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This quadcopter was dismantled on 19.6.2018 after 28 flights. I don't suggest making a copy of this setup. It's too heavy and underpowered as a result even after swapping the separate camera and vtx to a 2-in-1 combo. The custom prop size is also a pain especially when the prop breaks so easily. See Lantian L90 for a better alternative in the same size class.

Flex RC Mini Owl

Frame Flex RC Mini Owl
Motor DYS BE1104 6500KV x 4
ESC Favourite FVT LittleBee PRO 20A 4 in 1
BEC Integrated in FC and ESC
OSD MinimOSD integrated in FC
Prop RotorX RX2535W cut to 1.8“
Receiver FrSky TFR4-SB
Camera Tarot 520TVL FPV 1/4”
VTX Hawkeye 5.8GHz 200mW
Antenna Foxeer 5.8G RHCP, mini version
Battery Turnigy nano-tech 450mAh 3S 65C
Turnigy nano-tech 850mAh 3S 45C
Weight 131 g (without battery)
Flight time around 4 minutes



  1. Sand the sharp cut edges of all CF parts.
  2. JST connector will be replaced with XT30.
  3. Flashing the F3S AIO requires:
    • SPRACINGF3 firmware
    • “No reboot sequence” disabled
    • “Full chip erase” enabled
    • “Manual baud rate” set to 115200
  4. OSD setting in Arduino:
    • Arduino Pro or Pro Mini w/ ATmega328
    • 5V, 16 MHz
  5. The camera is in correct orientation when the wires come out from the bottom.
    • Those wires are of crap quality, caution is needed when stripping a cut wire.
  6. The camera is attached to the bottom plate in order to give the lens better protection while still achieving enough tilt.
    • The camera mount needed some removal of excess material in order to fit.
  7. The propeller cutter from Flex RC actually cuts 2.5“ propellers to 1.8” when the 1.9“ cutting hole is used.
  8. F3S AIO Pro FC has
    • the buzzer pads marked in inversed polarity
    • the four motor pads may also be in inversed order
  9. MWOSD needs to be updated if it shows really have current readings when disarmed.
    • The problem is caused by negative current sensor readings. Check scale and offset.
  10. Turnigy nano-tech 450mAh 3S 65C is the suggested battery by Flex RC.
    • Not good in reality, produces low voltage warnings early in flight.
    • Use Turnigy nano-tech 850mAh 3S 45C instead, even if a little heavier.
  11. The field of view of the camera is big enough not to require any uptilt.
  12. VTX video quality is rather bad even when flown outdoors and doesn't appear to be directly related to throttle levels.


Betaflight (3.0.0 RC13)

  • Align board
    • set align_board_pitch = 180
    • set align_board_yaw = 45
  • Reversed prop rotation
    • set yaw_motor_direction = -1
  • Voltage Sensor
    • feature VBAT
    • set vbat_scale = 110
    • set vbat_min_cell_voltage = 33
    • set vbat_warning_cell_voltage = 34
  • Current Sensor
    • feature CURRENT_METER
    • set current_meter_type = ADC
    • set current_meter_scale = x
      • the correct value should be around 360-380 due to the value of the resistor
        • these appear to give reasonable results at least with low rpm
        • set current_meter_scale = 690
        • set current_meter_offset = 98
      • the configurator is useless for calibration when usb is connected
      • possibly a flaw in fc design
  • No beeper when USB is connected
    • beeper -ON_USB
  • Switches
    • AUX1
      • Up: Angle
      • Middle: Horizon
    • AUX2
      • Up + Middle: Arm
      • Up: Air Mode
    • AUX3
      • Up: Blackbox
    • AUX4
      • Up + Middle: Beeper
  • Serial
    • serial 0 1 115200 57600 0 115200
    • serial 1 0 115200 57600 0 115200
    • serial 2 64 115200 57600 0 115200
  • Onboard flash for blackbox
    • set blackbox_device = SPIFLASH

BLHeli (14.6)

  • Programming by TX disable
  • PWM Frequency / Damped: DampedLight
  • Motor Timing: High
  • Beacon Delay: 5 minutes
  • PPM Min Throttle: 1020
  • PPM Max Throttle: 2000
  • Motor Direction Reversed for those motors that need reversing

MWOSD (1.6)

In Config.h

#define HORIZON
//#define MAPMODE
//#define MAVLINKREQ
#define AUTOCAM
#define USE_VSYNC
#define AUTOCELL


  • RGB leds have been left out from the final build.
    • No good attachement point available.
    • Saves some weight and wiring.
  • Only one end of a wire should have a connector if space allows.
  • Ideally, audio channels from vtx should be connected to ground when not used.
rc/flex-rc-mini-owl.txt · Last modified: 19.06.2018 19:34 by vergo