Audio problem: As Hawkeye and Trapper get into the chopper, you hear the engine powering up, and the blades slicing the air, but the blades aren't moving at all.
Audio problem: At the end when Borelli is getting ready to take off, in the close ups you can see the main rotor blades starting to turn, getting ready to fly. But the sound suggests that the rotors and engine are already at full power.
Answer: The C-42 was a military variant of the Douglas DC-2. Very few C-42's were built, so it's questionable that Potter would specifically have seen that particular model, but, given his military background, it's not entirely unreasonable that he might use the military designation even when the aircraft in question is actually a civilian DC-2.
Tailkinker ★