pylon rock oscillation
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pylon rock oscillation
Hi, everybody.Im trying to find some information about pylon rock oscillation, but find nothing really clear.
Could anybody clarify the origin of the oscillation to me??
Any other info regarding this issue, woul be helpful.
cheers
Could anybody clarify the origin of the oscillation to me??
Any other info regarding this issue, woul be helpful.
cheers
Iconoclast
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Pylon Rock. A new type of music.
To: rotorpol
I have to make a few assumptions regarding your question. You are talking about a Bell helicopter that has a transmission that is able to move relative to the fuselage. The major connection between the transmission and the fuselage is a “Lift Link”. From here on it takes a great deal of visualization. The lift link is not connected to the transmission in a direct line with the rotor mast. In fact it is attached at a point left and forward of the transmission center line.
When you pull collective the transmission will raise and because it is not attached at its’ center point the mast will lean forward and to the left. This is now its’ neutral point. Any cyclic input will cause the transmission to lean in the direction of the cyclic input. Because the pivot point is not on the centerline of the transmission the movement will be exacerbated giving the pilot the impression that the pylon is rocking which it is. If the lift link were attached at the centerline the transmission would still lean but the resultant feeling by the pilot would be much less.
I will now retire to my bunker as there will be twenty or more replies stating that I am wrong.
I have to make a few assumptions regarding your question. You are talking about a Bell helicopter that has a transmission that is able to move relative to the fuselage. The major connection between the transmission and the fuselage is a “Lift Link”. From here on it takes a great deal of visualization. The lift link is not connected to the transmission in a direct line with the rotor mast. In fact it is attached at a point left and forward of the transmission center line.
When you pull collective the transmission will raise and because it is not attached at its’ center point the mast will lean forward and to the left. This is now its’ neutral point. Any cyclic input will cause the transmission to lean in the direction of the cyclic input. Because the pivot point is not on the centerline of the transmission the movement will be exacerbated giving the pilot the impression that the pylon is rocking which it is. If the lift link were attached at the centerline the transmission would still lean but the resultant feeling by the pilot would be much less.
I will now retire to my bunker as there will be twenty or more replies stating that I am wrong.
Last time I looked, the lift link fitting on the bottom of the support case was in line with the mast (latterally anyways)
.
Typical cause for pylon rock is usually worn out friction mounts (below the aft transmission mounts) that do not do a good job of dampening transmission movement. These can be overhauled in the field by getting a refurbish kit from Lord (listed on their website).
.Typical cause for pylon rock is usually worn out friction mounts (below the aft transmission mounts) that do not do a good job of dampening transmission movement. These can be overhauled in the field by getting a refurbish kit from Lord (listed on their website).
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I've experienced it a few times in the 212.
I flew a 212 equipped with SCAS and hydraulic linear actuators (instead of electromecahnical), it was originally a CAA machine.
It use to get into pylon rock if you were slinging with the SCAS on. It was never a problem with the SCAS off. IT felt (in the hover) like someone was pumping the cyclic fore and aft and side to side.
It probably had poor transmission mounts.There was a check you could do in the hover to check the transmission mounts but I can't remember as its been years.
I flew a 212 equipped with SCAS and hydraulic linear actuators (instead of electromecahnical), it was originally a CAA machine.
It use to get into pylon rock if you were slinging with the SCAS on. It was never a problem with the SCAS off. IT felt (in the hover) like someone was pumping the cyclic fore and aft and side to side.
It probably had poor transmission mounts.There was a check you could do in the hover to check the transmission mounts but I can't remember as its been years.
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Another trip down memory lane .....
The Hueys with the stabiliser bars used to get an oscillation. When (in hover) the driver stirred the cyclic rapidly then centred the stick and counted the oscillations. This told you the condition of the dampers on the transmission rear mounts (I forget the number to look for but I think it was about six).
Let me be the first of the twenty: Lu is wrong. Run a pprune search and you will get a detailed discussion on this issue.
The Bell 212/205/204 series helicopter has it's transmission mounted by 5 transmission mounts. Like any mount, these allow a slight flexibility in movement and they provide a dampening effect on the vibrations of the rotor system so that they do not interact with the fusleage in an excited way (much the same as my wife's role preventing my excited interaction with other female bodies!). Not hard to visualise (despite Lu's comments) but the two bladed system of these aircraft has a certain vibration harmonic, just like every rotor system and other rotating part on earth. When that vibration harmonic is not effectively damped out by the transmission mount, you get "pylon rock".
The mounts can be tightened, or losened to provide the correct damping function, and can easily be tested in accordance with the maintenance manual. Simply sit at a 10 ft hover and move the cyclic rapidly fore aft until you feel a harmonic in the fore aft axis building up. At that point, freeze the cyclic in the hover position, and count how many more oscillations are felt before they stop. Typically, you should feel about 4 (from memory I think the limit is 3 to 6), and they get progressively weaker as they are damped by the mount. If you get less. the mount is too tight and will wear out quickly, if it is more, tighten the mount. If you cannot tighten the mount anymore, replace them.
Pylon rock also gets confused with SCAS induced oscillations in the 212 at low speeds, particularly in out of trim conditions. If this relates to your question, let us know.
Standby for Lu's 214 moving transmission story that has nothing to do with your question...
The Bell 212/205/204 series helicopter has it's transmission mounted by 5 transmission mounts. Like any mount, these allow a slight flexibility in movement and they provide a dampening effect on the vibrations of the rotor system so that they do not interact with the fusleage in an excited way (much the same as my wife's role preventing my excited interaction with other female bodies!). Not hard to visualise (despite Lu's comments) but the two bladed system of these aircraft has a certain vibration harmonic, just like every rotor system and other rotating part on earth. When that vibration harmonic is not effectively damped out by the transmission mount, you get "pylon rock".
The mounts can be tightened, or losened to provide the correct damping function, and can easily be tested in accordance with the maintenance manual. Simply sit at a 10 ft hover and move the cyclic rapidly fore aft until you feel a harmonic in the fore aft axis building up. At that point, freeze the cyclic in the hover position, and count how many more oscillations are felt before they stop. Typically, you should feel about 4 (from memory I think the limit is 3 to 6), and they get progressively weaker as they are damped by the mount. If you get less. the mount is too tight and will wear out quickly, if it is more, tighten the mount. If you cannot tighten the mount anymore, replace them.
Pylon rock also gets confused with SCAS induced oscillations in the 212 at low speeds, particularly in out of trim conditions. If this relates to your question, let us know.
Standby for Lu's 214 moving transmission story that has nothing to do with your question...
Iconoclast
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In response
To: helmet fire
Several years ago the US Navy experienced several engine drive shaft separations from the transmissions on their 212s. They traced it to loose or weak mounts but it was caused by the transmission lift link offset position. The movement is allowed by the weak mounts but it is caused according to their findings by the offset position of the lift link.
This is what I based my post on.
Several years ago the US Navy experienced several engine drive shaft separations from the transmissions on their 212s. They traced it to loose or weak mounts but it was caused by the transmission lift link offset position. The movement is allowed by the weak mounts but it is caused according to their findings by the offset position of the lift link.
This is what I based my post on.
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What????
First of all, they were UH1N's. Second, loose mounts are the "cause" (not the design of the offset like Lu says). Bottom line, replace stuff when you are supposed to, and the a/c will work as designed.
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We did the pylon rock check with 212's and if it failed we would change the mounts. I don't recall the mounts being adjustable. Don't even think about don't this in a 412! 412's don't have the 5th mount on he back of the transmission.
Iconoclast
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To: av8rbpm
There is another thread on rotorheads: "Ways to put it across".
This thread deals with the best ways to get a point across to students about the various aspects of flying helicopters. The poster suggests analogies and minimalist training aids. I will do the same. All it takes is imagery and visualization on your part.
Take a plywood sheet 4’ X 4’. Now take a metal plate 4’ X 4’ and lay the plywood sheet over it. Connect the respective corners of the plywood sheet and the metal plate with springs. Install a padeye at the exact center of the plywood sheet and connect a line to the padeye. Using some means of lifting device connect the line to the lifting device. Lift the plywood sheet and the springs will extend to the point that they will collectively lift the metal plate. The plywood and the metal plate will remain parallel to each other (within limits). Let’s assume that the springs stretched three inches.
Now install a mechanical linkage between the plywood sheet and the metal plate. This linkage is 2” long and installed left of the center and slightly forward of the centers of the plywood sheet and the metal plate. Again lift the plywood sheet. The mechanical linkage will immediately cause the metal plate to rise and in the process the plywood sheet will pivot about the connection point of the mechanical link so that they are no longer parallel to each other. As the plate and plywood sheet continue to rise the springs will extend in order to keep the metal plate and plywood sheet parallel to each other.
Now apply the same situation to the helicopter. The springs are the elastomeric supports. They can be compressed by the lifting force but to a limit. So, they allow limited movements of the transmission. However the actual mechanical lifting force is provided by the lift link which is not on the actual center of lift (Main rotor shaft). Any movement of the transmission due to control inputs will result in the transmission pivoting about the lift link connection. This applies loads greater than the actual lifting of the helicopter on the elastomeric supports causing them to wear and with the end result of pylon rock. With new elastomeric mounts the pylon rock is still there but it is subdued.
There is another thread on rotorheads: "Ways to put it across".
This thread deals with the best ways to get a point across to students about the various aspects of flying helicopters. The poster suggests analogies and minimalist training aids. I will do the same. All it takes is imagery and visualization on your part.
Take a plywood sheet 4’ X 4’. Now take a metal plate 4’ X 4’ and lay the plywood sheet over it. Connect the respective corners of the plywood sheet and the metal plate with springs. Install a padeye at the exact center of the plywood sheet and connect a line to the padeye. Using some means of lifting device connect the line to the lifting device. Lift the plywood sheet and the springs will extend to the point that they will collectively lift the metal plate. The plywood and the metal plate will remain parallel to each other (within limits). Let’s assume that the springs stretched three inches.
Now install a mechanical linkage between the plywood sheet and the metal plate. This linkage is 2” long and installed left of the center and slightly forward of the centers of the plywood sheet and the metal plate. Again lift the plywood sheet. The mechanical linkage will immediately cause the metal plate to rise and in the process the plywood sheet will pivot about the connection point of the mechanical link so that they are no longer parallel to each other. As the plate and plywood sheet continue to rise the springs will extend in order to keep the metal plate and plywood sheet parallel to each other.
Now apply the same situation to the helicopter. The springs are the elastomeric supports. They can be compressed by the lifting force but to a limit. So, they allow limited movements of the transmission. However the actual mechanical lifting force is provided by the lift link which is not on the actual center of lift (Main rotor shaft). Any movement of the transmission due to control inputs will result in the transmission pivoting about the lift link connection. This applies loads greater than the actual lifting of the helicopter on the elastomeric supports causing them to wear and with the end result of pylon rock. With new elastomeric mounts the pylon rock is still there but it is subdued.
Lu, you never cease to amuse. You take a simple cause/effect and create a complex analogy that so clouds the issue that you lose sight of the question - and then claim it is to simplify the visualisation. That is why you brought up 23 "simple" visualisation examples to prove the existance of centrifugal force - if you dont get the answer you want, just try another visualisation until you do!
Hopefully, any helicopter pilot or mechanic can actually visualise the rotor system without ply wood sheeting and springs.
By the way, I never knew the lift link was off set like you claim. Is it really?
You are not confusing this offset with the left forward bias of the rotor mast are you?
Hopefully, any helicopter pilot or mechanic can actually visualise the rotor system without ply wood sheeting and springs.
By the way, I never knew the lift link was off set like you claim. Is it really?
You are not confusing this offset with the left forward bias of the rotor mast are you?
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Seeing is believing
To: helmet fire
Take a look at the underside of a 204, 205 or 212 transmission.
One point that I forgot to bring out is the vertical movement of the transmission due to the two per rev. The amount of vertical movement is dependent upon the wear on the transmission mounts. When the transmission moves upwards there is a tension load applied to the lift link. This will cause the transmission to move in relation to the lift link connection point. When the transmission moves downward there is a compressive load applied to the lift link and the same thing will occur but in the opposite direction.
I have this phenomenon diagrammed out but the diagram is made using a program called Visio and can not be sent via e-mail unless the recipient has that same program.
Take a look at the underside of a 204, 205 or 212 transmission.
One point that I forgot to bring out is the vertical movement of the transmission due to the two per rev. The amount of vertical movement is dependent upon the wear on the transmission mounts. When the transmission moves upwards there is a tension load applied to the lift link. This will cause the transmission to move in relation to the lift link connection point. When the transmission moves downward there is a compressive load applied to the lift link and the same thing will occur but in the opposite direction.
I have this phenomenon diagrammed out but the diagram is made using a program called Visio and can not be sent via e-mail unless the recipient has that same program.
Last edited by Lu Zuckerman; 4th Feb 2005 at 00:15.
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pylon rock
As a Huey technical instructor and maintainer for over twenty years, here's the low down on huey pylon rock. The lift link is attached to the txmn directly in front of the mast. Pylon rock is what we call an extreme low frequency vibration (occurs less than one per revolution) and after inducing in a hover has a limit of 4-5 cycles before stopping. WE DO NOT adjust the mounts if incorrect, but replace the friction dampers which are mounted on the rear two txmn mounts only. The huey transmission in held in place by five mounts, with the fifth mount being between the two rear mounts. The friction dampers mounted to the rear mounts are to reduce pylon rock in the hover and transition while the fifth mount is used at higher forward speeds. The older UH-1 has a beem between the rear mounts to attach the fifth mount, but the upgraded txmn (Bell 212 txmn) has an integeral mount on the txmn.
Avoid imitations
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Great to see this old thread resurrected.
Some strong memories of the late Lu Zuckerman here, who tragically passed away earlier this year.
RIP LU, your memories linger on.
Some strong memories of the late Lu Zuckerman here, who tragically passed away earlier this year.
RIP LU, your memories linger on.
It really is vintage Lu.
We miss your complex visualisations and your stirring mate...........RIP
Jessie, I was mistaken re the adjustability of the dampers, and you are right. We used to refer to them as "too tight" or "too loose" depending upon the oscillation damping during the hover check.
We miss your complex visualisations and your stirring mate...........RIP
Jessie, I was mistaken re the adjustability of the dampers, and you are right. We used to refer to them as "too tight" or "too loose" depending upon the oscillation damping during the hover check.
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What do we call the new chopper vocabulary that you flailing blade drivers are using.?
What was it? "Pylon rock oscillation."
Hope we can continue to communicate intelligently!!
I've only tried to squeeze water out of the cyclic as the airspeed goes under the expected stalling V on
S-51s, a Hiller, a Sycamore and a Huey so my contribution to the intricasies of rock around the pylon would be miniscule.
What was it? "Pylon rock oscillation."
Hope we can continue to communicate intelligently!!
I've only tried to squeeze water out of the cyclic as the airspeed goes under the expected stalling V on
S-51s, a Hiller, a Sycamore and a Huey so my contribution to the intricasies of rock around the pylon would be miniscule.
Funny: when I saw this title my first thought was "Lu would have somethng to say about this", and so he has!
Jessie, you are correct about the oscillation damping out in the hover, but in some forward flight conditions (especially, high power low speed climb) it can become divergent with the 'assistance' of the stabilisation system.
Jessie, you are correct about the oscillation damping out in the hover, but in some forward flight conditions (especially, high power low speed climb) it can become divergent with the 'assistance' of the stabilisation system.
Jessie,
no I was talking AFCS. The degree of response varies between the types fitted and the effectiveness of the filtering they have.
no I was talking AFCS. The degree of response varies between the types fitted and the effectiveness of the filtering they have.