Pylon Rock
Guest
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Maybe it's better not to buy into this discussion, but...bugger it, why not!
We're only talking, fellas and girls, not buying and selling sheep stations.
Lu, I'm inclined to agree with your reasoning regarding the loss of lift when the blade is aligned with the longitudinal axis, although I'd call it a reduction rather than a total loss.
Also, there is probably a disruption when it passes over the rear cabin and tailboom area, where the downwash would be deflected and the induced flow would, I imagine, be decreased. The exhaust airflow must have an effect too.
All in all, the blades experience various different states of relative airflow all round the disc, so no wonder they flap up and down like crazy (just look at that footage from the 'Chopper' series, or whatever it was called, where a camera was attached looking outwards from the hub of a huey inflight, makes you wonder how the damn things stay on for 1 hour, let alone thousands!).
Naturally then, all the forces generated by this movement will be transferred to the transmission mounts, which will absorb and damp it.
We're only talking, fellas and girls, not buying and selling sheep stations.
Lu, I'm inclined to agree with your reasoning regarding the loss of lift when the blade is aligned with the longitudinal axis, although I'd call it a reduction rather than a total loss.
Also, there is probably a disruption when it passes over the rear cabin and tailboom area, where the downwash would be deflected and the induced flow would, I imagine, be decreased. The exhaust airflow must have an effect too.
All in all, the blades experience various different states of relative airflow all round the disc, so no wonder they flap up and down like crazy (just look at that footage from the 'Chopper' series, or whatever it was called, where a camera was attached looking outwards from the hub of a huey inflight, makes you wonder how the damn things stay on for 1 hour, let alone thousands!).
Naturally then, all the forces generated by this movement will be transferred to the transmission mounts, which will absorb and damp it.
Guest
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I stumbled into this thread a bit late, but have to chime in:
The blades all lift all the way around the mast. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue.
Pylon rock is a natural oscillation common to the Bell two bladed systems, where the natural rotor inplane frequency is matched by the fuselage and transmission. All rotors have natural frequencies based on the rotational speed (RPM) and the number of blades, and the natural damping (energy absorbing) of the system. In early C model hueys (and the Cobra, which shares the same rotor design) there existed a match between the natural transmission rocking mode (the transmission is suspended on rubber mounts for vibration absorbtion) and the rotor's natural frequencies. This is quite similar in concept to a wing flutter condition in an airplane.
At some flight conditions, especially high speed and increased load factor) the whole rotor transmission system could go to a large limit cycle amplitude (get big and not die away). The Cobra had (has?) electronic motion transducers on the transmission feet to sense when the transmission starts rocking relative to the fuselage. This signal is sent to the SCAS (stability control augmentation system)which sends out control movements to oppose the transmission rocking, and damp them out.
Finding and fixing such "Pylon Rock" is a part of all helicopter testing, and keeping these dynamic frequencies under control is a big part of developing helicopters and tilt rotors.
Often, the back-up structure is stiffened or loostened to move the rocking mode away from the rotor mode, or damping is increased in the main rotor (no luck here for a teetering "semi-rigid" system).
The V-22 flight controls have several strong "notch filters" in them to keep the flight controls from sensing and amplifying similar rotor/transmission/fuselage modes. They work quite well, as evidenced by the lack of headlines about such problems. These rotor/airframe modes caused the crashes of the early tilt rotors, and stopped their development until better stiffer structures and faster flight controls could be developed (along with the smarter design programs needed to understand it all).
The other Bell rotors with 4 blades don't share the pylon rock problem because their natural frequencies are much higher, not anywhere near the transmission mode, and so there is no amplification of the lower transmission frequencies. A natural part of the design of a nodal transmission mount system is to keep these frequencies separated so that pylon rock does not happen.
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The blades all lift all the way around the mast. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue.
Pylon rock is a natural oscillation common to the Bell two bladed systems, where the natural rotor inplane frequency is matched by the fuselage and transmission. All rotors have natural frequencies based on the rotational speed (RPM) and the number of blades, and the natural damping (energy absorbing) of the system. In early C model hueys (and the Cobra, which shares the same rotor design) there existed a match between the natural transmission rocking mode (the transmission is suspended on rubber mounts for vibration absorbtion) and the rotor's natural frequencies. This is quite similar in concept to a wing flutter condition in an airplane.
At some flight conditions, especially high speed and increased load factor) the whole rotor transmission system could go to a large limit cycle amplitude (get big and not die away). The Cobra had (has?) electronic motion transducers on the transmission feet to sense when the transmission starts rocking relative to the fuselage. This signal is sent to the SCAS (stability control augmentation system)which sends out control movements to oppose the transmission rocking, and damp them out.
Finding and fixing such "Pylon Rock" is a part of all helicopter testing, and keeping these dynamic frequencies under control is a big part of developing helicopters and tilt rotors.
Often, the back-up structure is stiffened or loostened to move the rocking mode away from the rotor mode, or damping is increased in the main rotor (no luck here for a teetering "semi-rigid" system).
The V-22 flight controls have several strong "notch filters" in them to keep the flight controls from sensing and amplifying similar rotor/transmission/fuselage modes. They work quite well, as evidenced by the lack of headlines about such problems. These rotor/airframe modes caused the crashes of the early tilt rotors, and stopped their development until better stiffer structures and faster flight controls could be developed (along with the smarter design programs needed to understand it all).
The other Bell rotors with 4 blades don't share the pylon rock problem because their natural frequencies are much higher, not anywhere near the transmission mode, and so there is no amplification of the lower transmission frequencies. A natural part of the design of a nodal transmission mount system is to keep these frequencies separated so that pylon rock does not happen.
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Guest
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I got the following from Bell Engineering. Included below is the question that percipitated the answer below.
The nodal beam system was originally developed by Bell Helicopter in the
early 1970s. It was designed specifically to reduce/eliminate the inherent 2
per rev "bounce" (vibration) found in the Bell two bladed main rotor
helicopter designs. Most of the models designed and brought to production by
Bell during the late 1970s and early 1980s (206L, 214, 222) incorporated
nodalization of the main transmission to isolate the main rotor vibration
from the airframe.
For a good technical description of the nodalized pylon concept and its
theory, we refer you to the American Helicopter Society for a copy of a
paper entitled "Fuselage Nodalization" which was presented by several Bell
engineers at the 28th annual forum of the Society in 1972. Unfortunately a
copy of this paper is not available from Bell Helicopter.
-----Original Message-----
From: Lu Zuckerman [mailto:[email protected]]
Sent: Monday, June 18, 2001 3:19 AM
To: Rodriguez, Leticia
Subject: Question
Dear Ms. Rodriguez,
I am a consulting engineer and when on assignment I sometimes
arrange with the local A&P school to teach a course in helicopter
aerodynamics. I once worked for Bell Helicopter International in Iran and it
was during my tenure as manager of technical assistance that the 214 was
introduced. This was the first time I had encountered the Nodamatic
suspension system. Technical literature on the system was limited but I was
able to figure out how it worked. However, I was never told why the system
was designed and why it was incorporated on the 214. I came to the
conclusion that the downward vertical movement of the transmission was due
to the decreased lift from the rotor system when the blades came into
alignment with the centerline of the helicopter in flight. As the blades
departed this position the lift increased and the blade began to lift
raising the transmission.
I have been laboring under this conclusion since 1976 and I really
don't know if I am being truthful when I discuss the flight characteristics
of a two blade rotor system with my students. Can you please have someone
contact me via email and set me straight.
With warm regards,
S L Zuckerman
RMS Engineering
[email protected] <mailto:[email protected]>
Since I received the answer I submitted another question about the blades losing lift when aligned with the AC centerline. It does however seem that the so called 2 per rev was caused by this phenomenon. When I get the answer, I will post it.
To: Nick Lappos
You said,"The blades all lift all the way around the mast. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue".
I have never been in a 206L or a 222 but I have flown in the 214. I had the opportunity to observe the transmission movement through an open inspection panel. In flight the ride was the smoothest I had ever had in a helicopter but with all this smoothness the transmission was going crazy.
It was moving up and down at least 2 inches or more at two times the rotor speed. To me the reason for this up and down movement had nothing to do with a vibration or coupled forces. The trannie was moving up and down due to variations in the lift of the rotor system. This is the reason for the 2 per rev.
If you have ever seen the blades on a 214 they look like barn doors when they spin up. They are capable of generating a lot of lift and at the same time any variation in airflow over the blades will result in the loss of lift resulting in the two per rev.
Regarding pylon rock, I made my comments on that in a post above. Check it out.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 June 2001).]
The nodal beam system was originally developed by Bell Helicopter in the
early 1970s. It was designed specifically to reduce/eliminate the inherent 2
per rev "bounce" (vibration) found in the Bell two bladed main rotor
helicopter designs. Most of the models designed and brought to production by
Bell during the late 1970s and early 1980s (206L, 214, 222) incorporated
nodalization of the main transmission to isolate the main rotor vibration
from the airframe.
For a good technical description of the nodalized pylon concept and its
theory, we refer you to the American Helicopter Society for a copy of a
paper entitled "Fuselage Nodalization" which was presented by several Bell
engineers at the 28th annual forum of the Society in 1972. Unfortunately a
copy of this paper is not available from Bell Helicopter.
-----Original Message-----
From: Lu Zuckerman [mailto:[email protected]]
Sent: Monday, June 18, 2001 3:19 AM
To: Rodriguez, Leticia
Subject: Question
Dear Ms. Rodriguez,
I am a consulting engineer and when on assignment I sometimes
arrange with the local A&P school to teach a course in helicopter
aerodynamics. I once worked for Bell Helicopter International in Iran and it
was during my tenure as manager of technical assistance that the 214 was
introduced. This was the first time I had encountered the Nodamatic
suspension system. Technical literature on the system was limited but I was
able to figure out how it worked. However, I was never told why the system
was designed and why it was incorporated on the 214. I came to the
conclusion that the downward vertical movement of the transmission was due
to the decreased lift from the rotor system when the blades came into
alignment with the centerline of the helicopter in flight. As the blades
departed this position the lift increased and the blade began to lift
raising the transmission.
I have been laboring under this conclusion since 1976 and I really
don't know if I am being truthful when I discuss the flight characteristics
of a two blade rotor system with my students. Can you please have someone
contact me via email and set me straight.
With warm regards,
S L Zuckerman
RMS Engineering
[email protected] <mailto:[email protected]>
Since I received the answer I submitted another question about the blades losing lift when aligned with the AC centerline. It does however seem that the so called 2 per rev was caused by this phenomenon. When I get the answer, I will post it.
To: Nick Lappos
You said,"The blades all lift all the way around the mast. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue".
I have never been in a 206L or a 222 but I have flown in the 214. I had the opportunity to observe the transmission movement through an open inspection panel. In flight the ride was the smoothest I had ever had in a helicopter but with all this smoothness the transmission was going crazy.
It was moving up and down at least 2 inches or more at two times the rotor speed. To me the reason for this up and down movement had nothing to do with a vibration or coupled forces. The trannie was moving up and down due to variations in the lift of the rotor system. This is the reason for the 2 per rev.
If you have ever seen the blades on a 214 they look like barn doors when they spin up. They are capable of generating a lot of lift and at the same time any variation in airflow over the blades will result in the loss of lift resulting in the two per rev.
Regarding pylon rock, I made my comments on that in a post above. Check it out.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 23 June 2001).]
Guest
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Lu,
Your confusion in this matter is that you somehow equate the vertical motion of the transmission (a fact, of course) to the mythological loss of lift of the blades as they pass around the mast. The blades produce relatively constant lift as they pass around the mast (plus or minus about 10 percent). Your belief that somehow the rotor blade loses lift when aligned with the fuselage is not based on fact! The tips of a Huey's blades are moving at 485 knots in a hover, and at 100 knots of airspeed on the helicopter, the blades are also doing 485 knots when they align with the fuselage, quite enough to produce their share of the lift.
The only time when blades no longer produce lift during their rotation is when deep retreating blade stall occurs, and that also produces strong rolling and pitching control problems as evidence of this lift imbalance.
What the rotor systems all do is to have "modes" of oscillating behavior due to the dynamic response of their components and of their soft mounts. In the case of the Bell transmission, the bouncing of the transmission is produced by this dynamic response. If you watch that purple bull with the bobbing head bouncing on the dashboard of a 1957 Chevy, you can't blame the bouncing on loss of lift, because it is producing no lift at all, but it still bounces, much as the transmission does.
The bounce also rocks the transmission, pitching it forward and aft, thus the use of the transducers to help the cyclic channel of the SCAS cancel the vibration.
That rotor/transmission bouncing is an important problem only if the natural period of the bounce matches a natural period of the rotor. In such cases, it must be damped or the control system must be used to help cancel it, or the mounts must be stiffened to quell it.
I have lots of experience with this stuff, I was the chief R&D test pilot for Sikorsky for about 15 years, and we had to handle several cases like this.
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Your confusion in this matter is that you somehow equate the vertical motion of the transmission (a fact, of course) to the mythological loss of lift of the blades as they pass around the mast. The blades produce relatively constant lift as they pass around the mast (plus or minus about 10 percent). Your belief that somehow the rotor blade loses lift when aligned with the fuselage is not based on fact! The tips of a Huey's blades are moving at 485 knots in a hover, and at 100 knots of airspeed on the helicopter, the blades are also doing 485 knots when they align with the fuselage, quite enough to produce their share of the lift.
The only time when blades no longer produce lift during their rotation is when deep retreating blade stall occurs, and that also produces strong rolling and pitching control problems as evidence of this lift imbalance.
What the rotor systems all do is to have "modes" of oscillating behavior due to the dynamic response of their components and of their soft mounts. In the case of the Bell transmission, the bouncing of the transmission is produced by this dynamic response. If you watch that purple bull with the bobbing head bouncing on the dashboard of a 1957 Chevy, you can't blame the bouncing on loss of lift, because it is producing no lift at all, but it still bounces, much as the transmission does.
The bounce also rocks the transmission, pitching it forward and aft, thus the use of the transducers to help the cyclic channel of the SCAS cancel the vibration.
That rotor/transmission bouncing is an important problem only if the natural period of the bounce matches a natural period of the rotor. In such cases, it must be damped or the control system must be used to help cancel it, or the mounts must be stiffened to quell it.
I have lots of experience with this stuff, I was the chief R&D test pilot for Sikorsky for about 15 years, and we had to handle several cases like this.
------------------
Guest
Posts: n/a
Thank God a voice of reason!
Further to the loss of lift point, from experience (both mine and others) the 212 tends to experience pylon rock most easily when at low speed and high torque. For example coming off a deck with a rather cack handed forward cyclic input, or when transitioning aay with a heavy underslung load. In both these cases there is negligeable forward speed and so the blade lift will, to all intents and purposes, be evenly distributed. ergo, the loss of lift theory doesn't hold much water, but then we all knew that anyway!!
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Another day in paradise
Further to the loss of lift point, from experience (both mine and others) the 212 tends to experience pylon rock most easily when at low speed and high torque. For example coming off a deck with a rather cack handed forward cyclic input, or when transitioning aay with a heavy underslung load. In both these cases there is negligeable forward speed and so the blade lift will, to all intents and purposes, be evenly distributed. ergo, the loss of lift theory doesn't hold much water, but then we all knew that anyway!!
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Another day in paradise
Guest
Posts: n/a
To: Nick Lappos
You said,” Your belief that somehow the rotor blade loses lift when aligned with the fuselage is not based on fact! The tips of a Huey's blades are moving at 485 knots in a hover, and at 100 knots of airspeed on the helicopter, the blades are also doing 485 knots when they align with the fuselage, quite enough to produce their share of the lift”.
The tip speed is constant but the relative wind generated by that tip speed is not. In a hover in a still air day the lift generated by the rotor is constant but at speed the relative wind is not the same across the disc. That is why I used the retreating blade stall analogy. In that comparison there is a loss of lift due to the rotational velocity of the blade at a given point being less than the air stream passing over the blade. I can’t speak from detailed engineering analyses but it seems to me that as the blades are aligning with the longitudinal centerline and as they pass through and slightly past that point the blades are in an airstream that is equivalent to the forward speed of the helicopter. That can be on some two blade helicopters as high as 150 + Knots. In retreating blade stall the retreating blade loses lift due to the reverse airflow on the blades. I believe that a blade with a tip speed of 485 Knots will lose some lift when passing through an airstream that is passing down the span of the blade which would reduce the lift generated by the blades.
You can address the end result and quote all types of engineering speak but until I can get this question resolved by Bell I will continue to believe that the 2 per rev bounce is caused by the intermittent loss of lift as the blades rotate. If Bell says it ain’t so I’ll go on to other pastures.
The reason this doesn’t happen on a multi blade system is that at least two of the blades are lifting and on helicopters like the CH-53 there are five or more blades lifting.
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The Cat
You said,” Your belief that somehow the rotor blade loses lift when aligned with the fuselage is not based on fact! The tips of a Huey's blades are moving at 485 knots in a hover, and at 100 knots of airspeed on the helicopter, the blades are also doing 485 knots when they align with the fuselage, quite enough to produce their share of the lift”.
The tip speed is constant but the relative wind generated by that tip speed is not. In a hover in a still air day the lift generated by the rotor is constant but at speed the relative wind is not the same across the disc. That is why I used the retreating blade stall analogy. In that comparison there is a loss of lift due to the rotational velocity of the blade at a given point being less than the air stream passing over the blade. I can’t speak from detailed engineering analyses but it seems to me that as the blades are aligning with the longitudinal centerline and as they pass through and slightly past that point the blades are in an airstream that is equivalent to the forward speed of the helicopter. That can be on some two blade helicopters as high as 150 + Knots. In retreating blade stall the retreating blade loses lift due to the reverse airflow on the blades. I believe that a blade with a tip speed of 485 Knots will lose some lift when passing through an airstream that is passing down the span of the blade which would reduce the lift generated by the blades.
You can address the end result and quote all types of engineering speak but until I can get this question resolved by Bell I will continue to believe that the 2 per rev bounce is caused by the intermittent loss of lift as the blades rotate. If Bell says it ain’t so I’ll go on to other pastures.
The reason this doesn’t happen on a multi blade system is that at least two of the blades are lifting and on helicopters like the CH-53 there are five or more blades lifting.
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The Cat
Guest
Posts: n/a
Nick,
Thank you.
Lu,
Clearly you have taught yourself lots of things about helicopters and good luck to you. Unfortunately, many of your empirically derived theories are not shared by those with far greater research facilities and technologies available to them. I have considerable experience as a helo maintenance test pilot, covering Bell products predominantly with a lot of others thrown in. I have read your prolific writings and am impressed by your zeal - unfortunately your dogged desire to be right all of the time has made a different impression.
Bell does not hold all the cards on heicopter dynamics - there is a rich academic and research literature out there. Please be careful to distinguish what is mere surmise from what is scientifically robust research - there are a lot of young folks out there who may believe your version of events, merely because they have yet to be exposed to better information.
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Stay Alive,
[email protected]
Thank you.
Lu,
Clearly you have taught yourself lots of things about helicopters and good luck to you. Unfortunately, many of your empirically derived theories are not shared by those with far greater research facilities and technologies available to them. I have considerable experience as a helo maintenance test pilot, covering Bell products predominantly with a lot of others thrown in. I have read your prolific writings and am impressed by your zeal - unfortunately your dogged desire to be right all of the time has made a different impression.
Bell does not hold all the cards on heicopter dynamics - there is a rich academic and research literature out there. Please be careful to distinguish what is mere surmise from what is scientifically robust research - there are a lot of young folks out there who may believe your version of events, merely because they have yet to be exposed to better information.
------------------
Stay Alive,
[email protected]
Guest
Posts: n/a
To: 4 dogs
Please note the following paragraph.
“You can address the end result and quote all types of engineering speak but until I can get this question resolved by Bell I will continue to believe that the 2 per rev bounce is caused by the intermittent loss of lift as the blades rotate. If Bell says it ain’t so I’ll go on to other pastures”.
The original theory of the blades decreasing lift, as they became disposed over the centerline is not mine. It is from the accumulated uttering of Bell test pilots and other pilots of Bell helicopters. Now, they may have been wrong in their conclusions and maybe I am wrong for espousing those theories but until the fat lady sings (Response from Bell service engineering) I have to stick with it.
To: All
Answer this question: What kind of vibration can cause a 600-800 pound transmission to move up and down at two times the rotor speed? What forces are at play that can cause this kind of movement? Where are those forces generated?
To: 212 Man
In the situation you described above you answered the question. In your description you stated that there was no forward speed and therefore, the blades were lifting equally. However when the helicopter comes up to speed the condition I described will (assumedly) take place.
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The Cat
[This message has been edited by Lu Zuckerman (edited 25 June 2001).]
[This message has been edited by Lu Zuckerman (edited 25 June 2001).]
Please note the following paragraph.
“You can address the end result and quote all types of engineering speak but until I can get this question resolved by Bell I will continue to believe that the 2 per rev bounce is caused by the intermittent loss of lift as the blades rotate. If Bell says it ain’t so I’ll go on to other pastures”.
The original theory of the blades decreasing lift, as they became disposed over the centerline is not mine. It is from the accumulated uttering of Bell test pilots and other pilots of Bell helicopters. Now, they may have been wrong in their conclusions and maybe I am wrong for espousing those theories but until the fat lady sings (Response from Bell service engineering) I have to stick with it.
To: All
Answer this question: What kind of vibration can cause a 600-800 pound transmission to move up and down at two times the rotor speed? What forces are at play that can cause this kind of movement? Where are those forces generated?
To: 212 Man
In the situation you described above you answered the question. In your description you stated that there was no forward speed and therefore, the blades were lifting equally. However when the helicopter comes up to speed the condition I described will (assumedly) take place.
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The Cat
[This message has been edited by Lu Zuckerman (edited 25 June 2001).]
[This message has been edited by Lu Zuckerman (edited 25 June 2001).]
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To: 212 Man
The problem does not manifest itself until the helicopter comes up to speed. In a hover assuming a still wind day the rotor speed generates the relative wind passing over the blades (disc). However, when the helicopter comes up to speed the relative wind is now equal to the forward speed plus the rotational velocity of the advancing blade and minus the rotational velocity of the retreating blade. That’s how you get into retreating blade stall. With the helicopter moving forward at 100-120 Knots the blades as they become aligned with the longitudinal axis have a constant rotational velocity but the airflow over the centralized blades have an airflow component of 100-120 knots flowing down the blade spanwise. This has to have some effect on blade lift.
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The Cat
The problem does not manifest itself until the helicopter comes up to speed. In a hover assuming a still wind day the rotor speed generates the relative wind passing over the blades (disc). However, when the helicopter comes up to speed the relative wind is now equal to the forward speed plus the rotational velocity of the advancing blade and minus the rotational velocity of the retreating blade. That’s how you get into retreating blade stall. With the helicopter moving forward at 100-120 Knots the blades as they become aligned with the longitudinal axis have a constant rotational velocity but the airflow over the centralized blades have an airflow component of 100-120 knots flowing down the blade spanwise. This has to have some effect on blade lift.
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The Cat
Guest
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Lu,
Whilst you maybe correctly describing something it isn't Pylon Rock. It occurs when 212man describes it, at low airspeed little more than walking pace and high torque.
Hopefully Bell will tell you in the awaited response.
[This message has been edited by Rotorbike (edited 25 June 2001).]
[This message has been edited by Rotorbike (edited 25 June 2001).]
Whilst you maybe correctly describing something it isn't Pylon Rock. It occurs when 212man describes it, at low airspeed little more than walking pace and high torque.
Hopefully Bell will tell you in the awaited response.
[This message has been edited by Rotorbike (edited 25 June 2001).]
[This message has been edited by Rotorbike (edited 25 June 2001).]
Guest
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danke schon mein freund,
There in lies the problem. I have tried to be as exact as possible in detailing the most likely scenarios when pylon rock will occur, and all we get in return is guff about blades losing lift when doing N knots with a bit of a cross wind.
What I have described is not a best guess guide as to when it might occur; it is empirical evidence describing when it does occur, and when it does not. I can tell you for a fact it occurs at low speed and high torque.
It is a dynamic response rather than an aerodynamic response and it has f*** all to do with precession or loss of blade lift.
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Another day in paradise
There in lies the problem. I have tried to be as exact as possible in detailing the most likely scenarios when pylon rock will occur, and all we get in return is guff about blades losing lift when doing N knots with a bit of a cross wind.
What I have described is not a best guess guide as to when it might occur; it is empirical evidence describing when it does occur, and when it does not. I can tell you for a fact it occurs at low speed and high torque.
It is a dynamic response rather than an aerodynamic response and it has f*** all to do with precession or loss of blade lift.
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Another day in paradise
Guest
Posts: n/a
To: 212 Man
It seems that we were talking about two different things. You were addressing pylon rock and I was addressing loss of lift. However I beg to differ with you about it only happening while lifting heavy loads and moving at walking speed. As I had previously stated when lift is generated the soft mounts flex until the entire load is suspended on the lift link. Because the lift link is not centrally located the transmission will shift as allowed by the flex mounts. This will cause the pylon to move forward and to the left. When the helicopter starts to move forward it will reach the point where the 2 per rev will start. When moving up and down on the soft mounts the reaction of the transmission forcing down on the lift link will cause the pylon to move to the right and to the rear. When the transmission rises the lift link geometry will cause the pylon to move forward and to the left. This same situation will happen but to a different degree when maneuvering causing the pylon to move in an elliptical pattern. This condition does not exist on four blade Bell helicopters nor, does it happen on a Bell with a Nodamatic suspension system.
I would still like to have someone answer my question about what forces are involved in causing a 6-800 pound transmission and an 800 pound rotor system (214) bounce up and down in excess of 2”. How and where are these forces generated.
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The Cat
It seems that we were talking about two different things. You were addressing pylon rock and I was addressing loss of lift. However I beg to differ with you about it only happening while lifting heavy loads and moving at walking speed. As I had previously stated when lift is generated the soft mounts flex until the entire load is suspended on the lift link. Because the lift link is not centrally located the transmission will shift as allowed by the flex mounts. This will cause the pylon to move forward and to the left. When the helicopter starts to move forward it will reach the point where the 2 per rev will start. When moving up and down on the soft mounts the reaction of the transmission forcing down on the lift link will cause the pylon to move to the right and to the rear. When the transmission rises the lift link geometry will cause the pylon to move forward and to the left. This same situation will happen but to a different degree when maneuvering causing the pylon to move in an elliptical pattern. This condition does not exist on four blade Bell helicopters nor, does it happen on a Bell with a Nodamatic suspension system.
I would still like to have someone answer my question about what forces are involved in causing a 6-800 pound transmission and an 800 pound rotor system (214) bounce up and down in excess of 2”. How and where are these forces generated.
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The Cat
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Lu asked:
I would still like to have someone answer my question about what forces are involved in causing a 6-800 pound transmission and an 800 pound rotor system (214) bounce up and down in excess of 2”. How and where are these forces generated.
Nick answered:
Lu, have you ever seen the video of the Tacoma Narrows bridge pulling itself down with horrendous twisting motions due to the dynamic response that was excited by the wind and the bridge's natural torsional frequency? Why do you think a small child can "pump" a swing until he is propelled up above head height? The mysteries of the tides, where billions of tons of water empty and fill twice a day is another example of dynamic response.
Can you see the parallels between these cases and the dynamic response of pylon rock? Why must you subscribe to loss of lift theories?
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I would still like to have someone answer my question about what forces are involved in causing a 6-800 pound transmission and an 800 pound rotor system (214) bounce up and down in excess of 2”. How and where are these forces generated.
Nick answered:
Lu, have you ever seen the video of the Tacoma Narrows bridge pulling itself down with horrendous twisting motions due to the dynamic response that was excited by the wind and the bridge's natural torsional frequency? Why do you think a small child can "pump" a swing until he is propelled up above head height? The mysteries of the tides, where billions of tons of water empty and fill twice a day is another example of dynamic response.
Can you see the parallels between these cases and the dynamic response of pylon rock? Why must you subscribe to loss of lift theories?
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Guest
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To: Nick Lappos
We are still talking about two different things. Pylon rock and the two per rev phenomenon. Although pylon rock can be excited by the vertical movement of the transmission and the interreaction of the lift link, I don’t think the comparison of Galloping Gertie and the oceans tides quite answers the question about what causes the vertical movement of the transmission and rotor system to a certain degree on some Bell Helicopters and especially those helicopters equipped with Nodamatic suspension on the transmission. You use the term dynamic response and I ask in response to what? I believe that it is caused by variations in lift as the rotor disc rotates now you can tell me what you think can exert the necessary energy to get a thousand pounds moving up and down at 500 times a minute. As indicated by Bell, the Nodamatic system was installed on the 206L, the 222 and the 214 in order to eliminate (Isolate) the vibratory energy introduced into the fuselage by the inherent 2 per rev that is common to two blade helicopters. In the process of isolating the vertical beat the Nodamatic system allows the transmission to move in relation to the fuselage. In the process of moving the Nodamatic system displaces heavy weights which absorb the energy. This is similar to the Bifilar system used by Sikorsky and the Pendular weights used by other helicopter manufacturers to eliminate vibratory forces from being transmitted to the rotorhead from the blades.
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The Cat
We are still talking about two different things. Pylon rock and the two per rev phenomenon. Although pylon rock can be excited by the vertical movement of the transmission and the interreaction of the lift link, I don’t think the comparison of Galloping Gertie and the oceans tides quite answers the question about what causes the vertical movement of the transmission and rotor system to a certain degree on some Bell Helicopters and especially those helicopters equipped with Nodamatic suspension on the transmission. You use the term dynamic response and I ask in response to what? I believe that it is caused by variations in lift as the rotor disc rotates now you can tell me what you think can exert the necessary energy to get a thousand pounds moving up and down at 500 times a minute. As indicated by Bell, the Nodamatic system was installed on the 206L, the 222 and the 214 in order to eliminate (Isolate) the vibratory energy introduced into the fuselage by the inherent 2 per rev that is common to two blade helicopters. In the process of isolating the vertical beat the Nodamatic system allows the transmission to move in relation to the fuselage. In the process of moving the Nodamatic system displaces heavy weights which absorb the energy. This is similar to the Bifilar system used by Sikorsky and the Pendular weights used by other helicopter manufacturers to eliminate vibratory forces from being transmitted to the rotorhead from the blades.
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The Cat
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Lu,
I think you are ducking the issue slightly. I possibly wasn't totally clear though in that I don't dispute that you can experience pylon rock at cruise speed but it is certainly more prevelant in the regime I have concentrated on. I was trying to ask how your explanation about it being a function of loss of lift came into play with no fwd speed.
I suspect that what you are refering to is simply the normal '2 per 1' vertical vibration experienced with a 2 bladed a/c. As Nick says, depending onthe harmonics being propogated and the various resonant frequencies of componants, you can get all sorts of "bits" bouncing up and down, including the pilots. Are you suggesting that the vertical bounce experienced in a badly track and balanced S76 is related to loss of lift? Of course not, no more than it is in a 212/206/214.
PS Sorry about the tone of my last; I was rather tired and emotional after a busy day in the class room (5 blades on this one!)
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Another day in paradise
I think you are ducking the issue slightly. I possibly wasn't totally clear though in that I don't dispute that you can experience pylon rock at cruise speed but it is certainly more prevelant in the regime I have concentrated on. I was trying to ask how your explanation about it being a function of loss of lift came into play with no fwd speed.
I suspect that what you are refering to is simply the normal '2 per 1' vertical vibration experienced with a 2 bladed a/c. As Nick says, depending onthe harmonics being propogated and the various resonant frequencies of componants, you can get all sorts of "bits" bouncing up and down, including the pilots. Are you suggesting that the vertical bounce experienced in a badly track and balanced S76 is related to loss of lift? Of course not, no more than it is in a 212/206/214.
PS Sorry about the tone of my last; I was rather tired and emotional after a busy day in the class room (5 blades on this one!)
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Another day in paradise
Guest
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I created several diagrams showing coning vs. flapping as well as diagrams showing what I believe is the main reason for pylon rock on UH1s and 212s. It may apply to the 412 but not including the 2 per rev beat. The diagrams can be found here:
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The Cat
[This message has been edited by helidrvr (edited 28 June 2001).]
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The Cat
[This message has been edited by helidrvr (edited 28 June 2001).]
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i might have to give up teaching theory.
the trasmissions on my 47's are bolted firmly to the engine and i have watched them thousands of times. sorry but you will have to pour molten lava in my eyes before i can subscribe to the loss of lift theory.
212 was correct, the argument was altered. sounds like a womens ploy, is lu short for
loise?
there are some great forums in rotorheads and this one is a classic
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your too high,your too low, your too fast your too slow
the trasmissions on my 47's are bolted firmly to the engine and i have watched them thousands of times. sorry but you will have to pour molten lava in my eyes before i can subscribe to the loss of lift theory.
212 was correct, the argument was altered. sounds like a womens ploy, is lu short for
loise?
there are some great forums in rotorheads and this one is a classic
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your too high,your too low, your too fast your too slow