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B Would a smaller fan blade be more efficient?

  1. Apr 15, 2016 #1
    Would a fan with half the blade length be more efficient? My reasoning is that there would be less mass to spin, also a shorter blade length would make the lever arm smaller thus reducing more of the torque the air would create on the blade.
     
  2. jcsd
  3. Apr 15, 2016 #2

    jbriggs444

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    How much energy does it take to keep a mass spinning once you have started it spinning?

    If you spin a blade that is twice as long at half the rotation rate, how does the required power change? What change in the rate of air flow would you expect?
     
  4. Apr 15, 2016 #3
    I think there probably would be no efficiency due to the rotational inertia, but I'm not sure since the lever arm is shorter in this case. Any fan of any size can be considered for this question.
     
  5. Apr 15, 2016 #4

    jbriggs444

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    Consider a fan blade that consists of a small vane at the end of a long arm. Ignore the mass and wind resistance of the arm.

    If you double the length of the arm and spin it at half the rotation rate, what happens to the speed of the vane?
     
  6. Apr 16, 2016 #5

    CWatters

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    Fan blades are essentially wings. The tip of a wing is usually less efficient than a bit in the middle of the wing. If you reduce the length of the wing does the tip represent more or less of the wing as a percentage?
     
  7. Apr 16, 2016 #6

    jbriggs444

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    That question is unclear. The obvious answer is no. If you scale down the wing in all dimensions, the "tip" retains the same percentage of the total area.
     
  8. Apr 16, 2016 #7

    sophiecentaur

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    That's only if you ignore the hub / bearing / shaft.
    I believe that the high airspeed of a large diameter blade can lead to inefficiency(?)
    Perhaps the conditions implied in the OP not are not precise enough for a proper answer.
     
  9. Apr 16, 2016 #8

    jbriggs444

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    Agreed. The vision I had was that with a larger diameter blade, one would scale down the rotation rate, thus having the same tip speed, flow speed and the same angle of attack, but it seems clear that other folks are making different assumptions.
     
  10. Apr 16, 2016 #9

    sophiecentaur

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    Unfortunately, this (Original Post) is that it contains a misapprehension. "Torque" is not particularly relevant because the fan could be driven with a suitable motor to produce that torque. What counts is Power, which is torque X rotational speed. Both the speed of the fan and the torque are relevant and a small fan would (optimally) have different shaped blades and rotate at a different speed.
    In most cases, there is a certain maximum allowable area for a fan to occupy, to provide a given volume flow of air. It's normal to use as wide a fan as possible, rather than using multiple small fans. That sort of implies that a big fan would usually beat a small one (loosely based on perceived practical evidence)

    As an example where size is not too much of a criterion, the 'best' fans for circulating air around a room tend to be large radius ceiling fans, rather than fast revving desk top fans. Also, the Chinook seems to be the only helicopter design which uses two separate rotors. All the others - even the heavy lifting jobs- seem to have just one (large) rotor. Again, this is only anecdotal evidence.
     
  11. Apr 16, 2016 #10
  12. Apr 16, 2016 #11

    CWatters

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    Agreed but the OP asked about the length of the blades and my answer addressed that.
     
  13. Nov 26, 2018 #12

    A.T.

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    No, it would be less efficient in terms of transferring momentum to the air for a given power. That's why glider planes have long wings, and human powered airplanes have long propeller blades. See also most wind turbine blades.

    That would be only relevant if you are switching the fan on and off all the time.

    But you also move less air with the shorter blade.

    You have to define the efficiency you are interested in quantitatively, instead of considering isolated quantities.
     
    Last edited: Nov 26, 2018
  14. Nov 26, 2018 #13

    sophiecentaur

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    And short blades need to move faster, creating more noise. The best fans for shifting air in a room are the large overhead ceiling fans. Their only disadvantage is that they are obtrusive and can injure your head if you stand on a chair.
     
    Last edited: Nov 26, 2018
  15. Nov 26, 2018 #14
  16. Nov 26, 2018 #15

    CWatters

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    Old thread.
     
  17. Nov 26, 2018 #16

    sophiecentaur

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    That could be said of a vast number of PF threads.
     
  18. Nov 26, 2018 #17

    DaveC426913

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    Maaaaan!

    Nuthin'
    like poring over 14 posts with interest, forming ideas and responses in one's head - only to find out at the last moment that it's years old...


    Who knows what's changed... CWatters and sophiecentaur might be so old and decrepit by now that they have to type using a stick in their mouths...
     
  19. Nov 26, 2018 #18
    The thread was brought back by a person who clearly does not understand that efficiency and power required are two different things. It seems to me that since PF threads are available for a long time, and we do not want to spread false information, then the post should either get response(s) or be deleted.
     
  20. Nov 26, 2018 #19

    sophiecentaur

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    Shame. I used to be one of your greatest fans, Dave.

    Edit: Necroposting is not, in itself a bad thing but it's true that OP's have a habit of dropping out. I can't see that being a serious problem unless the thread has been left with a nonsense line to follow. Perhaps a stronger notification about the age of a thread could be given - there could be an age rating / warning, writ large and in red. Alternatively, the text in ageing threads could gradually fade (as in Back to the Future) to a pale sepia until they become impossible to read.
     
  21. Nov 26, 2018 #20

    gneill

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    Without a specific definition of "efficiency" the discussion is bound to be fraught with ambiguities and misunderstandings. I note that the OP is still with us (logged in today in fact) but hasn't yet decided to weigh in. Perhaps they will grace us with the definition they had in mind at some point.
     
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