In one of the other threads, Igor B. and I were talking about the efficiency of flying around at partial throttle. I made a comment that I was seeing roughly 5% effects in power usage by flying at partial throttle as opposed to using something closer to WOT (of course at the same rpm level).
To make it clearer, what I mean, is suppose you can fly the full pattern at a rpm which makes you happy---using a 3s battery. Is there any penalty hit by replacing the 3s with a 4s battery and leaving the motor alone, and still targeting the same rpm level? Another way of looking at this is suppose you have a 4s battery, is there any reason to try and optimize the motor by choosing a different e.g. lower--kV motor to match the 4s voltage?
Just to check this out again, I setup a test bench in the basement with my Scorpion 3020-16 motor. I mounted an APC 11-5.5 prop and ran the setup in a pusher mode--just to keep the "breeze" blowing away from all my setup stuff. I used my standard Castle Creation Phoenix 35 running in rpm governor mode (I think set at 7950 rpm). It turns out that the 11-5.5 prop running statically more or less gives the same power levels (~250 watts) that my E-Nobler uses in level flight with the APC12-6 TE prop (at same rpm).
The following narrative refers to the attached plot.
I first connected a 4s2100mAHr pack to the setup and ran for about 4 minutes. The battery voltage is shown in the white trace. After that, I quickly took off the 4s and put on a 3s4200 pack and started it back up again. This I ran for about 7 minutes until I finally detected the rpm to start to drop. At that point I was presumably at WOT since the governor couldn't keep the rpm at the set value--the battery voltage had sagged. Unfortunately for this plot, my Eagletree data recorder had run out of memory, so you don't see the rpm sag, but please take my word that the battery voltage was only a 0.1V lower than what you see in the white trace at the end of the plot.
The power draw for both batteries is the light blue trace. You can see that when using the 4s pack, the average power was 248 watts, and with the 3s pack, the average power was 237 watt, or 11 watts different. I will also note that the average rpm for the 4s pack was 7969 rpm and the 3s pack was 7954 rpm (15 rpm lower). Ignoring this slight rpm difference and attributing the full 11 watts to the intrinsic inefficiency of the throttle (not completely obvious that this is ok--but it gives the maximum inefficiency), I see that the efficiency difference is ~5%, which I am thinking is pretty insignificant in the big scheme of things!.
The yellow trace is just the average current in amps. Since the ESC was basically at WOT at the ~11 minute mark, that says my motor current at ~7050 rpm is about 23 Amps.
Since the battery volts at 11 minutes is ~10.2V (at WOT), I think that means when I am flying a 4s pack with this motor, (pack voltage ~14V near end of flight), my throttle setting is just ~10V/14V or ~70% during level flight.
There are some curiosities in the plots that I don't fully understand, especially the lower power usage at the beginning of the flight at full rpm (seen with both packs). I am guessing it is some ESC heating but am not sure.
Also there are some other side effects to using a "mismatched" motor/battery pack, mainly in battery heating which isn't really shown in this data, but in general it certainly looks like we can getaway with reasonable mismatches in motor kV (too high) and pack voltage (also can be "too high").
Finally I note that I can't use a 3s pack in a real flight with this motor since I need the extra throttle capability to handle the higher power during the maneuvers. So any attempt to match the kV a little better would bring less than 5% since you always need that extra "oomph" of more throttle when you pull the nose up.