I just build a Tri based on ArduCopter. I had some issues with yaw and did some changes in the code. In this post I want to share my experience and I would like the developers to take my changes into the official release, that it will be easier for others to build a Tricopter.

First of all, this is the result:


One of the most interesting mechanical parts, the Yaw-Servo:

Now my yaw-issues and solutions, starting from code release 2.1:

1) Yaw-Servo not in middle position on "power on"

2) Yaw-Servo reverse with Mavlink RC7_REV

1) Yaw-Servo not in middle position on "power on"

Issue: On "power on" the Yaw-Servo moved into mechanical limit (bad sound, high current consumtion and reduced lifetime for the servo). After a few seconds it returned to the middle position.


In file "radio.pde" in function "output_min()":


    APM_RC.OutputCh(CH_7,   g.rc_3.radio_min);

By this:
    APM_RC.OutputCh(CH_7,   g.rc_4.radio_trim); // Yaw servo middle position
    APM_RC.OutputCh(CH_7,   g.rc_3.radio_min);

2) Yaw-Servo reverse with Mavlink RC7_REV

Issue: Depending on how you mount your Yaw-Servo the stabilisation of yaw can run in the wron direction. Reversing yaw on the remote doesnt help. You need to reverse the servo or change it mechanically. To reverse the servo:


In file "ArduCopter.pde" in function "fifty_hz_loop()":

    APM_RC.OutputCh(CH_7, g.rc_4.radio_out);

    APM_RC.OutputCh(CH_7, (   (-1 * (g.rc_4.radio_out - g.rc_4.radio_trim) ) + g.rc_4.radio_trim  ) );

I got this change from this Link!  Now you can reverse the yaw-servo by putting "-1" to the parameter "RC7_REV" via Mavlink.

Regards, Igor

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  • Developer


         I'd guess if it's rotating then it's either an input problem (for example the yaw input may not be centered - re-doing the radio calibration may fix this) or some motors/props are spinning in the wrong direction which makes the copter unable to maintain it's heading.  Impossible to be sure without a log file though.

  • "I had some issues with yaw" - can someone explain please, what sort of issues? My Y6 slowly drifts counter-clock wise. I calibrated magnetometer as well as I did compassmot which under 5% now (I'm using an external one). 

    Is it the same problem?

  • Developer

    Good stuff.  Leonard has on his list to built a tri and dig into any control issues that come up.  I suspect he will find some and we will probably make a bunch of changes.  Not sure if that will be for AC3.2 or AC3.3..but eventually it'll happen.

  • Found the location.  In the Advanced Parameters there is RC reverse (RC7_REV).

    Clicked on reverse from the drop down, and booyah!  It worked.  

    Now time to lower the gain and dial down this wobbly goblin to smooth it out.  I have not been this excited since I learned via a genealogy website I was a direct descendant of Darth Vader (though somehow not on the Luke side). 

  • Hello Randy,

    Thanks for your great work on all of these forums, which must keep you quite busy.

    Do you happen to know if there has been a fix for this yet?  I also have a tricopter and the yaw is reversed.  Once it starts moving, it only increases as it tries to self correct.  So far this consists of only setting up the system and testing it in the garage.  Everything seems so straight forward except for this one item.  With my RC Helis I can change this setting in the gryo with a small screwdriver.  Not so simple here.

    "As for #2, I'll ask Michael Oborne if he can add a check box somewhere in the mission planner to make it easier to reverse the yaw servo so you don't need to hunt around in the configuration screen."

  • Firmware 3.1RC5 seems to have solved the problem. The yaw servo no longer creeps over to one side on the bench and I've got level flight with the props on.

  • It creeps on the bench, with props/outside it leans over hard and yaws like crazy the more throttle I add.
  • Should I try the latest firmware (RC5)?

  • Developer

    Ah, it's only creeping is it?  That's normal..that's "I term build-up" because it's trying to control it's yaw direction but it can't because it's on the ground.  If you are in stabilize mode and hold the throttle down it should move back to the middle.

  • Yeah, I've tried reversing 4 and 7 and all combination. It'll just cause the servo to creep in the opposite direction. At least that proves it's software...I think.

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