It's generally accepted that everything else equal (same blade profile and airfoil) a two blade is more efficient than a 3 blade. A lot of this (if not all) has to do with the length of the blades. Just like a wing, the longer they are, or more specifically the higher the aspect ratio of the blades, the less induced drag. Since 3 blades of the same length also have more parasite drag, blade length of a 3 blade prop will be less than a 2 blade for the same engine. When you get north of 300 HP, two blades cannot absorb the power efficiently so 3 or more blades are needed despite the penalties but the further below 300 HP you go the more you lose with extra blades. On top of that, most 3 blade props weigh about 25% more than the 2 blade prop they replace. If the airplane is tail heavy this can be an advantage, but otherwise it cuts useful load and speed. Three blade props are also purported to be quieter, although I believe there is little truth to that concept. Instead, they produce noise with a 50% higher fundamental frequency which is generally easier to attenuate so the effect is less noise in the cabin and/or behind ANR headsets. And in some cases the reduced diameter provides a much needed increase in ground clearance. But more often than not the primary reason people put 3 blade props on engine/airplane combinations that don't require them is because they like the look.
That said, I believe some of the newer 3 blade designs are actually as or more efficient than the "standard" two blade.
Wow, great summary, Lance. Thanks. I knew that you needed more blades for higher power, but I thought the only reason you didn't use more blades at lower power was cost. Silly me.
I'll take a stab at why you need more blades for higher power since somebody asked. I'm sort of making it up as I go along, so please correct me if I go astray.
There's two ways to look at it. First, from the engine's point of view, power is equal to rpm times torque. The torque will be a function of the blade lift and therefore of the coefficient of lift of the blade, the rpm (again), and the blade area. The parasite drag will also be a function of blade area. The problem is, to get the torque up for a fixed rpm you need longer blades, but the longer the blades get the closer the tips get to the speed of sound. At something like Mach 0.8, you get a massive increase in prop drag without an increase in lift (i.e. thrust) because you start getting local areas of supersonic flow and shock waves.
The result is that you can only get so much power into a two blade prop without running into tip speed problems. A third (or fourth) blade means that you can get the blade area and therefore torque up without increasing the prop diameter, so you don't have the tip speed problems.
Another way to look at it is to consider that the power is also proportional to how much air you're able to throw backwards. What throws the air backwards is lift from the prop. To grab more air with your two blade prop, you need to increase the rpm or the diameter. But we've established that there's a limit on how much you can increase either without running into tip speed problems. Again, the answer is to add another "wing" to grab that air and throw it backwards.
I have no idea if that really makes sense to anyone else, but it more or less does to me.
Chris
