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Do the number of blades in a propeller matter?

  1. Aug 13, 2014 #1
    I was really wondering if the number of blades in a propeller influenced the power it has, are too many blades bad also?
    At last I will like to know how much force will you need to elevate a person 1 meter over the ground an mantain the person at that height?
     
  2. jcsd
  3. Aug 13, 2014 #2

    rcgldr

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    Using longer and fewer blades is normally more efficient than using shorter and more blades, but if the blades are too long, they will strike the ground, in which case shoter blades are needed, and more of them.

    The force needed is the person + aircraft weight. At 1 meter over the ground, the rotor would be operating in "ground effect", reducing the amount of power required to hover. The rotor would have to create an average pressure differential that when multiplied by the effective radius of the motor would equal the weight of the person + aircraft.

    If out of ground effect, impulse equation could be used, but determining the actual flow output by a propeller would be complicated. The impulse would be the weight of person + aircraft times one second. This would correspond to accelerating air downwards so that the average mass of the affected air per second would be accelerated to some average velocity v so that: weight (person + aircraft) x second = mass (affected air) x velocity(distance/second). Note that weight = mass x 9.80665 meters/second^2 if near sea level. Average velocity = ((mass vehicle) / (mass air)) x 9.80665 meters/second.

    You can find static thrust calculators, but they usually assume pitch is within a reasonable range and ignore. The thrust produced in a static situation (like hovering) isn't affected much by pitch if within a reasoanble range, but the power consumed is affected.

    For an actual example of an very efficient human powered "helicopter", here's a video. The vehicle uses four 2 bladed rotors:

    https://www.youtube.com/watch?v=syJq10EQkog
     
    Last edited: Aug 13, 2014
  4. Aug 13, 2014 #3
    Yes, I believe it has a big influence. Most propellers are chosen based on numerous variables and the most efficient propeller is normally anywhere from 2 to 4 blades. Power, rpm, pitch, air density, maximum noise, maximum diameter, intent are all relevant? If you want greatest power efficiency, this is likely different pitch than maximum thrust, and noise tolerance may make a big difference. In flying an ultralight, I have seen mostly 3 bladed propellers, but if you search "flying car and missionary," you will see that the 2 or 3 person Maverick uses a fairly large (car sized) power plant and a large diameter 6 bladed prop. I can guarantee a lot of thought went into the choice of this custom propeller. Just like a wind turbine, if the prop turns fast and has too many blades, the draft left by the previous blade can cause inefficiency or possibly stalling or "pulsations."
    For thrust needed at 3 feet - if you are trying to suspend a 400 pound load including power plant and pilot, you will want that much thrust. However, at only 3 feet, the ground effect will likely reduce this significantly.
     
  5. Aug 13, 2014 #4
    Thanks really this helped me a lot. Could you explain me a little more about the ground effect? Are there any propeller alternatives that could be more efficient like magnetism?
     
  6. Aug 14, 2014 #5

    Baluncore

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    Because the clean air must be “cut” by the blades efficiently, the number of blades is inversely proportional to the RPM.
    One blade needs a counterbalance weight.
    Two blades can oscillate about the blade axis, alternating in angle of attack.
    Three blades is stable and can usually be designed to be efficient without a gearbox.
    Four blades may have the same problem as two blades unless they are given an 85°, 95° offset.
    Five blades need a reduction gearbox or a heavy low RPM engine.
     
  7. Aug 14, 2014 #6

    rcgldr

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    I don't understand the point here. Two bladed fixed pitch propellers work just fine on model aircraft, including drones that use 3 or more propellers, and helicopters that use just a single rotor.

    RPM is also somewhat inversely proportional to blade length, but the limiting factor for the normal case is the power source, and the other limiting factor is keeping blade tip speed below the speed of sound.
     
  8. Aug 14, 2014 #7

    AlephZero

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    The rotordynamics of a 2-bladed propeller is more complicated than 3 or more blades, because the moment of inertia is large along the line of the blades and small at right angles to it. That creates new possibilities for shaft whirling and other instabilities for some ranges of RPM, compared with more than 2 blades.

    If the design never operates in those RPM ranges, that is not a problem, except that it's something else you have to consider at the design stage.

    The classic first paper on this was Smith, D. M., 1933, "The motion of a rotor carried by a flexible shaft in flexible bearings", Proc. R. Soc. London, Ser. A, 142, pp. 92-118.
    Smith wasn't explicitly considering propellers, and so didn't mention that the "shaft damping" might be created by the aerodynamics of the blades.

    The blanket statement that "the number of blades is inversely proportional to RPM" is a (small) proportion of the whole truth, IMO.
     
    Last edited: Aug 14, 2014
  9. Aug 14, 2014 #8

    Baluncore

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    YES.

    And yet it is the one foundation on which all other design features hinge.
    If you lengthen the blades you need longer legs on the undercarriage, until you reach the tip speed limit.
    For a fixed length blade the optimum number of blades will always be a function of RPM.
    If you look at single engine fighter aircraft at the end of WW2, over 2000 HP engines with 5 bladed props, a reduction gear and a torque converter were needed to handle the power, on very long undercarriage legs.
     
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