mAh, 168km/h with CNHL 1100mAh, and 172km/h with Tattu 1300mAh.. The GNB 530mAh was not suitable for the speed test because of its severe voltage sag when used alone, but 158km/h was achieved when two of them were connected in parallel.

I think my 23mm short motors from FPYCycle have finally settled in the right place. Even with heavy batteries, they give good performance from low to high speed.

Remaining Problem

I need to find a way to reduce the weight of the bottom plate without losing appropriate robustness and performance. My frame design is never ending.



Sign in to comment

Guides & Reviews

Jun 21, 2021

AirbladeUAV has done it again and this time they've brought long range to the 5" class! Based on the popular Transformer Mini, the new Transformer 5" Ultralight adopts a lot of the same design philosophies with larger props and more payload capacity. It can fly upwards of 20 minutes on a 4 cell Li-Ion battery pack and in ideal conditions it's got a range of over 4 to 5 miles. In this guide I'll walk..

Read more
Mar 04, 2021

With the release of the DJI FPV Drone cinematic FPV has become a lot more accessible, but you certainly don't want to crash a $750 drone! The QAV-CINE Freybott is a compact, lightweight cinematic FPV drone that can take a hit and keep going. It's a lot safer to fly indoors and around people. With a naked GoPro or the SMO 4k you can capture some great stabilized footage. In this guide I'll show you..

Read more