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Lets say aircraft has two ram air turbines that work as generators to produce electricity on each wing. One has two blades and one has 8 blades, both with the same diameter.

What is the aerodynamic reason why the turbine with 8 blades will rotate slower (lower RPM) then the one with 2 blades?

Often people say each blade adds drag, so drag reduces RPM but where is aerodynamic drag on blades if the resultant force is always tilted forward? A component in the forward direction is thrust, not drag. So what slows down the rotation?

I think there can be only 2 reasons to slow down rotation:

  1. if resultant force has lower magnitude or
  2. the vector is pointed more backward, then the question remains why this change in resultant vector happened at more blades configuration?

enter image description here

The tip speed ratio is the ratio between tip speed tangential velocity and wind speed that blows at the propeller. So for same wind speed one blade has the highest RPM... enter image description here

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    $\begingroup$ I don't think your assumption is valid. Adding more blades ought to make the generator spin faster. $\endgroup$
    – quiet flyer
    Jan 1, 2022 at 22:46
  • $\begingroup$ @quietflyer propeler with more blades will spin slower, but has higher torque. And higher torque is direct indicator of higher drag, but I need aerodynamic explanation why this happened... $\endgroup$
    – user628075
    Jan 1, 2022 at 22:58
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    $\begingroup$ I'd suggest editing the question to include some external source supporting this idea that in generator mode, adding blades makes the prop spin slower. $\endgroup$
    – quiet flyer
    Jan 1, 2022 at 23:01
  • $\begingroup$ Re lift and drag-- in relation to props, this can be confusing-- are lift and drag defined in relation to the direction of the external airflow, or in relation to the direction of the trajectory of a particular element on a particular blade? Is the torque that turns the prop mainly due to drag, or mainly due to lift? Anyway I'm still having trouble w/ this idea that adding more blades makes the generator spin slower. Will be interesting to see what answers are posted. $\endgroup$
    – quiet flyer
    Jan 1, 2022 at 23:03
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    $\begingroup$ DV & VTC - this question posits an unproven and intuitively questionable assumption & then asks why this unexpected result occurs. But, would it spin slower? I don't see any productive outcome here. If you change the characteristics of the blades & the generator enough, then all sorts of outcomes are possible, but all "ceteris paribus" (all-else-being-equal) is out the window. $\endgroup$
    – Ralph J ♦
    Jan 2, 2022 at 0:15

3 Answers 3

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The windmilling prop extracts energy from the mass flow through the propeller disk area. An 8-bladed prop can extract more energy from the mass flow, and does this at lower RPM than the 2-bladed prop. The extra energy it can harvest is due to lower tip losses.

enter image description here

Pic above is from Principles of Helicopter Aerodynamics by J. Gordon Leishman, and shows results from a combination of momentum and blade element theories (BEMT). With no viscous or tip losses to consider:

Notice that increasing the number of blades (or increasing solidity) does not affect the maximum efficiency of the turbine, but it does affect the tip speed ratio (or wind speed) where maximum efficiency is obtained. Too many blades ...gives a steep power curve that peaks at low tip speed ratios. This is not very useful in practice, besides the fact that the blades will also be approaching stall at these conditions (high wind speed).

So yes, a propeller with more blades turns slower when windmilling, because it has absorbed the maximum power at a lower rpm than the 2-bladed propeller. It is important to realise that the higher blade propeller turns slower by design - the blade angle is not fixed, but set according to wind speed, in such a way that the design tip speed ratio @ optimum is reached. And this is also the case with a windmilling propeller on an aeroplane, in fact the blade angle is set such that the prop does not turn.

In helicopter rotor measurements it has been shown that rotor solidity is a main design parameter: how much of the disk area is covered by blade area. How the blade area is distributed over the number of blades is of secondary importance, so a twin blade rotor performs much the same as a four-blade rotor with half the blade chord. Except for the noise and vibrations generated.

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  • $\begingroup$ You can add explanation what is aerodynamic reason why more blades slow RPM down, something must happened with local airflow over blades if RPM is lower... $\endgroup$
    – user628075
    Jan 2, 2022 at 10:15
  • $\begingroup$ It is not true that 8 blade can extract more energy from 2 blades, it just do it at lower RPM..second sentence need correct that.. $\endgroup$
    – user628075
    Jan 2, 2022 at 10:37
  • $\begingroup$ The picture is from theory without viscocity and tip loss effects, reality is different. $\endgroup$
    – Koyovis
    Jan 2, 2022 at 10:58
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    $\begingroup$ Are these turbines with different numbers of blades using otherwise-identical designs for each blade? I'm not clear whether the premise of the question is actually true for an all-else-equal comparison, e.g. take an 8-blade turbine and knock off 6 of the blades, then operate it in the same wind speed as before. (With no load?). vs. comparing two differently-designed turbines, designed to run with 2 vs. 8 blades in the same wind speed with some load, @JurgenM. $\endgroup$
    – Peter Cordes
    Jan 2, 2022 at 11:55
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    $\begingroup$ This answer would be correct if it mentioned that the slower 8-blade turbine is geared differently than the faster 2-blade turbine. This is possible because the 8-blade prop produces more torque for any given RPM in its effective speed range than a two-blade prop at the same RPM. The prop is spinning slower, but the generator may be spinning at the same speed as it would be with a faster, two-blade prop geared differently. $\endgroup$
    – dotancohen
    Jan 2, 2022 at 13:05
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What is aerodynamic reason why turbine with 8 blades will rotate slower then with 2 blades?

It won't, all else being equal.

The speed of the turbine is governed by the aerodynamic forces on the blades and the resistance in the turbine. (By resistance I mean non-aerodynamic forces like drag in the bearings and the actual generator.) In a two-bladed turbine (or any other) the normal speed is where the total of the aerodynamic forces and the resistance balance.

If we quadruple the number of blades, making each of the added blades identical with the originals, then according to naive, oversimplified, aerodynamics we are quadrupling the aerodynamic forces on the turbine (the ones driving it). The resistance is not quadrupling, so the turbine will spin faster.

It won't spin 4 times as fast for two reasons. Firstly, as the turbine speed increases the non-aerodynamic resistance increases, and they will balance at significantly less than four times the speed. Secondly the aerodynamic behaviour changes as the speed increases, including both an increase in the aerodynamic drag of each blade and aerodynamic interference between the blades, reducing their "lift". (Aerodynamic interference is why biplanes don't have twice the lift of monoplanes. It can actually reduce the many-bladed turbine's aerodynamic force below that of the few-bladed one.) That's what I meant above by saying that we are quadrupling the aerodynamic force only for naive, oversimplified aerodynamics. These two factors means the 8 bladed turbine will top out at well less than 4 times the speed of the two bladed.

It's also important to note that 8-bladed fans aren't designed by simply taking a two-bladed fan and adding six more identical blades. The blades will be designed differently. Turbines have an intended operating speed and the blades will be designed so the turbine operates at that speed for expected airflows. Higher turbine speed does not always mean more power - a multi-bladed turbine may be designed to run at a lower speed and generate the same power as a differently-designed two-bladed one.

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  • $\begingroup$ Your answer imply that using more blades means get more energy for given turbine diameter, that is not case in wind turbine theory. if this is case all wind turbine will have more blades not just 3 or 2... $\endgroup$
    – user628075
    Jan 2, 2022 at 9:26
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    $\begingroup$ Actually no I'm not. That's only according to the "naive, oversimplified" aerodynamics, and I explain why not in the paragraph after. Also there are other reasons to not increase the number of blades, or to make fewer larger blades rather than more smaller ones. $\endgroup$
    – DJClayworth
    Jan 2, 2022 at 9:57
  • $\begingroup$ @DJCalyworth This is very easy to test, just take two identical car ventilator, cut blades and leave one with 2 blades and one leave with 8 blades. Put of window and meassure RPM... $\endgroup$
    – user628075
    Jan 2, 2022 at 10:04
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Imagine a propeller being turned by an engine suddenly losing power. How to make it a good generator as well as a good propeller? Probably a more symmetric airfoil. As seen here, a good airfoil shape will help create more lift/thrust from the relative wind.

Interference will play a role in rotation speed. This is why more blades have a lower maximum rotation speed.

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