This is the moment a helicopter ‘magically’ starts to hover above the pad despite its blades remaining completely still. The video has been baffling viewers who can’t work out how the helicopter is able to operate without its motor. But things aren’t as they seem.
The strange phenomenon boils down to the camera’s shutter speed and frame rate, which can distort the appearance of spinning objects when synchronized. The video was published today by YouTube user Chris Chris, who wrote that “the helicopter seems to be hovering magically.” In the footage, the blades at some points can be seen shifting slightly – but, not nearly as much as one would expect for an aircraft that’s taking off.
As a video by HowStuffWorks explains, shutter speed is the time the camera spends gathering light every time it takes a picture. The more time spent gathering light, the more motion-blur will be apparent in an image. The video states: “Imagine you’re shooting a 24fps video of a helicopter rotor that spins one full rotation every second.”
“In the video, each rotation will thus be broken into 24 frames. You’ll see the blades rotating normally, just moving one-24th of their full rotation each time.”
“But, if the blades spin exactly 24 times each second, and you’re still shooting at 24 fps, each full revolution will be represented by only one frame. The blades will arrive back in their starting place each time the camera captures a frame, so they’ll look like they’re standing still.”
Frame rate can also cause blades to appear as though they’re spinning backwards, or even make it seem as though they’re bending in an S-shape.
So there you have it (it’s actually moving, you just can’t see it!).
This phenomenon was first observed during World War II when photographic and video editors noticed that pictures and films of fighter planes and bombers showed the rotors at a standstill or un-moving. In some films the blades seem to be spinning backwards.