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I have seen 2 rotor blades up to many and wonder when does having more rotor blades become less efficient? What helicopter has the most rotor blades?

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Related: Why don't helicopters prefer shorter rotors with more blades?

What is the definining line between a helicopter and a drone a person can ride in?

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    $\begingroup$ If the blades are longer it has to spin slower to keep the tips from going supersonic. They are also heavier and harder to control $\endgroup$
    – Ron Beyer
    Apr 22, 2019 at 1:59
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    $\begingroup$ @RonBeyer do you think redundancy of having more blades could allow the helicopter to fly with damaged blades? $\endgroup$
    – Muze
    Apr 22, 2019 at 2:04
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    $\begingroup$ Unlikely. Chipped/mildly damaged blades can still fly, but once they fail structurally the entire rotor is unbalanced an no amount of extra blades is going to make it stabilize. It will shake itself apart as soon as it becomes unbalanced enough. $\endgroup$
    – Ron Beyer
    Apr 22, 2019 at 2:09
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    $\begingroup$ when does having more rotor blades become less efficient? -- any number more than one. The problem is if you need more lift then you need to use longer blades but if you cannot increase blade length due to structural issues and/or storage issues then you have no choice but to sacrifice efficiency for more lifting power. A single blade is ALWAYS the most efficient number of blades. $\endgroup$
    – slebetman
    Apr 22, 2019 at 5:13
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    $\begingroup$ Is this blades per hub, or blades per helicopter? Tandem rotor (Chinook et al) or Coaxial rotor helicopters have several hubs, each with the same number/size of blades. $\endgroup$
    – CSM
    Apr 22, 2019 at 9:22

2 Answers 2

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Special mention for Raúl Pateras Pescara de Castelluccio (good article) who was fond of lots and lots of rotor blades, settling on sixteen for most of his designs, although his Model 3 had twenty.

Pescara's helicopters may look a little comical (and dangerous!) but they are an important part of early helicopter evolution, pioneers in the first fully controlled helicopter flights, as well as the first use of the what would be recognised as modern helicopter controls.

Here's a video of Pescara testing one of his helicopters in 1922 wearing a Homburg hat with about 5mm of clearance to his head. He had many setbacks, but persevered. It's visionary, brave guys like him who make advances in aviation.

Plus, loads of blades.

Here is his Model 3 from 1929: Pescara Model 3 (source1)

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    $\begingroup$ This is actually a comment. But only a heart of stone could refuse a +1 for that picture. $\endgroup$
    – Tim Lymington
    Apr 22, 2019 at 12:12
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    $\begingroup$ Wow. Counter-rotating biplane rotors. What could possibly go wrong? $\endgroup$
    – Bob Jarvis - Слава Україні
    Apr 22, 2019 at 12:19
  • $\begingroup$ @TimLymington you're right - answers with good/useful pictures earn more upvotes than plain text answers. Add images to answers for better scores. $\endgroup$
    – Criggie
    Apr 22, 2019 at 13:04
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    $\begingroup$ @TimLymington I have expounded the answer so it's hopefully less comment-like. $\endgroup$
    – Party Ark
    Apr 22, 2019 at 14:28
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    $\begingroup$ Mr Pescara did at least have the sense to fit a head guard to his machine. Very stylish guy, flight testing with that hat! $\endgroup$
    – Cpt Reynolds
    Apr 23, 2019 at 18:59
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The most blades I've seen are 8 on the Mi-26. Mi-26 with 737 in background
Source

But the highest theoretical lifting efficiency is achieved with the fewest blades and experiments have been done with single blade rotors (with a counterweight - there were vibration problems that couldn't be resolved).

So in practical terms, the most lift for the least power is achieved with a 2 blade rotor, but the need to absorb more power with a limited desirable disc diameter forces you to add more blades (or you may want a smaller rotor with lots of blades for maneuverability or other reasons, but it's sub-optimal from a horsepower efficiency standpoint).

Put another way, a helicopter like the Mi-26 could probably achieve a fair bit more lifting power for its installed horsepower with a 2 blade rotor, but each blade would seemingly extend off into the next township.

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    $\begingroup$ It's interesting to note that the tail rotor of Mi-26 also has large number of blades (5). $\endgroup$
    – trolley813
    Apr 22, 2019 at 9:36
  • $\begingroup$ @John K Could you maybe please provide some sources for your statements. Because for example the Ec145 helicopter recently got a new main rotor with five instead of 4 baldes and now it can carry more load with the same engine power verticalmag.com/news/… $\endgroup$
    – user33651
    Apr 22, 2019 at 13:56
  • $\begingroup$ Added a link to another ASE post where @Peter Kämpf responds with much greater authority than I on the theory. $\endgroup$
    – John K
    Apr 22, 2019 at 17:09
  • $\begingroup$ "The blades incorporate the latest airfoil design to produce more thrust than previous blades at the same power. While five blades have more drag than four blades in a rotor, Humpert said improvements to the aerodynamics of the rotor cuff of the blade had actually allowed the new H145’s blades to provide more lift. “This is the aerodynamic evolution which is technically behind it,” he said." Adding the extra blade in itself wasn't the benefit. It was a package of optimizations; blade airfoil, root design, etc. Fewer blades are still more efficient than more blades, all else being equal. $\endgroup$
    – John K
    Apr 22, 2019 at 17:33
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    $\begingroup$ Well jets is getting a bit out there but yes. It's well known in the airplane business that I'll get more static thrust out of a longer 2 blade prop than a shorter 3 blade prop with the same overall blade area, although the difference is small enough to make it worthwhile sometimes to use the 3 blade for the lower noise levels and better tip clearance. $\endgroup$
    – John K
    Apr 23, 2019 at 12:04

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