Talk about efficiency all you want. Number one, I dont think it makes THAT much difference on our R/C aircraft. You have to have a pretty large increase in efficiency to gain a small increase in speed. Number two, efficiency is not what propels your aircraft, thrust does. All efficiency is, is the ratio of power being produced by the prop to the power going into it. A two blade prop is capable of achieving higher efficiency than a three blade, but at the same time it uses less power and produces less thrust. If you operate a prop at a lower power setting than that where the efficiency is at it's peak, you achieve lower thrust and lower efficiency. Likewise, if you operate at a higher setting than peak, thrust will be higher, but efficiency will still be lower. The optimum power setting is where efficiency is at it's peak, we all agree on that one. BUT, if conditions require more thrust than is available at this power setting, then the power must be increased, and the prop efficiency begins to fall off. You finally reach a point where a prop operating at a power higher than that which results in peak efficiency actually has the same efficiency as a prop with more blades that is operating at less than optimum power. Any further increase in power favors the performance of the prop with more blades, because the prop with fewer blades is no longer operating at peak efficiency. Now, I'm not an engineer, just a dumb ole' "Stick Jockey" who cant be taught anything, but in 3,000 hours of flight time, I've flown quite a few two and three bladed aircraft. You give me two identical aircraft, one equipped with a two blade, one equipped with a three blade, and the three blade equipped aircraft will out perform the other in everything from time to climb, right on up to higher cruise speed. But, as I said before, most of this is a mute point when you are talking about R/C aircraft, at least it seems so to this dumb, uneducated "stick jockey". Oh, and by the way, reducing diameter is not the "ONLY" reason manufacturers use three bladed props. Two blade propellers produce an inherent once per revolution vibration that shakes the airframe. A three bladed prop eliminates this vibration, therefore reducing a small amount of airframe fatigue, reducing cabin noise, and making for smoother operation. Also in a single engine aircraft, the blade wake from a two blade prop beats on the windshield producing noise. Changing from two to three blades reduces the wake intensity and increases the frequency of the beat, and is percieved to be quieter in the cabin. Hmm, I guess maybe some of us dumb ole hick "stick jockeys" do know a little something.