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I have some random drone propellers which I don't have any information on. I looking to find a way to measure the pitch of the propellers to figure out if they will be well suited to a 5-inch drone I have. Is there any way to go about measuring prop pitch? (at least somewhat accurately)

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Theoretically the pitch of a prop is supposed to be the distance it would move in a full rotation (in an ideal world).

As most modern props vary pitch along their length, a single number is almost impossible to measure. Manufacturers tend to give a number which is an indication of how the user would expect the prop to perform.

Most propellers will work on any 5" drone, they usually have the pitch printed on the upper surface of the blade near the leading edge.

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  • $\begingroup$ The angle varies over the length of the blade, but the pitch should be more or less constant because the tips will be moving faster. $\endgroup$
    – Robin Bennett
    May 18, 2020 at 9:52
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There is a method for measuring on this page (Wayback Machine link); but, it might be a bit fiddly to get accurate measurements for small propellers as your margins of error become more significant.

His site copyright notice prohibits quoting material, so I will summarise:

  • Measure 75% along one blade, from the centre of the hub to the blade tip
  • At this point, measure the width of the blade as it appears from the top-down. Note, this is NOT the chord - your ruler should be parallel to the workbench, not the blade.
  • measure how far the leading edge and the trailing edge of the blade are from the bench. Subtract trailing edge from leading edge to give the difference between the two.

You can now calculate pitch using: $$ propeller\_pitch = \frac{edge\_height\_difference}{width} * 2.36 $$ 2.36 is 75% of pi ($\pi$). The diameter of the propeller is $2 * \pi * r$; one blade is therefore $\pi * r$ so 75% of one blade is $0.75 * \pi = 2.36$ (-ish. Near enough for practical purposes!)

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    $\begingroup$ Where does 2.36 come from? $\endgroup$
    – Robin Bennett
    May 18, 2020 at 9:37
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    $\begingroup$ More info on 2.36 added. $\endgroup$
    – Kralc
    May 18, 2020 at 9:43
  • $\begingroup$ That's a nice simplification over measuring the angle and using trig! $\endgroup$
    – Robin Bennett
    May 18, 2020 at 9:51
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    $\begingroup$ @Jacob The above reference lead me to this PhD thesis [Design and Analysis of Propeller Blade Geometry using the PDE Method ](etheses.whiterose.ac.uk/4168/1/uk_bl_ethos_569278.pdf) - University of :Leeds, 215 pages, 1993. $\endgroup$
    – Russell McMahon
    May 18, 2020 at 12:07
  • $\begingroup$ I don't agree with this. To wit, pitch = 2*pirtan(theta), where tan(theta) = y/x (which in this case is edge_height_difference and width, respectively). It's true that blade angles change with r, but that doesn't change the equation. For a given r, you have a given theta, and so the equation is Pitch = 2*pirtan(theta), without any modification. The 75% span is a common value amongst manufacturers because it's more representative. So to calculate pitch from 75% blade span, we have simply Pitch = 2*pi*r_75*tan(theta_75) $\endgroup$
    – Kenn Sebesta
    May 18, 2020 at 18:27

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