4 Blade Propellor

Discussion in 'Walkera Help!' started by Johan, Aug 1, 2020.

  1. Johan
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    Johan New Member

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    Hello,

    I have the Walkera Runner 250 Pro.

    Just because of curiosity I was testing a 4 Blade Propellor instead of the 2 Blade Propellor,
    to see what the drone was going to do.
    The only thing what was happening, was a loud and howling sound, when I tryed to take of.
    So I didn't dare to go any further.
    I used the HQProp 5x4x4V1S and the Emax prop nuts.
    What is the reason why this is happening, I like to know that very much.

    I wil also try the 3 blade Prop, I see on the internet that this is working.
    But I have to wait, because they are not delivered yet.

    To be sure: The 4 blade props were in the right position,
    and with the original 2 blade props the drone is working OK.

    Friendly regards, Johan Jansen Apeldoorn Netherlands

     
  2. bjr981s
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    bjr981s Moderator Staff Member

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    Do not change the propellors.

    The Statistics for Propeller selection.

    A propeller will move air based on its Pitch, Its Diameter and Its number of blades.

    An example is at a particular revolution the single blade propellor will cut a cycle in the air that covers the most distance. As you add blades the will cut multiple cycles in the air but require more power as they are moving more air. So for Speed the most efficient is a single blade. For Power the most efficient is more blades. The length of the cycle (the Screw Thread in the air) is based on the pitch of the blade. The greater the pitch the more air is moved but it requires more power to do so.

    For a manufacturer, they need to determine the correct propeller configuration based aircraft weight, Battery Size V(number of cells), Battery C rating (Max Current draw before brownout), Motor KV (number of revolutions it will attempt to reach per Volt of supplied battery power with no load, Max power rating in Watts, Max permissible Current draw an Amps, and Max Voltage. (Has to be => battery cell count at full charge and battery Chemistry (Volts Per cell). The Actual Speed of the motor at a given voltage will depend on the power of the Motor and the amount of air it is moving. Like driving up a hill in a car. The steepness of the hill = the amount of air you are trying to move. (Prop Diameter and Pitch.)

    Coupled with this information then then determine what Max speed the Aircraft needs to achieve and what power to weight ratio they want without exceeding any of the parameters above.

    The final task is to determine, at what Speed and Power they desire how long will the battery charge last. This compromise between performance and flight time will then determine the props to use.

    For a drone you need to divide the outcome into the number of Power Circuits by the number of Motors e.g. 4, 6, 8 etc. But with a single battery.

    Now what you experienced was a major error in this calculation.

    Your 4 blade pops were either too short a diameter, or too low a pitch.

    As a said above the greater the diameter and pitch of the propeller the more air is moved. The less diameter and pitch will allow the motor to achieve more revs.

    So you ended up with an amount of thrust, that was less than the weight of your drone.

    It was likely you did no damage to anything other than potential bearing damage of the motors by spinning them above the rated rotational speed.

    You will read on this site attempts of people that have gone to 4 blade props with too high a pitch or diameter. They suffer from destroying their ESCs.

    This is because electric motors have maximum torque at 0 revs. So if you put a high load on them by trying to generate more thrust the KV rating (Motor will attempt to achieve the revs per volt but dependant on its load) will pull more current through then ESC till it reaches that max current rating of the motor.

    If the max Current of the motor exceeds the max current of the ESC then the ESC will fry.

    Unless you have a valid reason like educational I would not change props on any Walkera drone. Its a complete end to end RTF / PNP solution that does not tolerate modification.

    If you do want to investigate the power train requirements, you will need specific tools.

    This is what I use for Prop measurements for Thrust, Current draw and battery performance. These test stands are set in Pairs for Dual motor interaction for Single battery operations. e.g. 1, 2 and 4 blade Electric model. Plus my Dual EDFs and Jet Turbines.

    IMG_3236.jpeg
     

    Attached Files:

  3. Johan
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    Johan New Member

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  4. Johan
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    Johan New Member

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    Thank you very much for your extensive explanation!!
    It is all clear now why this happened, and how it is all calculated together for the right setup.

    Friendly regards, Johan Jansen Apeldoorn Netherlands
     
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