The Thunderbolt training manual from 1945 has an illustration on page 44 that appears to show a trolley plugged into the fuselage of the aircraft. On that same page, the manual says:See that the crew chief is on hand with a fire extinguisher and a portable battery cart. If the plane has been standing for more than 2 hours, instruct the crew chief to pull the prop through four blades. One complete revolution is needed to clear the lower cylinders of oil.
Further into the text on start-up procedure, on page 45, the training manual instructs to:Turn on the main line battery switch as soon as the engine catches and signal the crew chief to unplug the power unit.
I don't know if auxiliary power units were used for every startup, but I'm sure they would have been used at all times during training and probably whenever possible at operational bases. Several important aircraft systems relied on the battery, including the radios and the AUTO propeller control, and reducing the load on the battery to keep it charged would have been a concern. The battery only had about one hour's worth of charge if the generator failed during flight.
Remember that the FSX version we have is a very early version of AccuSim, and lacks some of the brilliant features of the later products like wheel chocks, jack stands and the APU trolleys. And we don't have to pull the prop through four blades, or have a man standing by with a fire extinguisher, or for that matter ever worry that battery acid fumes or oil pools in the supercharger hood will cause our plane to suddenly burst into flames.
_________________I like Thunderbolts!
(Link to MySpace tribute page)120th Fighter Wing, Montana Air National Guard