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Mar 05, 2010, 09:05 AM
Stilwell Shipyard
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Electric for Dummies 101


I continue to be disturbed by questions posted on RCGroups. I believe there is a large component of the want-to-be electric pilots that have no clue. Not surprising because the capability to fly almost any model aircraft using an electric motor is new. Further there is no standard specification for motors. To complicate the issue many self proclaimed electric experts are dummies in disguise. I am requesting a "real" expert write "Electric for Dummies"

I would love to write the book but I do not have the depth in knowledge required. I use MotoCalc or WebCalc and sometimes for Ducted Fans I use FanCalc I am not sure any of these programs are very accurate but my experience is that MotoCalc will get you in the ballpark. Do not forget to search RCGroups for your setup I have also gotten good leads searching Google. Careful on Google sometimes you get answers for full sized aircraft. There is a good thread for the Midi ducted fan at https://www.rcgroups.com/forums/show...i+10lbs+thrust

Now let us talk watts. Watts is power 745.7 watts equals one mechanical horse power for purest click Watts volts multiplied by amperes equals watts in a direct current system. All of our battery operated systems use direct current (DC). Again for those who like to pick fly poop out of the pepper our motors run on AC (alternating current) The ESC (Electronic Speed Control) converts the battery supplied DC (Direct Current) to AC (Alternating Current) for the motor. Explaining AC is beyond the scope of this blog they use things like real and imaginary numbers.

We need to know watts because it is power that will fly the plane. My general rule of thumb is 100 watts per pound and it will fly. That assumes there is some sort of lifting surface. Again many "experts" will tell you that you do not need so much power.

So I got a battery and an ESC a motor and a propeller how many watts do I have? The best and quickest answer to that is BUY a WATTMETER. You can measure the current (amperes) and the voltage and multiply to get watts. It is easier to let technology do the job.

Turn the page back - If your very new in this world of electrons think of Voltage as water squirting up in the air from a hose, the height of the water is the voltage. The size of the column of water is current (amperes). Put the two together and you X gallons per hour. Oops I said gallons per hour same with electricity it is watts per hour, not just watts. Like horse power pulling a wagon at X miles per hour. Standing still the horse is not doing any work. 3S and 4S and 2S2P etc. Very confusing; somehow the electric model airplane community began to specify batteries by the number of cells the battery contains. The rest of the world specifies batteries by the voltage they produce. A single lithium (Li-poly) cell generates 3.7 volts of direct current (DC). A car battery (lead acid) generates 2.0 volts of DC. Six lead acid cells generates 12 volts DC, the old carbon zinc battery generates 1.5 VDC (Volts DC). A single cell nickel-cadmium battery generates 1.2 volts. If you want to know more about batteries do a search on Google. Back to the RC world a 3S lithium battery would produce 3.7 X 3 VDC or 11.1 volts. So it would be easy to think that if the circuit takes 10 amperes (volts X Amperes) we would have a 111 watt system. Wrong
You never get all of the battery nothing in this world is 100% efficient and our batteries are no exception.

Our batteries are rated in "C" the capacity of the battery to output it's power. A 10 C battery can output 10 times it's capacity. For example; a 3.000 mah battery rated at 10C can output 30,000 milliamphere hours. A milliamphere is 1/1000 of an amp. No free lunch here you still only get 3,000 mah out of the battery just faster*. In the early days of RC lithium batteries most capacity ratings were smoke and mirrors. Batteries now available are much better but do some research before you buy.

*I got lazy and did not finish the explanation. The duration the battery will output a give current can be calculated T (time) equals the battery capacity divided by the current. Just be sure to keep all the units the same.

Hurricane Larry

Readers please feel free to add your favorite reference.

Hurricane Larry
Last edited by lslewis; Mar 18, 2010 at 02:09 PM. Reason: Added discharge formula
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