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  1. #1
    Flight Cadet
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    Calculating motor thrust

    Hey, I have a hacker style brushless outrunner which has a rating of 924Kv and i have a 10x4.7 propeller. I need to calculate an estimate of the thrust. I came across this equation for the static thrust,

    T = [ (eta*P)^2 * 2 * pi *R^2 * rho ]^0.3333

    T = thrust (in Newtons)
    eta = prop hover efficiency = 0.7-0.8 is typical for unstalled low-pitch props
    P = shaft power = voltage * current * motor efficiency (in Watts)
    pi = 3.14159
    R = prop radius (in meters)
    rho = air density = 1.22 kg/m^3

    In this equation i'm not sure what to use for eta (i was thinking 0.75). I also don't know what I should use for current, voltage, motor efficiency and propeller radius. Should i just use the max current and voltage that the motor can take which is 17A and 11V respectively. Should I just assume the motor is around 75% efficient? Will the prop radius just be half the length of the propeller, in my case that will be 5 in (0.127 m).

    If i use the values i assumed above i get,

    T = [(0.75*11*17*0.75)^2 * 2 * pi * 0.127 *0.127 * 1.22] ^ 0.3333 (0.127 is the radius in m)
    T ~= 11.1 N

    Does this seem correct?

  2. #2
    Senior Pilot itod's Avatar
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    According to this converter:
    Newtons to Kilogram-Forces Conversion Calculator you should get 1131g of thrust, which should be very close what that setup should generate. I would say your calculation is right on target.

  3. #3
    Senior Pilot
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    A simple teststand can be quickly fabricated: a simple see-saw of equal length clamped to the side of a desk, with the motor haning over the free side the other end over a digital kitchen scale. Thurst = measured thrust + weight of motor and prop. Just make sure you can't stick any sensitive parts into the prop while testing, or that anything can come off of the motor and hit e.g. your computer screen and trash it....

  4. #4
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    May I force you over to this Thread

    I donīt know your ESC and I assumed you use the Hacker A20-20 and the APC Props thrust est. is 1002gramm - For that calculation I used http://adamone.rchomepage.com/calc_motor.htm

  5. #5
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    Hello, I am Prasanth. I saw this post and the great insights you people gave.
    If you could please spend a few minutes to answer my doubt, I would be extremely gratefully

    my doubt:

    I found

    Power (Watts) = 1.15 * D^4 * P * RPM^3 where D(feet) and P-pitch(feet) and RPM(in thousands).
    And iused the thrust formula you have provided.
    As i calculated the RPM with increasing thrust and voltage(the voltage reduces due to resistance and increase in current Vreal=Vmax-IR).
    but i saw that RPM increases with thrust and power.. But isnt Voltage = constant*RPM, so shouldnt RPM decrease..
    I know I am missing some important concept. PLEASE HELP me..
    Thank You In Advance

  6. #6
    Senior Pilot
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    wow talk about resurrecting a thread from the dead!

  7. #7
    Senior Pilot wooden's Avatar
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    Think about it - what can cause an increase in thrust? There are only two factors that can increase thrust - the propeller can change (increase diameter, pitch, twist, etc.) or the RPM can change. Since we're using fixed pitch props, the RPM must increase.
    push the envelope, watch it bend

  8. #8
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    Post Quadcopter

    Hello frnds!
    I am intrested in making a Quadcopter. Bt i dont know how to calculate thrust of a motor..
    I am using
    Motor : Park 250 Brushless Outrunner Motor 2200kv
    ESC :
    10-Amp Pro Brushless ESC
    Propeller :
    Slow Flyer Propeller, 8*6, SF
    Battery : 3200mAh 2S 7.4V 20C

  9. #9
    Senior Pilot Sicarius's Avatar
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    Hello Harshan, You might want to make your own thread in the beginner section, you're resurrecting a dead one now
    Try this:
    http://www.ecalc.ch/motorcalc.htm?ecalc&lang=en
    Fleet: AQ32 Y6 FPV, Mega v2.1.2 FPV quad, Blade mcp-X, 6' Skywalker 1900, 20" Flying wing, 100 mph 1:10 car
    Gear: GoPro Hero 2, Gopro hero 3 Black, Fatshark Predator, 600mw 5.8, 9XR, 850mw 1.2, scherrer lrs
    Carnage: This year: 1.2 TX, several ESC's, mostly due to polarity.. CHECK YOUR BATTERY LEADS!!


 

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