Calculating total Flight time.

[SSR]

I have some problem calculating my total flight time(Quadcopter). The formula i have read is

T(min)=Battery power(AH)*60/Total current drawn by motors.

1 motor=13.5A
4 motors=54A

Battery=4.4AH

Then T=4.89

I want to ask that the formula i am using is right and what is this 4.89. Either 4 min 89 sec or 4 hr 89min?

 

[jes1111]

Things about this calculation:

1) you need to allow for some efficiencies losses in the system - so your formula should really be Time(minutes) = BattPower(Ah) * 60 * 0.9 / TotalAmps(A) to factor in 10% losses.
2) you need to land well before the battery gives up!
3) make sure you're using the "hover" current, not the maximum current that the motor will take - i.e. if your motor pulls 13.5A maximum, then you'd have 4.89 minutes flight time at maximum throttle. But if it only pulled, say, 4A at hover, you'd get approximately 18 minutes of hover flight.

See: http://www.mikrokopter.de/ucwiki/MK3538#Thrust_over_current - this shows thrust and current for the MK3538 motor with various props. Divide the total flying weight of your craft by the number of engines to get the thrust you need (from each engine) to hover. Find that figure along the thrust scale on the graph, read off the current (for your chosen propeller). Then you can do the flight time calculation.

 

[SSR]

AlrightThanks jes.

 

[matwelli]

http://multirotor.files.wordpress.com/2011/03/multirotorcalc1.xls

gives a conservative flight time (based on 170 watts to lift 1kg, some super efficient systems will use as low as 120 watts to hover 1kg)

I always aim for using no more than 80% capacity, and thats what the above link also takes into account

Using more than 80% kills the battery life cycles

 

[SSR]

http://multirotor.files.wordpress.com/2011/03/multirotorcalc1.xls

gives a conservative flight time (based on 170 watts to lift 1kg, some super efficient systems will use as low as 120 watts to hover 1kg)

Dont you think 170 watts for 1kg is too much? As far i have read it takes me around 110-120 watts?

Correct me if i am wrong.....

 

[matwelli]

@SSR - its easy to change the figure in the spread sheet

real world experiance, i have measured one of my quads, running 10 by 3.8 apc props and it took 167 watts per 1kg to hover, i have tested the Lotus RC T580, and seen a number closer to 120 watts

Big, large slow moving props are more efficient, the faster you move the prop and the smaller it is, the lesser the efficiency (more watts) on the flip side smaller props moving faster seem to be less affected by wind

 

[SSR]

SO 167/4=42

Therefore each motor should be able to provide approx 42 watts.

Thanks for your help....

 

[matwelli]

yup, but make sure each motor is rated to 2-3 times that, for nice safe cool running