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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.
     
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  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.
     
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