Old 02-23-2008, 04:28 AM
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Your electric motor draws a certain amount of energy to do its job, which is to turn the propeller. With no prop attached it draws very little energy. If you put a big prop on the motor it draws a lot of energy.

This is similar to pulling a boat trailer behind your car. The car might get 20 mpg normally, but put a boat on a trailer behind the car and mileage will drop off to perhaps 15 mpg because the motor is using more energy just to maintain the same speed and travel the same distance. However as long as the boat and trailer are not too heavy, no real damage occurs, you just use more gas.

If you put too big a trailer behind your car, something will break. The motor may fail, the transmission may fail or something else. That is because you are asking the drive train to produce more work, use more energy then it was built to handle. Fuel mileage goes way down and then something breaks. You have over stressed things.

Back to your plane.

Your electric motor needs to "draw" a certain amount of energy in order to turn a given propeller at a given speed. Let's use a speed 400 motor as an example and let's say you have a 6X5 prop on it. That means the propeller is 6" across and has a pitch of 5" per revolution. Pitch indicates how far the prop would move forward through the air if there was no slippage. As either of these numbers go up, the motor is asked to do more work.

Now let's apply some numbers. These are made up numbers for illustration only. Don't assume that these are accurate for your motor in your plane turning your prop.

Let's say that, to turn that 6X5 prop your speed 400 motor draws 6 amps of electricity using a battery that delivers 10 volts, just to make the math simple. That would be 60 watts of energy that the motor consumes to turn that prop. (6 amps X 10 Volts)

If we go to a larger prop, say 7 inches and keep the pitch the same 5 inches, the draw might go up to 8 amps at 10 volts or 80 watts.

Likewise if we went to a 7X6 prop, the draw would go up again, say to 9 amps or 90 watts.

In each case we are increasing the amount of work the motor has to do to turn the prop. The harder it works the more electricity it draws. This is also placing an increasing amount of stress on the motor causing it to generate heat and placing more pressure on the bearings. If we push it too far, the motor will be unable to turn the prop fast enough to be useful in flying the plane and/or it will fail from stress, just like the car example above with the trailer that is too big.

What we try to do is to get the best balance of propeller and amp draw so that the motor operates efficiently without being over stressed.

Likewise if you have that same speed 400 motor and keep the prop at 6X5 but increase the electric pressure, volts, to 12 volts it will try to spin the motor faster causing it to draw more amps into the motor. This would be like putting a supercharger on your car's motor which forces more fuel/air mix into the car's engine. It will produce more power so it can do more work. However if we exceed the amount of power it was designed to handle, it will fail. It might not fail right away, but over a very short time it will start to degrade, perform badly and perhaps suddenly fail all together.

If we push the voltage up too high or the amp draw too high, we will over stress the motor and damage it.

The goal is get a good balance of propeller and power draw.


A comparison of Glow vs. Electric power

Electric Motors Described

MotoCalc will tell you everything you need to know: Amps, Volts, Watts, RPM, Thrust, Rate of Climb, and much more! It is a popular tool for predicting the proper motor, prop, battery pack for electric planes.

Last edited by AEAJR; 06-13-2011 at 02:28 PM.
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