pylon rock oscillation

[rotorpol]

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:O

[Lu Zuckerman]

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.




:E :E

[Encyclo]

Last time I looked, the lift link fitting on the bottom of the support case was in line with the mast (latterally anyways):confused:.

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).

[IHL]

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.

[sprocket]

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).

[helmet fire]

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...:ok:

[Lu Zuckerman]

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.

:E :E

[av8rbpm]

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.:rolleyes:

[S92mech]

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.

[Lu Zuckerman]

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.

:E :E

[rotorpol]

thank you everybody:O
It´s been very helpful.

[helmet fire]

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?

[Lu Zuckerman]

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.


:E :E

[jessie13]

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.

[ShyTorque]

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. :cool:

[helmet fire]

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.

[Milt]

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.

[212man]

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.

[jessie13]

212man,
Dont know too much about the 212 "stabilation system". Are you talking stab bar or AFCS?

[212man]

Jessie,
no I was talking AFCS. The degree of response varies between the types fitted and the effectiveness of the filtering they have.

[SASless]

Checking the Tranny mounts can be done on the ground during the preflight as well....takes two folks to do it properly. One makes like King Kong and shakes the head fore/aft and left/right...while the other guy feels for movement within the mount/bolts and gearbox flanges. Doing a climbing turn will also point out any mount problems by generating a shuffle sometimes.

As previously described....stirring the stick at a hover or in slow forward flight works as well.

A loose lift link caused by worn bearings will cause some problems as well. Have seen some bearings fall slap out of the link when the bolts were removed. The problem disappeared with the installation of new bearings.

(Bell 204/205/212/412/UH-1A,B,C,D,H,K,L,M,N models)


Question:

Why do most "Huey" pilots fly with all of the cyclic friction removed....but Sperry requires a "pre-set" minimum friction when their AFCS is installed?

Does it not make sense for some friction to be left applied ala Sperry.....and thus dampen out some of the oscillations that will be caused by "feedback" from out of balance condidtions and PIO?

[jessie13]

Thanks 212man. Had a pilot a few year ago who thought he could teach a huey to skate. Came in on a running landing on wet grass and didn't want to slow down real well. Pulled a little collective back on the cyclic to slow down just as one skid sank into soft ground causing the aircraft to start to go sideways. The aircraft continued for a little bit more before tilting up on one skid and falling down (at least he stayed upwright). The crewman in the back seat had a good view of the event. Anyway, he called it a heavy landing and we started on the inspections. First thing we found (very noticeable) was that every txmn mount had been crushed to such a degree that all the big disks (part of the mounts ) had been concaved like saucers, we gave the pilot one for his effort! Another situation we had was that one pilot got into such trouble that when he returned the aircraft to us, the upper wire strike had actually cut the bottom of both blades! Don't try this at home! The same pilot was going to get an award for a rescue but was denied because after the rescue (not the same day!) he suffered a blade strike on trees while recovering crab pots with the winch.

[NickLappos]

SASLess,
The friction is needed whan the sas is on (when you are not sasless....) because the sas actuators have to push against the friction to work the servos. If you have sas on with a fully limp noodle cyclic, some portion of the sas actuator motion will feedback toward the pilot, and that motion will be lost. This makes the sas very much less effective (it throws away about 20% of the total sas motion) and it also produces a slight and objectionable stick throbbing (the only throbbing folks of your age will ever again see). We call that "stick nibble".

Pylon rock is an artifact of the Huey family, as is collective bounce, both more prevelent in some models than in others. Pylon rock is basically a matching of the looseness of the transmission structure with a natural rotor in-plane motion. The transmission makes a slight fore and aft rocking motion that the rotor feels and amplifies, a sort of "air resonance". It is a whirl mode of the rotor that pumps the structure, and it nags the 540 system especially. It is often called the 'Huey Shuffle" and has a distinct low frequency beat as it pumps the pilots fore and aft at about 3 times per second.

Loose components can help make it big enough to be objectionable, so the advice above about mounts and lift links makes sence. In the Snake, the transmission had dampers built into its aft mounts, so the rocking of the transmission was absorbed (like the lag dampers on an articulated rotor blade). Also, the sas on a cobra had an input that came from the transmission (there were electronic transducers that read the pylon rock motion, and inserted that signal into the pitch sas so the sas would wipe the motion out by rocking the cyclic to cancel it.)

Why does the Huey have pylon rock and few other machines? Because it has no lag damping at all, so there is no resistance to the whirl mode at all, and that big heavy rotor system on the high mast has a ton of inertia that makes the mode frequency so low that it is easily felt by the crew. Also, the Huey is designed to ancient, very poor structural requirements, so that transmission is held in by very little structure. This makes it rock fairly easily, on a loose mounting structure. Before all the Huey-lovers jump on me, please recall what happens in a Huey if the rotor hits something solid - i. e. which crew member gets the transmission in his lap. Modern helos have much stronger transmissions, which stay in place when the rotor has the misfortune to hit something.