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Old 09-24-2008, 07:14 PM
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Join Date: Jan 2008
Location: Glenwood, GA
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Originally Posted by Dr Kiwi View Post

I think the connection is that guys were trying to determine how much of the observed drop in in-flight amp draw was simply due to battery sag (voltage) and how much was actually due to "prop unloading".

Cheers, Phil
I know the battery has an effect on motor speed and it always will, but it can be taken into account if you have a big enough battery with a good charge on it so that the load on the battery is not so high. Yes you will still have changes in battery voltage, but they will be much smaller and can be put on a chart to compare the change of voltage against prop unloading. The motor itself will cause a change of voltage with different loads. That's why I was talking about an IC engine instead of a battery powered one. It's much easier to tell if the prop is unloading with an engine then a motor, unless you can build a constant current, constant voltage power supply that can maintain voltage and current regardless of the load the motor puts on it. The problem most people have with voltage drop is because they have a battery that puts out 50 amps, but they are loading it to 40 amps. Try a 50 amp battery and put a 20 amp draw on it and the voltage sag is MUCH less.

I know the engine will speed up because of the fuel mixture changing during flight, but no where near 10,000rpm. Static rpm while adjusting the fuel flow goes to 23,500rpm and then decreases, so you back up the mixture to 23.000rpm. Now, on take off the motor starts to unload and speeds up, partly do to the mixture changing, but mostly do to the prop unloading. If the best you can get on the ground is 23,500rpm and it changes to 30,000rpm in the air, then that is not mixture change causing the rpm to increase that much, but prop unloading.

The point that I was trying to make is, prop loading or unloading has nothing to do with voltage sag, motor load does. If the chart has amps and voltage recorder on it and the rpm can be logged, then you have a way to measure the overall effect of prop unloading. You must chart rpm, voltage, and current to be able to tell if the prop is unloading. If the load on the motor decreases, then the amps will drop and the voltage will increase. This cannot be done very accurately with a heave load on the battery, you need a lite load, or the power supply I was talking about.

If a wind tunnel is used, then you must know the air speed of the plane and be able to control the air flow in the wind tunnel to the point that you can match that. Then you can use a GOOD power supply and run your test.

Now I will admit that I am new to electric planes and I have a lot to learn, but I have been flying IC models and full size planes since the 50's and I understand general flying and prop unloading. I know a prop changes load during and after take off. The only problem with proving it is using a proper power supply with the motor such that the battery is taken out of the circut altogether. As long as the battery is able to change voltage because of load, you will never prove prop unloading to the point of setteling the issue. The IC engine comes closer to that then a battery ever could.

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