Electric rc airplane power prediction
With glow engines, the manufacturer's recommended engine size is generally more than enough power for the airplane. Electric airplanes tend to be a much more critical balance of airplane weight and motor/battery power.
Also electric motor and battery systems are much less standardized than their glow counterparts.
It's useful to know in advance if an electric airplane [especially one that you are building from a kit or converting from a glow plane] will have enough power to fly as you would like it.
There are some rules of thumb that can help with this. [Warning: Math ahead.] First you need to calculate the power produced by your system. First, find the number of cells in your battery.
For each cell:
NiCad, about 1.2V
NiMH, about 1.1V
LiPoly, about 3.7V
Multiply number of cells by voltage per cell to get total voltage, eg:
7 NiCd cells = 1.2 V x 7 = 8.4 Volts
Then find or measure the current through the system. Data are available for several motor/propellor/voltage combos:
http://brantuas.com/ezcalc/dma1.asp
http://www.flyingmodels.org/motortest/Link_e.htm
If you can't find the current "draw" for your system, you may need to measure it on your workbench, using a tool such as Astroflight's "Whattmeter".
Once you know the current, then use: Power (Watts) = Current (Amps) x Voltage (Volts) Now find the total weight of your airplane, including motor, battery and radio components: everything that flies.
Divide power by weight (in pounds) to get the "power loading" or "watts per pound":
Power Loading(Watts/Pound) = (Power (Watts) x 16)/ Weight (ounces) Some general guidelines (known as Shaw's rule)
For an electric plane to rise off the ground (ROG), you need about 50 watts per pound.
For an electric plane to do basic aerobatics, you need about 70 watts per pound. [At about 100 watts per pound, you start to not need wings :)]