ORIGINAL: asupervee72
20720 assumes 100 efficiency which I know is not the case with any motor, especially a hobby king motor. So 20720 x .80 is 16576. I do have 2 3700 mah packs and several 4s 4000 mah packs but they will not fit.
Nope, you're confusing electrical efficiency with KV RPM's they are not the same thing.
Your motor will still run at it's KV rating even with a low efficiency. The big difference will be that more current is demanded from the battery to do that and much of that may be lost as heat.
20720 is actually a bit conservative since LiPo packs start off producing far more than their rated voltage and tend to operate around their rated voltages in normal operation.
As I said, all of this is ill suited to that plane, you should pull the electronics and install are more suitable motor and ESC.
Then put the motor into a bigger plane with a gear drive.