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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?

Muze
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    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 – Ron Beyer Apr 22 '19 at 01:59
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    @RonBeyer do you think redundancy of having more blades could allow the helicopter to fly with damaged blades? – Muze Apr 22 '19 at 02:04
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    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. – Ron Beyer Apr 22 '19 at 02:09
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    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. – slebetman Apr 22 '19 at 05:13
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    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. – CSM Apr 22 '19 at 09:22
  • @CSM I think so, I could not find the picture a second time for the reference. – Muze Apr 22 '19 at 17:08
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    How are we defining "helicopter" here? Your standard remote-control camera-drone toy creates lift in roughly the same way as a helicopter, using 4 rotors at the corners. Each rotor can have 2-6 blades or so, giving you a total of potentially 24 blades. I realize this is stretching the definition a bit, just wondering where you draw the line, since someone already brought up the Chinook 2-rotor varieties... – Darrel Hoffman Apr 22 '19 at 17:44
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    @DarrelHoffman As far as I'm aware, the are no passenger-carrying quad-rotors. Restricting this question to rotorcraft that can carry people (if only their pilot) is, I think, reasonable. – CSM Apr 22 '19 at 22:59
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    I wonder if there is an example of a helicopter carrying a load of other helicopter blades for some reason ... like a stack of them. Might win the contest... – Steve Apr 23 '19 at 02:26
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    @CSM The other question linked from this one (by the same person) shows a picture of exactly almost that: a passenger-carrying tri-rotor. I don't know if that's just a prototype or not actually functional or what, but there appears to be at least one... (Edit: looks like there's only 3 rotors on further inspection) – Darrel Hoffman Apr 23 '19 at 12:44
  • (Reminding school lessions) Another interest for more blades is limiting low-frequency vibrations, as the resonance frequencies get higher when more blades are installed. This greatly improves the flying experience for humans, in terms of comfort and flight safety, as low frequency impedds vision among other things. – Joël Apr 24 '19 at 07:49
  • @DarrelHoffman has mentioned drones and then admitted they may not count as they do not carry passengers. But that leads to the question of why don't we build helicopters that way. Multiple rotors clearly have may advantages (or drones would not be built that way) and they are a pretty obvious way of having unlimited power without excessive blade length, blade-per-rotor number or blade speed. The only reason I can imagine is the difficulty of transmitting power from the single engine. Once electrical transmission, or even batteries become viable, I assume all helicopters will look like drones. – David Robinson Apr 24 '19 at 10:49
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    Funny how it works the same way for power-generating rotors: Wind powerplants are also most efficient with very small number of blades, in practice basically only 2 or 3 are used nowadays. – yo' Apr 24 '19 at 15:59

2 Answers2

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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)

Party Ark
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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.

fooot
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John K
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    It's interesting to note that the tail rotor of Mi-26 also has large number of blades (5). – trolley813 Apr 22 '19 at 09:36
  • @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 https://www.verticalmag.com/news/airbus-helicopters-launches-5-bladed-h145-upgrade/ – user33651 Apr 22 '19 at 13:56
  • Added a link to another ASE post where @Peter Kämpf responds with much greater authority than I on the theory. – John K Apr 22 '19 at 17:09
  • "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. – John K Apr 22 '19 at 17:33
  • Than riddle me this why didn't they just take 4 optimised new blades instead of 5 if 4 according to your claims is more efficient. Yes you run into problems with wake with more blades, but with less blades you also need to increase the size of your baldes which brings other porblems. so it isn't just black an white there is a sweetspot in the middle for the blade number – user33651 Apr 22 '19 at 19:45
  • Probably because they discovered that the efficiency improvements resulted in more power available than could be absorbed by 4 blades, necessitating adding a 5th. – John K Apr 22 '19 at 19:49
  • If, theoretically, more thrust is achieved with the fewest blades - would that mean that, theoretically, a single blade prop is best for an airplane? And, even, theoretically, a single blade turbine is best in a jet engine? (a very wide jet engine, but this is theory!!) – Grimm The Opiner Apr 23 '19 at 10:44
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    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. – John K Apr 23 '19 at 12:04
  • Luck is approximately the last thing I want to be keeping a 737 in the air... – David Richerby Apr 23 '19 at 12:11
  • It's important to remember that the rotor blades of a helicopter are basically wings, and thus there is a finite amount of lift you can extract from them before they stall. More blades = more friction and lower efficiency, but also reduced loading on each individual blade (and thus reduced chance of stall). – MikeB Apr 23 '19 at 15:11
  • @DavidRicherby indeed, I'd rather have bloodymindedness :) – jwenting Apr 24 '19 at 04:48
  • If the most lift for the least power is achieved with a 2 blade rotor, what was Pescara doing? Why build it with (presumably not) the most power, for the least lift? – Mazura Apr 24 '19 at 11:31
  • He was trying to get the required blade area in a limited span with a rotor made from sticks spinning on a big pipe. You need a certain area, and you can increase span or chord or both, or add more blades or all of the above. Most helicopters use multiblade rotors to get the required blade area while limiting rotor diameter, because the benefits outweigh the theoretical penalty. Doesn't change the theoretical ideal of fewest blades for the required area. – John K Apr 24 '19 at 13:00
  • @JohnK I was thinking about the space between the blades. On smaller helicopters with less blades the distance between the blades tips are equal then the larger helicopter with more blades allowing having the same wake? – Muze Apr 24 '19 at 20:18
  • There's more to it than just the wake effects and spacing. On a two blade you have only two roots and tips with related losses. On a 5 blade rotor of the same total blade area say, you now have 5 roots and tips with related losses for the same lifting area. Put another way, a 2 blader is a little bit closer to the theoretical infinite span wing with no tip losses than a 5 blader. That being said, the benefits of a multi-blade rotor in many other ways more than make up for the difference on complex machines. For simplicity and the most lift on the least power however,2 blade is still best. – John K Apr 24 '19 at 21:14