Propeller - 2 blade vs 3 blade

I've been reading the DIY threads for about 2 months now and am motivated to get into this Arducopter flying.I have noticed that all copters use 2 blade propelers. Is there a technical reason why 3 blade propellers are not used? I would think they would provide more lift.

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  • Thanks for the link to the article. With only so much "power" available from the batteries, the most efficient propeller design is the most effective. I appreciate the explanation.
  • Two bladed propellers are more efficient than 3 or 4 bladed propellers so long as the propeller tips do not approach supersonic speeds.

    Three and four bladed propellers are used on model planes because of ground clearance and scale appearance considerations only.

    On a multicopter static thrust is the most important thing, in particular the thrust required to keep the copter in a hover.

    Ideally your motor / propeller combination would be most efficient for that circumstance.

    Generally speaking the larger the (2 bladed) propeller diameter the slower it will need to turn and the higher the efficiency will be for a given thrust.

    Ideally you would like to have the largest propeller diameter possible turning at the lowest RPM for a given thrust.

    Motors have KV ratings which indicate how many RPMs they will turn per volt of input energy and it is important that the maximum current ratings not be exceeded by trying to get too much power out of them by forcing them to turn too slowly or they will over heat.

    Basically the motor needs to be matched to the propeller that is used.

    If the propeller forces it to turn too slowly the motor will over heat and fail.

    That said the biggest diameter propellers you can use (without causing the motor to over heat) will result in the greatest efficiency and economy of battery usage.

    My ARF F450 Flamewheel came with cheesy 10" light weight not very efficient props.

    I replaced them with better designed 11" Carbon GemFans and acheived an easy 2+ extra minutes per flight and it is quieter and more stable.

  • They are quieter but less efficient. two blade APC slow fly types on lower kv motors work well in my experience.

  • Yes, they will give you more lift than a two blade, but at the coast of efficency. Theoreticaly, a one bladed prop would be the most efficient, but a bit hard to balance :-).

    The only reason three or more blades are used is because of  space limitation or because of too large prop diameter (the outer parts of the propeller will have a higher velocity then the central parts and, in the case of big aircraft propellers, come close to supersonic speed, which is also very bad for efficiency).

    take a look here:

    http://www.nar-associates.com/technical-flying/propeller/cruise_pro...

    regards

    http://www.nar-associates.com/technical-flying/propeller/cruise_propeller_efficiency_screen.pdf
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