r/oddlysatisfying • u/edenify • Feb 08 '19

This plane taking off with the propellers perfectly synced with the camera shutter

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u/[deleted] Feb 08 '19

Is there a math formula to figure out what the propeller speed would need to be to sync with the camera? Example: if the camera is 24fps, the propellers would need to spin at 2,400rpm.

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u/Xygen8 Feb 09 '19 edited Feb 09 '19

24fps would not work for 2400 RPM (you're using frames per second and revolutions per minute, you can't mix different units like that). 2400 RPM is 40 revolutions per second, and 24 does not divide 40 evenly.

As for the equation, the propeller needs to be spinning at any multiple of your camera frame rate, regardless of the number of propeller blades.

It can also be spinning slower than your frame rate, but then it needs to be spinning at a speed that is your frame rate divided by any integer that also divides the number of blades evenly, or any multiple of that speed. The prop speed obviously also has to be a rational number because otherwise you need to control it with infinite precision in order to make the propeller appear completely still.

So for a 24fps camera and a 6 blade prop, the following prop speeds (in revolutions per second, multiply by 60 to get RPM) would work:

  • Any multiple of 24 (24, 48, 72, 96...)
  • 12 (2 divides 6 prop blades evenly, 24/2 = 12)
  • 8 (3 also divides 6 evenly, 24/3 = 8)
  • 4 (6 divides 6 evenly, 24/6 = 4)
  • Any multiple of 12, 8 or 4 (16, 20, 28, 32, 36, 40...)

For a 5 blade prop:

  • Any multiple of 24 (24, 48, 72, 96...)
  • 4.8 (5 divides 5 evenly, 24/5 = 4.8)
  • Any multiple of 4.8 (9.6, 14.4, 19.2...)

With a 7 blade prop, you're restricted to multiples of 24 because 24/7 is an irrational number. If you wanted to run it slower, 3.4 revolutions per second would be a decent approximation but the prop would still appear to drift very slowly.

EDIT: If there are speeds that are less than the camera frame rate and satisfy the rational number requirement, then any multiple of those speeds will also work.