Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: syhprum on 19/11/2007 16:45:25

Title: Windmills
Post by: syhprum on 19/11/2007 16:45:25

Something less esoteric than blackholes, Gravitons and higgs what nots.
Why do wind generators have three blades ?.
Aircraft propellers vary from two to six while the oldie worlde wind pumps out on the prairie had about a dozen.

Title: Windmills
Post by: lyner on 19/11/2007 18:05:18

This seems to apply to bigger turbines. I wonder why.
I have a small,  2A 12V turbine on my boat and it has six blades.

Title: Windmills
Post by: daveshorts on 19/11/2007 18:10:15

a blade on a wind turbine leaves behind it a vortex, like the wing-tip vorticies you see on planes
 [ Invalid Attachment ]
These will be left behind the turbine in a spiral. The more blades you have the more these interact with each other and the other blades reducing the efficiency of the system. so if you are not limited in the area you can cover it is best to use fewer blades.

You get a similar issue with a propeller, but making the blades longer has 2 much larger disadvantages. The size of the propeller is limited by the height of the plane, and more fundamentally if you make the propeller large the blade tips will go through the sound barrier which is very inefficient, while the centre is still moving quite slowly. So for an aeroplane it is best to use more shorter blades.

Title: Windmills
Post by: syhprum on 19/11/2007 19:53:47

I remember during the war when the Americans got hold of our spitfires they kicked out the beloved 27 liter merlin and fitted a 40 liter radial.
This required a six blade prop to absorb the extra power

Title: Windmills
Post by: lyner on 19/11/2007 22:07:13

Quote

I remember during the war when . . .

I will have to start calling you Uncle Albert!