Here is the setup: AXI gold 2820/10, 3S 4200 mAh batt ( 3S recommended by AXI), JETI 45 Advance opto-plus ( 40 recommended by AXI), 11x6 e-prop ( recommended by AXI). Here is the problem: at full throttle, according to the watt meter, V=12.47, A=56, and W=610...all numbers are give and take a few. I'm not sure what the rated burst current is of the speed control, but for the motor it's 42A...I'm also not sure if the current is 56A at the motor, I dont think so, but not 100%. Will dropping the prop size down to lets say 10x6 reduce the amp draw enough to get those amps down? The motor and ESC were warm after the short full throttle run. Thanks for the input?

Pat

Pat -

I'd get the smaller prop and see what happens. Props are inexpensive enough that it is worth a try. The current will come down, but I don't know by how much.

- Jeff

Jeff,

Thanks for the reply. I'll give it a try and post what happens.

Pat

OK! The first prop used was a Master Airscrew e-Prop 11x6. I had on hand an APC e-prop 11x5.5 so I put it on just to see if there is any difference; honestly didnt think there would be, but it was worth a try. Using a fresh 3S 4200 at full throttle the readings were as follows: 11V, 45A, and 510W. ( NOTE: The volt reading in the first post was static voltage, not at full throttle, my bad!)

Could there be such a difference between the MAS and APC props? I certainly dont think a small change in prop pitch would make a 10A difference. Can anyone enlighten me? Thanks!

Some props are more efficient than others. The more efficient the prop, the lower the current requirement.

It is quite possible for two props of the same size but from different manufacturers to have very different efficiencies. So the results you are seeing are probably real.

To see some examples, try the Babcock calculator:
http://www.badcock.net/cgi-bin/powertrain/voltprop.cgi

You can select your motor and then try different props. You'll find differences in current consumption even when the prop sizes are the same. In some test results I saw on another forum, the MAS e-props were less efficient than APCs. But even within the same brand, certain sizes are more efficient than others.

It pays to test, as you are learning.

- Jeff

Jeff,

Thanks for the input and that webpage, I just added it to my favorites. I think I'm still going to try a 10x6 APC prop since the Amps are right at the max continuous for the ESC; I very rarely fly at full throttle except on takeoff ( sometimes not even then) but it's a little too close for comfort. According to the webpage a 10x5 will be right around 35 amps, which I can deal with; just hope the plane likes it .

Does anyone know the amperage delivered to the motor? Is it the same as what we read on the watt meter?

Thanks,
Pat

Pat -

The reading you see on the wattmeter is the current the motor/prop is drawing, minus the small amount of current drawn by the ESC. Think of it this way: When you throttle down all the way, current measures zero. When you throttle up, the motor/prop draws current from the battery. So the current you measure is the current being drawn by the motor/prop.

However, motors and props aren't 100% efficient, so some of the current drawn by the motor/prop ends up being used to create heat. In simplest terms, all of the current you measure is being drawn by the motor/prop, but not all of the power thus produced goes into moving the airplane. Some of the power ends up wasted as heat. The worse the efficiency of the system, the greater the heat produced. Any power wasted as heat is power that does not do anything to propel the airplane.

Think of current as something that the motor/prop draws from the battery, not something that the battery pushes into the motor.

- Jeff

Jeff,

Great information, thanks! I switched the prop to an APC 10x7e and here are the readings at full throttle: 10.85v, 41.7A, and 450W. Looks like I will be using the 10x7 as it is under the motor max amps and esc max continuous, even though it is close to the border line...I will just have to keep this in mind while flying. I will also be getting about 95W per pound, so still pretty good.

Test flight is tomorrow, so hope all goes well!

Until then,
Pat

I think you are doing the right thing testing with different props. One thing I don't see specified by you is the c rating of bat. that is also a n important factor. One more thing is the static tests will likely be higher than when in the air where it can unload. You should do fine.

TedMo,

Thanks for the info. I didnt realize that the system would be unloaded in the air, that's good to know. The C rating is 20.

The first flight went okay[X(] I had to adjust the cg a little more nose heavy. After that it was a real joy to fly. I flew three more times for 7 minute flights. Didn't charge the packs yet, but will be interested in how much the packs take after charging. I use the same packs with an E-Flite 25 and after 8 min. flights use just over 50%. I fly about 1/2 to 3/4 throttle most of the flight.

Thanks for the help and info. Jeff and TedMo!

Pat