Pylon Rock
 

Lu, one for you maybe???

A colleague of mine had a question come up in an engineering exam about Pylon rock, but said that he couldn't answer it because he didn't know what it was!!

Can anyone shed any light on this one??

Look forward to hearing from you guys.

Pylon, or mast, rock occurs in twin bladed a/c under certain load, torque and airspeed combinations. It is a lateral oscillation of the transmission on its mountings which can, in some cases, become divergent. It is usually seen at low speed and high torque such as on takeoff, and is even more common when underslung loads are involved. Often exacerbated by worn mounting dampers and an 'overcompensating seat-cyclic interface' (Agricultural pilot!). Some AFCS fits can compound the problem and will have 'anti mast rock' circuitry built in. Normally cured by relaxing on the cyclic whilst lowering the collective slightly.

That's the practical side, not too sure about the mechanics of it but would supect 'Hooke's joint' effect had a part to play.

------------------
Another day in paradise

To: HeliEng

On the Bell 212 and on the 412 the lift link is not on the centerline of the mast/transmission. If I remember correctly, it is located forward and left of center. When the pilot pulls collective the lift link transmits the aerodynamic lifting loads to the fuselage from the transmission. Because the lift link is not on center the pylon (mast) will move in a vector forward and to the left until it is over the lift link. This mast movement also causes the transmission to shift its’ positional relationship to the output of the combining gear box causing a shift in the centerline of drive. In extreme maneuvering conditions this shifting may cause a disconnect of the drive shaft if the drive shaft starts to whip.

------------------
The Cat

The manouvering required would be so extreme that you would probably be suffering airframe failures and overstresses elswhere too. The predominet feature is that it manifests itself in a lateral oscillation felt through the airframe. If allowed to divege unchecked it will cause damage and control problems. I won't say that the drive shaft wouldn't disconnect eventually but that really is not a serious consideration; things have really got out of hand by then.

------------------
Another day in paradise

To: 212 Man

The US Navy and the US Air Force suffered quite a few shaft separations on their 212s and their 412s. The phenomena of separations were attributed to movement of the transmission in relation to the combining gearbox. This misalignment would cause the shaft couplings to align with the respective attach points (Transmission and combining gear box) and the shaft would have to find a position of neutrality which caused it to bow in order to maintain alignment with the couplings. If in the process of this bowing the shaft mass went out of balance the shaft would begin to whip and in doing so, it would disconnect at one of the two ends and it would immediately begin to flail. Another thing that would contribute to this condition is if the shaft were already out of balance.

When the 214 was introduced they had problems related to the Noda Matic suspension system. The nodal beam allowed the transmission to move up and down in relation to the fuselage. This caused a misalignment between the engine and the transmission. The couplings on the short shaft would accommodate the misalignment. This movement of the couplings caused them to pump out the internal lubricating grease. This grease would plate out on the inlet bell of the engine and sand and other small-ingested material would stick to the grease. This build up would cause a disruption in the inlet flow and result in compressor surges or compressor stalls. I was associated with the 214 program for over two years and in that time they tried several fixes including a change in the internal “O” rings on the couplings. The only solution they found that worked was frequent engine washes and frequent refilling of the couplings. I don’t know how they eventually solved the problem.

------------------
The Cat

I don't see what any of that has to do with Pylon Rock, which was the original question.

212man's response is correct.

The condition of the transmission mounts will have an effect on the severity of the vibration, as will stirring the cyclic.

Furthermore, the 412 does not suffer this phenomenon.

IMHO Perhaps 'the man' would have been impressed by the foregoing very sensible discussion (no mickey take intended) but in an eng exam he probably wants to hear something like..
It's relative motion between the airframe and the transmission due to the flexibility of the transmission mounts. e.g. 212 box hangs on 4 (memory?) endwise-on Lord mounts with a rigid link taking the lift loads to the structure. To control the shuggling around two friction dampers are fitted at the flex mount positions. A rough check for serviceability of these is a 1-2inch cyclic 'stir' in cruise... this will generate a vertical 'lump' which (if dampers OK) will die out after one or two beats.
Recommendation is to approach this check with someone experienced (as a boy I did it with too much enthusiasm, both driver and self not happy with result!) http://www.pprune.org/ubb/NonCGI/redface.gif
All the above from 15 year old memory since last touch of a lovely helo, G-BAFN where are you now?, so if it's short on detail I'm sorry! ;)

------------------
Water, please...

Not sure that the above would suit the style of an engineering exam paper; a tad colloquial perhaps?

I think you'll find that the a/c hangs on the transmission, not the other way round, and there is a fifth mounting at the rear of the casing. As you say, the actual lift loads are passed through the lift link to the lift beam and then into the longitudinal I -beams. No idea where 'your' a/c is; slogging away in Nigeria no doubt and approaching the 30,000 hour mark.



------------------
Another day in paradise

To:212 man et al.

On the 212 the lift link is positioned left and forward of the mast centerline. When lift is applied the transmission will rise and due to the positioning of the lift link it will move in the same vector (left and forward). Because the 212 has a two blade rotor system each time the blades are aligned with the longitudinal centerline the transmission will fall in relation to the fuselage. This downward movement of the transmission will react against the fixed lift link and the transmission centerline will move to the right and rearward. As the blades pass the longitudinal centerline and lift is generated the transmission will rise in relation to the fuselage and react against the lift link and move forward and to the left. Other things such as worn mounts will add to this phenomenon.

The reason pylon rock is not present on the 412 is because of the four blade system that is constantly lifting. The 412 will however move to the left and forward when lift is generated and it will essentially remain in that position unless there is maneuvering and then the transmission will move in an orbit in relation to the optimal centerline of the mast. It is this movement on both airframes that can cause the separation of the drive shaft if the movement is excessive.

This vertical beat on the 212 or even the Huey or 206 was the reason for the development of the Nodamatic Suspension system. The transmission moves up and down in relation to the fuselage but the Nodamatic nodal beams and weights absorb the vertical beat. The pilots and passengers experience a very smooth ride and the transmission is moving up and down at a rate of two times the rotor RPM.


------------------
The Cat

Now I've heard everything: the blades on a two bladed a/c don't produce lift when the blades are longitudinally aligned with the fuselage, causing the transmission to drop with the sudden loss of lift.

Does this apply in the hover too? or is it a progressive decline in lift over the speed range?

Answers on a postcard to....

------------------
Another day in paradise

[This message has been edited by 212man (edited 16 June 2001).]

212 Man, Yr quite correct of course, the a/c does hang on the Xmsn (but then again, only when it's flying! We spanners spend more time in the 'on ground' condition, so you can see how I get confused, I hope).
As for colloquial, again yr prob right, but for some reason I was assuming an oral exam (showing my age again, remember orals?)
G-BAFN (Gor-Blimey, Another F~~~~~~ Nuisance), ex BAH Beccles, left my sight around 1985/6 bound for Australia, but not BHL so far as I knew. If only I could remember the S/N maybe someone could give her a pat for me for old times' sake?

(But did we answer the question?)

------------------
Water, please...

Arfro - S/N of G-BAFN is 30550. Though I have no idea where the airframe is now.

VH-NSA.

Lloyd Helicopters, OZ.

Damn - beat me to that info by three minutes, Cyclic!

To: 212 Man

I guess you are going to have to accept that fact. If you want to prove it to yourself take a ride in a Bell helicopter equipped with a Nodamatic suspension system. Open up an inspection port that will allow you to see the transmission. When the pilot pulls collective you will see the transmission move upwards. As the pilot introduces forward cyclic and the helicopter starts to move forward the trannie will start to rise and fall. As the speed builds up to cruise the transmission can be moving up and down by several inches depending on the helicopter type. The 214 is most pronounced.

When the blades are aligned with the longitudinal centerline and several degrees on either side of the centerline the blades lose lift as they are aligned with the relative wind. This decreases the lift and the transmission will fall. As the blades rotate past the center and several degrees on either side they generate more lift and the transmission will rise. I don’t know if they ever installed the Nodamatic on the 212 and if they didn’t then you can feel a vertical beat when flying. This is caused by the up and down movement of the transmission, which in the case of the 212 is limited, by the movement allowed by the transmission elastomeric mounts.

As the transmission moves in relation to the fixed lift link the transmission will also rock in a vector from forward left to rear right and when maneuvering, this relationship between the lift link and the transmission will cause the pylon or mast to move in an orbital motion and that as I see it is what causes pylon rock.

------------------
The Cat

Lu,
I don't dispute much of what you say, but statements that the blades aren't producing lift when aligned with the fuselage don't further your cause.

------------------
Another day in paradise

To: 212 Man

To prove my point let’s address retreating blade stall. There are several ways of describing this phenomenon one of which is that there is an imbalance of forces across the disc and gyroscopic forces cause the disc to perturb and the advancing blades rise and the retreating blades fall. The other more common way of describing retreating blade stall is that a portion of the blade is not creating lift because of the airflow over that part of the blade is greater than the rotational velocity of the blade. In this theory the blade can’t maintain the lift load and it stalls hitting the tail boom. This is total crap but it is the way POF for helicopters are taught. Whether it is crap or not the whole idea is that the blade produces less lift because of the reverse flow over the blade.

Now, if you buy into that, how much lift can a blade generate when it is aligned with the relative wind that can be far in excess of 100 to 150 Knots? Even though the rotational velocity is constant the relative airflow is not. If the blade can stall over the left side at a given rotational velocity why can’t it generate less lift when it is aligned with the relative wind.


------------------
The Cat

Hey guys!

I am really sorry that I have started this thread, I didn't want to start a slagging match off again.

I just wanted an answer to, what I considered, a fairly simple question for you guys to answer, and now it has turned to this.

'Soome days you are the statue and some days you are the pigeon.

To: HeliEng

It was not my intention to slag any body off. I was just making a point to 212 Man and I guess I got a bit carried away in my description(s) of retreating blade stall.

------------------
The Cat

I wasn't aware I was slagging either, but I really can't believe what I'm reading and won't bother to add anything further to this latest theory.

------------------
Another day in paradise