Coaxial v conventional MR + TR
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Coaxial v conventional MR + TR
Having read the previous items on Notars and Fenestron and conventional tail rotors I contemplate over the ratioale that all rotor systems havent become coaxial.
I understand that the notar/fenestron are most inefficient at hover, using more engine touqre than the conventional tail rotor blade system. But provide elements of safety for persons on the ground and quieter operation together with advantages in forward flight.
The coaxial appears to be the obvious answer to the power situation and be tailrotorless and thus as safe as you can get for the persons on the ground.
So why is the coaxial rotor system such a poor choice for the S92 and all modern helicopters?
I understand that the notar/fenestron are most inefficient at hover, using more engine touqre than the conventional tail rotor blade system. But provide elements of safety for persons on the ground and quieter operation together with advantages in forward flight.
The coaxial appears to be the obvious answer to the power situation and be tailrotorless and thus as safe as you can get for the persons on the ground.
So why is the coaxial rotor system such a poor choice for the S92 and all modern helicopters?
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So why is the coaxial rotor system such a poor choice for the S92 and all modern helicopters?
The first production helicopters started to appear just before and after the end of the Second World War. At that time, the German configurations were primarily side-by-side (including intermeshing), the Russian's were primarily coaxial and the American's were primarily main & tail.
After the war, Germany was not allowed to perform research and development on aircraft. Russian had lost 12-15 million people and it was economically destitute. Other countries, such as England and France had problems similar to those of Russia. America became the leader in aerospace; partially, thanks to the Germans.
Creative people, by nature, are a proud and 'bull-headed' lot. Therefore, why would the Bells (Youngs) and the Sikorskys want to change from that which they believed in and had spent much of their lives working on?
Subsequent research, lobbying and funding has been directed primarily toward the single rotor configuration. In my not so humble opinion, if the same effort had been directed toward twin rotor configurations, rotorcraft technology would be further advanced then it currently is.
To me, the V-22 is an example of a dichotomy in which there exists a frustrated client (the US military) wanting improved vertical aircraft, and an industry that is incapable of delivering.
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Head Turner,
While some "natural" properties seem to be favored, the rule "if it is stupid, but it works, it is not stupid" must be applied. One of the lessons that Igor Sikorsky learned was that the apparent "natural" symmetry of the coax was one of the reasons it took decades to develop a successful helicopter. It took that long to abandon the coax, with its complex packaging, blade meshing problems, high drag and soft yaw control to make the first several generations of helos.
While some "natural" properties seem to be favored, the rule "if it is stupid, but it works, it is not stupid" must be applied. One of the lessons that Igor Sikorsky learned was that the apparent "natural" symmetry of the coax was one of the reasons it took decades to develop a successful helicopter. It took that long to abandon the coax, with its complex packaging, blade meshing problems, high drag and soft yaw control to make the first several generations of helos.
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Coaxial helicopters USA
I may be wrong but I believe that the first USA attempt at development of a coaxial helicopter was the Hoppicopter developed by a friend of mine (Maurice Ramme) now deceased. It went through several stages of development starting with a strap-on version and then evolving into a single person helicopter with a rudimentary landing gear and a seat for the operator. Unable to get financing, Mr. Ramme sold his design to a British Company that was planning on further development and ultimate sale to individuals. That plan eventually went south and the program died.
Check this website:
http://avia.russian.ee/vertigo/hoppicopter.html
Check this website:
http://avia.russian.ee/vertigo/hoppicopter.html
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Thank you for your replies.
Aerodynamically what are the advantages/ disadvantages of the coax v Main + Tail rotor designs.
Surely the coax idea is the answer to power loss at low speeds that the fenestron and notar suffer.
Aerodynamically what are the advantages/ disadvantages of the coax v Main + Tail rotor designs.
Surely the coax idea is the answer to power loss at low speeds that the fenestron and notar suffer.
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Aerodynamically speaking;
Most coaxial rotors have three blades. The two 3-blade rotors are somewhat more aerodynamically efficient than a comparable single rotor with six blades. This greater efficiency is due to the counterrotating swirl of the two rotors and the vertical gap between them. The lack of a tail rotor also adds to the coaxial's efficiency.
Countering this is the fact that few, single rotor helicopters have six blades and a larger rotor disk with fewer blades is more efficient than a smaller rotor disk with many blades.
Countering this is the fact that few, single rotor helicopters have six blades and a larger rotor disk with fewer blades is more efficient than a smaller rotor disk with many blades.
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So what you are saying is that for vertical lift the coax is the best design, but for overall efficiency the three bladed main rotot coupled to a two bladed tail rotor benefits from the reduction in rotor drag and has more tail rotor authority/control.
I don't quite see how the rotor drag coming from the six blades being greater than in a conventional design can then be more efficient in lifting.
I can only guess that it's the tail rotor which is producing more drag than the extra three blades.
Anyboby have knowledge of this.
I don't quite see how the rotor drag coming from the six blades being greater than in a conventional design can then be more efficient in lifting.
I can only guess that it's the tail rotor which is producing more drag than the extra three blades.
Anyboby have knowledge of this.
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The most, perhaps ONLY, attractive co-axial rotor helicopter (IMHO) was the Sikorsky Advancing Blade Concept demonstrator from the mid-80s.
But this is more than just as aerodynamic issue. Quoting a certain Nick Lappos from Iteravia Jan 1986:
"Because of of its more complex control system and twin rigid rotors, the ABC is not as weight-efficient as a conventional helicopter. A penalty of about 10% would be likely in a production ABC."
But this is more than just as aerodynamic issue. Quoting a certain Nick Lappos from Iteravia Jan 1986:
"Because of of its more complex control system and twin rigid rotors, the ABC is not as weight-efficient as a conventional helicopter. A penalty of about 10% would be likely in a production ABC."
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Because of of its more complex control system and twin rigid rotors, the ABC is not as weight-efficient as a conventional helicopter. A penalty of about 10% would be likely in a production ABC.
Assuming that the rotors of a coaxial and an intermeshing helicopter are identical; the lateral offset of the intermeshing rotors will give a better figure of merit. Not only will this advantage overcome the mentioned 10% penalty of the coaxial, it will also be more efficient then MR + TR helicopters.
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The greatest loading on a rotor in forward flight is at the tip end of the retreating blades. The following web page very clearly shows that this area, on an intermeshing configuration, is outside the downwash of the other rotor;
Rotor - Disk - Downwash - Intermeshing in Forward Flight
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The Russians obviously favour the coax system having built the 'Chernaya Akula' attack helicopter, Ka - 50 (NATO designation = Hocum).
With a 170 Knot top speed and a carrying capacity of a 3 ton truck this was an awsom machine.
QUESTION. Is this technology lost for ever?
With a 170 Knot top speed and a carrying capacity of a 3 ton truck this was an awsom machine.
QUESTION. Is this technology lost for ever?
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Head Turner
A response to the questions on your post of 7th, Oct; which I missed.
For vertical lift, the coaxial may be best for extremely low disk loading. A number of universities are using this configuration, in an attempt to achieve man-powered vertical flight. The rotors are turning so slowly that their aerodynamic interaction is just about irrelevant.
For powered vertical flight, the optimal lift-to-power ratio will be achieved by the side-by-side rotor configuration.
This may answer your question about the; Required Power Comparison for Various Rotor Configurations, in Hover. Hope this helps.
A response to the questions on your post of 7th, Oct; which I missed.
For vertical lift, the coaxial may be best for extremely low disk loading. A number of universities are using this configuration, in an attempt to achieve man-powered vertical flight. The rotors are turning so slowly that their aerodynamic interaction is just about irrelevant.
For powered vertical flight, the optimal lift-to-power ratio will be achieved by the side-by-side rotor configuration.
This may answer your question about the; Required Power Comparison for Various Rotor Configurations, in Hover. Hope this helps.
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Head Turner,
Just in case your are seriously interested, the following 25-page report addresses your original question. It is excellent, and it was (and still may be) available on the net for free.
____________________________
A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research
Coleman, Colin P. (NASA Ames Research Center, Moffett Field, CA United States)
NASA Center for AeroSpace Information (CASI)
NASA-TP-3675 , 1997
The recent appearance of the Kamov Ka-50 helicopter and the application of coaxial rotors to unmanned aerial vehicles have renewed international interest in the coaxial rotor configuration. This report addresses the aerodynamic issues peculiar to coaxial rotors by surveying American, Russian, Japanese, British, and German research. (Herein, 'coaxial rotors' refers to helicopter, not propeller, rotors. The intermeshing rotor system was not investigated.) Issues addressed are separation distance, load sharing between rotors, wake structure, solidity effects, swirl recovery, and the effects of having no tail rotor. A general summary of the coaxial rotor configuration explores the configuration's advantages and applications.
No Digital Version Available - Order This Document
Just in case your are seriously interested, the following 25-page report addresses your original question. It is excellent, and it was (and still may be) available on the net for free.
____________________________
A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research
Coleman, Colin P. (NASA Ames Research Center, Moffett Field, CA United States)
NASA Center for AeroSpace Information (CASI)
NASA-TP-3675 , 1997
The recent appearance of the Kamov Ka-50 helicopter and the application of coaxial rotors to unmanned aerial vehicles have renewed international interest in the coaxial rotor configuration. This report addresses the aerodynamic issues peculiar to coaxial rotors by surveying American, Russian, Japanese, British, and German research. (Herein, 'coaxial rotors' refers to helicopter, not propeller, rotors. The intermeshing rotor system was not investigated.) Issues addressed are separation distance, load sharing between rotors, wake structure, solidity effects, swirl recovery, and the effects of having no tail rotor. A general summary of the coaxial rotor configuration explores the configuration's advantages and applications.
No Digital Version Available - Order This Document
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@ Head Turner
Russians and chinese have some days ago signed a contract to develop a new attack helicopter on base of the KA-50.
Due to the russian financial problems in russia only 4 KA-50 are built. The next stage is the KA-52. Now built two times.
Dave_Jackson is right. The intermeshing could be the winner.
For long liners the coaxial is the right deal. Lift to power ratio couldn't be reached by the others.
Is this technology lost for ever?
Due to the russian financial problems in russia only 4 KA-50 are built. The next stage is the KA-52. Now built two times.
Dave_Jackson is right. The intermeshing could be the winner.
For long liners the coaxial is the right deal. Lift to power ratio couldn't be reached by the others.
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Thank you all for your knowledge and thoughts.
From what you are saying, the coax is ok but the intermeshing is better.
Next Question.
Why then are there so few of the 'intermeshing' helicopters?
Q2. Is it the expected look rather than the right look that drives the sales of helicopters and therefore manufacturing?
From what you are saying, the coax is ok but the intermeshing is better.
Next Question.
Why then are there so few of the 'intermeshing' helicopters?
Q2. Is it the expected look rather than the right look that drives the sales of helicopters and therefore manufacturing?
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The KA-50/52 may be capable of 170 Knots, but you'll hear it coming at least 5 counties away - noiser than any Huey or Cobra or Chinook.
The KA-32 is an awesome machine for lifting loads and logging, but not one you'd want to work underneath, due to the downwash- even at flat pitch on the ground, you don't so much walk away from it as get pushed. Downwash is an issue with coaxial designs, and complexity of the rotor controls is nearly beyond imagining.
The coaxial designs also normally have featured pretty boxy fuselages and very draggy back ends for directional stability.
Everything is a compromise.
The KA-32 is an awesome machine for lifting loads and logging, but not one you'd want to work underneath, due to the downwash- even at flat pitch on the ground, you don't so much walk away from it as get pushed. Downwash is an issue with coaxial designs, and complexity of the rotor controls is nearly beyond imagining.
The coaxial designs also normally have featured pretty boxy fuselages and very draggy back ends for directional stability.
Everything is a compromise.
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Head Turner,
Five years ago, I started researching VTOL concepts; everything from weird dreams to functional craft. This ongoing research and development has averaged about 3,000 hours per year.
There has been a concerted effort to try to keep an open mind. In addition, no metal has yet been cut and therefore the commitment to a specific configuration is not as strong as that held by current helicopter manufactures.
Nick Lappos has said that "there is no free lunch" and he is absolutely correct. No one configuration can satisfy all VTOL demands; now and on into the distant future. However, in the second year I came to the realization that the underdeveloped intermeshing configuration offered the most. In the ensuing three years of research and development, nothing has come up to change this belief.
This web page may provide the answers to your question; ~ Intermeshing Information
Dave J.
Why then are there so few of the 'intermeshing' helicopters?
There has been a concerted effort to try to keep an open mind. In addition, no metal has yet been cut and therefore the commitment to a specific configuration is not as strong as that held by current helicopter manufactures.
Nick Lappos has said that "there is no free lunch" and he is absolutely correct. No one configuration can satisfy all VTOL demands; now and on into the distant future. However, in the second year I came to the realization that the underdeveloped intermeshing configuration offered the most. In the ensuing three years of research and development, nothing has come up to change this belief.
This web page may provide the answers to your question; ~ Intermeshing Information
Dave J.
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zalt,
Your concern about "meshed rotorcraft are not capable of cruise speeds typical of current helicopters" is a popular belief.
This misconception has come about because Charles Kaman decided to utilize the intermeshing configuration's superior Figure of Merit. He left United Technologies (Sikorsky) when he was told that they "already had one chief engineer and did not need another". It appears, his $5,000.00 bankroll was insufficient to take on the established United Technologies head-to-head, so he went for a niche market.
IMHO, everything points to the intermeshing configuration as actually being the BEST one for high-speed rotorcraft.
Without getting into the boring detail stuff, it might be noted that;
~ In the beginning, the intermeshing Flettner FL-282B had a top speed of 93 mph where as its single rotor contemporary, the Sikorsky R-4B, had a top speed of only 75 mph.
~ Since then, the Russians have produced fast coaxials, plus a side-by-side helicopter that held the rotorcraft speed record. The intermeshing configuration fits between these two.
~ For the future, the aerodynamicist Stepmewski's preference is an ABC Synchropter, with a cruise speed of up to 260 knots.
Your concern about "meshed rotorcraft are not capable of cruise speeds typical of current helicopters" is a popular belief.
This misconception has come about because Charles Kaman decided to utilize the intermeshing configuration's superior Figure of Merit. He left United Technologies (Sikorsky) when he was told that they "already had one chief engineer and did not need another". It appears, his $5,000.00 bankroll was insufficient to take on the established United Technologies head-to-head, so he went for a niche market.
IMHO, everything points to the intermeshing configuration as actually being the BEST one for high-speed rotorcraft.
Without getting into the boring detail stuff, it might be noted that;
~ In the beginning, the intermeshing Flettner FL-282B had a top speed of 93 mph where as its single rotor contemporary, the Sikorsky R-4B, had a top speed of only 75 mph.
~ Since then, the Russians have produced fast coaxials, plus a side-by-side helicopter that held the rotorcraft speed record. The intermeshing configuration fits between these two.
~ For the future, the aerodynamicist Stepmewski's preference is an ABC Synchropter, with a cruise speed of up to 260 knots.