"Seeing" moving fan blades

I’ve been looking at my floor-standing fan.

Obviously, the moving fan blades are a blur, but I’ve been noticing how they change in appearance according to the fan speed setting.

On the low speed setting, the blur is completely uniform and unchanging in the blade area circle. It’s all just a blur, and I cannot discern any movement or direction of rotation.

On the medium speed setting, I can clearly see rotational movement, with lighter and darker areas of blur going clockwise very fast - too fast to count the revolutions.

On the high speed setting, the fan blades look almost like the blurry petals of a flower, rotating at a relatively slow one or two revolutions per second.

What is this phenomenon?

Most likely, it’s a stroboscopic effect caused by a flashing light source such as fluorescent lights or LED lights.

Agreed. You almost certainly won’t see the effect from a natural light source.

Try watching a TV show through the fan blades for a real trippy effect.

Won’t you need a CRT TV for that?

While I hesitate because visual perception is a complicated subject, the human eye itself suffers from the wagon wheel effect at a rate that is about 13Hz

You don’t need a camera or a flashing light source to see this as described in this paper.

Yes you can see “flicker” at faster refresh rates but it seems that for searching for movement 10-15Hz is what our mind does with what it receives from the eye.

Let it autoplay or move slider.


Here’s a good video of the wagon-wheel effect.

There’s no strobe light going on, just light bulbs.

Yeah, the effect you describe isn’t happening with an LCD screen.

It’s interesting seeing the effect in your link and the video. It says the animation in the link is being rendered at 20 frames per second, and the camera in the video is recording at 24 frames per second.

Yet, apparently:

So, if one was actually there when the video linked to above was filmed, would we see the spinning wheel differently than what the camera was able to capture and play back?

I don’t know the specifications of my fan model, but here are some rpm figures for a not dissimilar fan I found on the internet:

Low speed: 975rpm (16.25 revolutions per second)
Medium speed: 1156rpm (19.26 revolutions per second)
High speed: 1311rpm (21.85 revolutions per second)

If they’re powered by alternating current, LEDs, fluorescent, and to a lesser extent, incandescent lights flicker.

Here’s an even better one.

Wow, that’s pretty amazing. Here’s a similar video with better picture quality.

In your video and mine, the rotor speed has to be very tightly controlled to be in lock-step with the camera’s frame rate like that. IANA rotorhead, but I’ll wager the speed is on automatic closed-loop control, with the computer calling for more/less fuel to the turbine as needed to keep the rotor RPM within an extremely tight range regardless of the amount of collective or cycling the pilot is commanding.

The viewer comments under the video you linked to are priceless. :smiley:

Nearly all helicopter rotors are of the constant-speed variety, with the allowable variation within only a few percent of the nominal speed.

But “very tight” control isn’t hard for a couple of reasons. The rotor’s enormous inertia tends to damp out any small or slow changes in drag and engine torque. Also, turbine engines respond quite slowly to throttle inputs even when they’re not attached to helicopter rotors. Both these things tend to keep the rotor turning at a very steady speed. (The helicopter in the linked video is indeed turbine-powered).

As an aside, a helicopter-pilot friend of mine says that many people prefer to learn to fly on piston-engine helicopters. Because piston engines respond much more quickly to throttle inputs than turbine engines do, beginners can add power to get out of sticky situations much sooner than they could in a turbine-engine helicopter).