Matt,

Yeah, I dont really understand the esc chopping effect either, but I think it does really make more sense to go to a lower prop size. Keep in mind that that constant rpm of a motor changes with battery voltage. So for example, a 1000KV motor runs at constant voltage of 11,100 rpm at 3S and 14,800 at 4S, but they are constant values regardless of prop size.

With regard to amp draw, it increases at higher cell counts rather than decreases. Here is a quote from a back and forth email discussion I had with Lucien Miller, which I found very informative:

"the power rating of a motor is simply a measure of how much heat it can handle while it does its job spinning the prop.

If you keep the same prop on a motor, going from a 3-cell battery pack to a 4 cell battery pack will result in a large increase in current, and an even larger increase in power. This is a compound effect, because the faster you spin a prop, the more drag it has and the more load it puts on the motor. Also, when you increase the voltage, the motor pulls more current, so these two effects multiply together when you go up in voltage.

For example, if we look at the SII-3014-1040 motor we spoke of earlier, here are a couple of data points from the prop chart with an APC 10x5E prop.

Running at 10.5 volts, 3-cell Li-Po equivalent

Amps - 21.8
Watts - 229
RPM - 9,625
Thrust - 45.3 ounces
Pitch Speed - 45 MPH


Running at 14.0 volts, 4-cell Li-Po equivalent

Amps - 35.3
Watts - 494
RPM - 12,100
Thrust - 74.7 ounces
Pitch Speed - 57 MPH

Looking at these numbers you can see some mathematical relationships develop. The voltage was increased by 33.3% going from 10.5 to 14.0 volts. The motor RPM and Pitch speed increase as a direct relationship to the increase in voltage. Going from 9,625 RPM to 12,100 RPM is an increase of 25.7%. Because the prop load increases and its speed increases, the 33.3% increase in voltage does not correspond to a 33.3% increase in RPM. The actual mathematical relationship is the voltage increase raised to the ¾ power.

If you look at the amperage increase, it goes from 21.8 amps to 35.3 amps, which is an increase of 62% from a 33% increase in voltage. This is roughly an increase of the square of the voltage increase.

The total power went from 229 watts to 494 watts, a whopping increase of 116%, which is roughly an increase of the cube of the voltage increase.

Because the motor heats up more at the higher power levels, its efficiency does go down a bit, so the actual increases are multiplied by the lower efficiency and you get a bit less of an increase than you would expect.

So going back to your original question, you can see how much the current increases with the same prop when you go from a 3-cell to a 4-cell battery. If you did try to keep the same prop, you would need to increase the size of your speed controller. This is why you normally use a smaller prop on a motor when you go to a higher voltage battery.

Looking back at the prop charts, if you run the motor on a 4-cell battery, you would need to drop from a 10x5 prop to a prop around an 8x5 to pull the same current. However, even if the current is the same, the total power will go up by 33% because the voltage went up 33% and total power is equal to Voltage x current.

Hopefully I did not lose you too bad in the math, but that is the relationship between voltage, current and power when you change things in a motor."


Hope all this helps (rather than just irritating):-)

Jon