Has there been any established or recommended maintenance schedule for the Phantoms? I'm particularly concerned about the hard working prop motors. It seems to me that inevitably one will have a mechanical failure, and it's only going to fail while flying -- which most likely means goodbye drone. Have these been out long enough to know when/how often the motors should be swapped out for new ones? Do owners do that as a precaution, say, every 2 years if they fly a lot? Just wondering.