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NUMBER OF BLADES
Basic Discussion

The number of blades in a wind turbine rotor can vary, and there is no "Truth" as to the best concept. It depends on what machine the rotor has to drive, at what wind speed you want to get started, and how you are able to produce the blades.

You can design wind turbines with just one blade. The problem is that this concept is hard to balance. It must run very fast in order to pick up the necessary lift. This creates noise from the tips and gives severe dynamical vibrations, hard to control in the blades and in the rest of the construction.

A two bladed concept is easier to balance, but still has some dynamical hangs. The rotor also yaws quite abruptly. It can be hard to get a two bladed rotor started in very low winds (3 m/s).

A three bladed rotor is easy to balance and yaws smoothly.

Two and three bladed rotors have only moderate starting torques, which, when it comes to smaller wind turbines, might create problems if you want to start constructions incorporating a transmission with a ratio higher than 1:5 in low winds. (When it comes to larger wind turbines, the controller either start them up as a motor, or they run idle until the rotor has enough power to match the generator load.)

Multi bladed rotors like old American water pumping windmills have blades with a strong twist at the tip. This is against the theory of making efficient blades, but it provides a big starting torque in order to get the rather heavy mechanical pump started. Once started the rotor keeps on working with a rather low efficiency. Efficiency, however, is not considered important in this case. A steady pumping of water is considered important.

This is not to say that multi bladed rotors can not be designed very effectively. In the theoretical windmill literature the maximum obtainable rotor efficiency is 0.6 . This is called the Betz efficiency, named after the man who calculated it on the basis of a rotor with an "infinite" number of blades. The rotor efficiency is also named Cp, and in real life the highest obtainable Cp values are around 0.5.

The 12 bladed Windflower rotor, for example, operates with a measured maximum Cp value of 0.47, which is very good. Having 12 blades the rotor do not have to run so fast in order to pick up the needed lift. This gives a very quiet rotor. It also gives a good starting torque, which makes it possible to start wind turbines operating a transmission of 1:10 (max) in low winds.

Two and thee blades rotors have rather thick blade profiles. If you want to do smaller rotors in in wood this does not mean anything, but if you want to do larger composite constructions (glass and coal fibres/polyester, vinylester, epoxy) you often end up with quite advanced and expensive constructions.

Multi bladed rotors have rather thin profiles, which allows small and medium sized composite rotors to be pressed as solid constructions.

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