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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
  22. Feb 3, 2016 #21

    cjl

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    Most model helicopters use nitromethane/methanol 2 stroke engines though, which have much lower efficiency than your quoted 35-40%, and typical flight times are rather short. Gas powered models are much more difficult to keep running optimally (they're still carbureted, so you have to fiddle with the fuel metering fairly frequently), and they can make quite a mess. Electrics are quieter, run nearly as long (for typical models, not record holding ones), and require far less maintenance to keep them in optimal running condition, so I'd take an electric model helicopter over a gas one any day.
     
  23. Feb 4, 2016 #22

    rcgldr

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    Consider the work performed on the air as output power, in which case the thrust times the speed of the induced flow is the output power.

    In a static (hovering) situation, the pitch, if within a "working" range, doesn't matter much, and the induced flow and thrust are about the same regardless of pitch, again if the pitch is within a "working" range.

    For RC helicopters, electric models are more popular and also dominate even the aerobatic 3D competitions, which have limited flight time in a contest situation. The term gas powered models isn't clear anymore, as it could refer to nitromethane/methanol based engines or gasoline based engines. Oil is mixed with the gasoline for both 2 stroke and 4 stroke model engines (that I'm aware of). The gasoline engines don't have the reliability issues of the "nitro" engines. I don't know if the 5 hour record quadcopter used nitromethane/methanol or gasoline.
     
  24. Feb 4, 2016 #23

    russ_watters

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    Yes, that's often the basis for a fan efficiency calculation (or the similar airflow * pressure), but pressing the point implies you are arguing against yourself.

    The goal here is to maximize force per unit of power (N/W), not mechanical efficiency. Indeed, the mechanical efficiency is often higher at higher pressure and velocity, which would mean a smaller rotor instead of a larger one.

    The other side of the coin for HVAC is what I do for work: Energy codes specify minimizing airflow per unit of power (CFM/W) for new systems and my energy efficiency projects are all about minimizing watts. For example, if you find a clean room can stay clean at a reduced airflow, you reduce the airflow, which reduces the watts. But it is usually the case that the fan's mechanical efficiency goes down - often it becomes spectacularly poor. But you still save energy by doing it.

    So that was exactly my point: "efficiemcy", as typically defined is mechanical/thermodynamic efficiency, and it is not a significant consideration here.
    That's fine, but again, my purpose for addressing all of the possible constraints was to ensure the OP would not incorrectly generalize what he learned.
     
  25. Feb 4, 2016 #24

    rcgldr

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    True, since the goal is mostly about hovering or relatively slow horizontal speeds, then force per unit power is the key, and as you mentioned before, the same force exerted on a larger amount of air flow (due to a larger propeller), requires less of an energy (speed^2) increase of the affected air.
     
  26. Feb 6, 2016 #25

    russ_watters

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    That is, of course, a typo: it is W/CFM that should be minimized (CFM/W maximized).
     
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