workhorse
Topic Author
Posts: 614
Joined: Sun Jul 10, 2005 11:35 pm

Benefits of a 2 blade rotor

Thu Jun 20, 2019 9:49 am

What are the benefits of a 2 blade rotor such as used, for example, on the Bell UH-1/204/205/212/etc, other than being cheaper and producing the iconic "Hollywood helicopter" sound?

The auto-rotation capabilities must be worse than with 3, 4 or more blades, aren't they?

Rotation speed must also be higher, leading to higher fuel burn and wear of the parts?

So why did Bell stick to this configuration for decades of manufacturing the Huey family?
 
Armadillo1
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Joined: Thu Apr 20, 2017 5:14 pm

Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 10:09 am

auto-rotation capabiliies also needed "stored" torque moment (dont know how to say in english)
may be some point here
 
workhorse
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Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 10:56 am

Armadillo1 wrote:
auto-rotation capabiliies also needed "stored" torque moment (dont know how to say in english)
may be some point here


I see what you mean. Logically, the heavier the rotor, the more inertia you get (of course within reasonable limits). Also, having more blades may help to alleviate eventual assymetry (don't know if it's a factor significant enough).
 
Bostrom
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Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 11:23 am

workhorse wrote:
What are the benefits of a 2 blade rotor such as used, for example, on the Bell UH-1/204/205/212/etc, other than being cheaper and producing the iconic "Hollywood helicopter" sound?


A rotor blade operates in the wake turbulence of the preceding rotor blade, which reduces efficiency. The more blades in a rotor, the closer the blades will be, hence more wake turbulence for each blade.
 
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SheikhDjibouti
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Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 11:40 am

Bostrom wrote:
workhorse wrote:
What are the benefits of a 2 blade rotor such as used, for example, on the Bell UH-1/204/205/212/etc, other than being cheaper and producing the iconic "Hollywood helicopter" sound?


A rotor blade operates in the wake turbulence of the preceding rotor blade, which reduces efficiency. The more blades in a rotor, the closer the blades will be, hence more wake turbulence for each blade.
:checkmark:
According to basic propeller lore, from an aerodynamic viewpoint the most efficient set up of all is a single rotor. :o
Of course, in reality there are severe balancing issues in such a case which is why you don't see them around much.
Nothing to see here; move along please.
 
workhorse
Topic Author
Posts: 614
Joined: Sun Jul 10, 2005 11:35 pm

Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 1:30 pm

Bostrom wrote:
A rotor blade operates in the wake turbulence of the preceding rotor blade, which reduces efficiency. The more blades in a rotor, the closer the blades will be, hence more wake turbulence for each blade.



Makes sense. I guess that as the helicopter becomes heavier, this benefit becomes less relevant because the necessary length of the 2 blades becomes too impractical. That's probably why most 2-blades machines are small ones.

The Huey family is special though because some of its members are quite heavy (the 212, for example, has a MTOW of 5 tons) and yet they still have a 2 blades rotor. I wonder why.
 
stephanwintner
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Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 4:48 pm

For storage, especially on a carrier, 2 blades have some advantage as well. Else you need a folding blade - which has of course been done, but adds complexity and hence weight.

Rotor inertia is NOT about weight. It is about weight distribution. A solid disk that is 1 inch thick and 12 inches diameter will have a lower inertia about it's axis than a solid disk of the same weight, that is 1/2 inch thick. The diameter will be greater so the inertia will be higher. I do not know if a 2 blade rotor will end up with a higher inertia than a 4 blade rotor for the same lift - that depends too much on rotor aerodynamics for me to estimate. But I suspect it would.
 
Bostrom
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Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 8:13 pm

workhorse wrote:
Makes sense. I guess that as the helicopter becomes heavier, this benefit becomes less relevant because the necessary length of the 2 blades becomes too impractical. That's probably why most 2-blades machines are small ones.

The Huey family is special though because some of its members are quite heavy (the 212, for example, has a MTOW of 5 tons) and yet they still have a 2 blades rotor. I wonder why.


There are a lot of factors to consider when choosing the amount of rotor blades. And as mentioned, easy storage is a great advantage for twobladed rotors. Could that be the reason for the Hueys being heavy and twobladed?

For larger helicopters the advantages of less wake turbulence are still there, but the disadvantage of a larger rotor becomes really large. The Mi-26 has a rotor diameter of 32 m and eight rotor blades, that's 128 m of rotor blade length. A rotor with 2 blades will probably be more efficient, but even with a 20% reduction in rotor blade length it would be about 100 metres in rotor diameter. A rotor that large will be hard to handle, but the big problem with large rotors is the speed of the blade tips, you don't want the tips to become supersonic.
 
hivue
Posts: 1891
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Re: Benefits of a 2 blade rotor

Thu Jun 20, 2019 8:27 pm

Cost is a major factor, right? Two blade semi-rigid rotors likely are cheaper but still work very well. Of course they are susceptible to mast bumping, but then three blade systems are susceptible to ground resonance issues that, I think, two blade systems don't have.
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