It's just allot of guessing and seeing what someone else did after making a few mistakes. You would think by now someone would at least come up with a KV to wing area chart.
kV has practically nothing to do with wing area, aside from the fact that it tends to trend down as motors get bigger because the number of cells in a pack trends up. kV says how fast the motor spins in response to voltage, but there's no physical reason it has to be one number or the other -- it really boils down to the fact that small planes are easier and lighter with a few cells, and big planes are easier and lighter with lots of cells.
Select the kV to get the RPM you want with the prop pitch and number of cells you have. Most people run around 9000 RPM; you need to juggle cells and kV to get that. Select numbers such that
kV > RPM / (0.8 * cells * 3.7V)
The 0.8 in there insures that toward the end of the flight, when the battery is sagging, you still have plenty of overhead to turn the prop. Note that kV and cells are both, at this point, free quantities -- so you can work it in reverse, and calculate
Note also that the 80% figure is for Hubin and Renicle timers. An Igor Burger timer may need more overhead than the 80% figure provides -- if you're using his timer, ask him, I don't know, it's not my job.
cells > RPM / (0.8 * kV * 3.7V)
For a five-cell battery pack and 9000 RPM, this works out to kV > 608 RPM/V. For an 800 RPM/V motor and 9000 RPM, this works out to kV > 3.8 cells (use 4 -- 3.8 cell packs are hard to find). The more "excess" kV you have the less efficient things get, but having one more cell than is "optimal" isn't the worst thing that could happen to you, and then you have more overhead.
Select the motor's top rated power by the ready to fly weight of the plane (including motor & battery). I once totted up all the planes in the "list your setup" and came up with 11 watts per ounce of plane weight:
PMAX = (weight) * (11 Watts/oz)
For instance, a 63 ounce plane would need a motor with a maximum power of 700 watts (give or take). Note that this is for a Hubin timer -- an Igor timer demands more energy, but I don't know how much.
From the same "list your setup" I came up with about 7 watts per ounce of weight, again with a Hubin timer:
Pav = (weight) * (7 watts/oz)
For that same 63 ounce plane, that works out to around 450 watts.
Figure out the average current:
Iav = Pav / (cells * 3.7V)
(Yes, Virginia, this varies by cell count -- live with it). For a five-cell, 63 ounce plane, this works out to 24.3A.
Multiply the average current by your desired flight time to get total charge:
Q = Iav * time
For a six-minute flight (1/10 hour) and a 24.3A average draw, this works out to Q = 2.43 amp-hours.
You want the battery to end the flight with
at least 75% charge remaining at least 20-25% charge remaining (unless you like buying lots of batteries).
25% charge remaining means 75% charge used, so divide the total charge by 0.75
or 0.80 to get the battery's rated capacity:
Qrated = Q / 0.75.
For our 2.43 A-h flight
and 75% of the charge used, this works out to 3.24 A-h, or 3240 mA-h. So, go shopping for 3300 mA-h, 5-cell packs.
If you have a motor with a kV significantly higher than 610 RPM/V, you may want to go with a 4-cell pack -- so, do all the calculations over again. Ditto if your motor has kV < 610 and you need six or seven cells (although, given what's commonly available that probably means that your motor is bigger than you need for any reasonable stunt plane).
Edited on 7-15-15, thank you Igor for rubbing my nose firmly enough in my error to make me actually look.