Yaw-Aileron Interchange in FW/ACRO Mode (VTOL Duo Tailsitter)

Hi all,
unfortunately the documentation for the VTOL Duo Tailsitter is quite sparse so I am currently stuck with fundamental installation issues. I installed the Pixhawk4 in my VTOL Duo tailsitter in such a way that the arrow points vertically up in MC mode and it points horizontally to the front in FW-mode. Therefore I set ORIENTATION_PITCH_90. I performed the “Level Horizon” calibration in MC mode.
For MC-mode ACRO and STABILIZED and for FW-mode MANUAL and STABILIZED, the RC-commands yield correct elevon-deflections (RC-elevator-stick movement moves both elevons up and down, RC-aileron-stick movement left-right move the servos in opposite direction, Yaw-RC-stick movement has no effect). But when switching to FW-ACRO-mode, the RC-aileron-stick movement does not have any effect but the RC-yaw-stick moves the elevons in opposite direction, so that it rolls around the longitudinal axis. It seems that Yaw-RC-command and Aileron-RC-command are interchanged. Is that as supposed to be or is that an error?

Check if the attitude displayed in QGC is correct first. If that’s correct, then check the servos and use the params to swap a servo direction if required with the params like https://dev.px4.io/master/en/advanced/parameter_reference.html#PWM_AUX_REV1.

The artificial horizon display in QGC is correct for all three axes when the vehicle is in MC-orientation (nose of aircraft pointing up, arrow on pixhawk pointing up as well). But the display is not correct when the vehicle is in FW-orientation (nose of aircraft pointing forward, arrow on pixhawk pointing forward as well). Is there a way to automatically switch the artificial horizon by 90° when doing the transition or will it always show the wrong orientation in FW-mode?

Or should the calibration be performed in FW mode? But then the artificial horizon is wrong in MC-mode.

PWM_AUX_REV* does not exist for “VTOL Duo Tailsitter”, is there any other way to reverse aileron direction but to maintain elevator direction or vice versa?


Hm, that should have been fixed a while ago in PX4: https://github.com/PX4/Firmware/pull/13249, and also QGC: https://github.com/mavlink/qgroundcontrol/pull/7943.

Hm, how do you mean not available?

@sfuhrer could you comment?

When I type “AUX” in the QGC-parameter-search, “RC_MAP_AUX” is the only parameter that is shown (for channels 1-6), but no parameter called “PWM_AUX_REV” is shown. It is also not listed unter “parameters/PWM Outputs”. I guess this parameter does not exist for the VTOL-Duo-Tailsitter configuration?

I am using QGroundcontrol v4.0.1 and Firmware 1.10.1. The fixes you mentioned above - they should be included in this version, right?


If you’re using the standard VTOL due tailsitter airframe then you have all actuators on the MAIN pins, which would mean that you’d have no AUX channel and you don’t see it, no? So the correct param would be PWM_MAIN_REV*.
But I anyway doubt that inverting the PWM will solve your problems. In the end, they move in the right direction in multicopter mode, don’t they?
Have you tried the same test in Stabilized flight mode? If not, please do so, and check if it’s correct there.

Correct - I am using thee MAIN-pins only, no AUX pins. Changing PWM_MAIN_REV* only inverts the servo directions but it doesn’t solve the problem that the channel assignment is wrong in FW-ACRO mode. The RC-YAW-stick moves the ailerons and the RC-aileron stick doesn’t move any servo. In all other modes the servo- and motor-controller behavior is correct.