I find this research log quite helpful (found it via google, not written by me)
Else, I’m not working on multicopter so I’m not familiar with the mixing, but i would suppose some combination of throttle, roll, pitch, yaw is used.
These are (at least in fixed wing aircraft) ATTC_Pitch/Roll/Yaw/Throttle. You can write directly to this message
Else, you can modify sensors.cpp like the following (where they inject a sweep sine into a particular channel)
