T-Rex 800 Pro DFC Params

anybody out there how can help me with some good PID Values for 3.3.3 to start?

My first two attempts were unsuccessful and that could be very expensive. If it's a secret, I would be willing to pay for it (Paypal) :-)

Mechanic: T-Rex 800 Pro DFC with stock components.
FMU: Pixhack from CUAV
Software: Arducopter 3.3.3

The mechanic is built very well and was also in the air with a Skookum Robotics SK720 GPS and a DJI Wookon H. I have also built a few small and large Multicopters with Pixhawk, but it is my first Helicopter with Ardupilot/Pixhawk...

Many Thanks!



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  • What way, be able to use a single connection Pixhawk sbus of sbus out to FBL system (Vbar, SK540 / 720) of sbus in it? Which parameters need to modify it?

    • I tried SBUS using Futaba R7008SB.

      In Arducopter heli 3.3.3 stable I had to set BRD_SBUS_OUT = 1 and BRD_SAFETYENABLE = 0 to get signals on the SBUS out of the Pixhawk.

      In Arduplane 3.5.3 stable I had to use BRD_SBUS_OUT = 4, using other values (1,2,3,5,6,7) I didn't get any signal.

      Results were not promising, don't think it will work without channel remapping.

      I got a Vbar NEO now, unfortunately it doesn't take the seperate channel inputs like the old Vbar. It would be great if we could have channel switching option for Pixhawk+FBL, hopefully you can convince a developer.

    • Sorry haven't yet tried using SBUS, but Tridge used it. Maybe I can try it as well.

  • Hi Jochen,

    here are the basic steps for Pixhawk+FBL :

    Arducopter Heli Version 3.3.3 stable

    Tip: calibration of accelerometer is much easier before you put the Pixhawk in the helicopter.

    WARNING !!! Remove blades or pinion or motor wires NOW.

    Insert the Pixhawk between RX and FBL. RC1 = aileron, RC2 = elevator, RC3 = collective pitch, RC4 = rudder, RC8 = ESC, RSC, throttle whatever it is called

    If necessary change the TX output (channel number, servo direction) to achieve the correct movements in Radio Calibration.
    In Radio Calibration make sure the bars move with your stick left and right for aileron and rudder.
    The collective pitch is called throttle and should correspond to your stick movement as well.
    The elevator must go down (lower numbers) for helicopter nose down stick movement and must go up (higher numbers) for nose up stick movement.

    WARNING !!! MissionPlanner tries to help you and changes your input to the recommended minimum or maximum value. Unfortunately these values are most of the time for multicopters and not for helicopters. Therefore it is recommended to use Full Parameters List/Tree and Refresh to check if the value is what YOU want.


    H_SWASH_TYPE = 1
    H_TAIL_TYPE = 1
    ACRO_TRAINER = 0 (this gives the FBL full control in ACRO mode)

    H_CYC_MAX = 4500 (you adjust this to limit the aileron and elevator output for your FBL setup)
    H_COL_MAX = 1900, H_COL_MIN = 1100 (you adjust this to limit the collective pitch for your FBL setup)

    H_SV1_POS = 90 (aileron)
    H_SV2_POS = 0  (elevator)
    H_SV3_POS = 180

    Check rudder and swash movement (collective, aileron, elevator) and adjust as needed using
    HS1/2/3/4_REV = 1 / -1 (1 or -1 to change direction)
    DO NOT use the TX to reverse servo direction !

    RATE_PIT_P/I/D = 0, RATE_RLL_P/I/D = 0, RATE_YAW_P/I/D = 0
    RATE_PIT_FF = 0.4, RATE_RLL_FF=0.4, RATE_YAW_FF = 0.3 (adjust as needed)
    ACCEL_Z_P = 0.3 (adjust as needed, many helicopters oscillate on collective using standard value 0.5)

    DISARM_DELAY = 0 (if you can't arm because after arming software disarms)

    WARNING !!! Remove blades or pinion or motor wires NOW.

    Check FAIL SAFE, what happens with TX off, what happens with RX off and Pixhawk on, etc.

    Check ACRO mode FBL correction direction before EVERY flight !
    Check STABILIZE mode correction direction before EVERY flight !

    Complete Parameter List — Copter documentation
    • Hi Ultrafuge,

      many many thanks for your detailed reply! I will give it a try und report you.

      One last question: i can use whatever FBL system and Pixhawk will do the rest. So when i want to fly only in stabilize mode, the FBL is responsible for flying the Heli, and Pixhawk will stabilize it, so when i for example roll to left, heli will go left, when i release the stick, heli will go back to horizontal position (FBL only would roll the heli as long as i have stick to left), is this korrekt? Is this working out of the box with our params?

      Thank you very much!

      Best regards


    • ....is this korrekt?

      Usually the answer is yes if everything works fine.

      Is this working out of the box with our params?

      You must for sure adjust H_CYC_MAX, H_COL_MAX, H_COL_MIN and
      check rudder and swash movement (collective, aileron, elevator) and adjust as needed.

      The following parameters can work, but should be tuned for your heli, in doubt lower.
      RATE_PIT_FF = 0.4, RATE_RLL_FF=0.4, RATE_YAW_FF = 0.3
      ACCEL_Z_P = 0.3

  • Hi Jochen,

    first you have to decide if you want to go pure Pixhawk or Pixhawk+FBL. I prefer Pixhawk+FBL but both work.

    I would use SK720 and set ACRO mode with Trainer=0 which gives you all the benefits of the SK720. You set all Rate_xx_PID = 0 and use Rate_RLL/PIT_FF=0.4 and Rate_YAW_FF=0.3 as a starting point for the other modes. Ofcourse you have to set the other parameters for FBL as well.

    What do you want pure or with SK720 ?

    • Hi Ultrafuge,

      that's a good idea! Is it possible to fly with this combination in stabilize-mode or PositionHold? I 'm not the best pilot (have begun only 3 years ago) is Stabalize then made by the SK720 or Pixhawk? I also have a Bavarian Demon HC3SX here, so this could do the job also? What is the other possibility, do you have working Params for pixhawk only? I Think pixhawk + FBL could by a solution for my turbine trainer...

      Best regards

      P.S. thank you for your help!
    • Hi Jochen,

      do you have a PC problem or I ? All your replies have a vertical line on the left and I can see only a part of your answer. I see the introduction till "...in stabilize-"  and then "Best regards...etc".

      Hope this is the answer to your question:

      The Acro mode with ACRO_TRAINER = 0 is basically a pass through mode, you are flying as if there would be the FBL only, without the Pixhawk.If your SK720 has a rescue function you could use it, but the Pixhawk doesn't help you because with this settings you tell the Pixhawk NOT to modify the RC input.

      If you switch to another mode like Stabilize or RTL the Pixhawk+FBL behaves as expected, i.e. stabilizes the heli or does a RTL (if everything works correct :-) ). Rumours are if you fly 3D it is expected that the EKF can't follow and switching to Stabilize mode etc. will NOT rescue the heli (I never tried it).

    • are you from the CUAV team? I have followed a discussion and seen a video about such a configuration...

      when the FBL works in acro mode the pixhawk can hold position or stabilize and so on? Sounds very good!

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