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Why most of the fans have three wings

  1. Sep 18, 2014 #1
    Why most of the fans have three wings, why not two, four, five and so on.
     
  2. jcsd
  3. Sep 18, 2014 #2

    Simon Bridge

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  4. Sep 18, 2014 #3
    The site says that they can have any number of blades depending upon certain factor.

    It doesn't explain why most commercial fans have three blades?
     
  5. Sep 18, 2014 #4

    Simon Bridge

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    That is correct - what are the factors that the number of blades depends on?

    The site is one example of many sites which talk about this - you learn best when you do the footwork yourself, go look for other places this question may have been discussed. hint: physics forums has a search function.
    You should start to get an idea of the decision making that goes into the design.

    ... are there any other factors which may influence the design of a commercial fan?

    Aside: how did you determine the number of blades that "most commercial fans" have?
    Ceiling fans usually have 4-5 but can go from 2 to 7 for example, my wee fan heater has a disk of about 20 blades in the blower and my desk fan has 5. These are all commercial fans.
     
  6. Sep 18, 2014 #5

    Doug Huffman

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    Aside from commercial design considerations, as the number of blades in an axial (compare ducted) fan increases so does their restriction of the flow rate. Gedanken; imagine an operating fan, change only the number of blades, in the limits the fan stops moving air axially.
     
  7. Sep 18, 2014 #6

    cjl

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    Not necessarily. The number of blades will be determined by the needs of the fan - fewer blades tend to be optimum when you want a high flow rate and efficiency, but do not need a large pressure change across the disk of the fan. If there is a large pressure change though, many more blades can be the optimum (look at the front fan on a turbofan jet engine for example).
     
  8. Sep 18, 2014 #7

    Simon Bridge

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    I'd like to hear from ajayguhan before we get too far into this.

    The bottom line is pretty prosaic - commercial fans tend to have three blades because those sell best.
    However - there are markets for fans with different numbers of blades. There are other reasons for considering making blades with few blades rather than lots, and why an odd number would be a better design than an even number, but the configuration that does not sell, no matter how great it is otherwise, would not get made.
    I'm hoping that ajayguhan will be able to identify the other constraints with a little effort.

    Note: As my guide for what "commercial fan" means, I am using the google image search on the term.
    This search produces mainly desk or floor standing fans and a few, small, portable, industrial designs.
    3-blades in a cage is the by far the dominant design with 18 examples in the first 28.
    4 blades was the next most common with 6.

    Note: no aircraft-engine fans.
     
  9. Sep 18, 2014 #8

    rcgldr

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    Doing a web search for household fans shows mostly 3 and 5 bladed fans (for fans with blades) at amazon. Also wiki.

    http://en.wikipedia.org/wiki/Mechanical_fan

    At our home, we a 12 inch 3 bladed fan with circular housing, and a 20 inch 5 bladed fan with a box housing. The compressor fan on our air conditioning unit has a 4 bladed fan.
     
    Last edited: Sep 18, 2014
  10. Sep 18, 2014 #9

    Simon Bridge

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    It can be fun checking assumptions - one of the things I wanted OP to do.
    You can check the blade shape and number against the obvious constraints - which I don't want to comment on yet.
    Hmm does the box housing all-but obscure the fan?
     
  11. Sep 18, 2014 #10
  12. Sep 19, 2014 #11

    rcgldr

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    No, just a thin plastic "grill" in an otherwise open front and back. Do a google search for "household box fan images", and you see a few similar to our 20 inch box fan. Ours is old (30+ years) and has square corners instead of rounded corners.
     
  13. Sep 19, 2014 #12

    AlephZero

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    You won't find 4-bladed fans made in China, because the Chinese think 4 is an unlucky number.

    Even in the west, 3 and 5 blades probably "look nicer" (i.e. less boringly symmetrical) than 4 when the fan is switched off.

    On the other hand, the fans on the front of most large jet engines have about 24 blades (plus or minus one or two) because that is the most efficient number for them to have, not because it makes them look pretty.
     
  14. Sep 19, 2014 #13

    Simon Bridge

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    @rcldr: Oh OK - I've been musing on how many-blade fans seem to be hidden.
    Hmmm... the google images for box fans most of them have 5 blades.


    Efficiency is much more important for a jet engine than for a desk or ceiling fan - but in both cases, the controlling restriction is "which makes us the most money". You can also get some good clues to design constraints by comparing with windmills.
     
    Last edited: Sep 19, 2014
  15. Sep 19, 2014 #14
    I had thought that the 3 blade fan( or even a multiple of 3 ) had a more even balance than a 2 or 4, especially when one considers the blade part is stamped from a sheet of metal, and thus the tolerance of the manufacturing process do not have to be of extra high precision.
     
  16. Sep 19, 2014 #15

    AlephZero

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    A 2-blade rotor has a different moment of inertia about different axes (along the blades and at right angles to them). That can create dynamic instability problems, but it's not likely to affect a low-speed room fan. It is a design issue for 2-blade aircraft propellers and helicopter rotors though.

    Four blades are no better or worse than and other number greater than 2, so far as rotor balancing is concerned.
     
  17. Sep 20, 2014 #16

    Simon Bridge

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    I understand that there are fewer resonances for an odd number of blades.
    The issue with low-speed room fans is the noise you get from the wobble.

    It can be surprising to a lot of people just how many every-day design concepts are not strongly constrained by engineering.
     
  18. Sep 30, 2014 #17

    OldEngr63

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    A three bladed fan has constant MMOI about any transverse axis. This is important in some high speed applications, particularly if the fan must yaw (such as a power generating windmill).
     
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