Pylon Rock

[Harmonic_Vibe]

I'm flying a 212 building towers right now. The machine is in fairly good shape but it's a beefy 6900 lbs empty... not what we would call "a good lifter" in Canada.

 

Two or three times I have been coming in with a load and just as I'm arriving the machine starts to rock and roll. I fly through but it's a very uncomfortable ride for a while until I get in the green on airspeed and then it dampens out.

 

Yesterday I was bringing in a 2400 lb compressor to a site at 6000 feet or so and it started to happen. I was not waving the stick around and was under complete control (well... as controlled as I ever get) and in the space of about 5 seconds I was bouncing in my seat and felt like I needed a training bra... or maybe that nice red one that Skully was referring to in an earlier thread. This time it took forever to dampen out and I was very close to saying "bah bye" to the compressor... pretty scary I must say.

 

Every time it has happened has been a very windy day... and the locations all suck. You couldn't pick worse locations on mountains with respect to windy conditions.

 

Do I have a bad mount or something? Or is it just a normal thing in some wind conditions? I remember about five years ago a pilot always complaining about pylon rock (or pylon whirl as some people say) in one of VIH's 205's... But I flew it and never noticed anything amiss. The consensus then was that there may be a bad mount or two but the "rocking" was directly proportional to the roughness of the pilot. I've got a fair bit of time doing this kind of thing and I would think that if I was so rough it would have turned up sooner... wouldn't it?

 

In this case the engineer can't find anything wrong but I seem to recall that there's some check you can do where you introduce a perturbation (today's word... there's a quiz later) and see how many oscillations you get before it dampens. I don't want to end up punching something off... That's not the way to become famous over here.

 

Any info is much appreciated.

 

HV

[carholme]

I'm flying a 212 building towers right now. The machine is in fairly good shape but it's a beefy 6900 lbs empty... not what we would call "a good lifter" in Canada.

 

Two or three times I have been coming in with a load and just as I'm arriving the machine starts to rock and roll. I fly through but it's a very uncomfortable ride for a while until I get in the green on airspeed and then it dampens out.

 

Yesterday I was bringing in a 2400 lb compressor to a site at 6000 feet or so and it started to happen. I was not waving the stick around and was under complete control (well... as controlled as I ever get) and in the space of about 5 seconds I was bouncing in my seat and felt like I needed a training bra... or maybe that nice red one that Skully was referring to in an earlier thread. This time it took forever to dampen out and I was very close to saying "bah bye" to the compressor... pretty scary I must say.

 

Every time it has happened has been a very windy day... and the locations all suck. You couldn't pick worse locations on mountains with respect to windy conditions.

 

Do I have a bad mount or something? Or is it just a normal thing in some wind conditions? I remember about five years ago a pilot always complaining about pylon rock (or pylon whirl as some people say) in one of VIH's 205's... But I flew it and never noticed anything amiss. The consensus then was that there may be a bad mount or two but the "rocking" was directly proportional to the roughness of the pilot. I've got a fair bit of time doing this kind of thing and I would think that if I was so rough it would have turned up sooner... wouldn't it?

 

In this case the engineer can't find anything wrong but I seem to recall that there's some check you can do where you introduce a perturbation (today's word... there's a quiz later) and see how many oscillations you get before it dampens. I don't want to end up punching something off... That's not the way to become famous over here.

 

Any info is much appreciated.

 

HV

<{POST_SNAPBACK}>

 

Harmonic_Vibe

Providing that all other factors were reasonably normal, goss weight etc., the first thing I would check are the two friction mounts which on the 212 are mounted under the aft main mounts. You can access the lower mount bolts for the friction adapters by popping the inspection caps on the sides of the pylon island. Aside from the friction adapters being worn, one of the first cases is often that the lower mounting bolts are worn or the small brgs are shot and the adapters work in the vertical plane without the friction forces being felt. If all is ok here then you move up to the main mounts and depending on experience and feel, moving the mast in all quadrants should initially identify if there is an very bad mount.

 

Regards

carholme

[twinstar_ca]

carholme, i deleted the post for you where it ended up being just a quote of HV...

[Winnie]

I don't have ANY medium experience...

 

However, I do believe that "Pylon Rock" or "Pylon Whirl" are occurences on the ground (Due to low RRPM and a little sink rate), However, you may be experiencing "Sympathethic Resonance" or "air resonance" since it seems to happen in the air. Not entirely sure though.

[matador]

Hi HV, glad to hear from you and find out you are doing what you like

Regarding to your question I bet you know how to do the test to find out about wearing of xmsn mounts, if not I'm sure some guy could describe it in your language better than me. Anyway we had same problem with another 212 a few years ago. The problems shows up when you are quite loaded and low speed, it doesn't matter how smooth you are on the controls, it only will appear later but it will come up. Sometimes have to do not with wearing of mounts (even they might be within limits) they might have worn a different amount: you might have a stiff one on the right and a soft one on the left

We sent the ship back to maintenance though the owner sustained that was flown before without problems We didn't blieve him anyway.

Buen vuelo and hope to see you around this summer.

P.D. I think you guys are supposed to go to a diferent contract this year (Castilla La Mancha).

[DGP]

I dont have much experience working on the 212 but lots of it on 205,as mentioned above check what Iam use to calling the 5th mount bearing and mounts...I have seen the hardware beat up pretty bad from logging.Also check around the lift link...have seen these crack and/or the bearings pop loose from there staking.Also,not to cut down the driver as I am one myself,bell 2 bladed systems have a tendency to shudder and shake when loosing translational lift with a heavy load especially with even marginally high rates of descent.Also and this may sound crazy but I had this happen once...as mentioned rock the mast towards the 4 corner mounts and take note of the mast deflection when the mounts are compressed.I had an engineer ask me once to go over his 205,as the driver was complaining about it being rough(the wrench involved was not medium endorsed...the driver was p/e on 205 but couldn't give his engineer the time of day).We went up on the roof and as I was showing him how to check the mounts as I pushed the mast hard right,it fell over to the right and stayed there.Now this scared the bejesus out of me...as this is not normal...investigation reveals...the 4 corner mounts are sitting there with the bolts dropped in place and the nuts on the bolts are finger tight.I got out the tools and got 13 full turns on these nuts before they torgued up...this my friends ended my 205 lesson for the day.I told this story to a bell factory instructor...he wouldn't believe that the trany had not departed with the rotor system...if the lift link had broken...we would have had another story to talk about...so just for the he// of it...check the torque on the corner mount bolts or at least give the trany a good shake and see if she is all together....play safe :shock:

[arctic_front]

I'm no medium expert, but am endorsed. The friction mounts are very hard to detect a 'soft' one, but a totally shot one should be easily detected. I agree with the torque on the bolts as a first step, then also check to see if the structure attaching the mounts. I have seen a few sheared rivets in that area. also any issues with the 5th mount should be looked at closely. btw, does your skid-gear shake a lot in flight?....as previously mentioned, a sympathetic harmonic might be what amplifies the the initial onset? good luck and better to drop something than break the helicopter.

[skullcap]

HV:

 

Looked through some old memos came across one from Don MacKenzie and Frank Ranger regarding 212's and pylon rock;

Pilots:

 

1) If pylon rock check shows dampening rate of 4 cylces+-1, carry out the following checkes prior to changing pylon dampers or mounts.

 

2) With vasic ship and crew of 2, accelerate to 20 - 30 kts, 25 -40 Q, at 500' AGL and excite the pylon the same as at hover.

 

3) During the dampening cycle shut off both hydraulic systems. If rock ceases immediately, engineers are to carry out the following checks.

 

Engineers:

 

4) Check cyclic cylinders for excessive servo calve seal breakaway forces(max. of 2 lbs allowable). If forces are excessive, replace servo.

 

a) 212. This check is done on the servo control input lever, ref. 212 Parts book, fig 26. Remover AN174-13 bolt, Item 27, and disconnect rod Item 28. Re-insert bolt Item 27.

 

With a spring scale attached to the bolt, check spring force required to move the pilot valve. Any force in excess of 2 lbs. is cause to replace the servo. A tight servo valve will cause the servo to motor.

 

5) Check for worn of loose cyclic cylinder uniball mounts.

 

6) Check for accumulated wear in control linkage from cylinders to main rotor head.

 

7) Check wear on U-joint on top of cyclic cylinders.

 

8) Check for bulkhead deflection at lower end of cyclic cylinders.

 

9) Check for proper built-in control friction.

 

10) If any of the above checks points show a discrepancy, this should be corrected.

 

11) If the pylon rock is still unacceptable, then the condition of the transmission mounts and dampers should be investigated.

 

Thought it might be of interest and are in remote local, good luck.

 

p

[Skids Up]

Ran into something that sounds kinda the same on our 205. My first thought was collective bounce as the machine was bouncing quite badly(!) in what appeared an up & down motion, however, when I took my hand off the collective it was completly still. I had a load of fuel for some cats in a sling, as was ready to punch it off as it was getting quite bad. But as I was closs to the ground I kept going and when the load hit the ground, it immediatley stopped.

 

I was pretty green on the machine at the time so was asking everyone I could what it might be, and tried all the tests that people suggested, (had one of the best engineers one could have, with me) until finally someone asked how much friction I was running on the cyclic. I was in the habit of keeping it quite loose (read none - 206 days) so he suggested running a bit and the problem went away. :up:

 

Turned out that withount being noticeable. I was inducing a slight for and aft motion which was coming from the machine and caused enough shaking and motion in the body and transfered through the leg to the arm to the cyclic.... :down:

 

Cured my problem - you could feel it start to build but with the higher friction, it never would go into it again.

 

As a side, during the winter maintenance, we found that transmission mounts were weak but not felt enough to cause the problem by themselves.

[LTE]

HV

 

I'm not sure what country you're play'n in these days but I'm sure the lingerie selection meets your tastes......careful nothing falls out of your trainer while leaning out the left side & you may want to keep the camera turned off for that one!

Safe, smooth flying

 

Flpr :up: