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  • Gareth Rens December 25, 2011 at 9:08pm

    Found this a while back if you want to do it manually:

    For all those of you who like me plan things ahead, here is some information you may find useful :

    To estimate your flight time (quite precisely in fact), the formula is :

    - take your quad UAW, divide by the number of motors, you get thrust needed to hover per motor.
    - mutiply this number by 2, it gives you max thrust needed fo effective braking and recovery manouvers (in order to chose the right motor for your machine)
    - make a bench test with motor at hover thrust and note power consumed (W)
    - divide power by nominal battery voltage (3s = 11.1V, 4S = 14.8V etc...) it gives you hovering curent per motor : Hc(A)
    - divide Battery capacity Bc(Ah) by (Number of motors x Hc(A)) and multiply this by 60 (minutes)
    - then, multiply this number by 0.8 (20% loss of efficiency)
    The result is your estimated flight time for hovering or gentle flying, not crazy 3D flying of course

    Example :
    your quad is 2000g AUW, so you need 500g thrust per motor (and your motor needs to be able to give roughly 1000g thrust at full throttle)
    You calculate you need 55W per motor, that gives you roughly 5A with a 3S LiPo
    Your battery is 5000mAh, so that's 5Ah
    You divide 5Ah by 4x 5A and multiply this number by 60 = (5/20) x60 = 15 minutes
    Then, multiply this number by 0.8 and you get 12 minutes

    voila! you have your estimated flight time

    If you want to know how much power your machine needs for hovering, you do the reverse calculation :

    If you can fly 15 minutes with a 5000mAH 3s battery, you consume :
    Bc(A) x 60(min) / Flight time (min) = (5 x 60) / 15 = 20A (or 5A per motor)
    So with a nominal 11.1V for a 3S battery, that's 220W (or 55W per motor)
    Your quad weight 2000g, that's 2000/220 = 9.10g/W (very good efficiency!)
    No need to alter the result with 20% losses, you are working on the real life data

    Simple version:

    2000g. AUW
    5000 mAh 3S

    Assuming an efficiency motor / prop of 7.5 grams per watt:

    2000 / 7.5 = 266 Watt

    5 x 11.1 = 55.5 Watt



    (55.5 / 266) * 60 = 12.5 minutes flight time

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