Encountered udp connection issue while using the mavlink library

Encountered udp connection issue while using the mavlink library
I can sent heartbeat/arm&disarm but I cannot sent pwm value to rc_channels, what should I do ? thx !

I use this in SITL, "arm_sendrc.txt"use pymavlink to arm throttle and send rc pwm value , it can work,
arm_sendrc.txt (1.5 KB)


I also can arm/disarm in C ,but I still can not send rc value
arm_disarm
this is complete code
arm_disarm.txt (6.9 KB)