I'm using the Dronekit-python API to control my APM-based drone and lately I've been facing a problem trying to arm it with the code vehicle.armed = True but I just get an error. I've also tried to arm it directly from Mavproxy using arm throttle and always gives the next MAVLik message: COMMAND_ACK {command : 400, result : 3}. That's the only way MAVLink have of returning a result, with a code. In this case 3 means "UNKNOWN/UNSUPPORTED" but it doesn't give more information like the cause of the error. And command 400 is because de order is arm/disarm.
After some trials tests I've managed to arm it if I previously overrive the third radio-control channel (the throttle channel) to some value other than 0 with rc 3 1000. The problem is that if I do that it disables that channel and I can't use that channel anymore unless I set it again to 0.
I've searched into the codes of other people and nobody seems to have the same problem. Can anyone help me? Thanks.