View Full Version : Motor choice / calculation

rebell

01-24-2006, 06:29 AM

I used motocalc to check what the amps should be on different battery/motor/prop combinations. It is a AXI 2208/20 with LiPo 2A 2 cell fully charged. I then did the actual test and rpm and volts are close to the calculation, but the amps calculation is 40% more than that of the actual measurement. (10.4A measured - 14.7A calculated) I used a second ampmeter and got the same results. Another calculator I got from the net give the same results as motocalc.

Is it a standard in these calculations to give a safety margin or what might be the reason for the difference?

TManiaci

01-24-2006, 07:24 AM

I used motocalc to check what the amps should be on different battery/motor/prop combinations. It is a AXI 2208/20 with LiPo 2A 2 cell fully charged. I then did the actual test and rpm and volts are close to the calculation, but the amps calculation is 40% more than that of the actual measurement. (10.4A measured - 14.7A calculated) I used a second ampmeter and got the same results. Another calculator I got from the net give the same results as motocalc.

Is it a standard in these calculations to give a safety margin or what might be the reason for the difference?

Rebell,

I believe the calculations are computing Output wattage. There is a reduction from the Input Wattage, representing the motor and propeller efficiency. On most outrunners, this differential is generally said to be 75-80%, but when a motor is pushed into or over it's peak ratings, efficiency falls off rather severely. Your numbers make sense, based on that. The Model Motors site shows motor efficiency falls to 72% at 13.4 amps. Now lose a little more with the prop power conversion...

Matt Kirsch

01-24-2006, 04:08 PM

Actually, he's saying that the calculated Amps are HIGHER than the measured Amps.

Traditionally, Motocalc has had some trouble accurately calculating outrunner motors. It was pretty bad on version 6. You could keep increasing the size of the prop, but the Amps would go DOWN?!?! Plus, it was quite a ways off from reality.

rebell

01-24-2006, 06:02 PM

Is there a calculator or a good set of formulas I can put on a spreadsheet available that is closer to reality and with a good range of motors, batteries etc included. I dont need all the fancy reports, all I need is to calculate the amps, rpm and thrust to expect from a given input without changing props all the time.

Thanks for your replies

TManiaci

01-24-2006, 09:06 PM

Is there a calculator or a good set of formulas I can put on a spreadsheet available that is closer to reality and with a good range of motors, batteries etc included. I dont need all the fancy reports, all I need is to calculate the amps, rpm and thrust to expect from a given input without changing props all the time.

Rebell,

I got frustrated with MotoCalc too. I two-step it now using WattageCalculator (http://www.coloradogliders.com/propellerloads.htm) to work Output Watts, then factor that by an estimated efficeincy of 75% (divide by 0.75) to get the battery side of the Wattage (higher value).

I recommend 5-8 Watts per oz for Trainer/mellow flight, 8-12 W/oz for pattern aerobatics, and minimum 12 W/oz for 3D flying. These are as measured at the battery, assuming normal efficiency ranges for the hardware. If you are running a "economy" motor, or a gearbox, shoot a couple watts/oz higher to be safe.

I assume a properly loaded, fully charged Lipo will pull down to the nominal pack voltage. For 3S, 11.1V is the number. This seeems to be accurate most of the time. Then, Watts = Amps x Volts... so Amps = Watts / Volts.

Then I use ThrustCalc (http://www.mmaa-modelairplanes.org/staticthrustcalculator.htm) to get bead on thrust, using a CF factor of 0.8 to get it to match my real-world numbers.

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