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Old Mar 30, 2015, 07:40 PM
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First time build need help

Hello everyone i currently own a 3dr Iris+ and have decided that i wanna build my first quad because my iris can only fly for about 12mins with gimbal. I want my new quad to be able to flight for atleast 30mins with video equipment but would like to fly longer of course. I will be using electronics from my IRIS and have bought a new frame which are the PIXHAWK +gps and telemetry. I have purchased a new frame which is the H4 680mm Alien.

So what i need is help with picking the motor(s), speed controller(s), and battery

with the H4 frame i think the max prop size is 16

I am estimating that weight will be around 1900-2100grams depending on motor and battery setup. the current weight with just the frame +controller+ video equipment is around 1100grams

Anyway your help would be appreciated
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Old Mar 30, 2015, 07:55 PM
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You might want to build a hexacopter instead.

And read this:

For any platform of any size and weight, the flight time is the result of:

time = whkg / (1000*R / eff_gw) * 60, where

whkg = battery energy density, in Watt hour / kg, ranging from about 150 for the high C packs to 270 for LiIons. 10C Multistars are 185.
R = ratio between AUW and battery weight
eff_gw = motor/prop efficiency, grams per Watt, taken from mfr data.
Be aware to get from table the efficiency for the intended thrust, and for 50% throttle !
Efficiency greatly depends on load, so don't use efficiency stated for 1kg, to compute flight time of 2-3 lg platform !
Also, don't use values for more than 60% throttle, the platform will be unstable, no more room for motors to do their control job....

As I wrote, this formula works for ANY size platform and energy source, lets do the math for some particular cases:

Case I Ph2
battery wh/kg = 5.2(Ah)*11.1V/0.37(kg) = 156wh/kg
R ratio between AUW and battery = 1250/370 = 3.4
propulsion efficiency = 11g/W
time = 156 / (1000*3.4 / 11) * 60 = 30 min

Case 2 the 129 min record
battery wh/kg = 266Wh/kg - typical LiIon energy density
R ratio between AUW and battery = 3.4/2 = 1.7
propulsion efficiency = 14.1g/W
time = 266 / (1000*1.7 / 14.1) * 60 = 132 min

And finally, my 30 min case
battery wh/kg = 165, I use a Zippy Compact, having a bit more energy than the average LiPo
R ratio between AUW and battery = 0.75/0.195 = 3.8
propulsion efficiency = 11g/W - using Phantom 2 clone
time = 165 / (1000*3.8 / 11) * 60 = 28 min

As you can see from the above, reasonable precision of all estimations, proving this formula is valid for any configuration, so now you have a tool that should tell you what is wrong in your setup, and where to work to improve flight time.

The biggest unknown for most users is the propulsion efficiency, for this reason I think is mandatory to not buy stuff without manufacturer precise specification of this parameter.
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Old Mar 30, 2015, 08:15 PM
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thanks for the post need afew minutes to understand the math

well i ordered the frame yesterday so im gonna build around that
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Old Mar 30, 2015, 08:44 PM
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Im stuck on how to get the propulsion efficiency im on hobbyking looking at every motor between 300-400kv but i dont see it anywhere
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