Why do some windmills have two wings and others three?

Question

Does two or three wings give the greatest efficiency? When do they choose two and when three?

Answer 1

Three wings cost more to fab but provide a smoother rotation with less vibration than two. Smaller and cheaper windmills will use and get by with two blades. Larger and better windmills will use three.

As a blade rotates it will come into alignment with the staff/pole. The pole tends to block the path of the wind and causes the blade to generate less force when it is aligned with the pole. Once the blade moves away from teh pole the normal wind force is reapplied.

This loading and unloading of force happens quickly for each blade as it comes into alignment. For that brief instance during alignment the total force of the blades become unbalanced and will cause a vibration and rapid change in loading on the bearings at the hub.

Two blades generate more vibration than three blades. Because with three blades the 120 degree seperation allows the forces to be spread out to either side of the pole and not just at the top. A two blade unit has all of the force change applied vertically at the top (opposite the lower blade)

The size of the windmill and number of blades will determine the sizing of the bearings. A cheaper unit can be made using two blades if the bearings can take the stress but there will be more vibration and noise generated than if three blades are used. At some point on a larger windmill design three baldes will be used as the cost of the extra blade will be less than the cost of larger bearings.

Answer 2

They are designed by engineers who set the length of the "wings" and the number according to set rules that will ensure greatest efficiency, and a weight ratio that will ensure the windmill is safe.

Have you seen the eggbeaters? They're neat but not as common.

Answer 3

the cost of windmill mainly depends on these aerodynamic shaped fibre glass blades. two blade windmills are cheaper compared to three blade windmill.
two balde wind mill (propeller type) are less efficient compared to Three balde windmill (sail type)
Market share of sail type wind mills is high due to its efficiency.