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
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