Well the answer to this is quite confusing, but can be very simple! In simple terms, a 3 blade prop for RC use generally run more effeiciently than a 2 blade prop. Before everyone starts jumping on me let me explain!

1. 3 blade props will generally require less energy from the engine to turn it do to a smaller thinner blade, thus less fuel burn. Now the down side is that a 3 blade prop doesn't hold it's rpm very well in aerobatic manuevers, thus not allowing the airplane to retain energy through the manuever.

2. A 3 blade prop is generally designed to run at a constant rpm for cruise flight. Do to aerodynamic energy, everytime you make a turn, the lighter weight prop will either bog down or speed up slightly. For most RC flight this is not that important, except for aerobatics.

3. 3 Blade props can genarlly be made to make an airplane run a little faster do to slightly less drag on the prop and slightly lighter weight.

4. A 2 blade prop is generally thicker in blade size, longer in blade length, and heavier for the same engine size requirements. Therefore, if you plan on doing an aerobatics, a 2 blade prop will carry a more constant rpm giving you airplane more torque through manuvers. This extra torque generally pulls an airplane through a manuver with less energy loss.

5. 2 blade props require more energy from the engine to maintain thus a higher fuel burn rate than a 3 Blade.

Now all of this being said, for RC use a 3 blade prop will make the engine run easier, faster, and with less fuel burn, but with a loss of avialable torque than a 2 blade prop. A 2 blade prop will generally give you better climb, stronger manuver performance, and more drag than a 3 blade prop.

Now if you want an airplane engine to run for longer, faster flights go with an appropriate 3 blade prop. If you want stronger, slower more aerobatic flight go with an appropriate 2 blade.

This is all assuming that you are using the appropriate 3 blade or 2 blade prop for your engine!

Reg