Westland Lynx (Merged threads)
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Siouxsie,
Thanks for your reaction! My question is not ment to doubt the FRC's and moreover the theory behind to which I for sure agree. In our (RNLN)Lynx fleet the cyclic markings were not always used as strictly as promulgated nowadays. Moreover the MPOG markings and the use of them were introduced in 1991. Therefore I'm still interested in the applied procedure of past and present Lynx pilot's.
Thanks for your reaction! My question is not ment to doubt the FRC's and moreover the theory behind to which I for sure agree. In our (RNLN)Lynx fleet the cyclic markings were not always used as strictly as promulgated nowadays. Moreover the MPOG markings and the use of them were introduced in 1991. Therefore I'm still interested in the applied procedure of past and present Lynx pilot's.
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Big Fish,
Sorry if I jumped down your throat. Over-reaction!
We started with the markings at about the same time as you. With us they are rigidly observed.
I don't know, but I heard that the RNLN Lynx ground-running accident last year was possibly attributable to the cyclic markings not being used????????Does anyone know for sure.
Till anyone comes up with a better idea, I stand by my original comments regarding the advisability of using the marks.
Sorry if I jumped down your throat. Over-reaction!
We started with the markings at about the same time as you. With us they are rigidly observed.
I don't know, but I heard that the RNLN Lynx ground-running accident last year was possibly attributable to the cyclic markings not being used????????Does anyone know for sure.
Till anyone comes up with a better idea, I stand by my original comments regarding the advisability of using the marks.
Guest
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Must admit I do make sure to set up the Min Pitch marks, but have noticed that having got the rotors up to 107% the aircraft requires to trim back to the left to sit level (via the AI). The aircraft is between the skid stops(Mk 7) and therefore not under stress.
Is this correct or is the MRH under stress?
Just something I noticed when the aircraft needs to nearly rock to R/H skid stop to line up marks prior to entering sub min pitch.
Is this correct or is the MRH under stress?
Just something I noticed when the aircraft needs to nearly rock to R/H skid stop to line up marks prior to entering sub min pitch.
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Gem.
The damage is done to the aircraft mainly in the fore and aft axis,when the majority of bending can take place. As you said in your post, as long as the lateral diplacement is confined to between the left and right stops no bending can take place, but, make sure you maintain the fore and aft position when rolling the A/C parallel to the slope.
The really critical time that damage can be done is when running in sub-min pitch.
Similar damage can be inflicted by using too much in slope cyclic when nose-up slope landings/takeoffs are made.
The damage is done to the aircraft mainly in the fore and aft axis,when the majority of bending can take place. As you said in your post, as long as the lateral diplacement is confined to between the left and right stops no bending can take place, but, make sure you maintain the fore and aft position when rolling the A/C parallel to the slope.
The really critical time that damage can be done is when running in sub-min pitch.
Similar damage can be inflicted by using too much in slope cyclic when nose-up slope landings/takeoffs are made.
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YEAH hehehe but how many off us can say we do it religiously at night in a hurry on goggles when starting up, not this kid, and I can quote sitting in the back of many watching the crew at night, nope no line ups on the floor done there. Do the navy guys do it on a rolling boat even when strapped down?
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Yeah, OK Buffy you're dead 'ard, real warry stuff. 'Cos you're goin to war.No time for checks or any of that namby pamby New labour tree hugging' Bullshi*.
Don't you bother to set the a/c up correctly, all your mates will be eternally grateful for the extra unnecessary fatigue life you impose upon the aircraft, 'cos you're so warry.
My hero.. Not.
Don't you bother to set the a/c up correctly, all your mates will be eternally grateful for the extra unnecessary fatigue life you impose upon the aircraft, 'cos you're so warry.
My hero.. Not.
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Siouxie,
Eat me, who said anything about going to war or being hard you ****,I said that I can put my hand up to not having done it all the time everytime, and have witnessed others not doing it, sorry your Mr Perferct of course and do everything (including pulling your pud) by the numbers don't you. Of course I/we don't stress it on purpose , or perhaps your one of those who have never been anywhere and had to do it in a hurry, hangar room pilot then. Get another job.
Eat me, who said anything about going to war or being hard you ****,I said that I can put my hand up to not having done it all the time everytime, and have witnessed others not doing it, sorry your Mr Perferct of course and do everything (including pulling your pud) by the numbers don't you. Of course I/we don't stress it on purpose , or perhaps your one of those who have never been anywhere and had to do it in a hurry, hangar room pilot then. Get another job.
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Buffer
Eat you, I don't think so.
But a very erudite reply nevertheless.
Hangar room pilot, woz' at!!
Yes you're(note spelling)quite correct, been nowhere done nothing.
Well I haven't been to the 'K' place. Otherwise been there done that. No T shirts though, they just get the boys toooo excited!
You sound like a bigger tart than me.
With people like you about I think I'll just get another job as you recommended.
Kiss, Kiss.
Eat you, I don't think so.
But a very erudite reply nevertheless.
Hangar room pilot, woz' at!!
Yes you're(note spelling)quite correct, been nowhere done nothing.
Well I haven't been to the 'K' place. Otherwise been there done that. No T shirts though, they just get the boys toooo excited!
You sound like a bigger tart than me.
With people like you about I think I'll just get another job as you recommended.
Kiss, Kiss.
Guest
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Phase Lag in Rigid Heads (Lynx)
Re: Phase angle etc. Let me explain about the Lynx.
The rotor head is rigid in the flapping axis and dragging axis, but not the feathering axis; hence it is often called a semi-rigid head. For all intents and purposes we can treat it as rigid with respect to the aerodynamics of flapping.
When a cyclic pitch change is input by the pilot, the blades flap with respect to the control axis. The displacement of the blades is creates a bending moment on the rigid head, which in turn creates a restoring force that is a function of the displacement. This is a key point, as the restoring force is 180 deg out of phase with the DISPLACEMENT. The result of this restoring force is that the control input must be made later, i.e. phase lag is reduced below 90 degrees. See http://www.av8.org.uk/phaselag.htm for a graphical proof.
This brings a problem. While the restoring force is constant for a given displacement, the aerodynamic forces resulting from cyclic displacement will vary with density altitude, in effect the system phase lag will vary with density altitude. This is obviously a problem for the designer, who must settle on an average phase lag when choosing the rotor system rigging angle, hence any difference between the actual density altitude and the design altitude will induce a control cross couple.
Next up come all up mass. As the AUM changes so the total power required changes and thus the coning angle changes. This will affect the restoring force and thus in this case it follows that AUM will have an effect on phase lag.
Next up, blades. If you change the blade characteristics then the aerodynamic performance of the blades will change. This will change the relationship between the bending moment and aerodynamic moments. Once again, therefore, a change in blade performance will change the phase lag.
Now, take the lynx, first envisaged will an all up mass of around 4200Kg, now cleared to over 5200kg, that’s nearly a 20% change in max AUM, imagine the effect on phase lag. Next add new BERB tipped blades with different aerodynamic characteristics, imagine what happens to phase lag.
So, the Lynx was correctly rigged to operate with a less than 90 degree phase angle. Changes in the max AUM, blades and differences from the design density altitude will all induce a small cross couple. It is not a problem ASE in or out.
Next up, acceleration cross-couple. Despite the above, in a dynamic control environment, i.e. after a control input but before the system has found equilibrium the aerodynamic moments are greater than the bending moments, for all sorts of reasons, one being because of the effects of airframe inertial forces fed back to the head. In short, if a HIGH rate demand is made then the acceleration and inertial forces will be out of phase, hence a control cross-couple can be induced (imagine that in a rapid accelerative state the head reverts to a phase lag of close to 90 degrees instead of 72ish). This means that, because of the rigging angle is designed for normal control input rates, if the pilot makes rapid, large cyclic pitch inputs a large couple can be induced. For example in a wingover left a quick forward check on the cyclic can induce a rapid roll left. This is not a problem because this is not a normal flight regime and can be easily avoided by lower RATE control inputs. Lynx pilots are taught to apply gentle control inputs in normal operations, as long as you move the stick gently you can demand rapid manoeuvres with large cyclic displacements without encountering the effects of acceleration cross-couples.
GA
The rotor head is rigid in the flapping axis and dragging axis, but not the feathering axis; hence it is often called a semi-rigid head. For all intents and purposes we can treat it as rigid with respect to the aerodynamics of flapping.
When a cyclic pitch change is input by the pilot, the blades flap with respect to the control axis. The displacement of the blades is creates a bending moment on the rigid head, which in turn creates a restoring force that is a function of the displacement. This is a key point, as the restoring force is 180 deg out of phase with the DISPLACEMENT. The result of this restoring force is that the control input must be made later, i.e. phase lag is reduced below 90 degrees. See http://www.av8.org.uk/phaselag.htm for a graphical proof.
This brings a problem. While the restoring force is constant for a given displacement, the aerodynamic forces resulting from cyclic displacement will vary with density altitude, in effect the system phase lag will vary with density altitude. This is obviously a problem for the designer, who must settle on an average phase lag when choosing the rotor system rigging angle, hence any difference between the actual density altitude and the design altitude will induce a control cross couple.
Next up come all up mass. As the AUM changes so the total power required changes and thus the coning angle changes. This will affect the restoring force and thus in this case it follows that AUM will have an effect on phase lag.
Next up, blades. If you change the blade characteristics then the aerodynamic performance of the blades will change. This will change the relationship between the bending moment and aerodynamic moments. Once again, therefore, a change in blade performance will change the phase lag.
Now, take the lynx, first envisaged will an all up mass of around 4200Kg, now cleared to over 5200kg, that’s nearly a 20% change in max AUM, imagine the effect on phase lag. Next add new BERB tipped blades with different aerodynamic characteristics, imagine what happens to phase lag.
So, the Lynx was correctly rigged to operate with a less than 90 degree phase angle. Changes in the max AUM, blades and differences from the design density altitude will all induce a small cross couple. It is not a problem ASE in or out.
Next up, acceleration cross-couple. Despite the above, in a dynamic control environment, i.e. after a control input but before the system has found equilibrium the aerodynamic moments are greater than the bending moments, for all sorts of reasons, one being because of the effects of airframe inertial forces fed back to the head. In short, if a HIGH rate demand is made then the acceleration and inertial forces will be out of phase, hence a control cross-couple can be induced (imagine that in a rapid accelerative state the head reverts to a phase lag of close to 90 degrees instead of 72ish). This means that, because of the rigging angle is designed for normal control input rates, if the pilot makes rapid, large cyclic pitch inputs a large couple can be induced. For example in a wingover left a quick forward check on the cyclic can induce a rapid roll left. This is not a problem because this is not a normal flight regime and can be easily avoided by lower RATE control inputs. Lynx pilots are taught to apply gentle control inputs in normal operations, as long as you move the stick gently you can demand rapid manoeuvres with large cyclic displacements without encountering the effects of acceleration cross-couples.
GA
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To: Grey Area
So that I can fully understand the problem can you please provide the following information.
When the pilot pushes forward cyclic which way does the swash plate tip?
What is the lead angle of the pitch horn in relation to the blade?
Here are a few examples so that you understand the questions.
On Bell single rotor helicopters the swash plate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 90-degrees. On other Bell single rotor helicopters the swashplate tips down towards the tail and the pitch horn trails the blade by 90-degrees.
On the Robinson the swashplate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 72-degrees.
On a Sikorsky helicopter the swashplate tips down 45-degrees ahead of the longitudinal axis and the pitch horn leads the blade by 45-degrees.
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The Cat
So that I can fully understand the problem can you please provide the following information.
When the pilot pushes forward cyclic which way does the swash plate tip?
What is the lead angle of the pitch horn in relation to the blade?
Here are a few examples so that you understand the questions.
On Bell single rotor helicopters the swash plate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 90-degrees. On other Bell single rotor helicopters the swashplate tips down towards the tail and the pitch horn trails the blade by 90-degrees.
On the Robinson the swashplate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 72-degrees.
On a Sikorsky helicopter the swashplate tips down 45-degrees ahead of the longitudinal axis and the pitch horn leads the blade by 45-degrees.
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The Cat
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Thanks Grey Area for opening this topic.
To Lu,
I wish I could send you a copy of a Lynx ACM. Swash plates in a Lynx?
The Lynx is equiped with a so called spider arm. It is connected to all the pp-rods and runs through the rotorhead to an area below the MRGB. Here all three control-servos are connected to the spiderarm. By either raising or lowering the spider arm a collective input is made. By moving the spider arm left-right or forward-aft (around 70 degrees offset) a cyclic input is made.
Simple eh?
All the problems started in my opinion when the composite rotorblades were introduced. There a gross handling difference between the old and new blades. Most probably caused by less flexibility in the new blades. This together with higher RRPMs and less coning angles changed the steering caracteristics a lot. Most noticable with ASE off. There we can agree. In no way, by my own experience, introduced this dangerous characteristics. It's just more unstabel to fly with the new blades.
The worst thing is the new blades introduced a massive rise in vibration levels. Also the new blades are very very sensitive to eg. salt or dirt, this makes the helicopter shake in as never experienced before.
[This message has been edited by have another coffee (edited 10 March 2001).]
To Lu,
I wish I could send you a copy of a Lynx ACM. Swash plates in a Lynx?
The Lynx is equiped with a so called spider arm. It is connected to all the pp-rods and runs through the rotorhead to an area below the MRGB. Here all three control-servos are connected to the spiderarm. By either raising or lowering the spider arm a collective input is made. By moving the spider arm left-right or forward-aft (around 70 degrees offset) a cyclic input is made.
Simple eh?
All the problems started in my opinion when the composite rotorblades were introduced. There a gross handling difference between the old and new blades. Most probably caused by less flexibility in the new blades. This together with higher RRPMs and less coning angles changed the steering caracteristics a lot. Most noticable with ASE off. There we can agree. In no way, by my own experience, introduced this dangerous characteristics. It's just more unstabel to fly with the new blades.
The worst thing is the new blades introduced a massive rise in vibration levels. Also the new blades are very very sensitive to eg. salt or dirt, this makes the helicopter shake in as never experienced before.
[This message has been edited by have another coffee (edited 10 March 2001).]
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The attached site has some wonderful closeup pictures of rotorheads. Three of them are of the Westland Lynx, which this thread is discussing
http://www.b-domke.de/AviationImages/Rotorhead.html
"On Bell single rotor helicopters the swash plate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 90-degrees. On other Bell single rotor helicopters the swashplate tips down towards the tail and the pitch horn trails the blade by 90-degrees."
Lu; correct me if I'm wrong, but would it not be correct to say that the swashplate tips down over the nose in both the above examples.
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Project: UniCopter.com
http://www.b-domke.de/AviationImages/Rotorhead.html
"On Bell single rotor helicopters the swash plate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 90-degrees. On other Bell single rotor helicopters the swashplate tips down towards the tail and the pitch horn trails the blade by 90-degrees."
Lu; correct me if I'm wrong, but would it not be correct to say that the swashplate tips down over the nose in both the above examples.
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Project: UniCopter.com
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To: Dave Jackson
"On Bell single rotor helicopters the swash plate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 90-degrees. On other Bell single rotor helicopters the swashplate tips down towards the tail and the pitch horn trails the blade by 90-degrees."
Lu; correct me if I'm wrong, but would it not be correct to say that the swashplate tips down over the nose in both the above examples.
First of all thanks for the info on rotorheads the pictures are fantastic but it only shows the head and not the swashplate.
Regarding the tipping of the swashplates on different Bell helicopters, it has been a long time since I was around them but I believe the AH-1 series works the way I described in that the swashplate tips down towards the tail. These helicopters are weapons platforms and as such have controllable horizontal stabilizers to keep the nose from tucking. I don’t know for sure but I believe the kinematics of the controls for the tail plane dictated that the swash plate tip down towards the tail as the controls for the stabilizer are linked to the swashplate.
Dave, go to the rotorheads website and click on the rotorhead for the Super Cobra. The pitch horn is on the rear of the blade.
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The Cat
"On Bell single rotor helicopters the swash plate tips down over the nose in line with the longitudinal axis and the pitch horn leads the blade by 90-degrees. On other Bell single rotor helicopters the swashplate tips down towards the tail and the pitch horn trails the blade by 90-degrees."
Lu; correct me if I'm wrong, but would it not be correct to say that the swashplate tips down over the nose in both the above examples.
First of all thanks for the info on rotorheads the pictures are fantastic but it only shows the head and not the swashplate.
Regarding the tipping of the swashplates on different Bell helicopters, it has been a long time since I was around them but I believe the AH-1 series works the way I described in that the swashplate tips down towards the tail. These helicopters are weapons platforms and as such have controllable horizontal stabilizers to keep the nose from tucking. I don’t know for sure but I believe the kinematics of the controls for the tail plane dictated that the swash plate tip down towards the tail as the controls for the stabilizer are linked to the swashplate.
Dave, go to the rotorheads website and click on the rotorhead for the Super Cobra. The pitch horn is on the rear of the blade.
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The Cat
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Lu,
The info you request is mostly irrelevant. For a start as HAC has pointed out there is no swashplate on a lynx. Second, as the system has 4 blades the well documented problem you are alluding to with the Bell configuration cannot occur.
But, as already stated, the system is rigged to about 70 deg or so.
The Lynx is a weapons platform (and a very successful one), yet there is no link between the rotor head and tail plane. In fact the aircraft is cleared for flight with the horizontal stabiliser removed. Because of its rigid head, the Lynx control power is immense, the moment of the virtual flapping hinge is huge. Iif you get the opportunity to watch the Blue Eagles (UK Army Air Corps) Display Team of recent years, you will see a back flip (360 deg pitch backward) from the hover at 500’ or so. You can’t do that in a cobra!
[This message has been edited by Grey Area (edited 10 March 2001).]
The info you request is mostly irrelevant. For a start as HAC has pointed out there is no swashplate on a lynx. Second, as the system has 4 blades the well documented problem you are alluding to with the Bell configuration cannot occur.
But, as already stated, the system is rigged to about 70 deg or so.
The Lynx is a weapons platform (and a very successful one), yet there is no link between the rotor head and tail plane. In fact the aircraft is cleared for flight with the horizontal stabiliser removed. Because of its rigid head, the Lynx control power is immense, the moment of the virtual flapping hinge is huge. Iif you get the opportunity to watch the Blue Eagles (UK Army Air Corps) Display Team of recent years, you will see a back flip (360 deg pitch backward) from the hover at 500’ or so. You can’t do that in a cobra!
[This message has been edited by Grey Area (edited 10 March 2001).]
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To: Grey Area
With no swashplate as used on conventional helicopters how do you get input from the stationary servos via the push pull tubes to the rotating elements of the dynamic system? Is the so called spider arm in effect a swashplate?
Regarding your diagram, I find it difficult to understand for several reasons.
1) What is the significance of the numerical scale at the left of the diagram?
2) What is the difference between the desired input and the required input?
3) Does the dotted line indicate blade flapping as a result of pitch input?
Regarding the power (interlock) of a rigid rotor helicopter I saw it demonstrated by a Lockheed 286 when I was working on the Cheyenne. In fact on the Cheyenne the cyclic was locked out while the helicopter was on the ground. The interlock was so strong that the helicopter could be tipped over if the cyclic was moved.
On the Super Lynx rotorhead they have lead lag dampers. Did they incorporate the capability to lead and lag to minimize the stresses on the blades and the rotorhead?
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The Cat
[This message has been edited by Lu Zuckerman (edited 11 March 2001).]
With no swashplate as used on conventional helicopters how do you get input from the stationary servos via the push pull tubes to the rotating elements of the dynamic system? Is the so called spider arm in effect a swashplate?
Regarding your diagram, I find it difficult to understand for several reasons.
1) What is the significance of the numerical scale at the left of the diagram?
2) What is the difference between the desired input and the required input?
3) Does the dotted line indicate blade flapping as a result of pitch input?
Regarding the power (interlock) of a rigid rotor helicopter I saw it demonstrated by a Lockheed 286 when I was working on the Cheyenne. In fact on the Cheyenne the cyclic was locked out while the helicopter was on the ground. The interlock was so strong that the helicopter could be tipped over if the cyclic was moved.
On the Super Lynx rotorhead they have lead lag dampers. Did they incorporate the capability to lead and lag to minimize the stresses on the blades and the rotorhead?
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The Cat
[This message has been edited by Lu Zuckerman (edited 11 March 2001).]
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Grey Area,
The Lynx rotor head is considered to be of a semi rigid design, I don't think this is because the only axis of movement is in the pitch axis. If you look at the close up of the picture in Dave Jackson's link you can see a damper between the blade attachment point and the end of the flex arm. The damper runs parallel to a round titanium bar which is aprox 2"-3" in diameter and this bar is designed to flex in both the vertical axis and horizontal axis.
Jiff
The Lynx rotor head is considered to be of a semi rigid design, I don't think this is because the only axis of movement is in the pitch axis. If you look at the close up of the picture in Dave Jackson's link you can see a damper between the blade attachment point and the end of the flex arm. The damper runs parallel to a round titanium bar which is aprox 2"-3" in diameter and this bar is designed to flex in both the vertical axis and horizontal axis.
Jiff
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Lu,
A quick explanation of the Lynx rotor head. At the centre you will note the star shaped rotor hub, this is where the flapping occurs. The long cylindrical structure (commonly referred to in the UK as the “dog bone”) is where lead/lag loads are handled, it does not absorb flapping loads. The dampers you refer to are fitted to all Lynx, except French Navy ones. Their role is simply to reduce stress on the head during rotor engagement./disengagement cycles, it has no role in flight and can be ignored for the purposes of this discussion.
The graph scale is arbitary. The dotted blue line labelled “blade displacement” indicates exactly that. The DESIRED INPUT is the force required to achieve the blade displacement, you will note it is exactly 90 deg in advance of the blade displacement (this fits both my flapping to equality argument and your precession argument). The restorative force causes a problem, as it will modify the forces resulting from cyclic input, therefore the cyclic forces must be applied such that when combined with the restorative force they will equal the DESIRED INPUT. The key point, therefore, is that the REQUIRED INPUT in combination with the RESTORATIVE FORCE must equal the DESIRED INPUT, and thus they must be applied less than 90 degrees in advance of the desired blade response.
GA
[This message has been edited by Grey Area (edited 11 March 2001).]
A quick explanation of the Lynx rotor head. At the centre you will note the star shaped rotor hub, this is where the flapping occurs. The long cylindrical structure (commonly referred to in the UK as the “dog bone”) is where lead/lag loads are handled, it does not absorb flapping loads. The dampers you refer to are fitted to all Lynx, except French Navy ones. Their role is simply to reduce stress on the head during rotor engagement./disengagement cycles, it has no role in flight and can be ignored for the purposes of this discussion.
The graph scale is arbitary. The dotted blue line labelled “blade displacement” indicates exactly that. The DESIRED INPUT is the force required to achieve the blade displacement, you will note it is exactly 90 deg in advance of the blade displacement (this fits both my flapping to equality argument and your precession argument). The restorative force causes a problem, as it will modify the forces resulting from cyclic input, therefore the cyclic forces must be applied such that when combined with the restorative force they will equal the DESIRED INPUT. The key point, therefore, is that the REQUIRED INPUT in combination with the RESTORATIVE FORCE must equal the DESIRED INPUT, and thus they must be applied less than 90 degrees in advance of the desired blade response.
GA
[This message has been edited by Grey Area (edited 11 March 2001).]
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It seems to me that if you change both the mass and flexibity of the rotor blades on the Lynx, you would also have to change the flexibility (to accommodate new displacement forces) and perhaps the resonance frequency of the flex arm to match the new rotor blades.
I would think both the flex arm and the rotor blades have to matched to other in terms of at least the flapping loads, to prevent both unwanted vibrations and unexpected behavior. In other words the flex arms need to be "tuned" to the rotor blades, or funny behavior will happen (which seems to be the case now).
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Safe flying to you...
I would think both the flex arm and the rotor blades have to matched to other in terms of at least the flapping loads, to prevent both unwanted vibrations and unexpected behavior. In other words the flex arms need to be "tuned" to the rotor blades, or funny behavior will happen (which seems to be the case now).
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Safe flying to you...
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From my perspective there is no particular funny behavior. I am certainly not complaining about the Lynx, old or new. All of the charactoristics are known and explained, none are dangerous. True, there is an increase in vibration in the hover and at low speed, but that is a result of high energy vortices rolling off the tips and interacting with following blades. It is not ideal for roles requiring long periods of hover, but it's much better at speed. As always with helos there is a trade off!