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My home ceiling fans have 5 blades to reduce the wopwop sound in a quiet room. It probably also moves air more efficiently at a lower spin rate. Can a helicopter operate properly with 5 blades?

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The most popular helicopter in the world has 5 blades. It's rare in the US, but elsewhere it's like the 737 of rotorcraft. Altogether, 5-blade designs account for about half of all helicopters outside of North America.

Odd vs even blade count doesn't matter much for ceiling fans. The slower the fastest point of the blade is, the quieter the fan. More blades allow for a slower rotation speed for given diameter. Also, if the diameter is decreased, the peak noise frequency moves up, with some extreme examples.

Odd blade counts are very common for Russian and European helicopters, as they make the noise quieter and less distinctive. The canceled Comanche "stealth helicopter" also used 5. Multi-blade rotors, especially with odd blade counts, are much more difficult to balance than simple 2-blade teetering rotors. However, once you get above 4 blades, it's a full balancing job anyway. Sikorsky easily switched from 6 to 7 blades in the CH-53E, and so did Mil from 5 in the Mi-6 to 8 in the Mi-26.

Even blade counts are more common in the US for historic reasons. While Sikorsky went with 3 blades from the start, as did Mil, but Bell, who preferred a simple 2-blade rotor, captured the market first. The Huey's success reinforced this. So 2 or 4 blades are seen as more conservative, even though the Apache's 4-blade rotor is as complex as any 3- or 5-blade one (it has dual teetering tail rotors though). American heavy-lift helicopters do use 5-7 blades.

For low-cost GA helicopters, there's no choice but a 2-blade teetering rotor - articulated hubs are too complex for that market.

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    $\begingroup$ I can almost imagine, though I may be wrong, that if you add enough blades it actually becomes easier to balance - though "enough" might a silly amount for a helicopter. After all, balancing a disk (such as a car wheel) is pretty trivial. $\endgroup$ Jan 23 at 8:51
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    $\begingroup$ @GrimmTheOpiner A car wheel doesn't rotate fast enough to affect its structural integrity. Rotors and turbine disks do, and balancing turbines is quite difficult. So I'd say the challenge increase slows down after 5 blades, but doesn't drop. $\endgroup$
    – Therac
    Jan 23 at 10:52
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    $\begingroup$ Is it odd blade counts that make things quieter, or just more blades? Would 6 blades be quieter than 4? Could you fairly easily balance 6 blades (3 teetering blades)? $\endgroup$
    – Jamin Grey
    Jan 23 at 17:13
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    $\begingroup$ @JaminGrey For a household fan, more blades. For a helicopter rotor, where blades are approaching Mach 1, odd number or uneven angle between the blades. As for balancing, the Apache has 2x2 teetering blades on its tail rotor, but its main rotor has 4 separate blades, requiring a full balance job, made only slightly easier by their orthogonality. $\endgroup$
    – Therac
    Jan 23 at 17:33
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    $\begingroup$ @GrimmTheOpiner It definitely gets easier to balance going from one blade to two. $\endgroup$ Jan 24 at 4:10
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Airbus H145 is a 5 bladed helicopter, recently certified by the FAA and EASA. https://www.airbus.com/en/newsroom/press-releases/2020-06-five-bladed-h145-receives-type-certification-by-easa

And there are 6,7, even 8 bladed helicopters.

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    $\begingroup$ Does that high a blade count not massively reduce the efficiency by putting each blade closer to the next blade's turbulence? $\endgroup$
    – Sneftel
    Jan 23 at 10:06
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    $\begingroup$ @Sneftel Yes and no. The rotor need enough wing area and airspeed for lift. Wide-chord blades create a lot of induced drag. Extremely long blades are heavy and flex excessively. For large helicopters, more rotors is the best solution, more blades the much cheaper second-best. $\endgroup$
    – Therac
    Jan 23 at 10:28
  • $\begingroup$ Most H145s and predecessors are 4-bladed though. $\endgroup$
    – M2FKXY
    Jan 23 at 12:22
  • $\begingroup$ @M2FKXY the switch to 5 blade was for the H145 a necessity due to the need to increase the mtow further (and to improve noise and performance in general). There were no more room for improvement with the old 4 blades design. The 5 blades rotor derives from the bluecopter platform. $\endgroup$
    – sophit
    Jan 23 at 12:28
  • $\begingroup$ Thanks a million. Went for an answer, stayed for an education! $\endgroup$
    – Lenny C.
    Jan 24 at 2:15

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