Originally Posted by
DaveReidUK
Actually, it's all about frame rate.
Even a minute variation in rotor RPM between the front and back rotors could produce the effect where one or other appeared to be moving in the opposite sense to actual.
Watch any film or video of a helicopter in flight and it's very difficult to discern the direction of rotation from successive frames.
None of the weird backwards blade effects apply to high speed footage with a frame rate far in excess of rotor RPM. The footage looks to be taken at maybe 120-240 fps (default options on most cell phones, gopro etc), the rotor RPM of a Ch47 is 225 or something, or just under 4 per second. In other words, even at the lower end estimate (120fps), the camera records 30 individual frames of the blades rotating, which means that there can absolutely not be any effect caused by the frame rate that would distort the visible rotation or make it seem opposite. A frame is taken every 12 degrees angle of rotation, or every 6 degrees at 240fps, while the blades are obviously spaced roughly 120 degrees from each other.
In fact one could easily determine the frame rate this was shot at based on the expected rotor RPM, by measuring said angle.