Pylon Rock

[HeliEng]

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.

[212man]

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.

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Another day in paradise

[Lu Zuckerman]

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.

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The Cat

[212man]

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.

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Another day in paradise

[Lu Zuckerman]

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.

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The Cat

[CTD]

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.

[Arfro Veesquared-Ess]

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! ;)

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Water, please...

[212man]

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.



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Another day in paradise

[Lu Zuckerman]

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.


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The Cat

[212man]

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....

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Another day in paradise

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

[Arfro Veesquared-Ess]

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?)

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Water, please...

[Speechless Two]

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

[Cyclic Hotline]

VH-NSA.

Lloyd Helicopters, OZ.

[Speechless Two]

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

[Lu Zuckerman]

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.

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The Cat

[212man]

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.

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Another day in paradise

[Lu Zuckerman]

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.


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The Cat

[HeliEng]

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.

[Lu Zuckerman]

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.

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The Cat

[212man]

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.

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Another day in paradise

[CTD]

", but I really can't believe what I'm reading and won't bother to add anything further to this latest theory.".......Ditto :rolleyes:

[Lu Zuckerman]

To: CTD

Which theory is that? The one about blades losing lift when aligned with the longitudinal centerline or, the one about retreating blade stall?

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The Cat

[Arm out the window]

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.

[Nick Lappos]

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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[Lu Zuckerman]

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).]

[Nick Lappos]

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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[212man]

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

[Lu Zuckerman]

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

[4dogs]

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]

[Lu Zuckerman]

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).]

[212man]

Lu,
you choose to ignore the case where no fwd speed is involved.

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Another day in paradise

[Lu Zuckerman]

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

[Rotorbike]

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).]

[212man]

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

[Lu Zuckerman]

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

[Nick Lappos]

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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[Lu Zuckerman]

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

[212man]

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

[Lu Zuckerman]

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:

http://pprune.homestead.com/files/pylon.jpg


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The Cat

[This message has been edited by helidrvr (edited 28 June 2001).]

[imabell]

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 :rolleyes:

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your too high,your too low, your too fast your too slow

[Lu Zuckerman]

To: imabell

Nothing was said about a Bell 47 helicopter. Although you didn’t see it, the engine / transmission combination moves up and down to the limits allowed by the two Dynafocal mounts attached to the engine /transmission support cage. The movement is minimal but it is there. There is however something common on the 47 that happens on all of the other Bell 2-blade helicopters and that is the 2-per rev, which gives you a vertical beat.

Also on the 47, the pylon (Mast) and engine will deflect to the limits of the Dynafocal mounts and if the pilot takes it beyond those limits he will come up against the sprague cables on the bottom of the engine. If he goes any further, the mast will bend and cause a growling of the gears in the transmission.


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The Cat

[Nick Lappos]

Lu:

It is quite clear that you take this forum very very seriously, and that you absolutely cannot admit that there is something you can learn.

Your loss of lift theory is simply untrue. Period. Would you like me to provide some blade bending data to show that the blades are all bent upward in level flight by the lift they are creating, all subjected to their share of the lift, and that this lift is a relative constant all around the mast?

What exactly will it take to get you to stop saying "I was once told by a Bell engineer..." as some kind of proof for this simply absurd premise?

An open mind is a wonderful thing!

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[Lu Zuckerman]

To: Nick Lappos

I do take the forum seriously and I have learned a lot from the participants on the forum. However I will continue to believe the theory of loss of lift until I am set straight by Bell Helicopters. I am also awaiting a copy of a paper delivered to the American Helicopter Society dealing with Fuselage Nodalization. I believe that the answer lies in that paper.

As a test pilot, an engineering graduate and a former Army Snake driver I would like to ask a couple of questions.

1) As a Snake driver did you ever ask yourself or anybody else what causes the 2-per rev bounce and why the frequency of the bounce was two time the rotor speed and not three or four times?

2) As an engineer can you explain in layman’s terms how the energy is developed that would lift and drop a 1000 pound dynamics system by 2+/- inches at a frequency of about 500 times per minute.

I quote,” What exactly will it take to get you to stop saying "I was once told by a Bell engineer..." as some kind of proof for this simply absurd premise?
An open mind is a wonderful thing!

I would like to paraphrase you question. What will it take me to stop saying “I was once told by a Sikorsky engineer…” as some kind of proof that my premise was absurd?

Here is a story about a Sikorsky engineer and his limited understanding of how a helicopter works. I was once involved in a comprehensive 14 month technical training program at Sikorsky. Aside from extensive classroom work I worked in every department of the company from engineering to flight-test. If a part or an assembly was made in an assigned department I built it, I tested it and I flew in it.

On one assignment I was working in the hydraulics test lab. While there I witnessed a hydraulics engineer cobble together a system that was supposed to drop the collective when engine oil pressure was lost. In order to work the collective was raised by a hydraulic piston until a spring was cocked and then the collective dropped. I asked him how the collective was raised and he stated that as hydraulic pressure built up the piston would lift the collective.

I told him that the system would not only not work it could cause serious damage to the airframe. He became extremely belligerent saying that I didn’t understand his device and that it would not damage the helicopter. It seems that his involvement with design was limited to hydraulics and he had no knowledge of theory of flight.

I told him that the hydraulic pump was on the transmission and as the transmission was brought up to speed the blades could not support lift. I further told him that at a slow rotational speed the blades would rise up and stall out falling and hitting the tail cone. The next day the device was gone and two days later, the engineer was gone.

I don’t know what bearing this story has our discussion other than that it takes a lot to change someone’s feelings about something that they hold dear.



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The Cat

[Nick Lappos]

Lu asked:
1) As a Snake driver did you ever ask yourself or anybody else what causes the 2-per rev bounce and why the frequency of the bounce was two time the rotor speed and not three or four times?

Nick answered:
That is because in the case we discuss, the 2/rev is the mode that tickles the transmission mounts. On some machines, it could be 1/rev or another that drives the limit cycle instability. On some Sikorsky's, it is about 2/3 per rev that makes things hum, and we work to damp this until it is gone. I guess you would believe that a 2/3 per rev mode is excited when the blade looses 2/3 of its lift, huh?

Lu asked:
2) As an engineer can you explain in layman’s terms how the energy is developed that would lift and drop a 1000 pound dynamics system by 2+/- inches at a frequency of about 500 times per minute.

Nick answered:

Lu, you should be able to explain this, as it is fundamental to understanding the nature of dynamic oscillations, a field of study for all engineers. Simply put, the vibration needs almost NO ENERGY, Lu. When a system is in resonance, the natural vibration is at that frequency, and very little energy is needed to make the oscillation keep on going. The trick is to understand that when the transmission is dancing up and down, it gets a down push just as it needs it, so only a little push is needed to keep the dance up.

Remember the analogy of the small boy on a swing. That boy could not lift himself up to head height through normal strength, yet he can do it on a swing by simply adding a little energy when the swing needs it, so his dynamic interaction with the swing is perfectly timed. For a boy to make a swing go from head height in one direction to head height in the other takes almost no energy, as the swing is simply converting the kinetic energy of speed at the bottom into potential energy of lift at the top.

Somehow, you confuse this vast impressive motion with a great need for energy, and you are wrong.

The energy needed to bounce the transmission up and down is small, I estimate that a 1,000 pound force might stretch the mounts an inch or so, so the potential energy would be about 1,000 pounds times 0.083 feet, or 83 foot pounds. This could be applied to the system each second by a small 1/6 horsepower electric motor!



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[Lu Zuckerman]

To: Nick Lappos

Thanks for the detailed explanation. Now I want you to carry it one step further. If a 1/6 horsepower motor were strong enough to excite a 1000-pound dynamic system and cause it to bounce up and down I would like for you to define the fractional HP motor as it applies the excitation process. Remove the motor from the equation and substitute the 2 per rev. The motor is powered by electricity. What is the motivational force behind the two per rev and what causes it?

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The Cat

[4dogs]

Lu,

The exciting force is most certainly aerodynamic in regards to what we called "verticals" and you most probably mean by 2/rev. However, in the case of that which we called "laterals" and you call pylon rock, the exciting force was mechanical.

Before Mr Chadwick Helmuth confused the issue and we used to make Miss Bell smooth by flag tracking and interpreting a vibration sensitive bum, we would minimise 2/rev verticals by making corrections to the tip path planes of the individual blades. These 2/rev vibes were speed sensitive because the blades responded differently to the forward-speed induced airflow changes - the blades twisted differently, bent differently and had slightly different lift curves because their rigging incidence was different.

If there was a significant split, the 2/rev vertical was obvious in the hover (no forward speed!!!) and we would roll the grips to smooth it out. If the blades flew out of track (the true cause of 2/rev verticals) then we would tab the blades by trial and error to minimise the vibration. I might add that I had the singular pleasure of test flying 5 brand new UH-1Hs from Bell and they were as smooth as many of the 3/4/5 bladers I have flown since - absolutely minimal levels of 2/rev anything.

"Laterals" were different again. Invariably, these vibrations were induced mechanically by mass balancing errors. At near zero lift, very small imbalances resulted in significant rotor swirl/pylon rock. There was no change in vibration level with forward speed in most cases - developing lift did not affect what was essentially a wobbling gyro. They were also characteristically 1-2/rev. If the vibration was of a higher order, say 3 or 4/rev, then the damping characteristics of the transmission mounting system were invariably at fault. Those we would isolate by a pylon rock check in the hover - we would deliberately excite the transmission mounting system and then analyse the time and nature of the damping.

At the risk of being repetitive, I have created or cured most of these characteristic vibrations and I can certainly vouch for the fact that fairly tiny amounts of lead shot in a blade bolt can certainly get a fairly massive and well restrained transmission moving in ways that would water your eyes. I never once worked out how to create extra lift at the front and back to achieve the same thing.

Now for a beautiful '91 Cabernet Sauvignon....

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Stay Alive,

[email protected]

[Nick Lappos]

Lu asked:
Thanks for the detailed explanation. Now I want you to carry it one step further. If a 1/6 horsepower motor were strong enough to excite a 1000-pound dynamic system and cause it to bounce up and down I would like for you to define the fractional HP motor as it applies the excitation process. Remove the motor from the equation and substitute the 2 per rev. The motor is powered by electricity. What is the motivational force behind the two per rev and what causes it?

Nick answered:
Every rotor produces a natural force wave form at N per rev where N is the number of blades. Basically, there is an oscillation of the lift caused by the natural dynamics of the rotor. I mentioned in a previous post that the magnitude might get as large as 10% of the blade's lift. The frequency is driven by the number of blades, and the amplitude of the oscillation drops very quickly as you get more blades, which is why many-bladed helos are very much smoother than those with few blades.
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[This message has been edited by Nick Lappos (edited 29 June 2001).]

[Lu Zuckerman]

To: Nick Lappos

I had stated previously that when the fat lady sang (response from Bell) I would either crow about being right or, I would eat crow. This afternoon I received a call form the senior aerodynamicist at Bell. We had a long conversation and I expounded on my views. He gave me a bit of support in stating that there is a variation in generated lift when the blade becomes aligned with the longitudinal axis but that was not the cause of the 2-per rev bounce. He told me that the 2-per rev bounce is generated when the blades are aligned with the lateral axis of the helicopter and it was due to the lift differential across the disc. Now this sounds reasonable but after a bit of thought I became confused. I had always believed that as the advancing blade was moving into the relative wind the pitch had decreased and the opposite was true for the retreating blade as it had higher pitch and was moving with the relative wind. With this scenario in place I was lead to believe that the disc loading was the same on both sides.

I was lead to believe that if there was differential of lift across the disc, the excess lifting forces generated by the advancing blade would result in a perturbing force and through gyroscopic action would result in a flap back (blow back) as this is what happens when you have retreating blade stall. Would you care to comment? Please don’t sprain you arm patting yourself on the back.

I can’t get my money back from the school of engineering as I was an Industrial Design Major.


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The Cat

[Dave Jackson]

There are some very knowledgeable people on this thread, so with trepidation, and for the fun of it, the following is humbly submited.
________________________

Lu, could it be that Nick is correct in that the thrust of the rotor disk is quite consistent, irrespective of the location of the blades?

Could it be that in forward flight with a 2-blade rotor the downwash from the blades is striking the fuselage at 2/rev. During hover, this downwash is coming from center of the rotor disk where it is minimal. As the helicopter's forward speed increases, the downwash striking the fuselage will become stronger, since it is now coming from closer to the leading edge of the rotor disk. This will give the fuselage a 2/rev vertical vibration. This segment of the downwash is initiated when a blade is pointing forward but it reaches the fuselage when the blades are out to the sides.

[i.e. The rotor thrust is constant and the parasitic drag oscillates at 2P.]

A continuation of the oscillations, particularly if there is harmonic excitement, will cause both the mast and the fuselage to oscillate. The rotors oscillations will be the greater by far since it has the least mass, but, the rotor's oscillations will be restricted to the mast, hub and inner portion of the disk. The tips will oscillate very little and the thrust will vary very little.

Just a guess.
_________________________

A test for pylon rocking is described on the following web page: http://www.adtdl.army.mil/cgi-bin/atdl.dll/tc/1-211/Ch7.htm


[This message has been edited by Dave Jackson (edited 30 June 2001).]

[This message has been edited by Dave Jackson (edited 02 July 2001).]

[Flight Safety]

Let me see if I can put to rest the "loss of lift" theory once and for all.

Consider the swept wing jet aircraft. The airflow over the wing does not flow with a velocity vector that is perpendicular to the leading edge of the wing, in fact far from it. Some jet aircraft have had wings with sweep angles up to 45 degrees (the MIG-17 comes to mind), and somehow the wings managed to generate lift. I believe the Concorde has a wing sweep even greater than this.

So if a rotor blade passing across the front of the helo, has a velocity component caused by its rotation and a velocity component caused by the helos forward motion, will the combined velocity vector by straight down the centerline of the blade's airfoil? I don't think so, and therefore the blade will still be generating lift.

The interesting part is that the "sweep angle" of the leading edge relative to this combined velocity vector will be constantly changing from the blade root to the tip, since the velocity vector caused by the blade's rotation will vary constantly from root to tip. Even at the blade root there would still be some airflow from leading edge to trailing edge, however the relative "sweep angle" at the root would be severe.

[This message has been edited by Flight Safety (edited 30 June 2001).]

[rotorque]

Lu,

You may be looking deeper into it than me at the moment, but could you be meaning to say the 'Angle of Atack' when you are talking about the "pitch Angle" in reference to symetry of lift.

Ie: the leading blade reduces its angle of attack as it flaps up not reduces its pitch angle.

On the point of sweep back, probably not relevent in the rotor blade scenario, but with a span wise flow of air across the blade when it's anywhere in the 12 to 3 oclock position, could the 'fineness' (thickness to cord ratio) be a more efficient producer of lift than when the blade is in the truly perpendicular position ?

It's the big words that make it fun.

[Lu Zuckerman]

To: rotorque

In addressing the pitch angle of the blade I was referring to the fact that the advancing blade in the 3:00 position is at it lowest point of pitch and the retreating blade at 9:00 is at its’ highest point of pitch. In theory the advancing blade with less pitch rotating into the relative wind will have the same degree of lift as the retreating blade which is rotating with the relative wind. In theory this equalizes the lift across the disc. In fact, the advancing blade will flap somewhat and further reduce its’ angle of attack in relation to the relative wind. I believe this flapping manifests itself in blowback or flapback whatever you wish to call it and it is countered with the forward movement of the cyclic stick.

However in order to create an advancing and retreating blade the helicopter must be moving in a given direction. In order to do that, the disc must be tilted in the direction of movement. To accomplish this tilting the lift differential must be greater on the left side (assuming forward flight) and the lift must be less on the right side. This differential of lift across the disc perturbs the rotating disc and the gyroscopic forces cause the disc to tilt up 90-degrees after the upward perturbing force. That is, if you believe in gyroscopic precession as this is not taught in the UK or, I believe in OZ.


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The Cat

[This message has been edited by Lu Zuckerman (edited 01 July 2001).]

[rotorque]

Good point Lu,

We are only taught how to ride our pet Kangaroo's to the airfield. Not how to make laser gyro's.