Dynamics - N-per-rev Vertical Vibration

[Dave Jackson]

There has been at least one thread in the past year on N-per-rev vertical vibration. The various methods of minimizing this vibration were discussed but I do not recall, and could not find, any reason(s) for this inherent vibration.

Information related to cause, or causes, of N-per-rev vibration will be much appreciated?

[Lu Zuckerman]

To: Dave Jackson

At the risk of being publicly chastised for my offering an opinion on this matter I offer the following:

A vertical vibration may be caused by an out of track blade but I believe that it is most likely caused by the undamped sinusoidal wave that travels down the blade from the tip to the root.

Some helicopters have pendular weights on the
blades or the blade roots as energy absorbers while on the Sikorsky rotor heads they use a Bifiler* System (SP) that absorbs this energy. Some Bell blades have a weight inside the blade with the weight being placed at the nodal point of the wave. Some French helicopters have a weight supported by a coil spring with this weight being installed on the top of the rotorhead while other helicopters have a system that works like a noise canceling head set which sends out a sine wave that is equal to but opposite in polarity. This type of device can be mechanical or electronic. This sinusoidal wave was not too much of a problem on earlier designs that used the NACA (NASA) 0012 airfoil. When the helicopters started using unsymmetrical airfoils the problem started to manifest itself.

*Bifiler if I spelled it correctly means two threads and I can’t determine why they would chose this name for the system.

I shall now assume the position.

[ 14 January 2002: Message edited by: Lu Zuckerman ]</p>

[Dave Jackson]

Lu

Thanks for the info.

As you said, it is assumed that the blades are tracking properly etc. My query relates to the aerodynamic source(s) of the low frequency vibration. You and others have provided quite a bit of interesting information about methods used to minimize or offset these vibrations.

Perhaps the sinusoidal wave of the blade, which you mentioned, is caused by a varying lift at different blade radiuses, azimuths and forward speeds.
Perhaps the source is the rotor's downwash striking the fuselage or tail.
Perhaps the source is ....?????


The illogical logic here is; if the source is eliminated then the remedy is eliminated.

[Lu Zuckerman]

To: Dave Jackson

“Perhaps the sinusoidal wave of the blade, which you mentioned, is caused by a varying lift at different blade radiuses, azimuths and forward speeds”.


Perhaps the source is the rotor's downwash striking the fuselage or tail.
Perhaps the source is ....?????

Response:

I believe the first part of your statement is correct and in addition I think the vertical movement of the blades from flapping might enter into it as well. Perhaps Nick Lappos would like to add to this.

[Nick Lappos]

Dave,
The N per rev vibration is a natural effect of the blade's motion around the head, and is intrinsic, as is the 1 per rev resonance of the blade in flapping. The rotor delivers a "root shear" to the head that sums at the head to the n per rev vibration in the static system.
These root shears are increased by stall, compressibility and blade dynamic responses. They have nothing to do with track and balance at all.

This vibration is intrinsic, and cannot be eliminated, only minimized or absorbed. The hinge offset increases the amount of root shear delivered by the blade, and the number of blades reduces the root shear delivered by each blade. Generally, the n per rev drops geometrically as the number of blades is increased, so that the natural n/rev vibration for 5 blades is a very small percentage of the magnitude of that for 2 blades.

see a brief explanation on page 12 of:
<a href="http://atrs.arc.nasa.gov/atrs/97/jacklin/977183/977183_jacklin.pdf" target="_blank">http://atrs.arc.nasa.gov/atrs/97/jacklin/977183/977183_jacklin.pdf</a>

In the rotating system, this vibration has two components, at N+1 and N-1 per rev. In other words, an absorber attached to a 4 bladed rotor head will see 3 per rev and 5 per rev vibrations, and the transmission will see the sum of these two as a 4 per rev.

Generally, the n/rev for a 2 or 3 bladed helicopter is high, and for more than 5 blades is very low.

PS Whoever rated me as listed in the column to the left - I am flattered, but also, isn't it a bit much? I'd settle for "Boyfriend of Liv Tyler" myself! <img src="wink.gif" border="0">

[ 14 January 2002: Message edited by: Nick Lappos ]</p>

[Wunper]

Nick has summed up very neatly a very complex area in rotorcraft dynamics the theory understanding and modelling of which is the preserve of very well educated men. Cool Link Nick

If I may add my pen'th to the bun fight.

Nper rev has very little or nothing to do with 1 per rev, it is however very much more noticeable if the 1 per rev is very low as on a well tracked and balanced machine, on a 2 bladed machine if the 1 per rev is too low the overall ride quality can be very poor. There is a trade off to be had here to ensure the perceived vibration is acceptable to the crew. Bell pioneered Nodamatic ride systems to improve Nper rev on their heavy two bladed machines viz the 214,222 etc.

Nper rev on the Lynx helicopter is very pronounced, it has a 4 bladed semi rigid rotor head and Main gearbox that is bolted directly, undamped, to the airframe. These are all design features that confer the huge control power that make the aircraft very good for what it was primarily designed for, deck landing (and staying on) the back of small ships in rough seas. These features come with a price in vibration terms.

Nper rev on the Lynx is controlled by scheduled maintenance inspections and actions as a result of in service vibration survey operations. Key contributors to the levels of Nper rev are the mechanical condition of the mountings of any of the heavy components on the airframe e.g. engine mount bearings, horizontal stabiliser (which lives in the downwash environment), cabin door latches, Instrument panel, and avionic box AVM, rescue hoist mounts etc. Any wear or slop in these and the Nper rev cocktail gets bigger.

The most significant driver to the levels of Nper rev on the Lynx is the Main gearbox top bearing pre-load figure. This if you like controls the free play between the rotor system and the airframe, too low a pre-load will allow excessive relative motion between the two systems and will contribute hugely to the Nper rev generated by the aircraft.

The Composite rotor blades with Berp tips also increased the Nper rev of the Lynx to noticeably higher levels than that experienced with the conventional shaped metal blades fitted to the earlier marks. The Horizontal stabiliser was cropped span wise and elastomeric engine mounts were introduced to compensate for this increase in vibration. The increased performance offered by the Berp blades also comes with a price in vibration terms.

Blade pass downwash acting on the upper surface of the fuselage is also a contributor to the Nper rev values on a helicopter; the rotor mast was reputedly lengthened on Blackhawk/Seahawk to reduce its Nper rev figures. Maybe Nick has the inside scoop on this?

Wunper

P.S this is not a dig at the Lynx! it is a very fine helicopter but serves the explanations for this thread well

PPS this is my first technical post so I will just put the tin hat on and zip up the Kevlar jacket,,,,,,, <img src="cool.gif" border="0"> <img src="cool.gif" border="0">

[HeliEng]

LU,

[quote] A vertical vibration may be caused by an out of track blade but I believe that it is most likely caused by the undamped sinusoidal wave that travels down the blade from the tip to the root.

Some helicopters have pendular weights on the
blades or the blade roots as energy absorbers while on the Sikorsky rotor heads they use a Bifiler* System (SP) that absorbs this energy.
<hr></blockquote>


I was under the impression that the Sikorsky Bifiler system was there primarily to reduce LATERAL vibration not vertical as you state.


Can anyone please explain how the Bifiler system works???? <img src="confused.gif" border="0"> <img src="confused.gif" border="0"> <img src="confused.gif" border="0">


"Some days you are the pigeon, some days you are the statue!"

[ 14 January 2002: Message edited by: HeliEng ]</p>

[Lu Zuckerman]

To: HeliEng

My statement about the Bifiler system was based on an article in one of the major aviation journals describing the system when it was first developed. In that article it indicated that the weights were capable of moving in plane which would support your statement about damping out lateral vibration. It also indicated that the weights were free to move in the vertical direction (up and down) which would act like the pendular dampers on other helicopters. If the article was wrong then I am wrong.

When the Apache was first designed the pendular dampers were similar to those on the model 500 which had two pendular dampers suspended on a common axis. The two dampers were of different sizes and weights which were tuned to specific frequencies of the standing wave on the blades in flight. I believe they may have removed one or both when the Apache went into service at least they were talking about it while I was still on the program.

As a matter of interest the MIL Mi-8 HIP has a system that looks just like the Sikorsky Bifiler damping system.

The Bifiler system and the pendular weight systems work in the same way. Both systems employ (Please don’t jump on me) centripetal and centrifugal force. The centrifugal force acting on the weights increases their resistance to movement and the weights are aligned with the blade. If any vibratory force such as the traveling wave moves down the blade from the tip to the root it will have to displace the weight of the pendular damper or the Bifiler damper. This weight will absorb the movement of the blade and damp out the frequency before it can enter the rotorhead or at least it will reduce the intensity of the force. On the Bifiler system the weight is free to move in plane so that any inplane vibration that feeds back to the rotorhead will be absorbed by the damping element.

I now stand open to criticism.

[ 15 January 2002: Message edited by: Lu Zuckerman ]</p>

[Hovering in the Wings]

My dear chaps,

After having browsed Prunne for some weeks, sometimes in awe, sometimes in bewilderment, sometimes even enlightenment, but oft times in amazement and disbelief..... you have finnally forced me out of the wings and to apply forward cyclic and subscribe to this site.....

To dave,

Firstly many thanks for raising this most interesting of topics. It has certainly perplexed me for many years and been the brunt of much unofficial investigation, hypothysising and speculation.

My first questions at this point in time is:

1. How do you feel thus far about your responses received and,
2. Now do you understand WHAT causes N per rev vibes??? If so, could you please reword it in simlisticish terms and post it so that us lay man can understand.....if not just say so, so we may continue the discussion further

Many thanks.....more to follow...over.

[Nick Lappos]

Helieng asks how a bifilar works.

Here are some concepts on vibration absorbers in general, then on the bifilar:

If you put a weight on top of a coil spring, the spring will bounce the weight at a specific frequency, based on the amount of mass and the stiffness of the spring. Think of the hula doll sitting on the car dashboard, with the head on a spring and eyes that light up when the brakes are pressed.

A stiffer spring makes a higher frequency, as does a smaller mass. By varying either or both, we can make the spring-mass system "like" (resonate at) any given frequency.

If we place this spring-mass system in a helicopter, and tune it to "like" the exact n per rev frequency of the helicopter, it becomes a vibration absorber. That means that it picks up the vibration energy from its mountings and easily transmits that energy to the mass. The mount and deck become quiet, and the mass bounces like crazy. We can orient the mass for vertical or horizontal vibrations, although it tends to absorb both to some degree.

The bifilar is a spring-mass vibration absorber, but the "spring" is the centripital acceleration field that the bifilar arm swings in. This is easy to picture, since the mass pivots on a roller so that it must swing closer to the hub when it swings off center. It behaves like a pendulum in the centripital field.

The great strength of a bifilar is that it automatically tunes to the frequency of the rotor, since its "spring" is based on the rotational speed. This is better than the spring-mass system we first described, where we must change spring or mass to retune. Since the rotor rpm changes slightly as we maneuver, a bifilar stays in tune and stays efficient across a wide band of rpms, where a spring mass absorber falls off its peak and can be quite ineffective. A bifilar equipped helo is smoother in autorotation and power changes than a fixed absorber equipped helicopter.

Bifilars were first used on engine cranks to smooth out the vibration of the engine.

Regarding the orientation of the absorber, the rotor head bifilar is quite effective even though it is aligned in the lateral axis. This is because the rotor root shears I mentioned in previous posts act both in-plane and vertically, so there is enough vibration to go around. In fact, even a vertical vibration at the head is often transmitted as a roll mode, since the airframe resists vertical motions by its whole mass, while its rolling motion has much less inertia. <img src="cool.gif" border="0">

[Dave Jackson]

God. Excuse me, <img src="wink.gif" border="0"> Nick

Thanks for providing information related to the sources of rotor induced vibration. The fact that vibration is an intrinsic part of a rotor, which is flying edgewise into the airflow, is also understandable.

The alternatives of being 'minimized or absorbed' is the intriguing part, especially when it seems that minimization would be the preferred method. I assume that higher harmonic control (incorporating efficient algorithms, parallel processing and rapid cyclic pitch actuators) is the primary area of research at present for minimizing vibration.

It would appear, from a relatively naive perspective, that more could be done with the actual rotor to reduce this vibration. Retreating blade stall can be eliminated by the advancing blade concept. Vortices from the previous blade might be reduced by blades with high rigidity, and the compressibility effect might be somewhat reduced by the combining of ABC, blade rigidity, a high lift airfoil and a slightly slower RRPM.

It also appears that primary source of vibration, the in-plane and out-of-plane moments, might also be reduced by the application of ABC and rotor rigidity. In fact, I have the audacity to think that the intermeshing configuration might be better than the coaxial. The intermeshing rotors have their centers of rotation laterally offset from the centerline of the craft and this means that the 75%R position on the advancing blades is closer to the craft's longitudinal centerline.

No doubt, it's a long trip from the scratch pad to the helipad, but on some trips, the journey is half the fun; ~ and theorizing sure is fun.

If you find fault with any of the above, please criticize: tenderly. <img src="smile.gif" border="0">

[ 15 January 2002: Message edited by: Dave Jackson ]</p>

[nucleus33]

Nick, I think that I'd rather be Liv Tyler's boyfriend than God too!

Hans Conser <img src="eek.gif" border="0">

[Hovering in the Wings]

To: Nickii,

Thanks for the NASA site, it does indeed answer some Q's but not I think, as clearly cut and dried as some people may think.

Some Q's for you if you don't mind...

1. Is the theory you have described:

"The N per rev vibration is a natural effect of the blade's motion around the head, and is intrinsic, as is the 1 per rev resonance of the blade in flapping. The rotor delivers a "root shear" to the head that sums at the head to the n per rev vibration in the static system.",

that of Sikorsky's (I assume Jerry Abbey's) or your own??

2. Can you elaborate further your explaination of a 1 per being a function of the blade flapping?? I was always under the impression that a 1 per was a function of an out-of-blance condition.

3. By "root Shear" (luv that term), I assume you mean a transmission of energy from the blade to the mast (via the root obviously)???

4. Is this "shearing" a vertical or a lateral vibration??

5. In your experience (or Sikorskys view) what flight regime are n pers most noticeable???

6. In your ref:
"In the rotating system, this vibration has two components, at N+1 and N-1 per rev. In other words, an absorber attached to a 4 bladed rotor head will see 3 per rev and 5 per rev vibrations, and the transmission will see the sum of these two as a 4 per rev."

7. Finally, what CAUSES the vertical component and lateral component of the N per rev vibe (not N+1, N-1 vibe)??
Are you referring to the Bifilar as the absorber of in-plane N+1 & N-1 per rev vibes - I assume so - not the n per rev absorbers typically fitted to the airframe??

For Wunper,

I agree with your observations, but now some Q's if you don't mind,

1. At what flight regime/s do you smooth the n pers in the Lynx??

2. Therefore, at what flight regime are N pers most noticeable i.e. when are your aiframe absorbers working the most??

All for now....more to follow ....over.

Bulls**t Baffles brains

[Dave Jackson]

Edited to move 'off-topic' post from here to appropriate thread
______________

Hovering in the Wings

To answer your questions;
<ol type="1"> They've been good. But don't tell anyone, 'cause everybody else may then stop posting. My understanding is improving, but your second post does raise a few interesting questions
[/list=a]

[ 15 January 2002: Message edited by: Dave Jackson ]</p>

[Nick Lappos]

Hovering in the Wings asks (denoted by &gt;&gt <img src="wink.gif" border="0"> :
&gt;&gt;1. Is the theory you have described:......
that of Sikorsky's (I assume Jerry Abbey's) or your own??

Nick sez: It is what Dynamicists (like Jerry!) tell me and is the current accepted theory within the industry.

&gt;&gt;2. Can you elaborate further your explaination of a 1 per being a function of the blade flapping?? I was always under the impression that a 1 per was a function of an out-of-blance condition.

Nick sez: I was referring to the blades flapping motion as an intrinsic 1/rev resonance. This is not a reference to any vibration, but to the natural flapping motions of the blade. Similarly, the n/rev vibrations are intrinsic outputs of the dynamics of the rotor, and are not the result of something broken that must be fixed or adjusted.

&gt;&gt;3. By "root Shear" (luv that term), I assume you mean a transmission of energy from the blade to the mast (via the root obviously)???
&gt;&gt;4. Is this "shearing" a vertical or a lateral vibration??

Nick says: exactly so, if you picture the blade as a kind of mechanical shaker attached to the hub arm. The shear forces are in both lateral (really lead-lag) and vertical directions.

&gt;&gt;5. In your experience (or Sikorskys view) what flight regime are n pers most noticeable???

Nick sez: Mostly in transition and at high speed.

&gt;&gt;7. Finally, what CAUSES the vertical component and lateral component of the N per rev vibe (not N+1, N-1 vibe)??
Are you referring to the Bifilar as the absorber of in-plane N+1 & N-1 per rev vibes - I assume so - not the n per rev absorbers typically fitted to the airframe??

Nick sez: The blade produces varying forces in all axies as it swings about the mast, due to the assymetrical conditions of upwind and downwind flight, the natural dynamic vibrations of the blade as it leads and flaps and also due to disturbances from other blades. The n/rev we feel is the sum of the forces from all the blades as felt in the stationary system.

The root shears passed to the head are in all directions, and produce responses in the airframe that can actually amplify (if the airframe has resonances at those frequencies). A absorber can be very effective even if not aligned with the root shear. Picture the rotor hub arm being shaken by a blade in the vertical direction. If a bifilar is on that rotor head, it will feel the resulting motion of the head as both a vertical and a lateral (sort of a rolling motion), and so will be effective even so. The bifilar is tuned to either n-2 per rev or n+1 per rev, usually n-1 since this lower frequency is more disturbing to the airframe. We have flown double bifilars to absorb both frequencies.

For Dave Jackson, who said: ....I have the audacity to think that the intermeshing configuration might be better than the coaxial. The intermeshing rotors have their centers of rotation laterally offset from the centerline of the craft and this means that the 75%R position on the advancing blades is closer to the craft's longitudinal centerline.

Dave, the intermeshing could be worse, because the blades directly fly in each other's wakes. Let me know how it works, OK?

[ 15 January 2002: Message edited by: Nick Lappos ]</p>

[Dave Jackson]

Nick

&gt;".. the intermeshing could be worse, because the blades directly fly in each other's wakes"&lt;

You have certainly hit on one of the primary concerns. There seems to be very little information available on this subject, for twin rotor helicopters. I have a 146 page Prewitt report on the intermeshing Flettner FL-282, which is mainly devoted to plots of the vibration under various flight conditions. Only one of the motions appears to exceed the pilots comfort level.

Would you be willing to give a quick comment or two on the rotor-rotor induced vibrations that were experienced in the S-69 (XH-59) ABC?


&gt;"Let me know how it works, OK? "&lt;

Hopefully, we live that long. <img src="smile.gif" border="0">

[Wunper]

Hovering in the wings

1
We didn’t directly smooth the Npers at any flight regimes in the Lynx I was involved with , we “controlled” their levels through regular vibration surveys and maintenance actions some of which I listed in my posting. One other significant check that I omitted were the main rotor flying control actuator linkages .

We would always attempt to get the ride quality best at cruise conditions 120~140kts but that effort was directed solely at 1R through blade tracking { pitch link and tab only we had no balance adjustables as all blades delivered from the blade facility were deemed “compatible” <img src="wink.gif" border="0"> }

We did not have absorbers in any shape of form fitted then as they carried a weight penalty which any naval aviator would rather trade fuel for!
( I believe later marks with some equipment fits may have a F/A absorber fitted now) .

The Army had rotorhead mounted spring absorbers to make life easier for the TOW sighting system (I think), as Nick said they are only effective for a narrow tuneable band of Nr, their effectiveness on the Lynx was limited as the engine control system was then hydro-mechanical governors with a static droop range of 107 to 102.5% Nr and their power on Nr was quite lively within that range so the absorber was only working effectively in stable regimes of flight when the crew had selected the Nr for which the absorber had been tuned. A Fadec equipped machine would probably control the Nr significantly better and thereby keep it in step with the Absorbers tuned frequency band.

2
On the metal bladed Lynx (I can only speak from experience on these) once clear of the transition in forward flight the Lateral and vertical Nper rev figures rose almost linearly with increasing airspeed the F/A levels rose almost exponentially however. The characteristics may well be different with the composite bladed machines with the cropped stabilisers and elastomeric Engine Gimbal bearings.

The worst steady regime of flight that I remember for vibration was doing Para-drop type max rate climbs at about 60kt but I think that was the (original shape) stabiliser taking a beating and as I never had any kit fitted for those flights could not say for sure whether it was Nper 1R or some cocktail in and around those frequencies.


Wunper {8 <img src="smile.gif" border="0">
<img src="cool.gif" border="0"> :)

[Dave Jackson]

Hovering in the Wings & Wunper: Thanks for the in-depth questions and answers.

There is more to be uncovered about the sources of rotor induced vibration, but to deviate slightly, for the moment;

At appears that the aerodynamic sources of the N-per-rev vibration, which are summed at the hub, are only one half of the problem. The other half is the dynamic resonance(s) that amplifies this vibration. If it is assumed, for this argument, that the sources of this vibration cannot be reduced, then it would appear that a reduction of the resonance is preferable over that of offsetting the vibration.

Much is done to assure that the components within the fuselage are 'out of tune' with the vibration. The blade is the primary source of aerodynamic vibrations, so what is done to assure that the blade does not resonate?

In this regard, I would think that a highly rigid blade would be advantageous since its frequency will then be far above those of the sources of the vibration. Lu previously mentioned " ... undamped sinusoidal wave that travels down the blade from the tip to the root. ". A rigid blade should result in a local disturbance at one blade element being greatly dampened by the other elements of the blade, before it get to the hub.

[ 15 January 2002: Message edited by: Dave Jackson ]</p>

[Hovering in the Wings]

To Helieng/Nikkii

To shed some light on the mystical Bifilar, (and to elaborate on its actual purpose as opposed to its principle of operation as Nickii has down previously), the following is a typed quote from a Sikorsky US Army Users Conference of UH60 BlackHawk Main Rotor 4/rev Vibration, held at Corpus Chriti dated March 21-24 1995.(Jump in here any time Nickii if the Sikorsky theories have changed over the last 6 years):

ROTORHEAD-MOUNTED BIFILAR PENDULUM 3/REV ABSORBER.

1. Designed to reduce rotor blade 3/REV INPLANE ROTATING SYSTEM FORCES (these words were underlined - must be important??)

2. Bifilar has NO impact on blade 5/rev inplane OR blade 4/rev vertical components

3. Designed to be self-tuning (via pendulaum geometry) to follow rotor speed changes

4. The four pendulums swing to counter-act rotor system 3/rev blade forces.

5. Minimal Maintenance Required. (Lubrication of the bushings is for corrossion control only; too much lubricant may Actually Degrade bifilar operation).

The conference notes go on to further descibe the use of airframe absorbers in the battle against 4/rev vibes...

Food for thought.......

For Nickii,


Ref your response to my previous Q2, I thought that the flapping motion of the blade is solely dependant upon the source of excitation e.g forward airspeed to induce flapping to equality, or coning angle versus Relative Airflow or indeed any of the suggested sources of excitment suggested by the NASA paper by Stephen Jacklin.

Either way, irrigardlesss of method of excitation, I find it dificult to agree with your statement that the resonant flapping freq of a blade is a 1 per rev since the blade does many variations on the "classic" flapping action per rev.

This is witnessed by the famous hub video taken of a rotor blade in flight which I'm sure everyone flying helicopters has seen. (Nikki, does Sikorsky have any similar footage of in-flight blade flapping tests by any chance and if so is there any way we can get copies of it???) It can actually be seen the changes in the flapping action as the blade sees a number of sources of exitations as it conducts 1 revolution.
I can agree that the blade will describe one sinusoidal evolution in 1 rev as a reaction to forward airspeed (ie flapping to equality).

I cannot agree that ALL flapping has a resonant frq of 1 per rev. This is evidenced by the hub mounted video footage of the helicopter blade flapping in flight. I saw this some years ago on a BBC doco called the "Chopper Story" - most enlightening and thoroughly recommend it.
I agree that the blade has a natural resonant freb, but I do not agree that all flapping has a resonant freq of 1 per rev - it must surely be dependant upon the source and type of excitement which instigates the particualr flapping the blade is performing - watch the video...

In fact I think Prouty suggests a natural resonant freq of the average blade (I would assume this is dependant upon the aeroelasticity of design/manufacture...pertinent to DAve's last post ref blade rigidity??), being "just under three times per revolution".

Ref response Q3, May I suggest another example to you to which may possibly better demonstrate/illustrate the point you are trying to describe.

Imagine you have a piece of rope with a ring attached at one end. You place that ring over
a stake placed in the ground. I now raise the rope (i.e. a blade has reached its maximum
vertical flapping displacement- over the nose of the aircraft if we are using the flapping to equality as the source of excitment of the rotor blade in this case.)

Now you bring the rope down rapidly (representative of the blade passing the nose of the aircraft in forward flight and beginnning its downward flapping motion).

How does this anology sound so far??

Question: What happens to the ring over the stake?? Does it not move up the stake (should we say mast??).

Now further to this....lets fix the ring to the stake so that it cannot move vertically up the stake.

Repeat the same exercise.

Question: What happens to the stake?? Does it not attemp to move vertically up and ALSO sideways (laterally??) as a reaction to receiving
this "flapping" energy?? Ultimately, our stake will actually pull itself out of the ground if this excercise is repeated often enough. Have we not all seen this phenomena in our garden hoses and we have used this same "flapping" energy to release the hose from around some obstacle (maybe its just me in my obstacle strewn garden???).

Surely then the flapping motion of the blade in response to flapping to equality MUST impart this energy BOTH vertical and Lateral every time a blade passes over the nose (and the tail) whenever the aircraft is in forward flight. This energy or vibration increasing in magnitude with airspeed since the flapping action of flapping to
equality is greatly increased with increasing IAS (exponentially I would think - or close to it - you may be able to help me there - may be dependant to a degree on the aeroelasticity of the particular blade??)


Ref response to Q5, I agree with you with higher IAS, and I can certainly say that I have been subjected to the "Sikorsky shuffles" passing translational, but I am not convinced that the "Shuffles" are really 4/revs but as Wunper suggests - a possible cocktail??. Are they 4 per rev vibes or are they a cocktail mix of a 3 or 5 per revs prevailling in this flight regime??? Do you have access to a vibration spectrum of the "Shuffles" at Sikorsky and could you confirm from the vibe print out in that translational flight regime, the make up in magnitude and Nper, of the vibe cocktail for us??

If not, can you do me a favour??

I no longer have access to a BlackHawk in which to do a little experiment. Could you, the next time you do a balance excercise on an aircraft at Conneticut, hold a decelerative flare or hold the machine in translational (in the "shuffles") long enough to get a vibe spectrum for us. I would be very interested to see what freq are manifesting themselves in this flight regime.

Perhaps Wunper could do the same if he has access to a Lynx - it would be interesting to see the composition (if possible) of the vibration
spectrum in relation to n per.

Also Nikkii, have you ever observed the cabin absorbers thru this translational flight regime???? Are they working at their max (as they should be if experiencing 4 per revs eg @
140 kts). I have not I must admit but would love to know the answer - perhaps you could help us with that Nikkii, next time you fly a Hawk - let us know on this post or email me???


Ref response to Q7, I have enclosed above an extract from a Users conference written by Jerry Abbey. It seems to indicate that the Bifilar has absolutely NO effect on 4/rev vertical vibration. With my limited experience, I would have to agree with him. There is stuff all vertical movement in the Bifilar in which to accomodate absorbtion of the vertical component. Only enough to allow allignment to small angular change in plane of the lateral vibration. Its effect on any vertical component would be minimal - if any at all.

Your thoughts much appreciated.....

OOOOhhh....almost forgot...

A final Q Nikkii while I think of it, do you beieve that individual blade downwash interaction with the fuselage casues n per rev vibes???

CUL8r
<img src="smile.gif" border="0"> <img src="wink.gif" border="0">

[Nick Lappos]

I must say, Hovering in the Wings, you like to write!

Who is this Nickii that you direct your verbose questions to?

[Lu Zuckerman]

To: Hovering in the wings

The movie that you alluded to (there may be several) was made by attaching a 16 mm Camera to the head of an S-51 which was mounted on a special whirl stand that allowed the introduction of both cyclic and collective inputs. This movie had a major impact in two areas. When many helicopter pilots saw the movie they elected to never fly helicopters again and even more important it caused a major modification in the thinking of Sikorsky engineers. Up until that time they had believed that due to profile drag and relative wind speeds the blades lagged on the advancing side and led on the retreating side but the movie proved the leading and lagging to be opposite of conventional thinking at that time.

As I had mentioned in a previous post the blade was moving in a sinusoidal wave that traveled from the tip to the root much like the tethered string example that you had made in your post.

[Hovering in the Wings]

Dave,

I have tried for many years to get a straight answer from both Helicopter nad Vibe analysis manufacturers into the CAUSE of n per revs. Never received a straight answer yet.

My best answer has come from reading "Practical Helicopter Aerodynamics" by R.W. Prouty. Chap 4 discusses blade rigidity and the N per rev problem. Other chapters cover vibrations and design. It provides answers for N per Rev with IAS. I think the paper by Stephen Jacklin provides some answers for the harmonics of N per in flight regimes at lower IAS.

If you haven't already read it, would recommend it.

Hope it helps in your Quest....

Back to Hovering :)

[Nick Lappos]

Hovering in the wings,

I feel bad that you feel that you "Never received a straight answer yet" regarding Dave's question about n/rev vibration. Where I come from, a straight answer is one that is true, so I infer from your gripe that you feel nobody has told you the truth yet.

I think you have had two folks ernestly try to describe what occurs when a small number of blades attempt to average out the lift of the machine, while each individually vibrating, circling up-wind near Mach 1 and down-wind near stall. If I can find a better reference for the issue, I will post it. Until then, I suggest that you might look inward when seeking to blame someone for your inability to grasp technical subjects that are beyond your grasp.

[Dave Jackson]

Hovering in the Wings

Thanks for the leads. Unfortunately, Prouty's "Practical Helicopter Aerodynamic" is out of print. I will look into the paper by Stephen Jacklin.


The first couple of paragraphs of Nick's posting, combined with other information that he has previously provided, go a long way toward describing some of the sources of nP vibration.

It appears that the primary source is due to the asymmetrical airflow during forward flight and this might (nothing is impossible <img src="eek.gif" border="0"> ) be an intrinsic feature. The angle of attack of the blades' elements will be different at different locations on the disk. Increasing a blade's angle of attack causes the coefficient of lift and the coefficient of drag to increase as well. Since any increases in these two coefficients are not quite equal, both the lift and the drag can not be made equal on the advancing side and the retreating side. If the lift is equalized between the advancing and retreating sides, then the difference in drag will result in a nP vibration.

The web page link, which Nick gave, lists some secondary sources such as retreating blade stall, advancing tip compression and vortices from previous blades. I suspect that these are temporal and can be eliminated or greatly minimized.

The fuselage, stabilizers, and the tail rotor (for those who must put up with such a handicap <img src="smile.gif" border="0"> ) are, no doubt, additional sources.

Perhaps an electronic, or a mechanical means beyond the swashplate, will or are advancing the control of the rotor above 1P.
________

The above may have many errors and is certainly incomplete. Corrections and expansion, please.

[ 17 January 2002: Message edited by: Dave Jackson ]</p>

[helmet fire]

Thanks for the great topic Dave, and the user friendly explanation Nick (or is it "Nickii" <img src="wink.gif" border="0"> ?). Always wondered about the "Sikorsky shuffle". I thought it was to do with main rotor vorticey interacting with the tail rotor at that speed rather than the extra Nper vibration that most aircraft feel going through translational. The UH-60 certainly got that extra Nper feel, but it "shuffled" in yaw as well - or is my memory suspect?

Hovering in the wings: You have used my favourite word: irregardless. You said: "Either way, irrigardlesss of method of excitation".

Irrigardless. Does that mean:
"without regard to"
"without without regard to"
"without regardless to", or
"with regard to" (due to resolving the double negative)?

Sorry, at my grammatical level I cannot point ANY fingers, but I couldn't resist my favourite word. :)

Lu: I saw the Chopper film too, and it was mounted on a UH-1H in flight. No chopper pilot I know quit flying because of it, but it did give us a great story to impress the bar fraus with!! :) :)

[Hovering in the Wings]

Dave,

Sorry to hear Prouty's book is out of publication. If you are interested, email me a fax number and I'll fax a couple of relevant pages to you which I think may help shed light....


Nick,

It would be great if we could someday get together and discuss vibe issues in a bar (with a whiteboard) over a few beers where all the problems of the world are solved <img src="wink.gif" border="0"> . Until then I guess I will have to learn to live with my "inability to grasp technical subjects that are beyond (my) grasp"....

Been fun,

Back to Hovering

[Nick Lappos]

Hovering in...

I'll take you up on that beer, I think n/rev becomes crystal clear by about the 4th, as does my grasp of particle physics....

Dave,
try searching for Ray Prouty's book at the used book web site:

<a href="http://www.abe.com" target="_blank">http://www.abe.com</a>

I also recommend Stepniewski and Key "Rotary Wing Aerodynamics" Dover press.

[Dave Jackson]

Nick

Thanks for mentioning the used book site. Will look into it, and also look deeper into 'Rotary Wing Aerodynamics'.

Coincidentally, I ordered Prouty's new book a couple of weeks ago. Its called 'Military Helicopter Design'. As you know, he has a very clear way of writing so it should make for interesting reading.

[ 18 January 2002: Message edited by: Dave Jackson ]

Correction to previous post.
The book is not new. It was originally published in 1989, with corrections in 1998 reprint, but it looks very interesting.

[ 18 January 2002: Message edited by: Dave Jackson ]</p>

[Nick Lappos]

Dave,
The Stepniewski book has a good first section that describes why 1/rev is the flapping resonance frequency (as varied slightly by damping and hinge offset) and also describes the ways that the n/rev root shears are affected by forward flight. Gave me new insight, as did this thread (always does!).

[helmet fire]

Nick, any response to my Q on the "Sikorsky Shuffle" above?
Thanks. Now about that bar............

[Nick Lappos]

helmet fire,
Early H-3's had some "tail shake" due to rotorhead wash going into the tail, solved with a carefully shape main rotor pylon collar (the horse collar). In my first days in the Pilot's Office, I had been told this was the "shuffle" and that it was not intrinsic to any other Sikorskys. I have always thought the "Sikorsky Shuffle" was more of a conveniently aliterative name than an actual condition.

[helmet fire]

I had to look a few of those words up Nick, but thanks for the answer. :)

[Nick Lappos]

helmet fire,
As long as you don't look up the spelling, I'm safe! :)

[Dave Jackson]

The discussion to-date has address the probable sources of nP vibration during high-speed flight, but little has been said about this vibration during translation.

Could this vibration be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?

Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?

[ 19 January 2002: Message edited by: Dave Jackson ]</p>

[Nick Lappos]

Dave Jackson asked (Nick added paragraph letter headings):

A) Could this vibration (vibration during translation) be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?

B) Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?

. .Nick sez:. .A) I believe that translational vibration is mostly due to aerodynamic duisturbence of the blade by the previous blade wake, and is especially a problem on some helos during approach when the descent allows more BVI (blade vortex interference). Ever fly a BO-105? The panel is shock mounted to prevent atomic dissassembly of the gages, I think.

OTOH, I do believe that stabilizer vibration can affect n/rev anywhere in the envelope, as at least one Sikorsky model has n/rev that is strongly influenced by main wake on the tail at mid cruise speed.

B) I don't believe this has much affect, Dave. I have flown many machines for vibration tuning where we adjust the CG (and therefore the steady flapping angle) with no appreciable change in the n/rev. In fact, flapping is highest at forward CG, and for many Sikorsky models, forward CG is the smoothest.

[CTD]

Dave,

The horizontal and vertical stabs will certainly have an effect on n/rev in many aircraft.

On the Bell 412 for example, horizontal stab rigging will affect the 4-per felt in the cabin.

Similarly, if the 4-per on the 407 is noticeable at 60kts, moving very slightly out of trim with right pedal will considerably reduce it.

[Dave Jackson]

Nick

Thanks for mentioning the BVI (blade vortex interference) and the fact that it is particularly noticeable on some helicopters when descending during an approach.

This enhances the concern regarding BVI in helicopters with twin rotors, such as the coaxial configuration, where the lower rotor never gets out from under the vortices of the upper rotor.

Your point B) is also noted.. .________

CTD

Prouty appears to confirm what you are saying. He mentions that the "Designers at Bell generally placed horizontal stabilizers forward on the tail boom so that they are in the wake during hover and never have an awkward transition going from outside the wake to inside it."

[Wunper]

Dave

There is a very good round up on this whole topic in the latest edition of AHS Vertiflite Vol 47 No.4 page 40 titled Dynamics 2000-2001 by Cynthia Callahan of Boeing. None of the topics are in any great depth but it could point you to some useful contacts as it gives a good idea as to who is researching what in the fields of Individual Blade Control /Active Twist Rotors / Smart airframe response etc.

If you don’t have a copy AHS will ask you $20US for one tel (703) 684 6777 or email [email protected]

Happy hunting

Wunper <img src="wink.gif" border="0"> . . <img src="cool.gif" border="0">

[Dave Jackson]

Wunper; ~ Will read the article. Thanks