Old 10-09-2011, 04:03 AM
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Location: Wisconsin, USA
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Originally Posted by CT4 View Post
I have been flying for 40 years I am an instructor and I am authorized to issue permits to fly for oversize aircraft (anything over 12 Kg but under 25Kg in Australia). I am however a total newbi to electric flight with aircraft. I have been reading endless threads on line but I am having trouble understanding prop size to motor to amp draw. In order to try and understand this I have started using either the CC ICE data logger or the Eagle Tree logger.
The results I am getting are so far from those published by the manufacturer of the motor I am really confused and would appreciate some help.
Scorpion SII-3020-1110Kv (V2) motor CC ICE 75 ESC 3 cell and 4 cell 25c battery

Scorpion says with a ACP 10x5E prop on 3s I should pull 29A and with 4s should be 48A
I get 37A and 56A respectively. If I change to a SP 10x3.7 prop the current draw jumps to 71A

I have tried similar other setups with different motors and in every case the current draw has been a lot more than the figures published.

If anything what am I doing to get such a different result to the manufactures published results. I have tried Motorcalc but could not get it to work due to the data base not having ICE ESC or any of the current Hyperion batteries in its data base. Any help would be appreciated.
I've not used the Scorpion motors, but have found that the various $$$$ Hacker motors are fairly close to their published value.

The CC ICE ESC will have little effect on the actual current pulled by the motor/battery/prop combination. By far, with a given motor, the propeller and cell count in the battery will have the greatest effect.

So, rather than using a Hyperion battery, find another battery with similar capabilities in the motocalc battery database.

Now, for the Scorpion motor. What I've done is take a current/voltage/rpm reading off of the motor with a given propeller. Then compare that reading to the measured values that you've taken.

Then, go to the Scorpion motor specifications, save it as a different name, such as Scorpion my tests. Then tweek the KV numbers of the motor to match your actual tests. Now you've got a baseline on your Scorpion motor to try different props and so on in motocalc.

You will find it doesn't take much of a change in the KV numbers to really make a big difference in how much current that motor/prop/battery pulls. And, just a few percent more in battery voltage under load (with a higher quality battery) can also make a big difference in the current pulled by the motor.

Take a look:
Thread on 70 size glow engine conversion to electric


And, if you decide to go for real high powered electric models, you will find that the cost skyrockets! And, you've got to find a way to field charge the motor battery. It takes either an AC source, or a PAIR of 120 Ampere Hour deep cycle batteries. And those deep cycle batteries won't last much more than a year or two. Personal experience.
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