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Hi,

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.

Solution:

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

Replace:

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

By this:
    #if FRAME_CONFIG ==    TRI_FRAME
    APM_RC.OutputCh(CH_7,   g.rc_4.radio_trim); // Yaw servo middle position
    #else
    APM_RC.OutputCh(CH_7,   g.rc_3.radio_min);
    #endif

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:

Solution:

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

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

by:
    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