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Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 9:49 am
by workhorse
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?

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 10:09 am
by Armadillo1
auto-rotation capabiliies also needed "stored" torque moment (dont know how to say in english)
may be some point here

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 10:56 am
by workhorse
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).

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 11:23 am
by Bostrom
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.

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 11:40 am
by SheikhDjibouti
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.

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 1:30 pm
by workhorse
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.

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 4:48 pm
by stephanwintner
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.

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 8:13 pm
by Bostrom
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.

Re: Benefits of a 2 blade rotor

Posted: Thu Jun 20, 2019 8:27 pm
by hivue
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.

Re: Benefits of a 2 blade rotor

Posted: Thu Jul 18, 2019 12:16 pm
by workhorse
stephanwintner wrote:
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 wrote:
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 wrote:
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.


Better late than never, thank to all of you for your answers!


Since then, I took the time to do a little comparison between two almost identical helicopters with the number of blades being the only difference: Bell 212 and Bell 412.

- both have the same engines (P&WC PT6T-3) producing the same power: 900x2=1800hp

- the rotor of the 212 is 2 blade with a diameter of 14,64 meters. The rotor of the 412 is 4 blade and only slightly smaller: 14,0 meters.

- the 212 is slightly (117kg) lighter than the 412: 2962kg vs 3079kg. I guess, that's the weight of the 2 extra blades.

- however the MTOW difference is 317kg (5080kg vs 5397kg) which means that the 412 can take 200kg more payload.

- both cruise speed and max speed of the 412 is 20 knots higher (120kn vs 100kn cruise and 140kn vs 120kn max).

- operational ceiling of the 412 is 2600 feet higher than the 212: 20 000 feet vs 17 400 feet.

- but the big difference is the range. The 412 has double the range: 529nm vs 237nm. This might be partly due to the 200kg extra payload, although I don't know how much bigger are the 412's fuel tanks. Some of it is probably due to the higher cruise speed. Is there also an advantage of the hourly fuel burn at cruise?


To conclude, it looks like the 4 blade rotor has some clear advantage from the efficiency point of view. The easiness of storage and the cost are probably the main drivers for operators who choose the 212 over the 412.

Re: Benefits of a 2 blade rotor

Posted: Thu Jul 18, 2019 7:52 pm
by WPvsMW
"ease of storage"
There are many non-native EN speakers reading the forum.

Re: Benefits of a 2 blade rotor

Posted: Fri Jul 19, 2019 7:23 am
by kitplane01
workhorse wrote:
stephanwintner wrote:
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 wrote:
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 wrote:
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.


Better late than never, thank to all of you for your answers!


Since then, I took the time to do a little comparison between two almost identical helicopters with the number of blades being the only difference: Bell 212 and Bell 412.

- both have the same engines (P&WC PT6T-3) producing the same power: 900x2=1800hp

- the rotor of the 212 is 2 blade with a diameter of 14,64 meters. The rotor of the 412 is 4 blade and only slightly smaller: 14,0 meters.

- the 212 is slightly (117kg) lighter than the 412: 2962kg vs 3079kg. I guess, that's the weight of the 2 extra blades.

- however the MTOW difference is 317kg (5080kg vs 5397kg) which means that the 412 can take 200kg more payload.

- both cruise speed and max speed of the 412 is 20 knots higher (120kn vs 100kn cruise and 140kn vs 120kn max).

- operational ceiling of the 412 is 2600 feet higher than the 212: 20 000 feet vs 17 400 feet.

- but the big difference is the range. The 412 has double the range: 529nm vs 237nm. This might be partly due to the 200kg extra payload, although I don't know how much bigger are the 412's fuel tanks. Some of it is probably due to the higher cruise speed. Is there also an advantage of the hourly fuel burn at cruise?


To conclude, it looks like the 4 blade rotor has some clear advantage from the efficiency point of view. The easiness of storage and the cost are probably the main drivers for operators who choose the 212 over the 412.



I would bet a large amount of money that the 412 has different rotors than the 212. Maybe more chord, or a thicker profile, or rotate at a different RPM.

Basically, for a given amount of lift the most efficient scheme has the fewest blades. But if you have fewer blades, you might require a larger diameter. Larger diameters can cause problems such as
- Need a larger landing space
- Interference with the tail
- aero-elastic problems (flutter)

Re: Benefits of a 2 blade rotor

Posted: Fri Jul 19, 2019 8:02 am
by Armadillo1
your talk about the same lift for 2 and 4 - where 2 much longer.

but as we see with this example, >2 simply allows to build bigger total lift (blade length, etc), like larger wing.

Re: Benefits of a 2 blade rotor

Posted: Fri Jul 19, 2019 8:02 pm
by vikkyvik
I don't know this for a fact, but I would think that more blades also allows a lower rotor RPM, which might allow a faster helicopter speed, as the receding blades will suffer a decreased loss of lift.

Re: Benefits of a 2 blade rotor

Posted: Thu Jul 25, 2019 7:23 am
by kitplane01
vikkyvik wrote:
I don't know this for a fact, but I would think that more blades also allows a lower rotor RPM, which might allow a faster helicopter speed, as the receding blades will suffer a decreased loss of lift.


That's backwards. Lower RPM makes the retreating blade stall problem worse, not better.

Re: Benefits of a 2 blade rotor

Posted: Thu Jul 25, 2019 1:59 pm
by vikkyvik
kitplane01 wrote:
vikkyvik wrote:
I don't know this for a fact, but I would think that more blades also allows a lower rotor RPM, which might allow a faster helicopter speed, as the receding blades will suffer a decreased loss of lift.


That's backwards. Lower RPM makes the retreating blade stall problem worse, not better.


Ooooh, I clearly didn't think that one through! Thanks.