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Since we are on the subject of power systems, I have a beater sport plane that has a KD 3616M, apc clone 12x6e, and a turnigy Kforce 40amp. I needed to discharge some batteries last night and I played around with it.
The acceleration on it was much faster then the airboss/torque combo. At first I didn't believe it but I tested it over and over again and the yak just spooled up and was almost instaneous on the accel. The airboss/torque lagged a little bit. I remembered I had some youtube videos when I was trying to figure out the braking difference between the Kforce and Airboss. here they are http://www.youtube.com/watch?v=u2lottVDC0 http://www.youtube.com/watch?v=9iAQB4Um4Wc You can hear it in the pitch of the prop, but do smaller motors really spool up faster then the big ones or am I just hearing things? The little motor pulls about 10 more amps then the torque also and puts out about 120 more watts. Don't know about thrust as I don't have a fish scale. 


Joined Jan 2004
3,389 Posts

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People have long wanted the e quation to be as simple as nitro powered, it actually is for me when you consider the battery is the powerplant producing the horsepower. ie 749watts equal 1hp, so my formula is 10g battery produces 25 watt power or .015HP So in the end 3D performance preffrence for a given RTF weight range will show a needed battery for a preffered prop, which in turn will show me a given HP into a specific prop. Using the EF models is a great place to start as everything is built to exacting weights with only the prop prefference as a variable. Want to do an interesting spreadsheet concerning HP needed for given weight range models? Go to Horizon Hobby and you will see that starting in the ultra micro series that for every 35grams increase in RTF weight they increase the battery size/weight/KV/prop while in some cases its the same motor. You can go all the way up thru the model weight range and see what gram weight of the battery recommended and using my formula you will see how it compares in HP to its equivalent nitro counterpart which most are dual rated. The e motor specs would merely be capable of handling the battery wattage produced. Its probably best to PM with any more questions as to not hijack/confuse thread anymore than needed..02 

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But the fact is that this is the same oversimplification that we're trying to move beyond. I know that I need about 1500 Watts of power (2 HP) to turn a 16x8 APC propeller 8800 RPMs, and your formula says I need a 600g battery, when a 450g battery will easily give me the volts and C rating I need. There are also heavier batteries that will actually deliver less volts at WOT and therefore not provide the power I need, and also make it impossible to balance the plane properly. I wish it were that simple. I also wish it were as simple as multiplying Amps time volts, or volts time Kv, but no such luck.



Joined Jan 2004
3,389 Posts

Well on the EF page concerning the 4016 motor and 1400W into a 15X8 it is recommended 5s 38505000 MAH so here is a quick spreadsheet, while my formula is not an exact lab formulation it gets me close to the bullseye from 1oz micros all the way to 12S 40% models, without looking for anomally batteries as you have referenced.
ie Have a look: http://www.hobbyking.com/hobbyking/s...5000&location= 
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I was just doing some flight time calculations on the 48 inch Edge thread and trying to make a linear relationship, so I went back to Lee’s data for some insight. I suddenly realized something interesting that we're going to have to deal with eventually....
The comparisons between the halfthrottle settings across the board show something interesting. I'll probably have to plot the data and stare at it for a while, but here's a quick synopsis... Half throttle settings for all six samples generates between 22 and 24 percent of the WOT Amps and between 67 and 69 percent of the WOT RPMs. Any thoughts? 



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But the Torque 4016 + 15x8 prop + 5s 3850 do not give 1400 W, not even 1100 W. Battery is NOT power, it's the energy source, and you need the motor to convert electrical power to mechanical power. Mechanical power = force x velocity = work per unit time Electrical power = voltage x current. The battery weight alone cant do anything helpful. Vien 




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Similar to driving a car down the road. It takes much more power at higher speeds as you loose efficiency. JC 




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In electric motor, the story is different. The power loss increases because resistance of the whole power system increases at higher operating temperature. Friction also increase btw. Vien 




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Good stuff, and also falls in line with the fact that the higher magnetic field, reverse voltage, harder working FETs, etc, will contribute to resistance as well, making it an uphill climb between half and full. And since heat is a function of current^2 and resistance, all negative components increase at higher throttle ratings.
So while the Amps are increasing more than 300%, representing the increase in electrical Watts, the RPMs, which represent the mechanical Watts, are only increasing by 33%, but the output is the same using the two different formulas: Electrical Watts at HalfThrottle = volts * Amps 23.62v * 14.3A = 337.8W Mechanical Watts at Half = pK * ppitch * plenth^4 * (RPM/1000)^3 1.08 * 0.67 * (1.25^4) * (5.76^3) = 337.6W 







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So for the APC prop, that's 17 RPMs per Watt for the first half of the throttle curve and 2.5 RPMs per Watt for part2. When I have some time, I'll run the numbers for the rest of the prop samples from Lee's data so we can compare to the reproduced data that should be coming soon. 





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