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i am a beginner and am working on a fan animation for a small project but it keeps turning from anticlockwse to clockwise and then back to anticlockwise. i have set the fan to rotate on the z axis from 0 to 72000 degrees and interpolation mode is quartic and easing mode is set to ease out. this happens from frame 54 and can be seen if you look at each keyframe carefully. it would be a great help to know why this is happening and would love to know how to resolve it. Thank you

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    $\begingroup$ Could it be the Wagon Wheel Effect $\endgroup$
    – batFINGER
    Jun 3 at 15:23
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    $\begingroup$ @batFINGER that’s what I thought, though for a moment there I was mentally pronouncing wagon with a German accent, thinking it was a name; trying to be too big for my britches, lol $\endgroup$
    – TheLabCat
    Jun 3 at 15:27
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I downloaded your .blend and opened it up. When scrubbing through the animation, the rotation value on the left kept only going up.

EXPLANATION

What you are experiencing is something called the Wagon Wheel Effect.

To summarize, this is when it appears that something switches direction when it actually doesn't. If you have ever driven on a highway, you have likely seen how it appears that the other car's wheels are spinning in reverse.

Something similar is happening with your blender file. The animation speeds up enough that each frame it is turning somewhere between 180 and 360 degrees. This means that it appears that it is spinning in reverse.

Now that you are aware that it is not actually changing direction, you could be satisfied.

If not, keep reading.

FIXING IT

To change how it appears, you need to change the interpolation type.

To do this, hover over your timeline and press Shift+F12. This will switch your window type to the dope sheet editor.

Now, to change the interpolation mode, press A to select all, Numpad . to zoom to selection, and T to change interpolation mode.

Now, press linear on the dropdown that appears. This will make it so the fan spins at a constant speed the entire time.

If this is not satisfactory, experiment with the other interpolation modes until you get what you want.

Finally, if this was helpful, please upvote, and if it answered your question, please mark as accepted answer.

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  • $\begingroup$ If going linear what can also help is using a prime number. The value 72000 and the common frame rate of 24 have many common factors. As explained in wiki article it depends also on symmetry of spinning object. An 18 spoke wagon wheel will appear stationary if each frame rotates an exact multiple of 20 degrees, overlaying the perfectly onto previous, next etc. A little more or less will give the illusion of forwards backwards. $\endgroup$
    – batFINGER
    Jun 3 at 17:26
  • $\begingroup$ @batFINGER I hadn't thought of that. Good idea. Thanks for your suggestion! $\endgroup$
    – REMCodes
    Jun 3 at 17:52
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    $\begingroup$ Via drivers blender.stackexchange.com/a/139145/15543 $\endgroup$
    – batFINGER
    Jun 3 at 17:56
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    $\begingroup$ What I would do is make a keyframe at the point that you want the fan to get to full speed, and one at the beginning. Then you can make the interpolation between the first 2 keyframes a curve and then the rest to be linear. Do the same at the end if you want it to stop. $\endgroup$
    – REMCodes
    Jun 4 at 16:17
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    $\begingroup$ A touch of motion-blur might help. Or, at least, it would look like it does in the westerns. :) $\endgroup$ Jun 4 at 16:46
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i found that first making the interpolation mode of graph quartic and easing mode as ease in and then after a few frames after the fan speed considerably fast and starts having the 'wagon wheel effect' inserting a key frame at that position(60th frame in my case) and then making the graph from the 60th to the end as linear gives me almost the desired result. hope it was helpful.

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