Published in the Gruniad - again!

I have had several letters and and answers to the 'Notes & Queries' feature published in the 'Guardian' over the years. Readers send in questions to N&Q, and one was about wind turbines. This morning, my answer to that query was published:


Why do wind turbines have only three blades? Wouldn't four be more effective?

Like many design considerations, the number of blades on a wind turbine is a compromise. Each blade disturbs the air for the following blade, so ideally you want as few blades as possible to maximise efficiency. You need to have at least two to keep the turbine balanced, and the longer the blade the more energy it will harvest from the wind. But if too long, the blade tips will approach the speed of sound as they rotate, which curtails efficiency. Also, the longer the blade the taller the mast has to be to give sufficient ground clearance and to ensure that the entire blade 'disc' (the area swept by the blades) is high enough to be in the stronger winds away from the ground. It has been found that three blades gives a good compromise; not too much air disturbance for the following blade, tip speeds below transonic, reasonable mast height, and a reasonable amount of energy gathered from the airflow and delivered to the electrical generator atop the mast.

Vince Chadwick, Wilmslow, Cheshire


This is similar to the considerations regarding aeroplane propeller design, and indeed I sent the Guardian more than they published. The second paragraph drew the parallel with propellers but I was pretty sure they wouldn't print that as they are short of space and it's not strictly relevant to the question.

Here's the entire text I sent to them:

Like many design considerations, the number of blades on a wind turbine is a compromise. Each blade disturbs the air for the following blade, so ideally you want as few blades as possible to maximise efficiency. You need to have at least two to keep the turbine balanced, and the longer the blade the more energy it will harvest from the wind. But if too long, the blade tips will approach the speed of sound as they rotate, which curtails efficiency. Also, the longer the blade the taller the mast has to be to give sufficient ground clearance and to ensure that the entire blade 'disc' (the area swept by the blades) is high enough to be in the stronger winds away from the ground. It has been found that three blades gives a good compromise; not too much air disturbance for the following blade, tip speeds below transonic, reasonable mast height, and a reasonable amount of energy gathered from the airflow and delivered to the electrical generator atop the mast.

Similar considerations are given to aircraft propeller design, but the problems of ground clearance are more acute because the propeller is close to the ground when the aircraft is not airborne. For this reason, aircraft propellers have up to six blades to absorb engine power, the designer accepting a small loss of efficiency, or contra-rotating propellers are used (two propellers, one behind the other, and rotating in opposite directions). These are aerodynamically efficient as the air from the front propeller makes the rear propeller more effective; but there are losses in the more complex mechanical drive system of this arrangement.

However, I would not be surprised to see contra-rotating wind turbines developed for use in particularly windy locations. Perhaps they already have been?

Nice to see that they printed my first paragraph absolutely verbatim!

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