Why do blades appear curved in videos?

[Gladius]

I did a search through this forum, but didn't turn up anything.

When you watch videos of RC helis in flight, the blades always appear curved. Can anyone explain to me why that happens? I know that they appear relatively stationary due to the relationship between the framerate and headspeed, and that they appear to spin slowly forward or backward when the framerate is not an exact factor of the headspeed. I can guess that the curved effect is related to this, but haven't figured out how.

Thanks.

[Goldenhour]

Wiki CMOS artifacts. That answers the curved question, why they appear to move backwards I'd like to know too.

[toti3d]

it only happens on camera with cmos sensor.

you can google "rolling shutter"

or search youtube.

Why rolling shutter effect is on CMOS (0 min 21 sec)

Rolling shutter versus Global shutter (0 min 10 sec)

[Gladius]

Thanks for the answers guys. The Wikipedia article on rolling shutter even had a picture of a helicopter with curved blades!

Originally Posted by Goldenhour

why they appear to move backwards I'd like to know too.

I'll give you a very oversimplified example. Let's say that you have a video camera that shoots 24 frames per second. Imagine that your head speed is 72 rpm. Since 72 is exactly 3 times 24, the head of your heli would rotate exactly 3 times between each image that your camera takes. The blades would appear to be in exactly the same place in each image, and your blades would appear to be stationary. If your headspeed was 73 rpm, they would rotate slightly faster, and in each successive image, the blades would be slightly ahead of where they were in the previous image, and you would have the appearance of a very slow forward rotation. Similarly, if your headspeed was 71 rpm, each blade would not quite complete 3 rotations, and so each blade would be slightly behind where it was in the previous image, and the motion would appear to be moving backwards.

[StatiC]

What I don't understand is why there aren't more CCD cameras being pushed for RC AP use. I am thinking about getting a cheap 3 year old CCD camera on ebay for hobby use instead of dealing with trying to get out as much of the wavy video effect out of a CMOS video camera(like go pro, rclogger, keychain camera, etc) or any number of other video cameras that are pushed for hobbyist AP use.

[markxp51]

I believe it is called digital picture aliasing.

[Dorkmaster33]

I always wondered why they looked curved. Thanks guys!

[nickyd]

Originally Posted by xStatiCa

What I don't understand is why there aren't more CCD cameras being pushed for RC AP use. I am thinking about getting a cheap 3 year old CCD camera on ebay for hobby use instead of dealing with trying to get out as much of the wavy video effect out of a CMOS video camera(like go pro, rclogger, keychain camera, etc) or any number of other video cameras that are pushed for hobbyist AP use.

I got myself a really cheap used Panasonic LX3 for this reason. I do find that when I have vibes then I get a blur, rather than cmos jello.