I press the safety switch, make sure that the motor interlock switch is in the right position and give the stick command for arming. I get the beep/armed light and GCS says ARMED except about a second later it disarms itself with no messages to the GCS. I can’t get the motors to spin during that second either.
Looks like you have the motor interlock enabled. So to arm you’ll need to have the interlock so that the motors don’t spin. Then after arming you need to flick the switch so that they’re allowed to spin.
The vehicle is arming but then disarming about 5 seconds later. I think our motor interlock, when enabled, halves the regular disarm-delay time. If that seems odd, talk to Rob Lefebvre! The motor interlock was mostly added for helicopter users I think. Maybe you want to use the kill switch instead? It’s very similar but a little less complex. It’s explained on the wiki a bit on the “auxiliar function switches” page. http://ardupilot.org/copter/docs/channel-7-and-8-options.html
I didn’t realize that with the interlock you need to arm in one position then flick the switch to the other position. The documentation doesn’t really say that clearly.