Cool rotor illusion

[Henkrijder]

Hi all,

 

check out this cool Mi-24 vid. Quote from original site.

 

'The propeller is really rotating. Russians fix a magnet on their helicopter blades. The device sends a signal to synchronize a movie camera, allowing to visualize efforts and deformations on the blades'

 

http://www.dumpert.nl/mediabase/933951/1fd48eea/coole_propeller_illusie.html

[sobek]

Hi all,

 

check out this cool Mi-24 vid. Quote from original site.

 

'The propeller is really rotating. Russians fix a magnet on their helicopter blades. The device sends a signal to synchronize a movie camera, allowing to visualize efforts and deformations on the blades'

 

http://www.dumpert.nl/mediabase/933951/1fd48eea/coole_propeller_illusie.html

 

Yeah, that already floated around the forum some time ago, although the explanation with the magnet is hokum IMHO.

[Feuerfalke]

Yeah, that already floated around the forum some time ago, although the explanation with the magnet is hokum IMHO.

 

+1

 

This explanation is ridiculous.

[miguez]

If not yet explained, I'd submit the idea that this is simply the effect of a governor on the helicopter doing a good job of maintaining the rotor RPM where it is. Then, couple that with the coincidence that the RPM happens to be a multiple of the camera refresh rate, so that every time the camera records a new frame, the rotor is in the same position.

 

Some people refer to it popularly as the "wagon-wheel effect".

 

In this case, we could have the camera recording at 60 frames per second, and the rotor RPM at 300 or 360. So for every frame recorded, each blade of the rotor would do five or six rotations, and be back to pretty much the same position by the time the camera records the next frame.

[Boberro]

This trick is funny ^^, sometimes I regret i can't set my FPS camera manually :D

[AlphaOneSix]

The normal rotor rpm on the Mi-24 is 240 rpm, or 4 revolutions per second.

[miguez]

Cool, so that would still work, 240 being a multiple of 60.