"Rod Speed" wrote in message
...
Paul wrote
Brian Gaff (Sofa) wrote

How can you tell, I've no idea what your picture shows, does it have a
blade missing and by the way why do turbines not use more blades?

NASA made an experimental turbine, with just
one blade, and a counterweight. Someone has
tested that idea.

They do have turbines with two blades. The tower
wobbles more, when the top blade is in full wind, the
bottom blade is shielded by the tower. That torques
the hub and tower.

Apparently, adding the third blade only adds 3% more power. But the
torque ripple in the tower is less.

Additional blades would only add more mass and drag.

But doesn't explain why the old windmills as opposed to windmill water
pumps normally had 4.

Maybe its as basic as that with the old windmills it was
more practical to have two very long timber beams
which crossed at the middle. Much easier to do it
like that than to do 3 sails.

The next number of practical blades would be five.

That's not right.
One article on wind turbines, mentions that the design
is all about "number of flexures". And the blades are only
good for about 100 million flexures. Each rotation counts
as a flexure for the blade, it is constantly being worked
(and ruined). Same goes for the tower, it takes a lot of
abuse too.

The Dutch windmills are different. There's an entire article
on "Windmill Sail" design and mechanisms for automating the furling in
high wind (spring-loaded shutters).

But doesn't explain why they had 4 and not 3.
https://en.wikipedia.org/wiki/Windmill_sail