Question on RCn Function on Quadplane

Hoping someone can confirm for me:

If I am using channel 8 for my flight mode selection (ie. FLTMODE_CH = 8) then can I still use an RCn function on the corresponding port (Main 8) to control something else? In my case I have a quadplane setup with RC8_Function = 36 (right rear motor) and seems to fly OK but sometimes that motor appears to behave a little differently on arming. But that might just be a calibration issue or a “sticky” motor.

Related to my query above, I have another question: will the PWM parameters for RC8 (Max, Min, Trim) affect the right rear motor in any way? The existing values are correct for controlling the flight mode but they are different to the PWM values (Max. Min, Trim) for the other three motors. Do I need to be concerned about this? Or perhaps these have no effect when RCn function is used?


Just bumping this…anyone?


Please help.


Sticking my neck out a bit here, but on the first question, I believe that FLIGHTMODE_CH must logically be an RC input, while RC8_Function is an RC output. There should be no direct relationship between RC inputs and RC outputs., unless you set the RCx_FUNCTION to “passthrough” These parameter could probably be be better named.

For the PWM, it is pretty hard to fathom how they work. Logically since they are named similarly to RC outputs, they should apply to the RC Outputs, but if you say they are “correct for the flight mode”, that would imply they apply to the RC inputs.

This whole area of ArduPilot is a mess and could certainly be improved.

yes, that’s right. They are independent.

yes, and one of the big items I am working on for 3.8 is to do that.
It will be:
for output it will be:


then RCn_MIN, MAX, TRIM will be input side.


I am aware that you have been working on this bit of ArduPilot and I’m hoping that it will be possible to have a generic actuator output mixer , when it is completed. I understand that you are planning a mixer scheme based on the OpenTX system?

For RC outputs I personally prefer the term actuator to servo, since it covers the set of servos, ESC’s, switches, relays etc.

As an alternative to “RC inputs” I found the term joystick input most descriptive.

Also defining the the RC inputs using names “pitch”, “roll”, “yaw”, “throttle” rather than using numbers might also help understanding

Thanks gents, good to know that input and output are independent of each other. I’ve exhausted all the servo ports on my Pixhawk, so it’s good to know that I can use FLTMODE_CH = 8 and also use main 8 port for RCn function on my quadplane.