Get a baseline of what is going on. Get an AC voltage reading from the prop to a good grounding pin at the outlet. Be careful as the full 120vac could be on the prop. The AC power cord could be grounded to some metal at attach points or where screws are used. Another area to look at is the charger itself. The 120Vac internally has connected to the 12Vdc ground either through chafing or a loss of the insulation on the dc ground wire (excessive loads in the past or excessive heat in the charger). Good luck.