Here is an alternate method:

First you need to figure out what you want to end up with:

1. What prop rpm do you want to run? A small prop at 12k rpm or a large prop at 10k rpm?

I'm going to pick 10k rpm because they work better for 3d flying.

2. How much performance do you want? 125 watts/lb to 225 watts/lb going from mild to ridiculous.

I'm going to pick 200 watts/lb.

3. Now that we know what we want performance wise we need to figure out how much total power we want:

I'm going with 200 watts/lb and I'm looking at a 40 size airplane at 5lbs so I'm looking for 1000 watts.

4. Now we need to pick what size battery to run. Here are a couple of different options:

volts x amps = watts.

3s battery = 11.1 volts

4s battery = 14.8 volts

5s battery = 18.5 volts

6s battery = 22.2 volts

So with my 1000 watt target I can do any of the following:

1000 watts / 11.1v = 90 amps

1000 watts / 14.8v = 64.57 amps

1000 watts / 18.5v = 54 amps

1000 watts / 22.2v = 45 amps

So now that I know how much current (amps) I need from each battery, I can figure out the minimum pack size based on which battery I want to run. So if I go for a 20c battery (I never want to discharge faster than 20 times the battery size) I get:

(3s) 90 amps / 20c = 4.5 amp/hr or 4500mah

(4s) 64.57 amps / 20c = 3.2 amp/hr or 3200mah

(5s) 54 amps / 20c = 2.7amp/hr or 2700mah

(6s) 45amps / 20c = 2.2 amp/hr or 2200mah

Now discharging at 20C is reasonable for a light 3d airplane, but will give short run times. You can double this for a sport airplane and get really long run times:

So for a 10c discharge you get:

(3s) 90 amps / 10c = 9 amp/hr or 9000mah

(4s) 64.57 amps / 10c = 6.4 amp/hr or 6400mah

(5s) 54 amps / 10c = 5.4amp/hr or 5400mah

(6s) 45amps / 10c = 4.5 amp/hr or 4500mah

As you can see, weight and run times are a direct trade off.

I have some 4s3900 packs on hand for another airplane so lets use those.

5. Now that we know what battery we are going to use now we pick out the ESC.

This is really simple, we need something that can handle 70-75 amps on 4s.

6. Now we can pick out the motor:

To get the kv we just divide the target rpm by the volts:

10k rpm / 14.8v = 675kv motor

To get the size we figure a good motor can do about 3 watts per gram:

1000watts / 3watts = 333g motor.

7. Now that we know the battery, motor, watts we just need to find the prop to use. I like to use a calculator for this:

http://www.ecalc.ch/motorcalc.htm?ecalc&lang=enSet the field elevation to 0 and OAT to 65F

Put in a 4s4000 30c battery

Choose a motor that is around 675kv (I used a scorpion SII-3032-690)

Then start playing with props until you get something around 1000watts

I found the APCe 14x7 to come out at 818W and the APCe 15x7 to come out at 967 watts.

That is your starting point. I would probably pick up a 14x7 a 15x6 and a 15x7 and use the watt meter to fine tune.

Hope that helps,

schu