Consider the watts per pound guideline:
You can determine the power requirements of a model based on the â€˜Input Watts Per Poundâ€™ guidelines found below, using the flying weight of the model (with battery):
50-70 watts per pound; Minimum level of power for decent performance, good for lightly loaded slow flyer and park flyer models
70-90 watts per pound; Trainer and slow flying scale models
90-110 watts per pound; Sport aerobatic and fast flying scale models
110-130 watts per pound; Advanced aerobatic and high-speed models
130-150 watts per pound; Lightly loaded 3D models and ducted fans
150-200+ watts per pound; Unlimited performance 3D models
So, if you have a plane that weighs in at 25 ounces. That works out to roughly 1.5 pounds. To fly this at sport aviation levels, it will take 1.5 X 110 watts or 143 watts. If you use a two cell LiPo, that's 7.8 volts (nominal). 7.8 volts works out to a battery capable of supplying at least 18 amps. So, a 2000 mah pack is pretty much minimum for this plane, and you have to consider how you want to fly it at that and that's where the C factor comes in.. how long and how much burst power do you want/need? A three cell Lipo will need less current.. but you get the idea, i'm sure.
It's not as simple as just putting a battery on the plane and trying it out. You chance burning up both the motor and the ESC with the wrong selection.
Remember that when making weight measurements (not estimates, unless you know exactly what each component weighs), everything has to be considered: airframe, motor, esc, battery pack, receiver, servos, everything.
Next, there is a prop load to consider. OpJose posted a very good outline about this. This is a cut and paste from his post:
Actually it's quite simple...
To get 1:1 you want around 120-130+ watts per pound maximum ( burst ) power.
So if your plane is going to weight 15.2oz ( as projected by the manufacturer ) then take a figure OVER this and multiply by 130watts...
So let's say the plane will weight 20oz AUW just for safety....
1.25lbs x 130w = 162 w
So a 180 watt motor will do the trick.
Assume that you'll use an 11.1v LiPo pack...
180w / 11.1v = 16.3A
A 20A-25A ESC will work fine
Thust HP says that a 8x6 prop spinning @ 11000 RPM produces about .234 HP
1HP = 745w
.234 * 745w = 174w ( right in the ballpark! )
So we can use an 8x6 prop and spin it at 11000 RPM.
11,000 RPM / 11.1v = 990KV or approximately 1000KV
I'd use an 1100KV motor to add a bit more power.
Finally let's find the battery we'll need... we'll want to drive it only to a maximum of about 15c if it is rated for 20C.
16A will be our maximum draw
16A * 20/15 = 21A @ 20c
So a 1000mAh to 1200mAh 11.1v 3S pack would be the smallest you can use... I'd go with something around 1300mAh+ for safety. 1800mAh would give you the flight times you want but adds weight.
So there you have it.
1300mAh+ 3S 11.1v LiPo
20-25A ESC with BEC
1100KV Motor rated up to 180-200watts burst for 15-30seconds. ( look for a 150w motor or so ).
I don't want to oversimply this and insult your intelligence, but, here is the basic math for watts, amps, and volts:
Ohm's law (algebra) :
Volts X Amps = Watts
Watts/Volts = Amps
Watts/amps = Volts
Easier stated, multiply volts x amps you get watts. 11.1 Volts - three cell LiPo multiplied by 20 Amps drawn = 222 Watts
222 Watts divided by 11.1 Volts = 20 Amps
And so on.
I believe this is a German site, but the information is there and you can use the calculator to determine prop size.
The person that said electric flight was easy must have gotten a A in college Calculus... [X(] [&o]
I hope this helps.