What Causes the Propeller Illusion on Video During Flight?

[virtualetters]

During a flight today, I took a video of the propellers spinning and noticed this strange result:


I was using a Samsung Vice cell phone's video camera.

Now, it makes sense that the frame rate of the camera as compared to the rotational velocity could make for some weird effects, namely the slowing. The propellers were impossible to distinguish by eye; just look at the blur left by fan blades to get an idea.

But what causes the outrageous appearance of bending? Is it the pressure gradient across the propeller blades, which results in a density gradient of the air and hence bends the light.

My other guess is that the propeller was actually spinning in the opposite direction as it appears in the video. In this case, the parts furthest from the center are moving much faster than the parts of the propeller inside. I feel like this might, in combination with the finite (but short) shutter speed of the camera, possibly explain the bending. But I'm not sure how to complete this thought.

 

[gtoguy97]

It's possible that the bending is due to a combination of both factors. The pressure gradient resulting in a light-bending effect, as well as the slower shutter speed of the camera capturing the motion of the propellers, could both be contributing to the strange appearance.

 

[sir-pinski]

Thank you for sharing your experience with the propeller illusion during your flight. It is certainly an interesting phenomenon to observe and try to understand. As you mentioned, the frame rate of the camera and the rotational velocity of the propellers can create the illusion of slowing or even stopping. This is known as the stroboscopic effect, where the frequency of the camera's frame rate and the frequency of the propeller's rotation match up, causing the propellers to appear to be stationary.

As for the bending appearance, there are a few possible explanations. One is the pressure gradient across the propeller blades, which can create differences in air density and cause light to bend as it passes through. Another possibility is the rolling shutter effect, where the camera captures different parts of the propeller at slightly different times, resulting in a distorted image. Your idea of the propeller spinning in the opposite direction is also a valid explanation, as the outer parts of the propeller would be moving faster than the inner parts, creating a bending effect.

Overall, the propeller illusion is a fascinating example of how our eyes and cameras can interpret motion differently. It also highlights the importance of understanding the technical aspects of photography and how they can impact the final image. Thank you for sharing your thoughts and observations on this topic.