Electric Conversion Step 2
Electric motor KV rating
The second step is to determine the KV rating you want.
The KV rating means RPM per VOLT of input power. A 1000 KV motor turns 1000 rpm on 1 volt of battery power or 10,000 rpm on 10 volts. This is critical because it influences everything else you need including the number of battery cells, prop size, esc rating, etc. It is really a balancing act between the different needs including prop ground clearance.
For a starting place I'm going to assume you want an approximately normal prop size for the nitro engine that is being replaced. Let's say a 10 X 6 for a 40 size conversion ( 500 watts ). the normal RPM range is about 12,000. So to keep the same range we need a KV rating times battery voltage that equals 12,000.
If we want to use a 3 cell lipo battery the working voltage under load is around 10 volts so 12,000 divided by 10 volts equals a 1200 KV rating motor rated for at least 500 watts
If we want to use a 4 cell lipo battery the working voltage under load is around 13.7 volts so 12,000 divided by 13.7 equals a 875 KV rating motor rated for at least 500 watts
If we want to use a 5 cell lipo battery the working voltage under load is around 17.1 volts so 12,000 divided by 17.1 equals a 700 KV rating motor rated for at least 500 watts
The same math will work for any size motor. Volts times KV rating = RPM
Of course lower voltage batteries are considerably cheaper, as are the ESC's (electronic speed control) which replace your engine servo. The choice is all yours!
If you want or need to use lower voltage batteries there are options but basically you need to compensate for the lower rpm with a larger diameter higher pitched prop or find a high KV rated motor. Unfortunately high KV motors in high power levels are very hard to find!
A handy formula for prop pitch is (rpm in thousands) times (pitch in inches) times .95 = top speed of the airplane. Examples: 8000 rpm and 8" pitch prop. 8 X 8 = 64 X .95 = 60.8 mph. 10000 rpm and 6" prop 10 X 6 = 60 X .95 =57 mph
Increasing the prop diameter from a 10 X 6 to an 11 X 7 will give the same thrust and speed at a lower rpm of 10,000 instead of the 12,000 we used in the above example. Most 4 stroke engines run at 8500 to 9000 rpm and use larger props to compensate. You can do the same thing with electric motors. You may have to modify your landing gear or tire size to get ground clearance but it's possible and a lot cheaper than 5 cell lipo's!
Lets say your stuck with a Power 60, 250 KV motor. To get our desired 4 stroke equivalent 8000 rpm we will need a 10 cell LiPo battery or 2, 5 cell LiPo's in series to make the needed 37 volts.
Going to a 3 bladed prop does something similar but without the ground clearance problem. To convert from a 2 to a 3 bladed prop just subtract one inch from the diameter and add 1 to the pitch. A 10 x 6 converts to a 9 x 7 3 blade for example. My 40 size Uproar will hang vertically on a 12 x 8 3 bladed prop @ 7000 rpm with a 3 cell battery, and that was $10.00 conversion. Yes I am cheap!