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Heads up: This came from Wikipedia's page on contra-rotating propellers and it explicitly says citation needed:

However they can be very noisy ... It is also helpful if the two propellers have a different number of blades (e.g. four blades on the forward propeller and five on the aft).[citation needed]

Any truth to this? Can a variable number of blades per propeller really help reduce noise? If so, how?

(It doesn't explicitly say if it's piston prop or turbo prop. AFAIK, both can be used for contra-rotating props, and I have no idea which one would be more noisy, or if this supposed variable-blade idea will work on one but not the other.)

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  • $\begingroup$ I always thought this was a power recovery strategy, not a noise reduction one. Paging @PeterKämpf? $\endgroup$
    – egid
    Jan 28, 2016 at 5:51

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It does help a little, yes.

One source of noise is the fluctuation in dynamic pressure experienced by a blade of the rear propeller when it cuts through the wake of a blade of the forward propeller. Friction in the boundary layer on the first blade will reduce the dynamic pressure locally and create pressure fluctuations on the second blade, and noise.

By using a different number of blades, the pressure fluctuations will not happen all at the same time, but sequentially, which reduces the peak power of the noise. The effect of the second blade cutting through the wake of the first is still there, but is distributed over time.

Thanks to @MSalters for pointing out another effect. I copy straight from his comment: The effect is not just distributed over time, but also shifted up in frequency. At 6000 RPM (100 Hz), with 4 blades per propeller they pass each other 8 times per revolution (800 Hz). If the second propeller has 5 blades, this moves up to 4000 Hz.

An-70 with D-27 propfan

P-27 prop on the D-27 turboshaft of the An-70 transport, picture by Marina Lystseva. The forward propeller has 8 blades and the rear propeller has 6. For optimum noise reduction, the rear propeller should have just one blade less than the forward propeller. The leading edge sweep helps to stretch out the contact of a single blade with the wake over time, which again reduces peak noise.

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    $\begingroup$ The effect is not just distributed over time, but also shifted up in frequency. At 6000 RPM (100 Hz), with 4 blades per propeller they pass each other 8 times per revolution (800 Hz). If the second propeller has 5 blades, this moves up to 4000 Hz. $\endgroup$
    – MSalters
    Jan 28, 2016 at 8:02
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    $\begingroup$ @MSalters: Right! I forgot, will edit the answer. $\endgroup$
    – Peter Kämpf
    Jan 28, 2016 at 8:26
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    $\begingroup$ This is fascinating but for optimum reduction, I don't see why the rear prop should have 1 less blade than the front. Why not the front having 1 less than the rear? Why not any combination of different prime numbers? $\endgroup$
    – DrZ214
    Jan 29, 2016 at 0:00
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    $\begingroup$ @DrZ214: The rear blades work in a flow swirled by the forward propeller which gives them a better efficiency and higher loading when operated at the same incidence as the forward blades. If you want to neutralize torque, it is better to reduce the blade count on the rear prop. $\endgroup$
    – Peter Kämpf
    Jan 30, 2016 at 15:20
  • $\begingroup$ Logic would suggest you want the number of blades on each propeller to be coprime in order to shift the frequency maximally. If you also want the number of blades to be similar, using a difference of 1 blade is probably the optimal solution. $\endgroup$
    – abligh
    Dec 24, 2017 at 19:04
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Yes!

For c-r fan's tonal noise (one kind of noise and another one is broadband), it can be reduced a lot by set the modal mismatch index to be 2. For instance, if the no. of blade of one rotor is 7, the other one should be 9 or 5 for least radiated noise.

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  • $\begingroup$ Neat. But what is tonal noise and what is broadband noise? What is the difference between them? $\endgroup$
    – DrZ214
    Sep 29, 2017 at 15:06

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