Is a tail rotor really needed?
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A question for any of you guys flying fenestron T/Rs (or anyone who knows). Are they set up like conventional t/rs, thrust changed with blade pitch, or do they adjust the speed? Also I assume that they consume more power in cruise because they can't benefit from etl. true?
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To: Nick Lappos
“Unfortunately, Lu is wrong yet again. A spinning wheel cannot counter the torque of the main rotor, because once the wheel is up to speed, it takes no torque to spin it, so it can't counter the torque of the main rotor. The main rotor is continuously making lift, so it has the drag that creates torque (torque is really rotating drag)”.
Please read the first words of my comment on the spinning wheel.
“Let’s ASSUME that your concept is FEASIBLE”
I started from that premise and expanded on it to show the problems that would result from the concept.
Don’t be so picky.
“Unfortunately, Lu is wrong yet again. A spinning wheel cannot counter the torque of the main rotor, because once the wheel is up to speed, it takes no torque to spin it, so it can't counter the torque of the main rotor. The main rotor is continuously making lift, so it has the drag that creates torque (torque is really rotating drag)”.
Please read the first words of my comment on the spinning wheel.
“Let’s ASSUME that your concept is FEASIBLE”
I started from that premise and expanded on it to show the problems that would result from the concept.
Don’t be so picky.
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Lu said:
I started from that premise and expanded on it to show the problems that would result from the concept.
Earlier Lu aid:
The wheel would have to be of a large mass and spin at a high rate of speed in order to counter the torque developed by the spinning main rotor.
Nick sez:
Lu, you even critiqued the idea as needing a large mass! Then you duck what you wrote by pretending your confusing understanding was hypothetical. You are hopeless! You cannot remember what you said, how can we expect you to remember what we said.
I started from that premise and expanded on it to show the problems that would result from the concept.
Earlier Lu aid:
The wheel would have to be of a large mass and spin at a high rate of speed in order to counter the torque developed by the spinning main rotor.
Nick sez:
Lu, you even critiqued the idea as needing a large mass! Then you duck what you wrote by pretending your confusing understanding was hypothetical. You are hopeless! You cannot remember what you said, how can we expect you to remember what we said.
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I'd like to try a different explanation that should make this clearer (I hope).
It helps to look at torque as an accelerating force. Torque is very similar to linear acceleration, except that it acts in an arc (i.e. in a rotating direction).
Regarding the flywheel as a means of countering torque, the Hubble Space Telescope serves as an excellent example. It uses flywheels on all 3 axis to point the telescope in the proper direction. To change the attitude along an axis, that particular flywheel is accelerated (or decelerated, which is "acceleration" in the opposite direction) by torque from an electic motor. The opposing force from the motor is applied directly to the body of the telescope which starts a motion (acceleration from no motion) through that axis. To stop the motion along that axis, opposite torque is applied by the motor to accelerate the flywheel in the opposite direction.
For this application, the use of flywheels to control torque works great, since it's operating in the vacuum of space, and there is no constant application of torque, otherwise all of the pictures would be streaks of light. But for a helicopter this is an entirely different story.
First of all gravity is an accelerating force like torque, except that it's a linear force with an acceleration of 32fps/s. If a man falls from an airplane, he will accelerate at this rate until the wind resistance against him, causes his speed to stablize in balance with the accelerating force of gravity. So gravity is still trying to accelerate him against the wind resistance to increase his speed.
In a helicopter, the main rotor in trying to balance the linear accelerating force of gravity with a constant application of lift. The lift is being generated by the rotation of the main rotor, which has to have torque (an accelerating force acting in an arc) applied to it by the engine(s). So the main rotor is being accelerated by torque to both balance the lift of the helo against gravity, and to overcome the aerodynamic drag of the rotor blades. The engine(s)s in turn are applying torque to the body of the helicopter.
The problem with using a flywheel to control torque in this application, is that the torque is being constantly applied (instead of being momentarily applied as is the Space Telescope). Since torque is an acceleration force, a flywheel would have to be constantly accelerated in the opposite direction to counter it. This could work only very briefly until the flywheel was spun so fast that it burst.
So in summary for the helo application, gravity is a linear acceleration force, countered by the lift produced by the torque (rotational acceleration) of the main rotor, which applies torque to the helo body, which is countered by the horizontal "lift" of the tail rotor. A flywheel just won't work here since all of these acceleration forces are being constantly applied.
Hope this helps...
(edited for typos)
[ 01 December 2001: Message edited by: Flight Safety ]
It helps to look at torque as an accelerating force. Torque is very similar to linear acceleration, except that it acts in an arc (i.e. in a rotating direction).
Regarding the flywheel as a means of countering torque, the Hubble Space Telescope serves as an excellent example. It uses flywheels on all 3 axis to point the telescope in the proper direction. To change the attitude along an axis, that particular flywheel is accelerated (or decelerated, which is "acceleration" in the opposite direction) by torque from an electic motor. The opposing force from the motor is applied directly to the body of the telescope which starts a motion (acceleration from no motion) through that axis. To stop the motion along that axis, opposite torque is applied by the motor to accelerate the flywheel in the opposite direction.
For this application, the use of flywheels to control torque works great, since it's operating in the vacuum of space, and there is no constant application of torque, otherwise all of the pictures would be streaks of light. But for a helicopter this is an entirely different story.
First of all gravity is an accelerating force like torque, except that it's a linear force with an acceleration of 32fps/s. If a man falls from an airplane, he will accelerate at this rate until the wind resistance against him, causes his speed to stablize in balance with the accelerating force of gravity. So gravity is still trying to accelerate him against the wind resistance to increase his speed.
In a helicopter, the main rotor in trying to balance the linear accelerating force of gravity with a constant application of lift. The lift is being generated by the rotation of the main rotor, which has to have torque (an accelerating force acting in an arc) applied to it by the engine(s). So the main rotor is being accelerated by torque to both balance the lift of the helo against gravity, and to overcome the aerodynamic drag of the rotor blades. The engine(s)s in turn are applying torque to the body of the helicopter.
The problem with using a flywheel to control torque in this application, is that the torque is being constantly applied (instead of being momentarily applied as is the Space Telescope). Since torque is an acceleration force, a flywheel would have to be constantly accelerated in the opposite direction to counter it. This could work only very briefly until the flywheel was spun so fast that it burst.
So in summary for the helo application, gravity is a linear acceleration force, countered by the lift produced by the torque (rotational acceleration) of the main rotor, which applies torque to the helo body, which is countered by the horizontal "lift" of the tail rotor. A flywheel just won't work here since all of these acceleration forces are being constantly applied.
Hope this helps...
(edited for typos)
[ 01 December 2001: Message edited by: Flight Safety ]
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Lu
I've edited one of your earlier posts.
Would you PLEASE stop making childish comments or 'digs' at other people.
Your 'digs' obviously amuse you. They equally obviously irritate other people, and risk spoiling good discussions.
Heliport
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I've edited one of your earlier posts.
Would you PLEASE stop making childish comments or 'digs' at other people.
Your 'digs' obviously amuse you. They equally obviously irritate other people, and risk spoiling good discussions.
Heliport
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Tandem rotor helicopters despite their complexity offer superior lifting power and a wide CG range.
I think you'll find that due to interference losses there is no performance gain for tandem systems - only a weight penalty and some operating advantages (rear ramp etc). The biggest lifters in the world are "conventional" designs.
I think you'll find that due to interference losses there is no performance gain for tandem systems - only a weight penalty and some operating advantages (rear ramp etc). The biggest lifters in the world are "conventional" designs.
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Dave Jackson,
Thanks for the steer to the Kamov web site. That is a fine discussion presented there. I ran S-76 numbers to check, and the S-76B uses about 8% of its total power for the tail rotor in an OGE hover at Max Gross.
The rest of the Kamov discussion beats around proving that co-axials are the "best" configuration, but there are lots of shades of gray in the discussion.
One thing I have learned in this industry is the difference between theory and fact. As Mr. Sikorsky said, "There will be many times in your career, young gentlemen, when the theory and the facts do not agree. I heartily implore you to respect the facts!"
The actual payload of a helo relative to the size of the engines is almost a constant for all the various configurations, due to all the practical factors that complicate the basic overly simple discussion. As in all human nature, folks often fail to tell you all the facts to help serve their argument, too.
The Kamov is a case in point. While a single rotor helo must waste power in the tail rotor, the co-axial must have shorter blades (self mid-air collision is a real design problem. Two Ka-50 have been destroyed due to blade to blade impacts). The shorter blades and the height of the stacked diska allow a better flapping angle while avoiding blade contact. Unfortunately, the smaller Kamov rotor disks make their hover power efficiency less than that of a typical single rotor helo, so what they gain in one side (no tail rotor losses) they lose in another (higher disk loading wastes power.)
Nature is perverse enough to even out all the competition.
Thanks for the steer to the Kamov web site. That is a fine discussion presented there. I ran S-76 numbers to check, and the S-76B uses about 8% of its total power for the tail rotor in an OGE hover at Max Gross.
The rest of the Kamov discussion beats around proving that co-axials are the "best" configuration, but there are lots of shades of gray in the discussion.
One thing I have learned in this industry is the difference between theory and fact. As Mr. Sikorsky said, "There will be many times in your career, young gentlemen, when the theory and the facts do not agree. I heartily implore you to respect the facts!"
The actual payload of a helo relative to the size of the engines is almost a constant for all the various configurations, due to all the practical factors that complicate the basic overly simple discussion. As in all human nature, folks often fail to tell you all the facts to help serve their argument, too.
The Kamov is a case in point. While a single rotor helo must waste power in the tail rotor, the co-axial must have shorter blades (self mid-air collision is a real design problem. Two Ka-50 have been destroyed due to blade to blade impacts). The shorter blades and the height of the stacked diska allow a better flapping angle while avoiding blade contact. Unfortunately, the smaller Kamov rotor disks make their hover power efficiency less than that of a typical single rotor helo, so what they gain in one side (no tail rotor losses) they lose in another (higher disk loading wastes power.)
Nature is perverse enough to even out all the competition.
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Nick
Thanks for your comments on the coaxial configuration.
The coaxial and the intermeshing (synchropter) helicopters represent an interesting dichotomy of ideas. They have relatively similar configurations, and yet Kamov elected a small disk area and high disk loading, whereas Kaman chose a large disk area and low disk loading.
Today's stiffer rotors should minimize many of their differences.
Thanks for your comments on the coaxial configuration.
The coaxial and the intermeshing (synchropter) helicopters represent an interesting dichotomy of ideas. They have relatively similar configurations, and yet Kamov elected a small disk area and high disk loading, whereas Kaman chose a large disk area and low disk loading.
Today's stiffer rotors should minimize many of their differences.
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Good observation, Dave. Charlie Kaman's tilted syncropter disks are much less susceptable to blade contact, so he can make the disks very large, while Kamov designs must trim the blades accordingly. It never dawned on me before. This forum is keen! 
The stiffer rotors might not be applicable, because the higher mast of a coaxial needs even more beef to stand the high forces of a rigid stiff blade, The KA-50 illustrates this. The lower disk has 7% hinge offset (its much closer to the transmission, so the high moments from that offset need a shorter shaft to carry them to the airframe). The upper rotor has a measly 2.5% offset, so its long mast can stay light and skinny.
[ 01 December 2001: Message edited by: Nick Lappos ]
The stiffer rotors might not be applicable, because the higher mast of a coaxial needs even more beef to stand the high forces of a rigid stiff blade, The KA-50 illustrates this. The lower disk has 7% hinge offset (its much closer to the transmission, so the high moments from that offset need a shorter shaft to carry them to the airframe). The upper rotor has a measly 2.5% offset, so its long mast can stay light and skinny.
[ 01 December 2001: Message edited by: Nick Lappos ]
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Nick:
The comment on the KA-50's differing hinge-offsets is interesting. I understand that the Sikorsky ABC had fairly stiff rotors, but that it was necessary to increase Gamma as the forward speed increased, to maintain an acceptable blade to blade clearance. Another characteristic of the ABC was that its rotor assemblage constituted 50% of the parasitic drag.
To me, talking from a position of limited knowledge mind you, it appears that an intermeshing configuration with very short masts will allow for greater stiffness in the rotors. As well, the integrated fuselage and hubs should reduce the parasitic drag.
[ 02 December 2001: Message edited by: Dave Jackson ]
The comment on the KA-50's differing hinge-offsets is interesting. I understand that the Sikorsky ABC had fairly stiff rotors, but that it was necessary to increase Gamma as the forward speed increased, to maintain an acceptable blade to blade clearance. Another characteristic of the ABC was that its rotor assemblage constituted 50% of the parasitic drag.
To me, talking from a position of limited knowledge mind you, it appears that an intermeshing configuration with very short masts will allow for greater stiffness in the rotors. As well, the integrated fuselage and hubs should reduce the parasitic drag.
[ 02 December 2001: Message edited by: Dave Jackson ]
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Vfrpilotpb
i dont think the spining wheight would work because there is nothing for it to "twist against" once it has got up to speed( to use it for anitorque from the rotors it would have to be accellerating continuously). the rotors are spinning in the air so you need somthing in the air to counter act it unless the wheight is soooo big you wouldnt lift it of the ground anyway.
if the rotor was perpendiculare to the mast it would turn the torque from the engine into a rolling moment which the pilot could stop with cyclic but this would make everything else so hard to control.
i dont think the spining wheight would work because there is nothing for it to "twist against" once it has got up to speed( to use it for anitorque from the rotors it would have to be accellerating continuously). the rotors are spinning in the air so you need somthing in the air to counter act it unless the wheight is soooo big you wouldnt lift it of the ground anyway.
if the rotor was perpendiculare to the mast it would turn the torque from the engine into a rolling moment which the pilot could stop with cyclic but this would make everything else so hard to control.
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Dave said:
I understand that ABC had reasonably stiff rotors, but that it was necessary to increase Gamma as the forward speed increased, to maintain an acceptable blade to blade clearance.
Nick sez:
Yea, the rotors were stiff, about 25% hinge offset! The Gamma reset in flight was used, more for keeping the moment down (the rotors would oppose each other to some extent) and the controls square, but bett6er tip clearance was also had. The Boeing UTTAS YUH-61A had adjustable gamma, as does the Comanche, due to the high offset. This is simple proof that the rotor rigging angle, Gamma, is set by many factors, not gyroscopic precession. (Get ready for the DRIVEL attack, Dave!)
Dave said:
Another comment about the ABC was that its current rotor assemblage constituted 50% of the parasitic drag.
Nick sez:
That may be right. The head drag is a big fraction of a helo's drag, and two heads are not better than one. An old paper by Mikheyev of Kamov estimated that the co-axials lost 10% speed and range due to extra drag.
Dave said:
To me, talking from a position of limited knowledge mind you, it appears that an intermeshing configuration with very short masts will allow for greater stiffness in the rotors. As well, the integrated fuselage and hubs should reduce the parasitic drag.
Nick sez:
The intermeshing configuration could allow less stiffness, because tip clearance is not a problem at all, I think. Drag would be higher than a single rotor (more stuff in the breeze), but lower than a co-axial, because there is no long mast and upper swashplate arrangement.
I know that you favor the intermeshing configuration, Dave. There are advantages, but that transmission and head arrangement is still a handfull. Fly By Wire would help, however. Keep pushing for your dreams!
I understand that ABC had reasonably stiff rotors, but that it was necessary to increase Gamma as the forward speed increased, to maintain an acceptable blade to blade clearance.
Nick sez:
Yea, the rotors were stiff, about 25% hinge offset! The Gamma reset in flight was used, more for keeping the moment down (the rotors would oppose each other to some extent) and the controls square, but bett6er tip clearance was also had. The Boeing UTTAS YUH-61A had adjustable gamma, as does the Comanche, due to the high offset. This is simple proof that the rotor rigging angle, Gamma, is set by many factors, not gyroscopic precession. (Get ready for the DRIVEL attack, Dave!)
Dave said:
Another comment about the ABC was that its current rotor assemblage constituted 50% of the parasitic drag.
Nick sez:
That may be right. The head drag is a big fraction of a helo's drag, and two heads are not better than one. An old paper by Mikheyev of Kamov estimated that the co-axials lost 10% speed and range due to extra drag.
Dave said:
To me, talking from a position of limited knowledge mind you, it appears that an intermeshing configuration with very short masts will allow for greater stiffness in the rotors. As well, the integrated fuselage and hubs should reduce the parasitic drag.
Nick sez:
The intermeshing configuration could allow less stiffness, because tip clearance is not a problem at all, I think. Drag would be higher than a single rotor (more stuff in the breeze), but lower than a co-axial, because there is no long mast and upper swashplate arrangement.
I know that you favor the intermeshing configuration, Dave. There are advantages, but that transmission and head arrangement is still a handfull. Fly By Wire would help, however. Keep pushing for your dreams!
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RW-1, the NOTAR tail has a slower responce then a conventional tail. It has the same authority, but it takes longer to achive it. You have to chase the tail at times, correct what you just corrected because it is slow.
Senis Semper Fidelis
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Good morning Rotoheads, I have been away for a couple of days, but I am indeed pleased at the amount of replys from our worldwide collecion of heli-brains, I have quickly read through the post's and must say thank you , you have all added information that I just could not see, but I now see the failing of my pre senile rambling's, for any counter rotating weight to stand any chance it would have to stay in tandem with every move of the main rotor, that I now understand, nor had I considered the effective weight that a flywheel would need to be able to counter the spinning mass of the M/rotor at all various pitch's. So I guess that we ordinary mortals are going to be stuck with " Boring old Single Rotors"( typed with a beaming smile on his face),
However in the same vein of Rotor/counter spinning rotors, the achilles heel , so to speak, is the linkage( or so I have read ) consider this, the Avro Shackleton and various other FW type's with Counter spinning props all used a hydraulic system to alter the pitch change, this was called "The Translation Unit" and worked by pushing and pulling extremely well engineered solid steel bars, which were fixed to the rotor heads, would this system work on helicopters,or do we always have to have a physical link, for this translation unit system would do away with a lot of tiny (by comparison) control rods and connections. A swash plate would still be there, but that would work both rotors anyway.
Thank you all for your input
My Regards
Edited to remove early morning problems of co-ordination PB
[ 03 December 2001: Message edited by: Vfrpilotpb ]
However in the same vein of Rotor/counter spinning rotors, the achilles heel , so to speak, is the linkage( or so I have read ) consider this, the Avro Shackleton and various other FW type's with Counter spinning props all used a hydraulic system to alter the pitch change, this was called "The Translation Unit" and worked by pushing and pulling extremely well engineered solid steel bars, which were fixed to the rotor heads, would this system work on helicopters,or do we always have to have a physical link, for this translation unit system would do away with a lot of tiny (by comparison) control rods and connections. A swash plate would still be there, but that would work both rotors anyway.
Thank you all for your input
My Regards
Edited to remove early morning problems of co-ordination PB
[ 03 December 2001: Message edited by: Vfrpilotpb ]
Baranfin, looks like you got lost in the perennial Lu/Nick conflict (personally I think Danny should give them their own forum).
Anyway, fenestrons operate the same as conventional T/Rs, ie change pitch to alter power. If you look at fenestron aircraft, they've got large tailfins which produce sideways force in the cruise, so the pitch on the fenestron blades can be reduced same as T/Rs.
Anyway, fenestrons operate the same as conventional T/Rs, ie change pitch to alter power. If you look at fenestron aircraft, they've got large tailfins which produce sideways force in the cruise, so the pitch on the fenestron blades can be reduced same as T/Rs.
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Thanks for the response MightyGem, Another question about the fenestrons. I remember reading somewhere that ducted fans produce more thrust for their size than un-shrouded. Is this true? why? I would think it would be less efficient because the tip vortices getting all confused and bunched up at the edges.
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How about this for an idea?
A tail rotor that rotates 90 degrees which during cruise points backwards and produces forward thrust and therefore unloads the head which would allow a higher forward speed. A offset tail fin which is controllable (rudder) for yaw control at cruise speed and obviously the angle of the tail rotor thrust direction would be proportional to speed.
I know there is nothing new about a pusher prop on a helicopter (Cheyane) but the mechanics and control of this mechanism wouldn't be to difficult.
Jiff
A tail rotor that rotates 90 degrees which during cruise points backwards and produces forward thrust and therefore unloads the head which would allow a higher forward speed. A offset tail fin which is controllable (rudder) for yaw control at cruise speed and obviously the angle of the tail rotor thrust direction would be proportional to speed.
I know there is nothing new about a pusher prop on a helicopter (Cheyane) but the mechanics and control of this mechanism wouldn't be to difficult.
Jiff
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To: Jiff
Your idea about the folding tail rotor was given some consideration in the design of the Cheyenne but it was decided to go with a tail rotor and the rear propeller. If I remember there was a Beta controller that distributed power to the prop and also controlled its’ pitch for increased or decreased thrust. The propeller was used during ground taxi and direction was controlled with differential braking just like a fixed wing aircraft. The Cheyenne unlike a conventional helicopter could not taxi using disc tilt. Cyclic was not available to the pilot until the weight on wheels switch indicated the helicopter was airborne and then, the cyclic control was enabled and power had been previously shifted to the tail rotor. Once forward speed was built up the tail rotor was no longer needed and the beta controller shifted power to the propeller and the thrust was adjusted. Once forward flight was established and the Beta control had shifted most of the power to the propeller providing forward thrust the helicopter was now an autogyro. Control of the helicopter was through cyclic input but the blades were operating at minimum pitch.
Of course that was 32 years ago when I worked on the program and I may have confused the facts.
Your idea about the folding tail rotor was given some consideration in the design of the Cheyenne but it was decided to go with a tail rotor and the rear propeller. If I remember there was a Beta controller that distributed power to the prop and also controlled its’ pitch for increased or decreased thrust. The propeller was used during ground taxi and direction was controlled with differential braking just like a fixed wing aircraft. The Cheyenne unlike a conventional helicopter could not taxi using disc tilt. Cyclic was not available to the pilot until the weight on wheels switch indicated the helicopter was airborne and then, the cyclic control was enabled and power had been previously shifted to the tail rotor. Once forward speed was built up the tail rotor was no longer needed and the beta controller shifted power to the propeller and the thrust was adjusted. Once forward flight was established and the Beta control had shifted most of the power to the propeller providing forward thrust the helicopter was now an autogyro. Control of the helicopter was through cyclic input but the blades were operating at minimum pitch.
Of course that was 32 years ago when I worked on the program and I may have confused the facts.