Previously the concepts solidity and tip speed ratio were revised in which it was
learned that as the number of blades is increased, the tip RPM and tip speed of the
rotor blade is reduced, but what other consideration must be taken into account for
selecting any given number of blades?
If look around carefully, we will see that most medium and large turbines are made
with rotors of tree blades, rarely having two or four or more, we have learned that
the RPM increases with less blades turbines, right?, that is correct however they're
also mechanical stability and structural integrity considerations in selecting not
even number of blades.
As a brief example we will consider a two blades against a 3 blades rotor.
We know from the previous pages that the wind normally increases with height, so
we will have two different forces hitting the rotor, one at the top blade and one
at the bottom blade, now let see what happens in a two blade rotor:
Based on what we know, F2 will be > than F1 as consequence F2 will push the blade
harder when in the upper vertical position causing a bigger bending as compared with
the one at the lower position, as expected this will cause vibration on the whole
rotor and in the turbine pole that can be highly destructive.
If we compare the tree blades rotor depicted below we can see that when one blade
is at the top vertical position, the other two will be at lower point, but they still
will be higher if compared with the lower blade in a two blades rotor visualized
in doted lines. This means that the blade in the upper position will receive a wind
force that is less different in power that the other two in the lower position, This
situation causes less bending difference in the lower blades as compared with the
ones in the upper position, as they will in a two blades rotor.