Defining propeller pitch

Discussion in 'Props' started by gonzo, Feb 12, 2022.

  1. gonzo
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    gonzo Senior Member

    There have been some discussions about propeller pitch. Like the onset of planing it has created a fair amount of polemic. However, as planing, it appears to have different definitions. I have found that it is often used interchangeably with camber. Also, that there is confusion between nominal pitch (what is stamped in the propeller), and local pitch along the blade as it twists. How do you define pitch?
     
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  2. bajansailor
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    bajansailor Marine Surveyor

    I suppose in it's most basic form it is the distance travelled forwards in one complete revolution of the propeller, assuming that there is no 'slip' at all.
     
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  3. TANSL
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    TANSL Senior Member

    I and Wikipedia agree with you. Nothing to do with camber.
    upload_2022-2-12_18-18-10.png
     
  4. hoytedow
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    hoytedow Carbon Based Life Form

    Good question. I agree with the answers given by bajansailor and TANSL.
     
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  5. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Indeed. Same used here. I first encountered the concept in the early 90s, after I'd given up on classic sailboats, and become obsessed with aircraft. Something that was readily apparent from the description of the progressive twist down the length of the blade, and as the cord length to blade thickness ratio gets lower and lower, there gets a point where the blade has rotated enough that the body force on a given segment is rotated past the disk of the prop. The early ww2 props, for example, had tiny hubs. The builders just couldn't see clearly enough to realize that the inner sections of the blades only made drag.
     
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  6. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    s-l400.jpg

    republic-xp-72-no-1-roll-out.jpg
    See? I had a discussion a while back on a different thread, about improving efficiency by making a larger hub. Up to a point.
     
  7. Barry
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    Barry Senior Member



    According to more than one explanation, the pitch is taken at 2/3 of the radius of the blade. A prop blade has a varying "pitch" from hub to tip ( the purpose of the increasing pitch toward the root is to reduce drag)

    So Wiki does not appear to have it correct in that it is not the outermost tip "pitch" that determines what gets stamped into the prop
     
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  8. DogCavalry
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, unless it's a varied pitch prop, it's the same pitch from root to tip. The local angle of the blade is different at every point, from root to tip, otherwise it would be a different pitch at every distance. The angle to make a "square" prop, say a 24/24, is very different at 12" from the axis than at 24" from the axis, or at 2" from the axis
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Nor is it anything to do with a helix:

    upload_2022-2-13_15-55-48.png

    As any helix plot will demsotrate:

    upload_2022-2-13_15-59-34.png

    However....

    With reference to the question, propeller pitch, one can use several ways to define the pitch, all depending upon the objective:
    1. Nose-tail pitch,
    2. face pitch,
    3. effective or 'no-lift' [pitch,
    4. hydrodynamic pitch.

    All shown pictorially below:
    upload_2022-2-13_15-58-41.png
     
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  10. jehardiman
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    jehardiman Senior Member

    Actually, most standard marine propellers have decreasing pitch as you get closer to the hub, that is why pitch at 0.7 r is typically defined as the pitch of the wheel. This has to do with real world hub length and blade strength issues. 0.7 r is effectively the center of the working pressure face. It is rare to have a wheel with the same pitch from root to tip, especially if you have a full on wake adapted wheel.
    For all the wheels I have designed (using blade element methods) I have defined the "pitch" at that radius as the geometric angle that the foil generation baseline makes with the axis of advance (i.e. the co-angle of the helix angle of the baseline). I then adjust the "pitch" to get my desired inflow alpha for my wake adapted Va and cord to get my elliptical distribution over the blade. Therefore the "pitch" varies radially continuously on the blade as needed.
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    That's correct, generally referred to as: typical pitch or nominal pitch.

    The first few prop's i designed, I did it that way.
    Now, I just make sure I can get the correct gearbox ratio and largest diameter without causing cavitation, and simply select accordingly... i no longer delve into those nuances.
    I still get the desired result :D
     
  12. sandhammaren05
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    sandhammaren05 Senior Member

    Here are the only correct definitions of pitch and camber:

    In thin wing theory camber is specified on the mean camber surface, which generates all of the lift and induced drag; the symmetric thickness adds strength and creates thickness and creates form drag, which dominates induced drag in surface piercing.

    Start with a helicoidal blade. The pitch is constant over the entire blade, that's the property of the helicoid. When camber is added then pitch is measured from trailing to leading edge with the big gauge foot at a fixed radius. Pitch measured with a small foot will now vary along a radial arc. That is what prop shops mean by progressive pitch. This is all measured on the high pressure blade side. One could ask if it is better, following thin wing theory, to specify pitch and camber along the mean camber surface. We have tried that with a CNC prop, and for surface piercing there are reasons why it's better to specify everything on the high pressure blade side and then add thickness, ignoring the mean camber surface.
     
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  13. gonzo
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    gonzo Senior Member

    The tangents of the camber are not a measurement of pitch. Prop shops stamp a Diameter x Pitch on the hub.
     
  14. bajansailor
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    bajansailor Marine Surveyor

    To summarize, if a propeller has a stated pitch of say 20 inches, then it will travel 20" forward with one complete revolution, assuming no slip whatsoever?
    I don't think that the average person is interested in getting into more detail than this, when they ask what the (for example) 30" x 20" stamped on the hub means.
     
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  15. gonzo
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    gonzo Senior Member

    @bajansailor: that is the nominal pitch.
    @Ad Hoc: I agree that there are several ways of measuring the pitch, and there is no confusion as long as you state which is being used.
     
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