Underperforming Lipos or something else ?

[John Olsen 1]

That set of curves is rather like the ones for power transfer from a source with internal resistance into a load, for the excellent reason that the situations are similar. In the motor/ESC case, the source resistance is of course made up of multiple factors, lead resistance, resistance of the MOSFETs in the ESC and so on, and it would include the resistive losses in the motor, but the point is that maximum power into the load will occur when the power into the load is equal to the power being dissipated in the source resistance. So that implies that the several hundred Watts going into the load would be equalled by the same amount being lost in the circuitry. So John P and PatMc are very right when they point out that the right hand side of the graph would be pretty academic. In fact anything much to the right of the rated operating point will probably only be seen very briefly, eg until the system is obscured by smoke.

The efficiency in itself does not tell us much, a system with an output of 90 Watts with 100 Watts total going into the circuit would be quite efficient (eg 90%) but would not be delivering enough power in this case. We would want a lot more throughput even if the efficiency was a little lower.

Going back to the original problem, if you want the maximum power and the maximum possible current is fixed by either ESC or motor ratings, then for the different possible battery Voltages you are going to need different propellor sizes to load the motor correctly. Since the maximum current is fixed, the possible power is going to be proportional to the Voltage, and with this type of motor the speed will also be proportional to the Voltage. But the power absorbed by a propellor is not proportional to the speed, it is proportional to the square of the speed. So if you halve the Voltage, you will halve the speed, but the propellor will only absorb one quarter of the power...hence the current will also halve. Since we are actually allowed to keep the current the same, we can increase the propellor size to absorb more power,

This may seem counterintuitive, eg the same motor and ESC will need a bigger propellor to get the maximum power when using a lower voltage battery. (At the same maximum current.) Note that the propellor size will not need to be double, in fact the increase will be quite small since the power absorbed by a propellor is proportional to the fourth power of the diameter.

I just had a case where I inadvertently bought a 300kV motor when I should have bought the 500kV model. That meant that the motor was only turning at 3/5 of the speed it should have been. To get enough power I was able to increase the prop size from 13 to 16 inches, which doubled the power, giving good performance. Luckily the larger prop was able to be accomodated! An alternative might have been to go to a higher Voltage, but it is already 6S and the ESC would not want much more.

So a bigger prop size will certainly give more performance, but the maximum will be determined by how much current you can safely allow.

John O

[Steve Colman]

I'm overwhelmed guys and certainly didn't expect to see such in depth analysis.

Can I then concur from all of the above that it would be worth trying a larger prop, or two ? Seems to bo the simplest and cheapest option at the moment.

Thanks for all your input gentlemen.

Steve.

[PatMc]

Yup.
But do a ground run using the props with an ammeter connected making sure it doesn't draw over about 40A at WOT before flight testing.

[Steve Colman]

Thanks PatMC,

Will pay a visit to my LHS later this afternoon to see whats available. Luckily it's just 100m from my house.

[Steve Colman]

Update,

Just a quickie to say a big thank you to all who contributed to my questions,

I have since replaced the motor and now have all the power I need on the same lipos.