Helicopter rotor speed matches camera shutter speed
= new reply since forum marked as read
BTW, as with carriage wheels, it's not necessary that the camera capture the rotor returning to the same position each frame. There are five blades, and they are all equivalent, so effectively the same view is repeated every fifth of a rotation. So at 360 RPM (supposedly a typical rate) or 6 RPS, the view does not appear to repeat every 1/6 of a second, but five times faster -- i.e. every 1/30th of a second. So his guess of 6 RPS looks pretty good, and he is probably capturing the rotor in the same position each frame -- though if it were 7 RPS, I doubt it would look much different, owing to the symmetry of the rotor.
Also note that for 360 RPM the frame rate needs to be a divisor of 30fps, not a multiple of 6fps, as the videographer stated. At 12fps, for example, every other frame would show an image of the rotor rotated 180 degrees, and you would see ten ghostly blades where there are really only five. At 25fps you would see five, non-coincident, copies of the rotor, or 25 blades ! It's the time per apparent repeat of the rotor position (1/6 s, i.e. 5/30 s, or 4/30 s, or 2/30 s, etc.) that needs to be a multiple of the time per frame (1/30 s), which is the inverse of the rate.
It's possible to think about things too much sometimes. 
