Why wind turbine with more blades rotate slower(lower RPM) than turbine with just one or two blades, if both has equal diameter on same wind speed?

Often people say more blades have more drag, but where is aero drag on blades if resultant force is always tilted forward, component in forward direction is thrust, not drag. So what slows down rotation?

Only reasons to slow down are if resultant force has lower magnitude or vector is pointed more backward, then still stay question why this happened at more blades configuration? enter image description heretip speed ratio is ratio between tip speed tangential velocity and wind speed that blows at propeller. So for same wind speed one blade has the highiest RPM...

enter image description here

  • $\begingroup$ Check this post: engineering.stackexchange.com/q/48998/10902 $\endgroup$
    – Solar Mike
    Commented Jan 1, 2022 at 10:22
  • $\begingroup$ This looks like homework so you must show your work, what you have found and where you are stuck. $\endgroup$
    – Transistor
    Commented Jan 1, 2022 at 10:47
  • $\begingroup$ Also, why are you looking for maximum RPM? Surely you want maximum power. $\endgroup$
    – Transistor
    Commented Jan 1, 2022 at 11:17
  • $\begingroup$ @Transistor For sure I need max power=max efficiency. But I want to know what is aerodynamic reason why more blades rotate slower? $\endgroup$
    – 22flower
    Commented Jan 1, 2022 at 12:02
  • $\begingroup$ That's not quite what your question is asking. $\endgroup$
    – Transistor
    Commented Jan 1, 2022 at 12:06

1 Answer 1


Short answer:

enter image description here enter image description here

This should be a good reading material for you. https://www.nmmesa.org/wp-content/uploads/2019/10/Aerodynamics-of-Wind-Turbine-Blades.pdf

Aerodynamics of Wind Turbines: Drag

Both aircraft engineers who build aircraft wings and propellers, and wind turbine engineers who design rotor blades are concerned with aerodynamic drag. Drag is always important when an object moves rapidly through the air. Aircraft should have good fuel economy, and wind turbine rotor blades must have high tip speeds to work efficiently. Therefore it is critical, that both aircraft wings and rotor blades have low aerodynamic drag.

  • Drag Increases with the Area Facing the Wind

  • Drag Depends on the Shape of the Object

  • Drag Increases with the Square of the Wind Speed

and so on... http://www.xn--drmstrre-64ad.dk/wp-content/wind/miller/windpower%20web/en/tour/wtrb/drag.htm

  • $\begingroup$ Each blade produce thrust not drag, look at my question I edit it $\endgroup$
    – 22flower
    Commented Jan 1, 2022 at 22:04
  • $\begingroup$ I can only say the increase in drag, which involves quite a few factors, and increases in efficiency by adding blades are non-linear and negatively correlated. Did you go over the entire article? $\endgroup$
    – r13
    Commented Jan 1, 2022 at 22:09
  • $\begingroup$ Saying that blade in generator mode produce drag is same as saying that aircraft wing produce thrust component during flight.. think about it.. $\endgroup$
    – 22flower
    Commented Jan 1, 2022 at 22:47
  • $\begingroup$ Same effect. Drag presents when motion starts. That's the area you need more understanding. Too broad, couldn't help, sorry. $\endgroup$
    – r13
    Commented Jan 1, 2022 at 23:11
  • $\begingroup$ I think answer must be in interference between blades that change local airflow,so resultant force is reduced or is tilted more toward back of plane of rotation, this reduce RPM $\endgroup$
    – 22flower
    Commented Jan 1, 2022 at 23:28

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