Hi Gang,
So what you have is a battery that forces you into a 3S-2P, 9.6 V, 4400 mA-H battery OR into a 6S-1P, 19.2V, 2200 mA-H battery.
What we have been running otherwise is a 4S 4200 LiPO at 14.4V with an AXI 2826/10 with a Kv of 920. All harmonizes decently with this combo and a 12-6 prop at 9000 RPM give or take a few hundred.
Igor's 6S setup was inefficient, as he says, because he needed a motore with a Kv of closer to 690, not 920. 920 X (14.4V/19.2V)= 690.
Then it would have been happy with a 12" prop.
Conversely, a 3S A123 will need a Kv of around 1440 to run the 12-6 at exactly the same R's. 920 X (14.4/9.6)=1442. Well, the 2820/8 has a Kv close by at 1500. There is a Neu motor thaat would be better, and close to twice as pricey, but that's another matter.
Assuming there is no big mismatch of battery voltage, Kv, then the total energy available in the battery and the all-up weight dictates the maximum prop diameter. As Igor said, the AXI 2826/10 was not a good match to the 6S A123. You'd need a 2826/14 or 15.
later,
Dean