Originally Posted by
Uplinker
At about 1:22 on the #18 video, you can see one of the rail rotors suddenly change angle in the rotational plane. Instead of being 180° opposite the other blade, it moves round to about 150/210° from the other blade.
That's more from the rolling shutter on digital cameras. Rolling shutter scans the sensor line by line from top to bottom. At certain scan rate/rpm, the blades will appear to be curved.