Helicopter Propeller - is it an optical illusion?

Question

When a helicopter propeller spins clockwise very fast, after a while it appears to be turning anti-clockwise very slowly. Why is that?

Answer 1

Have people never seen a real helicopter close by.... of course the rotors appear to go backwards... It is an optical illusion but it is exactly the same as a spinning bike wheel. When it reaches a certain speed the eye See's a part of it slow down and then it appear to go slowly backwards...... until it's just a blur again....

I will go look for what the term for this is called and will be back with an edit shortly ;o)

OK Back. I found this....
Imagine a clock with just a minute hand. If we were to look at the clock every 59 minutes it will appear to have gone backwards by one minute each time and if we look every sixty minutes it will appear not to have moved at all! So to accurately observe the minute hand in motion we must look at the clock more frequently than every sixty minutes, in fact we must look at it every thirty minutes. This is known as Nyquist's theorem.

Graham

Answer 2

Why do people only think this effect happens on film?

It happens to the naked eye too. If you disbelieve me then find a bike, turn it upside-down and pedal it fast... let go and watch the wheel as it starts to slow down... the same thing occurs.

The reason is because the brain picks up messages from the eyes and can only go so fast. The brain will also try to concentrate on one point and when the reaction speed of the eye/brain meets the revolution of whatever is going round, bike wheel or rotors, it will seem like it stops, then reverses....

It is obviously an optical illusion..... something that appears to do something while it is actually doing something else is an illusion.

Good question by the way.

Charlie boy

Answer 3

Persistence of vision is correct, though someone gave the first guy a thumbs' down.

"Persistence of vision" is the theory that explains the effect described in the question.

The theory describes a situation where a visual image is held by the brain/retina for a brief moment. Then this image is released, and an updated image is "seen".

That means that what you visually see is similar to how a movie projector shows a series of pictures, except at a much greater speed.

The delay may be caused by the brain interpreting the image.

The previous image is overlapped by the next image, which may include the next blade in a position slightly behind where the previous blade was. This causes the appearance of backspin.

If this theory is incorrect, it's hard to see how anyone could have a "flicker fusion threshold". (defined as the frequency at which an intermittent light stimulus appears to be completely steady to the observer.. like a flourescent light fixture )

Answer 4

In a way, it is an optical illusion. What happens is that the way/speed the eye, optic nerve and the brain recieve information as a system and is translated as an image and the speed of the rotor are meeting and passing a threshold of information load/overload, depending on several factors. If you watch the rotor speed up beyond its first apparent reversal of direction, it will reverse itself again. Finally it will all but disappear when the speed approaches a threshold of where your visual system cannot keep up at all. Animals have different visual "speeds" which is why some cats respond to images on the TV while others don't. A normal cat's system is too fast and it can't see a complete image so it makes no sense to them and they ignore it while a cat that has an unusually slow visual system can, and responds to the same images that you and I see.

Answer 5

If you're talking about a helicopter you've seen in a movie or on television it is due to the image being composed of a number of frames of still pictures. The rate of the rotor spinning is such that it completes a near revolution in the time between still images. In comparing the images, it appears the rotor has gone backwards slightly since its leading blade is a little farther behind it's pictured position in the previous frame. Once the film is shown up to speed, this progression of slightly farther back rotor positions gives the illusion of backward movement. It is a common illusion also very noticeable with wagon wheel spokes in westerns.

Answer 6

this is has been explained rather well by some of the responses. i would like to reiterate that this alot do with human's ability to visualise fast moving objects. the phenomenon can be likened to the stroboscopic effect, and what we see on the bicycle wheel or car wheel or helicopter propeller are real life phenomenons NOT RESTRICTED TO HOLLYWOOD.

the effect occurs when the observer cannot process changing images at least as fast as the image is changing. when this effect is seen in movies, it is due to the camera not being able to snap images faster than the rate at which a helicopter propeller blade passes a certain point.

the only reason we can witness this effect on TV is because the camera caused it, and that we can see faster than the 30 frames per second that the film plays back at.

Answer 7

This is a stroboscopic effect. If the following blade gets into the same place as the previous one in the time between frames on the film or the time between pulses of light from a stroboscope the blades will appear stationery.
If the time difference is not quite perfect the blades will appear to rotate slowly forwards or backward.
This can be a problem with machinery under fluorescent lights. It may seem stationery until you put your hand on it!
RoyS

Answer 8

Ok - I can understand the strobe effect on a film, the same thing happens under an electric light, especially a fluorescent tube. However, would someone care to explain how it also can be seen in broad daylight, outdoors in the sunlight, with no inkling of an artificial light source anywhere ?

Answer 9

Only on TV or Film. If you're watching with the MKI eyeball then it's just a blur.

On TV you'll see 50 frames each second (in the UK) - so if in 1/50 of a second the blade has moved enough so that it's now behind it's original location - it looks like it's moving backwards.

Answer 10

Persistance of vision is the best way to describe the answer to this.
'Dr Greg' & 'Deadguy' got it just about right.

The 'TV/Film' thing does exagerate it quite a bit due to the fact that the camera's 'persistance of vision' is less then our own.

As to the 'strobing outside' thing.... strobing occurs when a bright light is flickering at a particular resonance.... and there is no brighter light nearby than Sol - our own pet star! The amount of variance or movement in the stobed image depends on many variables... intenisty of light, proximity of light source, depth of feild of vision of observer, eye accuity of observer, speed and frequancy of object breaking the light's contact with observer's retina.....