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Propellers on an RC Quadcopter?

  1. Jan 28, 2016 #1
    Hey forum,

    So I plan on creating a remote controlled quadcopter and I have a query regarding its propellers' sizes. I read that small propellers require motors with a high KV (RPM per volt), and large propellers need motors with a lower KV. There seems to be a way of determining how to match propellers with motors. Now I understand why small propellers need powerful motors, but why is it that large propellers need less powerful motors?

    Is it necessary that they have slower motors, or was that only said to maximise efficiency? I read that small props + high KV = acrobatic quadcopter, and large props + low KV = efficient quadcopter.

    Does this have anything to do with pitch at all?

    Why do small propellers at a high KV cause the quadcopter to be 'acrobatic'? I'd like to understand the physics behind that.

    Thanks.
     
  2. jcsd
  3. Jan 28, 2016 #2
    I think its because of surface area, larger displacement at lower RPM would produce greater lift.
     
  4. Jan 28, 2016 #3

    russ_watters

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    Rpm per volt has basically nothing to do with power. Rest assured, larger propellers use more power.

    The key difference is the linear speed of the blades is a function of propeller diameter.
     
  5. Jan 28, 2016 #4
    Russ is right, a bigger propeller would give greater vertical lifting force at lower rpms, but the greater mass and air resistance would cause greater strain on the motor.
     
  6. Jan 28, 2016 #5

    rcgldr

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    Larger propellers are more efficient, so they take less power, but more torque at a lower rpm. Smaller propellers have less angular momentum, so their speed can be changed quicker. There are/were some quad rotors with variable pitch propellers, similar to model helicopters that allowed for quick aerobatic maneuvers.
     
  7. Jan 28, 2016 #6

    russ_watters

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    Just to clarify, that's less power for the same airflow as a smaller propeller. The efficiency difference isn't enormous: if you want much more airflow, so you use a much larger propeller, it will use much more power.
     
  8. Jan 29, 2016 #7

    rcgldr

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    For aerobatic quadcopters, the speeds of the propellers need to be changed quickly, so smaller propellers are used, even though they need more power. Flight times are shorter than the less aerobatic models with larger propellers, assuming same weight and battery power. The alternative is variable pitched propellers, but those are fairly rare and more expensive.
     
    Last edited: Feb 4, 2016
  9. Jan 29, 2016 #8
    Thanks for the feedback everyone.

    Why does a lower RPM provide a greater lift for larger propellers? Surely a higher RPM means more vertical lift in all cases.

    So quadcopters with smaller propellers are more aerobatic, because smaller propellers can have their speed changed far quicker than bigger propellers?
     
  10. Jan 29, 2016 #9
    Greater lift than a smaller props at lower rpms, of course the faster you spin a prop the more lift you get but the smaller one has to spin faster to produce the same force.

    Air displacement, Larger surface area of the prop, pushes more air. Think ceiling fan. They spin slower but move more air then some of their smaller counter parts at the same speed. That's why with a small fan to produce more air flow it has to spin much faster.

    Addendum: The trade off is in power to weight, bigger prop more weight, bigger mass, more centripetal force, greater torque, more air resistance = more power drain.
    Smaller prop less weigh, less torque, faster speed, more agility do to less air resistance= a power drain of equal or lesser value. They may just cancel each other out. its advantages vs disadvantages. IMO
     
    Last edited: Jan 29, 2016
  11. Jan 29, 2016 #10
    So it is suggested to use a lower KV motor for larger propellers purely for efficiency?
     
  12. Jan 30, 2016 #11
    That's probably just for battery life so you'd get longer flight time, more bang for the buck I'd suppose.
     
  13. Jan 30, 2016 #12
    Thanks for the confirmation.
     
  14. Jan 30, 2016 #13

    cjl

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    This isn't really true - for the same force required (so the same mass of quadcopter), a larger propeller will require less power. This is because it is more energetically efficient to have a larger massflow of air with a lower downwash velocity (to achieve the same overall thrust).
     
  15. Feb 1, 2016 #14
    So what would be the most efficient set up? Give us an example of a large prop set up its advantages, and disadvantages. And a small prop setup.

    I'm assuming it will be in longer flight time and stability for larger props.

    Smaller props maneuverability speed and agility.

    personally I think Gas powered model helicopters are the best option for rotatory model aircraft but are vastly much more expensive.
     
  16. Feb 1, 2016 #15

    russ_watters

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    We had that discussion - please continue reading for full context.
     
  17. Feb 1, 2016 #16

    cjl

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    I read the entire thread, and I saw it stated several times that larger props = higher power, and never was it conclusively stated that larger props = lower power. That's why I made sure to clarify.
     
  18. Feb 1, 2016 #17

    cjl

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    Larger props = longer flight time and lower power requirements, but higher torque required, which can mean larger/heavier motors or different gearing. Response time will be somewhat application specific, but it wouldn't surprise me if smaller props had better response time.

    Why do you think gas powered model helicopters are better than electric, out of curiosity?
     
  19. Feb 1, 2016 #18

    rcgldr

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    Energy density (MJ is mega Joule):

    Gasoline: 32.4 MJ/kg
    Methanol: 19.9 MJ/kg
    Nitromethane: 11.3 MJ/kg
    Lithium Ion battery: 1.8 MJ/kg

    Gas type engines lose a higher percentage of energy to heat, and end up around 35% to 40% efficiency. Gas engnes are also heavier than electric motors with the same power output. Electric motors are 80% or more efficient, not enough to compensate for the lower energy density of batteries. Fuel based models have about 2 or more times the flight duration of battery powered models. The duration records for optimized quadcopters are around 2 hours for battery models, 5 hours for fuel models, but these aren't carrying a payload like some type of camera. Typical flight times for battery powered models are around 12 to 40 minutes.
     
    Last edited: Feb 1, 2016
  20. Feb 1, 2016 #19
    Basically flight time more bang for your buck.
     
  21. Feb 1, 2016 #20

    russ_watters

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    Well, not going through every post, I see it in #1, 2, 5, 7, & 8 for a start.

    The issue here, as is often the case, is that the OP contains unstated assumptions and different people answered differently based on different choices for those assumptions - sometimes stating them and sometimes not. Whether a certain assumption is more useful here or not, it worries me to see unstated assumptions, so I like to state the assumptions and provide the different (or alternate) answers using other assumptions. That way the OP doesn't come away thinking the wrong thing (if people guessed wrong about the assumptions) or thinking the answers that apply with the particular unstated assumptions apply with all unstated assumptions.

    All that said, there is another issue that was stated confusingly in the OP and I stumbled on: the issue of "efficiency". Mechanical efficiency is usually output power divided by input power and shouldn't be of issue here (though it is true that larger rotors tend to be slightly more efficient than smaller ones). Since the output power of a hovering copter is zero, the efficiency is zero. But for an "efficiency" that is really power per unit of force for a hovering copter, larger diameter is better.

    So to put a fine point on it:
    -For a larger rotor of the same pitch, higher rpm means higher power, thrust and airflow at the same velocity.
    -For a larger rotor of the same airflow, higher rpm means lower power, velocity and thrust.
    -For a larger rotor of the same thrust, higher rpm means lower power and velocity and higher airflow.

    To avoid confusion, I wouldn't use the term "efficiency" to describe any of that.
     
    Last edited: Feb 1, 2016
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