The APM “ESC” advanced parameter puts the copter in ESC calibration mode at next restart. So I guess you could:
- Remove all propellers in case this doesn’t work!
- Configure your joystick and ensure it appears as RC input.
- Set the ESC parameter to 1, “write parameters”, then reboot/restart APM.
- Connect and move the throttle up on the joystick, wait for some beeps, then back down and if you get the other beeps it worked.
- Moving the throttle up should then power up all motors at the same time (now synchronized). If one motor starts early/late then it didn’t work.
I haven’t tested that without a receiver, but in theory it should work. Also check the DIY Drones forum about the “ESC” parameter.