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Propeller or wind turbine often has only 3 blades,reason is wake behind each blade interfere with next blade.

I think wind that comes to propeller must clear this wake,so faster you go the air for each blade has better quality.

How then wake interfere with blade?

(Same like in Formula 1, there is no wake for car that drives behind car if cross wind is blowing. Or there is not turbulence at airfield when plane take off if cross wind is blowing)

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  • $\begingroup$ Your question contains a false statement, and a presumption. Not every propeller or wind turbine has only three blades, and you are presuming that they do interfere with each other, while at the same time offering a valid reason why they wouldn’t. $\endgroup$
    – Michael Hall
    Commented Nov 12, 2021 at 16:40
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    $\begingroup$ This is a good question though. Might benefit from some rewording, but as it induced (at least one) good answer(s), can't be all that bad. Most questions, in fact, stem from inadequate understanding of things and phenomena. $\endgroup$
    – Jpe61
    Commented Nov 13, 2021 at 11:27

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Aerodynamic induction works like electric induction since both can be calculated by the same laws. Those laws say that the influence of a vortex will decrease with the inverse of the square of the distance to the vortex line. But it will never completely die down, at least in subsonic flow.

It is not the wake (the mass of air washing over the preceding blade) itself but the pressure field it creates which affects the next blade.

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    $\begingroup$ At least one book mentions wake avoidance in staggered rotors as a reason for noise reduction. I know not all books get everything right, but perhaps expanding on the terminology/interaction may help? For example: What effect does the pressure field have? And why doesn't it also follow the same inverse distance law as the wake? Thanks! $\endgroup$
    – user14897
    Commented Nov 13, 2021 at 9:13
  • $\begingroup$ @ymb1 Propeller blades are in one plane while staggered rotors allow downwash from the forward to flow over the rear rotor. Same for pusher propellers: For noise reduction their axis must be lifted above or below the trailing edge so the blade cuts through the wake not all at once but gradually. Since air is carrying the turbulence with it, no inverse square law applies here. $\endgroup$
    – Peter Kämpf
    Commented Nov 13, 2021 at 9:18
  • $\begingroup$ "[it's] the pressure field [...] which affects the next blade", is the answer to be understood as: no, there is no negative interaction from one blade on the next one and the number of blades is not limited for this reason. Or does the pressure field prevent to use more blades and/or is this effect reduced as aircraft speed increases? (the two underlying questions in the post) $\endgroup$
    – mins
    Commented Nov 13, 2021 at 11:55
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    $\begingroup$ @mins No, there is definitely interference, just like with the wings of a biplane. A single-bladed prop is very efficient but only a good choice with a very low disk loading. $\endgroup$
    – Peter Kämpf
    Commented Nov 13, 2021 at 21:32
  • $\begingroup$ The second comment is the main basis of my answer (upon retrieving it after it flew over my head). $\endgroup$
    – Robert DiGiovanni
    Commented Jan 3, 2022 at 23:32
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There may be 2 cases worthy of consideration: the stationary and relatively slowly rotating turbine and the rapidly spinning propeller with forward motion.

how then wake interfere with blade?

The downwash from the previous blade reduces the angle of attack on the next blade, thereby limiting its thrust. This helps explain why wind turbines with more blades have a lower maximum speed.

Turbulence from the previous blade would also have a negative effect, but not so much at lower angles of attack.

Vultures and eagles actually take advantage of the downwash effect with the arrangement of multiple slotted wingtips, but with rotors, downwash is best avoided.

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