The wheels on the plane goes round and round

S

sxauer

New Member
I have been trying to find out the proper term, why, and hows of somthing, but cant find any info on it...perhaps someone in here can shed some light on it....
My question is this: Why do things that spin really fast tend to appear to start spinning backwards as they spin up? For example, watching a prop on an airplane, if you look at it for a while, it kinda looks like it starts spinning the other way. Has anybody else noticed this, or am I just weird or somthing? Btw, this is in real life, not on tv or video or somthing....anybody got any insight into this?
 
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For example, watching a prop on an airplane, if you look at it for a while, it kinda looks like it starts spinning the other way. Has anybody else noticed this, or am I just weird or somthing?

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I know what you mean. Perhaps, it is just our eyes playing tricks on us as the propeller starts to rotate very fast. Otherwise, I don't know.
 
cool, so i'm not alone. Anybody know the term? Btw, I think I saw a post in the squawkbox that someone had to go to the eye doctor tomorrow/next day? If so, if you could ask for us you would be the man....or the woman
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well...lets just say your coolness rating would go up or somthing.
 
It is to do with how quickly your brain can process pictures. Much like Camera's only take 24 images per second, your brain only does so many too, which causes effects like this.
 
I think it has to do with the focal point of your eyes. Basically along the lines of what Iain posted. I believe it also depends on what sort of motion the vehicle or object is in. I have noticed that if I am right abeam a car, I don't get that effect.
 
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It is to do with how quickly your brain can process pictures. Much like Camera's only take 24 images per second, your brain only does so many too, which causes effects like this.

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"I'm not a doctor but I play one on the flight line."


(All good fun, Iain.
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A term for the phenomenon is aliasing. In the signal processing field, if you sample a signal at less than twice the frequency of the signal, you will get anomalous results. Imagine looking at a normal sine wave. If you only sample (see) twice per cycle, you'll just see the top and bottom of each wave - just enough to reconstruct the signal. Any less and you can't figure out what made it.

If you are looking at a spinning propeller under lights that emit light at a certain frequency (like 60 Hz, the normal frequency of AC in the USA) and the propeller is spinning at an even fraction of 60 Hz (3600 rpm, 1800 rpm, 900 rpm), the propeller will appear to stand still.

I don't know what would cause this during the day, as the light from the sun is pretty much DC.
 
Yeah I believe that your brain processes a picture from your eye ever 1/10 of a second.

Thats why if youre watching a video or playing a video game dont notice much "jerkyness" at around 10 frames per minute.

I think I learned this in physyology or something.

When the prop gets going fast enough your brain is taking a pic when the prop is a little behind where it was last which would make it look like its going backwards.

You would have a similar (reverse) affect when increasing the speed of a strobe light while pointing it at a spinning fan.

Either I just remembered alot from physyology or I just made all this stuff up.
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It kind of makes sense though.

Regards

Tom
 
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yes, but doesn't light have a certain "frequency"?

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Yes, but it is orders of magnitude (way lots) higher than spinning props. Your eyes (and brain) perceive light as steady, unless it is modulated by something else (like a strobe light).
 
I think that I understand it now...still doesnt take away the fun of playing with the strobes in the clouds at night
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Actually humans are capable of noticing up to 60Hz as a 'flicker' but any two pictures that have less than approximately 1/10th of a second between them are construed by the brain as a moving picture.
 
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