Bifilar damper on UH-60

[Lu Zuckerman]

To: NickLappos

Refer to: http://www.tpub.com/content/hseries/TM-1-1520-265-23/css/TM-1-1520-265-23_72.htm

It appears on this drawing that the arms of the Bifilar damper are not disposed 45-degrees like the pitch links. Why is this? And, which vibrations does it dampen or eliminate?

:E :E

[NickLappos]

"like the pitch links" what does that mean? The two things are not connected by function or concept, except that they are bolted to the same place.

Do you think they behave together?

[Lu Zuckerman]

To: NickLappos


"like the pitch links" what does that mean? The two things are not connected by function or concept, except that they are bolted to the same place.

Stop trying to misinterpret and misconstrue what I say. I referenced the pitch links because according to the drawing they are disposed by what appears to be 45-degrees in relation to the blades. I know that the Bifilar damper and the pitchlink functions are not related. I was simply asking why the Bifilar dampers are disposed by what appears to be more than 45-degrees. I was expecting you to respond that the Bifilar dampers are disposed at XX-degrees in relation to the blades and why. I also expected you to respond that the Bifilar dampers react to such and such vibration.


Do you think they behave together?

No I do not nor did I ever think that.

Now, do you want to respond to the original questions?



:E :E

[SASless]

Lu,

Apply some of your engineering analysis ability here....and try to reverse engineer the design for us. That should be fairly easy to do...similar method you applied to the R-22 rotor system.

Give us your thoughts on the matter please...the world wants to know. We sit here with bated breath....!

[NickLappos]

Lu,

What else do you "expect?"

[Lu Zuckerman]

To: SASless and NickLappos

What else do you "expect?"

At the least I expected you to provide a technical answer to a legitimate question. As in the past you have evaded answering lots of questions that I posed to you.



Give us your thoughts on the matter please...the world wants to know. We sit here with bated breath....!

Are you looking to be hired by Sikorsky as a test pilot?

Here is the reason I asked the question. I recently finished assembling a 1:18 scale model of an Air Force HH-60. It is accurate in every detail except the Bifilar damper was mounted on the rotorhead with the arms aligned with the rotorblades.

I contacted the company that makes the model explaining the problem and they asked for a picture so that they could turn it over to their "experts" to determine what I was talking about. I wanted to get some technical information from the horses’ mouth. Instead I got a lot of drivel from the horses' :mad:

:E :E

[SASless]

Now...Now...Lu....don't get yer knickers in a twist here....if you had explained the reason for your interest....the answer could have been a lot easier to obtain. It sure sounded like a trolling expedition.

Yes...Lu...if offered...I would love to fly at the Sikorsky place....even if was just to do run ups....I would have to learn a new language though....that place being north of Richmond.....wonder if they have country ham, grits, and redeye gravy there?

[NickLappos]

Yep, and pretty good BBQ, but even better Italian, Polish, German and other ethnic foods.

And we have biflars, too!

[Ascend Charlie]

Chin up, Lu, they are just pulling your p155er.

The bifilars are to reduce certain rotational vibrations. With a 4-bladed system, you would expect 4:1, maybe 2:1 and some 1:1 vibes, but these are tuned to remove 3:1 and 5:1!

And no, they don't rely on precession to do it, there is a really kinky combination of moving weights sliding around on keyways and cams with specially shaped holes. Must have driven Nick mad to get them tuned, but they seem to work. But some operators pull one set of four off and just fly with the other set (in S76 anyway).

[ShyTorque]

AC,

I think Lu is very well aware of the design and reason for bifilars. He is fishing for something.....Eh, Lu? :suspect:

[Lu Zuckerman]

To: ShyTorque

I think Lu is very well aware of the design and reason for bifilars. He is fishing for something.....Eh, Lu?

No, I explained why I wanted the information. I could have expanded on the original question by asking why the damper arms are not disposed at 45-degrees as opposed to how they are disposed at least according to the drawing.

As far as Bifilar dampers and their function I am very familiar with their use on large radial engines. I never worked on a helicopter that used them.

Here is an oddity and I hope I get it right. On the S-58 T they reduced the hydraulic pressure to the two servo systems and at the same time added a Bifilar damper to the head. I had a lot of experience on the S-58 and its' variants and it never had any vibration problems requiring inplane vibration isolation.

Another bit of education. Did you know that Bifilar means double threaded?

:E :E

[John Eacott]

Am I the only one who spring loads to this (http://www.helicopterservice.com.au/photos/Ski/Give-A-fmeter.gif) when Lu tries to slag off at Nick?

:rolleyes: :p

[Double Echo]

I've read a fair number of threads on here (As a guest, however I have now entered the fray), but lads really, any chance of putting the catapults and erasers to one side for a second??:ouch:



I understand the rivalry between the two would be protagonists, but really guys.. there are a few people out here who would have actually been interested in an explanation to what is a mildly interesting question, especially to those of us who aren't fortunate enough to have worked with this aircraft.. So, Mr Lappos, as an interested bystander, I'd like to hear the answers too if you have them???:ok:

[Homer_Jay]

John E,

No, you are not the only one. I think we should organise to get these two together so they can settle this the old fashioned way....bare knuckles.

If we all chip in I'm sure we can raise the money for airline tickets to a neutral location. What do you say Pruners? (Incidentally, my money's on Nick).

After the bout we can see an end to these personality based, antagonistic threads.

[NickLappos]

I helped develop the bifilars on two of our models, and understand them a bit:

They are simple swinging masses that are resonent near the N/rev frequencies (4 per revolution in the Black Hawk and S-76)- they love to hum along at the same beat frequency as the blade passages. They are actually tuned to N-1 and N+1 because they are in the rotating system, where the blade passage frequency is made up of the sum of those two frequencies.

The bifilar is much lighter than other vibration absorbers, and right at the source. Also, there is no spring on a bifilar, they achieve their frequency by CF, so they automatically tune to the rpm they are at, allowing large rpm changes without large vibration changes.

Bifilars work in-plane, the most effective direction. Generally, if only one is needed, the one bifilar is tuned to 3 per rev (N-1) which is the bigger, more objectionable component. Nobody removes bifilars by themselves, because they are part of the airworthiness of the machine, not an option. The vibrations at many airframe stations, not just the people, depend on them, so just tinkering with them might make some nice piece of equipment very sad without showing up on the normal maintenance gear. We want our engines to be very very happy, don't we?

Bifilars were invented for piston engines decades ago, and are not rocket science.

The orientation of the arm over the space between the hub arms is specifically designed to allow ease of maintenance and to prevent interference with blade flapping. There is no issue with the exact orientation, the weights all act to absorb the in-plane vibrations.

The only vibration they act on is 4 per rev. NO other frequency is helped by the bifilar. If one weight is badly slung, and does not swing properly, some odd frequencies can be flet, because the other weights conspire to upset the vibration balance. Typically, a 3 per rev is felt if the bifilar is badly maintained.

Any connection between the bifilar and hydarulic pressure drop exists only as coincidence, there is no inter-connection between the two.

[Double Echo]

The only vibration they act on is 4 per rev. NO other frequency is helped by the bifilar.


There's something I didn't know..:ok:


The orientation of the arm over the space between the hub arms is specifically designed to allow ease of maintenance and to prevent interference with blade flapping. There is no issue with the exact orientation, the weights all act to absorb the in-plane vibrations.

Thanks again, common sense I suppose, however, I appreciate the post. :ok:

[Collective Bias]

Thanks Nick

Very helpful when trying to understand and correcting vibrations on helicopters having these devises.

:ok:

CB

[S92mech]

Nick, I have a question for you,

When we converted out S-76A's to A++'s we removed one set of bifilars. We have been told that the C30 engines have a rotor induced resonance in the PC air system that the second set of bifilars helped lessen and the 1S1 engines don't have this problem. Is this true?

[shak'n]

Aaaahhhh, the old Nper rev debate resurfaces.

Its remarkable that the helicopter owes it's very capabilities to rotor blades and how they perform - and yet they (the rotor blades) and their effects on the helicopter are proabaly the least understood thing about the aircraft by those within the industry.......truly remarkable.

Nick of course is correct particularly about the operation of the bifilar, but the explaination of the N per rev vibration problem can be expanded a deal further.

The principal of the bifilar has been used widely in automotive engines for some time - as Nick says. Those who remember the Astron engine in Mitsubishi cars of the '80's would remember the "revolutionary" silent shafts which were simply shafts with hollow compartments containing masses free to find their own equilibrium and tuned to roughly the operating regime of the crankshaft. The silent shafts were located close to the crank shaft in order that the countering (absorbing) effect of the free-to-move masses operated closer to the source of excitation. I'm sure there are many other models/manufacturers using the same principal.

But more on the N per rev vibration problem which appears to be grossly misunderstood throughout the helicopter community...

The N per rev always has 2 x components - a lateral & vertical component. The Bifilar (in Sikorsky's case) does take out (or tries to) ONLY 4/ rev in plane (ie predominantly 4/rev lateral) vibe. The vertical component is then absorbed or cancelled (or tried to be) by the cockpit(nose)/cabin absorbers and the interaction between them. If you ever get the chance, have a look at the cabin absorber/s working in flight or coming thru translational - very impressive little hummers. The Bifalars are not tunable by the operator - how they are delivered from the factory is how they shall remain - unless you are exceedingly brave or stupid.

On the other hand the nose/cabin absorbers (vertical n per rev) are tunable. They need to be as there are many variabls which can effect the magnitude and resonance of the vertical component of the N per rev vibration. Things such as loose/worn/frozen absorber bearings, loose pilot seat mounts, loose oleo busshings, loose cargo hook bearings, ESSS - note the fitment of the wing stub absorbers on all UH60L and above with the recommendation that they be retrofitted to models prior to the Lima if they were to be fitted with ESSS.


There is a huge mystique and mystery perceived around helicopter vibration - most of it borne from ignorance. To understand the Nper rev you need to know what causes it......

Try visiting http://www.rwas.com.au/rtb.html

I have found it gives a good general insight into RTB and other helicopter vibration issues. While not extensive, it does expose a few myths and shows the basics very well. There is certainly scope for the education of the entire helicopter community in the ways of RTB and vibes in general. I can only hope the site above continues to expand its information


Hope you find as interesting as I.....


forever Shak'n