Why is 2 propellers better than 3 propellers?

Answer 1

How do you mean that it is better? Each propellor is selected to produce the thrust based on the RPM and Torque output of the engine. A 3 blade propell will produce more thrust than a 2 blade in most general cases. Or another way to say it is that the 3 blade propellor can turn slower than a 2 blade to produce the same amount of thrust. Also, the 2 blade propellor is easier to design and build. My experience is with Helicoper rotor blades. The earlier Bell Helicopter designs were almost entirely 2-blade "teetor rotor" design. This was much easier to design the blade attachment, i.e. the rotor grip and trunnion mount, etc. Whereas Sikorski Helicopters preferred multi-blade designs and usually odd number: 3 or 5. The multiblade design requires more precise engineering. The multiblade for helicopters have to be designed to allow movement of the blade in all 3 axis and must have a dampener for each axis. This dampener can be a rubber absorber but they usually require a hydraulic shock absorber in the axis of rotation. The point being is that it is more complicated than a 2 blade design. A 2-blade helicopter blade doesn't have dampeners because of the special requirements of helicopter designs. Therefore the 2-blade propellor IS better because it is cheaper. If you mean two props (like, say, a DC-3 or a King Air) instead of three props (like a Ford Trimotor)... two props means two engines. It means 33 percent less maintenance, because it's easier to work on two engines than three. It means if someone gets stupid and noses-over the airplane on landing you have fuselage repairs to accomplish and NOT the replacement of an engine and a prop. It means you save the weight of an engine, so your payload can be higher. It also means the pilot isn't looking through a propeller.