Aerodynamics ~ The Coaxial Myth?
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Aerodynamics ~ The Coaxial Myth?
For many decades, the textbooks on helicopter aerodynamics stated that coaxial rotors require up to 41% more induced power then two single rotors in isolation.
Blade element theory shows that a 4-blade single rotor requires 27% more total power [approx. 33% more induced power] then two single rotors in isolation.
From the above, it is apparent that the 4-blade main-rotor / tail-rotor configuration is more efficient then the two 2-blade coaxial rotor configuration. Perhaps this is a reason for Igor's asymmetrical main-rotor / tail-rotor being a preeminent configuration, at least in western helicopters.
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Interestingly, the latest textbook now shows that the coaxial rotor is actually much more efficient then previously assumed. Experiments show it to require only 16% more induced power then the two single rotors in isolation. Perhaps, this should have been obvious, since there appears to be little difference between a single 4-blade rotor and two closely spaced counter-rotating 2-blade rotors.
In other words, the coaxial configuration is now shown to be more efficient than the 4-blade single rotor/ tail rotor configuration, by approximately 10%; and this, by no coincident, is the power consumed by a tail rotor.
Perhaps this 'aerodynamic revision' will cause developers to reassess the advantages of symmetrical twin-rotor helicopters.
Perhaps the world's first production helicopter, the symmetrical Flettner FL-282 will reappear as symmetrical V-22s and others. 
Edited to change 'induced' to 'total' in 2nd paragraph, thanks to Barannfin's question.
Blade element theory shows that a 4-blade single rotor requires 27% more total power [approx. 33% more induced power] then two single rotors in isolation.
From the above, it is apparent that the 4-blade main-rotor / tail-rotor configuration is more efficient then the two 2-blade coaxial rotor configuration. Perhaps this is a reason for Igor's asymmetrical main-rotor / tail-rotor being a preeminent configuration, at least in western helicopters.
______________________
Interestingly, the latest textbook now shows that the coaxial rotor is actually much more efficient then previously assumed. Experiments show it to require only 16% more induced power then the two single rotors in isolation. Perhaps, this should have been obvious, since there appears to be little difference between a single 4-blade rotor and two closely spaced counter-rotating 2-blade rotors.
In other words, the coaxial configuration is now shown to be more efficient than the 4-blade single rotor/ tail rotor configuration, by approximately 10%; and this, by no coincident, is the power consumed by a tail rotor.
Perhaps this 'aerodynamic revision' will cause developers to reassess the advantages of symmetrical twin-rotor helicopters.
Perhaps the world's first production helicopter, the symmetrical Flettner FL-282 will reappear as symmetrical V-22s and others. Edited to change 'induced' to 'total' in 2nd paragraph, thanks to Barannfin's question.
Last edited by Dave Jackson; 29th Sep 2002 at 16:51.
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But I like my tail rotor, how else am I going to cut the hedges?
I am a little confused about the induced power reqs. on the single rotor does that 27% include the tail rotor pwr? or is that added on.
I am pretty sceptical about any big changes occuring in the industry. I think most of the manufacturers are pretty set in their ways, and would probably be reluctant to switch to a new format without significant benefits. Or would the 1% improvement in effeciency actually justify it? I don't claim do know very much about aerodynamics or the industry, just my opinion on it.
I am a little confused about the induced power reqs. on the single rotor does that 27% include the tail rotor pwr? or is that added on.
I am pretty sceptical about any big changes occuring in the industry. I think most of the manufacturers are pretty set in their ways, and would probably be reluctant to switch to a new format without significant benefits. Or would the 1% improvement in effeciency actually justify it? I don't claim do know very much about aerodynamics or the industry, just my opinion on it.
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What about co-ax's in autorotation ??
Way back when I was in ground school we saw an old video that showed a model of a 47 in a windtunnel. They were showing the helicopter in various phases of flight to show how the air flows around/through the rotor system. During the segment on autorotation, the smoke failed to penetrate the rotor disk. For those in the know, how would the co-axial rotor system work in autorotation ?? Or would it even work ?? I tried asking the one person I met who flew a Kamov 32, and he told me they didn't practice dual engine failures.
Cheers
Randy_G
Cheers
Randy_G
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Barannfin,
Yes, the 27% includes the tail rotor power. It includes all power requirements. This percentage change comes from Prouty's 'Blade Element Calculations for Hover'.
The actual improvement is much better than 1%. It's up to 10%. If the dry weight to payload ratio was 50/50, then the theoretical increase in the payload, for the coaxial over the single rotor, is up to 20%; and that's fairly significant.
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Randy_g,
I understand that the co-axial rotor system works the same as the single rotor system in autorotation. The only difference is that the yaw flight control mechanism automatically switches the pedal inputs.
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widgeon,
As you say, synchronization is critical for the intermeshing configuration. It's not a serious concern in the coaxial configuration though.
Dave J
Edited to remove erroneous statement to widgeon; thanks to a correction by a knowledgeable reader.
Yes, the 27% includes the tail rotor power. It includes all power requirements. This percentage change comes from Prouty's 'Blade Element Calculations for Hover'.
The actual improvement is much better than 1%. It's up to 10%. If the dry weight to payload ratio was 50/50, then the theoretical increase in the payload, for the coaxial over the single rotor, is up to 20%; and that's fairly significant.
_____________
Randy_g,
I understand that the co-axial rotor system works the same as the single rotor system in autorotation. The only difference is that the yaw flight control mechanism automatically switches the pedal inputs.
______________
widgeon,
As you say, synchronization is critical for the intermeshing configuration. It's not a serious concern in the coaxial configuration though.
Dave J
Edited to remove erroneous statement to widgeon; thanks to a correction by a knowledgeable reader.
Last edited by Dave Jackson; 1st Oct 2002 at 21:58.
It's even better than that....
Dave et al,
Representatives of the Kamov company have been present at the last two big UK Helicopter technical get-togethers and on both occassions have claimed that the co-axial rotor configuration, when properly designed is infact more efficient than a single main rotor and tail rotor configuration. To the tune of about 15%!
They are understandably cagey about the exact reason for it but usually say...
The wake contraction of the downwash from the main rotor causes air to be drawn in to the second rotor from the side hence increasing the effective rotor diameter and reducing induced power requirement. Furthermore, there are addition beneficial effects of the vortex wake interactions between main rotors.
The key parameter required to achieve these performance gains are the spacing between the main rotors - required to be approximately 10% of MR diameter.
The earlier post about the directional control in autorotation is also correct. The only directional control a co-axe has in autorotation is derived from the moveable tail-planes - hence very marginal directional control especially at low forward velocity.
I understand that this is quite a fluffy explaination - but they really don't give too much away!
DAVE - Which book do you refer to? 'Latest Textbook'
Cheers
CRAN
Representatives of the Kamov company have been present at the last two big UK Helicopter technical get-togethers and on both occassions have claimed that the co-axial rotor configuration, when properly designed is infact more efficient than a single main rotor and tail rotor configuration. To the tune of about 15%!
They are understandably cagey about the exact reason for it but usually say...
The wake contraction of the downwash from the main rotor causes air to be drawn in to the second rotor from the side hence increasing the effective rotor diameter and reducing induced power requirement. Furthermore, there are addition beneficial effects of the vortex wake interactions between main rotors.
The key parameter required to achieve these performance gains are the spacing between the main rotors - required to be approximately 10% of MR diameter.
The earlier post about the directional control in autorotation is also correct. The only directional control a co-axe has in autorotation is derived from the moveable tail-planes - hence very marginal directional control especially at low forward velocity.
I understand that this is quite a fluffy explaination - but they really don't give too much away!
DAVE - Which book do you refer to? 'Latest Textbook'
Cheers
CRAN
Last edited by CRAN; 30th Sep 2002 at 18:03.
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The KA-32 is pretty impressive as a vertical lift machine. Even on the ground at flat pitch, the downwash is so strong it literally pushes you away from the machine.
There's a reason why it does so well with underslung loads and mostly vertical lifting- the two disks act like the first two stages of an axial compressor.
It's not just the horsepower that makes this machines so good for the logging mission, or makes the tandems so popular as well.
There are a lot of structural advantages to the tandem or coaxial machines when it comes to vertical lifting- no tailboom twisting, etc.
There's a reason why it does so well with underslung loads and mostly vertical lifting- the two disks act like the first two stages of an axial compressor.
It's not just the horsepower that makes this machines so good for the logging mission, or makes the tandems so popular as well.
There are a lot of structural advantages to the tandem or coaxial machines when it comes to vertical lifting- no tailboom twisting, etc.
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CRAN,
The textbook referred to is Principles of Helicopter Aerodynamics by Leishman. In section 2.4.6 he covers the coaxial and tandem configurations. He references experimental measurements for closely spaced coaxial rotors by Harrington (1951) and its review by Coleman (1993).
In addition, a comment by Leishman and the graph in Figure 2.20 suggest that even the revised calculations of the coaxials performance may still not fully portray its advantage over the single rotor. Perhaps Kamov's 15%, which you mention, is closer to the truth?
This anomaly has bugged me for a number of years. I have felt that perhaps the initial 1.41 factor came from comparing the twin 2-blade coaxial rotors against a 2-blade single rotor, and not against a 4-blade single rotor.
The 10% gap and wake contraction comments may be relevant to the intermeshing configuration, as well. This is because approximately half of the intermeshing's upper blade thrust (the outer half) is subject to similiar conditions, The intermeshing has, obviously, the additional advantage of a larger total disk footprint.
Thanks for the information.
______________________
My comment about yaw control in the coaxial was not very clear. The coaxial has excellent yaw control, which is done by varying the lift, and more importantly the drag, between the two counter-rotating rotors, while maintaining a constant total lift. During autorotation, the airflow through much of the disks is reversed. A particular pedal input would now cause a yaw in the opposite direction, if it were not for 'pedal switching linkages' that automatically take place in Kamovs at the onset of autorotation.
Dave J.
Symmetry is beautiful
The textbook referred to is Principles of Helicopter Aerodynamics by Leishman. In section 2.4.6 he covers the coaxial and tandem configurations. He references experimental measurements for closely spaced coaxial rotors by Harrington (1951) and its review by Coleman (1993).
In addition, a comment by Leishman and the graph in Figure 2.20 suggest that even the revised calculations of the coaxials performance may still not fully portray its advantage over the single rotor. Perhaps Kamov's 15%, which you mention, is closer to the truth?
This anomaly has bugged me for a number of years. I have felt that perhaps the initial 1.41 factor came from comparing the twin 2-blade coaxial rotors against a 2-blade single rotor, and not against a 4-blade single rotor.
The 10% gap and wake contraction comments may be relevant to the intermeshing configuration, as well. This is because approximately half of the intermeshing's upper blade thrust (the outer half) is subject to similiar conditions, The intermeshing has, obviously, the additional advantage of a larger total disk footprint.
Thanks for the information.
______________________
My comment about yaw control in the coaxial was not very clear. The coaxial has excellent yaw control, which is done by varying the lift, and more importantly the drag, between the two counter-rotating rotors, while maintaining a constant total lift. During autorotation, the airflow through much of the disks is reversed. A particular pedal input would now cause a yaw in the opposite direction, if it were not for 'pedal switching linkages' that automatically take place in Kamovs at the onset of autorotation.
Dave J.
Symmetry is beautiful
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The following is a link to; Required Power Comparison for Various Rotor Configurations, in Hover:
, for the few who might be interested.
Comments or corrections appreciated.
Dave J.
, for the few who might be interested.
Comments or corrections appreciated.
Dave J.
