coriolis effect & static droop
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coriolis effect & static droop
Hopefully Lu will pick up on this as I would like anyone to put forward their opinions. Firstly, Is there something other than coriolis effect which will make the rotor blades speed up with a reduction of disc diameter when increasing loading during flight? The reason I ask is that my examiner for the Instructors rating reckoned that a change in C of G of such a small amount wouldn't cause the increase in speed.
Secondly, whilst studying hard for my commercial exams today I came across something called static droop. I think I know what it means but would be grateful if someone could clarify.
Many thanks and I agree with everyone that Lu contributes a hell of a lot of knowledge to people like myself, just starting outin the helicopter industry.Don't know if the scare tactics on R22's is a good idea though. My scan now seems to include a sneaky peak above my head when flying. Thankfully the rotor head has been there each time I've looked so far.
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[This message has been edited by helimutt (edited 11 January 2001).]
Secondly, whilst studying hard for my commercial exams today I came across something called static droop. I think I know what it means but would be grateful if someone could clarify.
Many thanks and I agree with everyone that Lu contributes a hell of a lot of knowledge to people like myself, just starting outin the helicopter industry.Don't know if the scare tactics on R22's is a good idea though. My scan now seems to include a sneaky peak above my head when flying. Thankfully the rotor head has been there each time I've looked so far.
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[This message has been edited by helimutt (edited 11 January 2001).]
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Well, static droop, is just that, static, wouldn't be a factor in flight as to the discussion as far as I can tell. It's useful to know the static droop, as when it's spinning down one can avoid having them hit something, such as a pax.
Coriolis force is what causes the accel/decel of the blades.
With a large arm out to the spanwise CG of the blades, even a small chage an generate a response. I'm not aware of anything else that would cause an overall accell/decel in the context of the discussion (loading and unloading the rotor)
Of course, head design also comes into this, if it is a teetering head, it may be underslung to reduce the actual CG movement in/out at flight rpms.
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Marc
[This message has been edited by RW-1 (edited 11 January 2001).]
[This message has been edited by RW-1 (edited 11 January 2001).]
Coriolis force is what causes the accel/decel of the blades.
With a large arm out to the spanwise CG of the blades, even a small chage an generate a response. I'm not aware of anything else that would cause an overall accell/decel in the context of the discussion (loading and unloading the rotor)
Of course, head design also comes into this, if it is a teetering head, it may be underslung to reduce the actual CG movement in/out at flight rpms.
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Marc
[This message has been edited by RW-1 (edited 11 January 2001).]
[This message has been edited by RW-1 (edited 11 January 2001).]
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Helimut:
Static droop is a function of engine governing (as opposed to permanent droop being a built in rotor droopcharacteristic of the rotor sytem type).
As torque is increased, the tendancy is for the N2 and hence the Nr to droop. The governor compensates by automatically increasing N1 to keep the N2/Nr constant. Conversly when decreasing torque, the N2/Nr will tend to temporarily (static) increase (overspeed) thus commanding the governor to decrease N1 to compensate.
Hope this helps.
Cheers, OffshoreIgor
Static droop is a function of engine governing (as opposed to permanent droop being a built in rotor droopcharacteristic of the rotor sytem type).
As torque is increased, the tendancy is for the N2 and hence the Nr to droop. The governor compensates by automatically increasing N1 to keep the N2/Nr constant. Conversly when decreasing torque, the N2/Nr will tend to temporarily (static) increase (overspeed) thus commanding the governor to decrease N1 to compensate.
Hope this helps.
Cheers, OffshoreIgor
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I think you'll find that what Offshoreigor is referring to is actually transient droop, so named because it is not permanent. As he says, the governing system senses N2 variation (or Nr if piston) and makes and adjustment to restore it to a preset value. To assist with this most systems use some input from the collective, either mechanical (droop compensation cam) or electronic (anticipator) to'forewarn' the system of an impending change and to start adjusting before the change occurs, so minimising the effect.
Static droop is when the Nr decays to a new value after application of power, and does not return to the governed speed. This may be due to a fault if pronounced, however some degree of in-built droop is a requirement of hydromechanical systems to aid their stability. ie to prevent them oscillating about the datum. I'd like to say more, but it's late, I'm tired and I'd prefer to quote from more authorative sources to ensure accuracy. If you can get access to Simon Newman's book on Helicopter Aerodynamics, he has a good chapter al about Turbine engine governing.
Regarding the coriolis effect, don't forget that it is more concerned with the flapping of the blade in forward flight ie the advancing blade rises, so speeds up then slows down as it flaps down on the retreating side. Hookes joint effect has a similar effect as do periodic drag changes.
Hope this tallies with your understanding.
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Another day in paradise
Static droop is when the Nr decays to a new value after application of power, and does not return to the governed speed. This may be due to a fault if pronounced, however some degree of in-built droop is a requirement of hydromechanical systems to aid their stability. ie to prevent them oscillating about the datum. I'd like to say more, but it's late, I'm tired and I'd prefer to quote from more authorative sources to ensure accuracy. If you can get access to Simon Newman's book on Helicopter Aerodynamics, he has a good chapter al about Turbine engine governing.
Regarding the coriolis effect, don't forget that it is more concerned with the flapping of the blade in forward flight ie the advancing blade rises, so speeds up then slows down as it flaps down on the retreating side. Hookes joint effect has a similar effect as do periodic drag changes.
Hope this tallies with your understanding.
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Another day in paradise
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To: Helimutt
Corriolis force (conservation of angular momentum) is the force that causes the blades to lead and lag. The change in the blade mass is often diagrammed out showing one blade in various angular positions relative to the flapping hinges. The diagram shows the relationship of the blade mass at each position relative to the rotational axis. The diagram only shows one blade flapping up and the other blade is in the pure radial position and this can’t be. Another means of explaining this phenomenon is to have you picture an ice skater that brings her arms closer to her body and as a result speeds up in rotation and when she moves her arms outwards slows down. Speeding up and slowing down is due to her changing he center of mass.
Regarding what your instructor stated about the small change in blade mass CG not being sufficient to cause the leading and lagging I think he is not considering the blade mass due to centrifugal force. On a large helicopter the blades have a very large weight due to centrifugal force and could weigh in excess of 70,000 pounds at the lead lag hinge. So at this weight it does not take a very large change in mass CG to exert the energy to move the blades about the lead lag hinge.
Now, this is the way I learned it at Sikorsky over 45 years ago. It is difficult to explain without drawing diagrams but here goes.
First, something to consider: Discounting mast tilt to compensate for tail rotor drift or forward tilt on a Sikorsky mast that adds forward cyclic, visualize the disc as being horizontal and rotating about a fixed axis. This is the drive axis. In order to have lead and lag you must have an offset hinge. On the Robinson the cone hinges are the offset hinges. For arguments sake the rotor head does not move relative to the mast (difficult if you think of a Robinson head. Assume that the blades are coned up at the flapping hinge when you pull in collective. This will form a “V” with a flat bottom. Now lets look back at the explanation above that describes the blade in the flapped condition(s) and showing the blade mass CG moving closer to the center of drive creating forces that cause the blade to speed up or in other words, lead. This is totally false, as the blades do not lead when in a hover discounting tilt due to tail rotor compensation.
The blades lead and lag only when you introduce cyclic pitch change. Lets go back to the “V” with the flat bottom. When cyclic pitch is made the disc will tilt in the respective direction. This will make one leg of the flat-bottomed “V” move up and the other down in relation to the rotor head. Initially we discussed the drive axis. That was when you had a flat-bottomed “V”. Once the cyclic input was made the disc was displaced and now the blades are rotating about the driven axis. Due to the laws of conservation of angular momentum the blade wants to move forward from its’ position about the drive axis to the position scribed by the rotation about the driven axis. Since the blade can’t move linearly because the lead lag hinge anchors it, it will do what it can under the restrained condition. The tip of the blade will move forward on the advancing side. Now, when that same blade is on the longitudinal axis the forces diminish and the blade goes to the radial position and it wants to stay in that position. So, when it passes over the longitudinal axis, the blade will tend to hang back or actually move slower than it is being driven due to the change in mass CG position, which is lower. This is per the Ice skater illustration. It will hang back until it is over the left side of the helicopter and then return to the radial position. If you could look down on the tilted disc a four-blade rotor system would look something like a peace symbol.
While my wife and I went out for Chinese the other guys chimed in and covered droop as it applies to the rotor and the engine but there is another type of static droop and it applies to the drooping of the ailerons on a wing. When I worked on the CL604 and the Regional Jet flight control systems I discovered that after the flight control system is rigged and the Power Control Units (Servos) are turned on the pilot valves in both aileron servos are biased to move the ailerons down from the rigged position. I believe this was to both provide a bit more of lift and that aerodynamic forces would tend to move the ailerons to the rigged position and stiffen the system because the servos were always trying to get them to extend to the drooped position.
To:212man
Regarding your comment below:
Regarding the coriolis effect, don't forget that it is more concerned with the flapping of the blade in forward flight ie the advancing blade rises, so speeds up then slows down as it flaps down on the retreating side. Hookes joint effect has a similar effect as do periodic drag changes.
Hope this tallies with your understanding.
The advancing blade flaps down and the retreating blade flaps up. According to your descrioption you would be flying backwards. Your comparison to a hookes joint is valid. That is why they have constant velocity joints on a front drive car.
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The Cat
Corriolis force (conservation of angular momentum) is the force that causes the blades to lead and lag. The change in the blade mass is often diagrammed out showing one blade in various angular positions relative to the flapping hinges. The diagram shows the relationship of the blade mass at each position relative to the rotational axis. The diagram only shows one blade flapping up and the other blade is in the pure radial position and this can’t be. Another means of explaining this phenomenon is to have you picture an ice skater that brings her arms closer to her body and as a result speeds up in rotation and when she moves her arms outwards slows down. Speeding up and slowing down is due to her changing he center of mass.
Regarding what your instructor stated about the small change in blade mass CG not being sufficient to cause the leading and lagging I think he is not considering the blade mass due to centrifugal force. On a large helicopter the blades have a very large weight due to centrifugal force and could weigh in excess of 70,000 pounds at the lead lag hinge. So at this weight it does not take a very large change in mass CG to exert the energy to move the blades about the lead lag hinge.
Now, this is the way I learned it at Sikorsky over 45 years ago. It is difficult to explain without drawing diagrams but here goes.
First, something to consider: Discounting mast tilt to compensate for tail rotor drift or forward tilt on a Sikorsky mast that adds forward cyclic, visualize the disc as being horizontal and rotating about a fixed axis. This is the drive axis. In order to have lead and lag you must have an offset hinge. On the Robinson the cone hinges are the offset hinges. For arguments sake the rotor head does not move relative to the mast (difficult if you think of a Robinson head. Assume that the blades are coned up at the flapping hinge when you pull in collective. This will form a “V” with a flat bottom. Now lets look back at the explanation above that describes the blade in the flapped condition(s) and showing the blade mass CG moving closer to the center of drive creating forces that cause the blade to speed up or in other words, lead. This is totally false, as the blades do not lead when in a hover discounting tilt due to tail rotor compensation.
The blades lead and lag only when you introduce cyclic pitch change. Lets go back to the “V” with the flat bottom. When cyclic pitch is made the disc will tilt in the respective direction. This will make one leg of the flat-bottomed “V” move up and the other down in relation to the rotor head. Initially we discussed the drive axis. That was when you had a flat-bottomed “V”. Once the cyclic input was made the disc was displaced and now the blades are rotating about the driven axis. Due to the laws of conservation of angular momentum the blade wants to move forward from its’ position about the drive axis to the position scribed by the rotation about the driven axis. Since the blade can’t move linearly because the lead lag hinge anchors it, it will do what it can under the restrained condition. The tip of the blade will move forward on the advancing side. Now, when that same blade is on the longitudinal axis the forces diminish and the blade goes to the radial position and it wants to stay in that position. So, when it passes over the longitudinal axis, the blade will tend to hang back or actually move slower than it is being driven due to the change in mass CG position, which is lower. This is per the Ice skater illustration. It will hang back until it is over the left side of the helicopter and then return to the radial position. If you could look down on the tilted disc a four-blade rotor system would look something like a peace symbol.
While my wife and I went out for Chinese the other guys chimed in and covered droop as it applies to the rotor and the engine but there is another type of static droop and it applies to the drooping of the ailerons on a wing. When I worked on the CL604 and the Regional Jet flight control systems I discovered that after the flight control system is rigged and the Power Control Units (Servos) are turned on the pilot valves in both aileron servos are biased to move the ailerons down from the rigged position. I believe this was to both provide a bit more of lift and that aerodynamic forces would tend to move the ailerons to the rigged position and stiffen the system because the servos were always trying to get them to extend to the drooped position.
To:212man
Regarding your comment below:
Regarding the coriolis effect, don't forget that it is more concerned with the flapping of the blade in forward flight ie the advancing blade rises, so speeds up then slows down as it flaps down on the retreating side. Hookes joint effect has a similar effect as do periodic drag changes.
Hope this tallies with your understanding.
The advancing blade flaps down and the retreating blade flaps up. According to your descrioption you would be flying backwards. Your comparison to a hookes joint is valid. That is why they have constant velocity joints on a front drive car.
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The Cat
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Sorry Helimutt, I am just going to throw a wobly in on this thread, hopefully it will get back to your topic reasonably soon.
Lu: I have heard you quote the flapping arument before, the one that involves the leading blade flapping 'down'. I am under the impression that you may be refering to the result of the cyclic reaction that occurs in forward flight only. But in this scenario we must seperate the reactions taking place. I believe that 212man is correctly refering to the flapping 'up' of the advancing blade due to the increase in lift resulting from the increased forward speed when the blade advances. If you think in basic terms (which is the only way I can do it) The whole reason we have a reaction from conservation of angular momentum is because as the advancing blade flaps up the CoG moves inboard and the blade ties to lead, this leading may seen as a flexing of the blade or an actual movement from a hinge or a bearing. On the other side of the disk we have the retreating blade reducing its lift due to slower relative airspeed and therefor it flaps 'down', this in turn throws the CoG outward and the blade tries to slow down therefore we get the lagging of the blade. This is why we see it as a peace sign when viewed from above.
All of the above is refering to forward flight but Lu mentioned that we won't see the same sort of things in the hover. We do, especially when we pull in pitch. The blades cone up and we get conservation of angular momentum, seen as a leading of all the blades at the same time, the only problem is that at the same time the Nr is trying to droop through drag so we don't get any benefit from the reaction. The most important thing to remember in this scenario is that the reaction is only taking place while there is displacement (movement) of the blades from a previous position.
One last thing for Helimutt. You will and do see a reaction in flight when we get an increased or decreased load on the disk. Especially now that you guys are flying Robbinsons with govenors fitted. Take a look at the manifold pressure gauge when you get some gusty conditions, notice the change in power as the loads increase and decrease with the gusts, this is due to the governor working to maintain a constant RRPM. The coning angles change as a result of the gusts and therefore the RRPM tries to change as well (don't get me wrong there are heaps of things going on but this is highlighting what we are talking about).
Any way Helimutt, this thread should be fun from here. Hope you can keep up.
Cheers mate.
Lu: I have heard you quote the flapping arument before, the one that involves the leading blade flapping 'down'. I am under the impression that you may be refering to the result of the cyclic reaction that occurs in forward flight only. But in this scenario we must seperate the reactions taking place. I believe that 212man is correctly refering to the flapping 'up' of the advancing blade due to the increase in lift resulting from the increased forward speed when the blade advances. If you think in basic terms (which is the only way I can do it) The whole reason we have a reaction from conservation of angular momentum is because as the advancing blade flaps up the CoG moves inboard and the blade ties to lead, this leading may seen as a flexing of the blade or an actual movement from a hinge or a bearing. On the other side of the disk we have the retreating blade reducing its lift due to slower relative airspeed and therefor it flaps 'down', this in turn throws the CoG outward and the blade tries to slow down therefore we get the lagging of the blade. This is why we see it as a peace sign when viewed from above.
All of the above is refering to forward flight but Lu mentioned that we won't see the same sort of things in the hover. We do, especially when we pull in pitch. The blades cone up and we get conservation of angular momentum, seen as a leading of all the blades at the same time, the only problem is that at the same time the Nr is trying to droop through drag so we don't get any benefit from the reaction. The most important thing to remember in this scenario is that the reaction is only taking place while there is displacement (movement) of the blades from a previous position.
One last thing for Helimutt. You will and do see a reaction in flight when we get an increased or decreased load on the disk. Especially now that you guys are flying Robbinsons with govenors fitted. Take a look at the manifold pressure gauge when you get some gusty conditions, notice the change in power as the loads increase and decrease with the gusts, this is due to the governor working to maintain a constant RRPM. The coning angles change as a result of the gusts and therefore the RRPM tries to change as well (don't get me wrong there are heaps of things going on but this is highlighting what we are talking about).
Any way Helimutt, this thread should be fun from here. Hope you can keep up.
Cheers mate.
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Thanks rotortorque.
Lu, find yourself a photograph of an S61 or HH 53 or similar. Try and find one with the blades 'frozen': you will see the advancing blade flapped up with low pitch and the retreating blade flapped down with lots of pitch QED.
I also think, unless I've mis-read your description, that what you are actually describing is Hookes-joint effect, and not coriolis effect.
If you want to be pedantic, the blade mass does not change, nor does its weight, only the forces exerted on it.
Anymore from anyone on droop? If my PC skills were up to it I'd show static droop curves. These are produced on a test bed with the engine at varying speeds and then the N2 plotted vs load, to produce a curve. The steeper the gradient, the less the droop. This will result in hard damping though which can lead to over swinging of N2. A shallow curve results in slow return to the datum. It is possible to artificially alter the effect mechanically to have soft damping coupled with minimum droop.
Having said all that, the static droop to which the question refers is almost certainly that commonly found day to day, where, for instance, you set 100% Nr at flat pitch then lift in to an 80% Q hover and find the Nr is 98% and needs beeping up. On the 76 we set 101% Nr before lifting and this gives 100% in the hover.
Anyone with more detail to offer? it's a while since I read up on this.
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Another day in paradise
[This message has been edited by 212man (edited 12 January 2001).]
Lu, find yourself a photograph of an S61 or HH 53 or similar. Try and find one with the blades 'frozen': you will see the advancing blade flapped up with low pitch and the retreating blade flapped down with lots of pitch QED.
I also think, unless I've mis-read your description, that what you are actually describing is Hookes-joint effect, and not coriolis effect.
If you want to be pedantic, the blade mass does not change, nor does its weight, only the forces exerted on it.
Anymore from anyone on droop? If my PC skills were up to it I'd show static droop curves. These are produced on a test bed with the engine at varying speeds and then the N2 plotted vs load, to produce a curve. The steeper the gradient, the less the droop. This will result in hard damping though which can lead to over swinging of N2. A shallow curve results in slow return to the datum. It is possible to artificially alter the effect mechanically to have soft damping coupled with minimum droop.
Having said all that, the static droop to which the question refers is almost certainly that commonly found day to day, where, for instance, you set 100% Nr at flat pitch then lift in to an 80% Q hover and find the Nr is 98% and needs beeping up. On the 76 we set 101% Nr before lifting and this gives 100% in the hover.
Anyone with more detail to offer? it's a while since I read up on this.
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Another day in paradise
[This message has been edited by 212man (edited 12 January 2001).]
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Thank you gentlemen for enlightening me with your wisdom. It's going to take me a while to get this all straight in my head but im off out tonight to see if I can cause my own type of droop with a few shandies!!!
My question wasn't meant to cause arguments but it seems to have gotten everyones grey matter working. The actual question I was asked about static droop was as follows:-
Static droop results from:-
a/. increasing collective pitch
b/. decreasing collective pitch
c/. changing the cyclic pitch
Am I missing something here? Are we all talking about the same thing?
I sit in anticipation!
Thanks guys.
My question wasn't meant to cause arguments but it seems to have gotten everyones grey matter working. The actual question I was asked about static droop was as follows:-
Static droop results from:-
a/. increasing collective pitch
b/. decreasing collective pitch
c/. changing the cyclic pitch
Am I missing something here? Are we all talking about the same thing?
I sit in anticipation!
Thanks guys.
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Helimutt,
Quite simply, your answer is a).
Put simply, it refers to rotor RPM (Nr). More collective pulled = more static droop. This is very apparent on the older AS-330J Puma(with a mechanical FCU using flyweights and needle-valves to control fuel flow) where the flat pitch Nr is higher than the hover Nr.
However, on later versions such as the AS-332 Super Puma the Makila FCU is more sophisticated. At flat pitch on the ground the Nr sits slightly low and is artificially increased by the FCU when the lever is raised for lift-off to the hover.
We can now all discuss which way is positive droop and which way is negative droop
Quite simply, your answer is a).
Put simply, it refers to rotor RPM (Nr). More collective pulled = more static droop. This is very apparent on the older AS-330J Puma(with a mechanical FCU using flyweights and needle-valves to control fuel flow) where the flat pitch Nr is higher than the hover Nr.
However, on later versions such as the AS-332 Super Puma the Makila FCU is more sophisticated. At flat pitch on the ground the Nr sits slightly low and is artificially increased by the FCU when the lever is raised for lift-off to the hover.
We can now all discuss which way is positive droop and which way is negative droop
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Oh God, all that effort over a question like that?
ShyTorque is referring to the artificialy altered droop characteristics I alluded to. The inherent nature of the system is to droop along a curve, however the gradient can be altered to give more favourable characteristics, and even made positive as indeed the 332 is ( I think it goes from 260 Nr at MPOG to 265 in the hover?).
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Another day in paradise
ShyTorque is referring to the artificialy altered droop characteristics I alluded to. The inherent nature of the system is to droop along a curve, however the gradient can be altered to give more favourable characteristics, and even made positive as indeed the 332 is ( I think it goes from 260 Nr at MPOG to 265 in the hover?).
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Another day in paradise
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While everyone here is in the 'govnr' mode. A quick question on B206 gov. Which gov do you consider better for droop compensation characteristics. (ie stays close to 100% throughout collective increases and decreases) I remember flying Seiko fitted 206's eons ago that were pretty good. These days (post AS332/350 driving) they seem more 'lazy'. Bendix or Seiko guys 'n gals ??
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Yes there is a reason for the increase in rotor rpm when increasing the load factor on the disc in maneuvering other than coriolis effect. The best way to make sense of it is: say you are in auto rotation. As you are gliding down if you could some how add weight to the aircraft the rpm would increase and you would have to start pulling pitch to keep the rpm constant as you take on weight. As you loaded a larger amount of weight you would get to the point where your collective would be pulled pretty high. Adding weight to the helicopter is basically the same as increasing the load factor by turning or pulling aft cyclic. Increasing load factor in power on flight does the same thing to rpm as it does in unpowered flight. RPM does not increase just because of coriolis effect. Coriolis effect only increases rpm for the time when the coning angle is increasing, once the coning angle stabilizes there is no more coriolis effect and the rotor still must overcome the same amount of drag as before the coning was increased if it is to maintain rpm. If coriolis was the only force trying to increase rotor rpm, then when you rolled into a turn torque would momentarily decrease and once you were stabilized in the turn torque would return to where it was. We pilots know this does not happen.
Now for the explanation. It will help if you dig out a helicopter book that shows the airfoil section of a blade with all of the lift and drag vectors. They should show one for powered flight and one for autorotation. If you look at the one for autorotation you will see that the total force vector is perfectly vertical. That means that there is nothing opposing rotation and nothing to increase it. Now look at the diagram for powered flight, you will see that the total force vector is tilted aft trying to slow the rotor down. The torque generated by the engine of course overcomes this. Now imagine what happens to the relative wind vector if you increase the load factor. It will become more upward actually increasing the angle of attack. But look what happens to the total force when you change the relative wind.
Remember the drag vector is always parallel to the relative wind and the total lift vector is always 90 degrees to it. The total force is the lift and drag added together.
So if the relative wind is more upward then the total force is more forward, whether it is powered flight or unpowered.
Bottom line: when you increase load factor at any collective setting, you change the relative wind to be more upward and that angles the total force forward which is a decrease in drag and rpm increases or torque is reduced in a governed engine.
Now for the explanation. It will help if you dig out a helicopter book that shows the airfoil section of a blade with all of the lift and drag vectors. They should show one for powered flight and one for autorotation. If you look at the one for autorotation you will see that the total force vector is perfectly vertical. That means that there is nothing opposing rotation and nothing to increase it. Now look at the diagram for powered flight, you will see that the total force vector is tilted aft trying to slow the rotor down. The torque generated by the engine of course overcomes this. Now imagine what happens to the relative wind vector if you increase the load factor. It will become more upward actually increasing the angle of attack. But look what happens to the total force when you change the relative wind.
Remember the drag vector is always parallel to the relative wind and the total lift vector is always 90 degrees to it. The total force is the lift and drag added together.
So if the relative wind is more upward then the total force is more forward, whether it is powered flight or unpowered.
Bottom line: when you increase load factor at any collective setting, you change the relative wind to be more upward and that angles the total force forward which is a decrease in drag and rpm increases or torque is reduced in a governed engine.
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helisphere, if you are saying that once the coning angle stops increasing the rotor rpm drops back to its original value, I'd have to disagree. Nr should remain at its accelerated value till the coning angle decreases. perhaps I misunderstood.
[This message has been edited by lmlanphere (edited 13 January 2001).]
[This message has been edited by lmlanphere (edited 13 January 2001).]
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To:Rotorque, Helisphere and 212man
This might get a bit mixed up because intermixed with your comments about corriolis forces you were addressing static droop.
One of you made a comment about leading of the blades when you pulled collective in countering a statement I had made about a diagram. Whoever made that statement was correct. The leading would be limited and controlled by the amount of blade cone. If the blades were returned to flat pitch the lead would diminish to the point that the blades were in the normal position. It would be assumed that the blades in the flat pitch condition would be in the pure radial position and assuming a four-blade system the blades would form a cross. In actuality, due to the inertia of the blades instead of the blades forming a cross it would be more like a very mild X and as such the blades all would lag behind the pure radial position (Cross) and all leading and lagging would be behind the radial position. The only time the blades ever cross the radial position is during auto rotation when the blades are driving the system as opposed to the other way around.
Regarding the leading of the blades causing the rotor system to speed up (for a very short time) I don’t think this is true. Because the blades are mounted on a hinge and connected to the rotor head by a soft damper any movement of the blades about the lag hinge would not be reflected on the rotorhead. That is, unless the rotor system is in autorotation when the dampers are bottomed out and the aerodynamic forces on the rotorhead are back driving the gearbox.
If what was said above were true then the leading and lagging of the blades would be reflected in the dynamic system and the power train and be very noticeable by the pilot. The only time the pilot is aware of this condition is when he has a bad damper and he feels it as a lateral beat. But although the lateral beat stems from leading and lagging it really is a mass balance problem as the effected blade is out of balance with the other blades relative to their respective positions during rotation
Regarding the terms flapping up and flapping down there is a problem of interpretation. These terms are usually discussed in teaching helicopter aerodynamics and it causes many many problems. It is true on a two bladed rotor system if one blade flaps up the other will flap down. Assuming you are in a hover in a Bell and discounting any cyclic adjustments for tail rotor translation here is what happens. In a hover the blades are disposed at 90-degrees to the rotor shaft. When the pilot pushes forward the aerodynamic and precessional forces will cause the disc to dip down over the nose and rise up over the tail. In order to do this the advancing blade must flap down over the nose and due to the construction of the system the other blade flaps up over the tail. Now to what you were referring to. The advancing blade meets the oncoming relative wind and flaps up and the other blade flaps down. When the rotor system flaps relative to the fixed swashplate there is pitch coupling and this restores the blades to the commanded position therefore any tendency to flap out of the commanded disc position the blades immediately return. However when they use the flap up/flap down illustration they never address pitch coupling.
Regarding the comment about seeing the blades flap up on the advancing side and flapping down on the retreating side of an S-61, I would advise you to look at the tip path change when you move the cyclic. The disc will tip down in the direction of cyclic movement and, although you can’t see the rear of the disc I will assure you if one part goes down the opposite side of the disc will go up. Even on a 7 bladed CH-53.
In one post I mentioned Hookes joint effect in agreeing with a post made previously. On a hooks joint there is not a constant velocity in the rotation of the input Vs the output and that is when I mentioned a constant velocity joint being employed on a front drive automobile as opposed to using a Hookes joint in the drive system. The leading and lagging is a function of the law of conservation of angular momentum or as it is known as Corriolis forces. To minimize the effects of Corriolis forces on a single rotor helicopter they underselling the rotor system to lower the center of mass relative to the drive point on the rotor shaft. Speaking of center of mass, the center of mass on the blades does not move in or out relative to the root or tip of the blade. However it can move up or down relative to the flapping hinge and this is what causes the Coriollis force to kick in. The mass of the blades increases due to centrifugal force and does two things it increases the stiffness of the blade and it increases the moment of inertia of the disc system providing the forces necessary to cause precession when a perturbing force is appplied.
Now lets discuss the exceptions to the statements above. I previously stated that with a soft inplane rotor system the leading and lagging forces are not transmitted to the rotorhead and then into the drive system. A major exception to this is the Robinson head. The Robinson head has cone hinges, which are in effect, offset hinges that allow flapping in relation to the rotorhead itself. As I stated previously when a blade flaps it leads and lags. The Robinson head does not provide for leading and lagging brought about by flapping. Nevertheless, the tendency to lead and lag is there. These forces are so great that any lead lag motion is reacted by the cone hinges and then into the teeter hinge and then into the drive shaft and transmission. This fact is borne out by the high replacement frequencies for the respective hinges/bushings as they are worn into an egg shape. The Robinson head like the Bell head is underslung to minimize the tendency to lead and lag but the Robinson head unlike the bell head has the ability to flap and the Bell doesn’t.
Hopefully, I have covered all of the points in your respective posts. If not let me know as I am not going anywhere.
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The Cat
This might get a bit mixed up because intermixed with your comments about corriolis forces you were addressing static droop.
One of you made a comment about leading of the blades when you pulled collective in countering a statement I had made about a diagram. Whoever made that statement was correct. The leading would be limited and controlled by the amount of blade cone. If the blades were returned to flat pitch the lead would diminish to the point that the blades were in the normal position. It would be assumed that the blades in the flat pitch condition would be in the pure radial position and assuming a four-blade system the blades would form a cross. In actuality, due to the inertia of the blades instead of the blades forming a cross it would be more like a very mild X and as such the blades all would lag behind the pure radial position (Cross) and all leading and lagging would be behind the radial position. The only time the blades ever cross the radial position is during auto rotation when the blades are driving the system as opposed to the other way around.
Regarding the leading of the blades causing the rotor system to speed up (for a very short time) I don’t think this is true. Because the blades are mounted on a hinge and connected to the rotor head by a soft damper any movement of the blades about the lag hinge would not be reflected on the rotorhead. That is, unless the rotor system is in autorotation when the dampers are bottomed out and the aerodynamic forces on the rotorhead are back driving the gearbox.
If what was said above were true then the leading and lagging of the blades would be reflected in the dynamic system and the power train and be very noticeable by the pilot. The only time the pilot is aware of this condition is when he has a bad damper and he feels it as a lateral beat. But although the lateral beat stems from leading and lagging it really is a mass balance problem as the effected blade is out of balance with the other blades relative to their respective positions during rotation
Regarding the terms flapping up and flapping down there is a problem of interpretation. These terms are usually discussed in teaching helicopter aerodynamics and it causes many many problems. It is true on a two bladed rotor system if one blade flaps up the other will flap down. Assuming you are in a hover in a Bell and discounting any cyclic adjustments for tail rotor translation here is what happens. In a hover the blades are disposed at 90-degrees to the rotor shaft. When the pilot pushes forward the aerodynamic and precessional forces will cause the disc to dip down over the nose and rise up over the tail. In order to do this the advancing blade must flap down over the nose and due to the construction of the system the other blade flaps up over the tail. Now to what you were referring to. The advancing blade meets the oncoming relative wind and flaps up and the other blade flaps down. When the rotor system flaps relative to the fixed swashplate there is pitch coupling and this restores the blades to the commanded position therefore any tendency to flap out of the commanded disc position the blades immediately return. However when they use the flap up/flap down illustration they never address pitch coupling.
Regarding the comment about seeing the blades flap up on the advancing side and flapping down on the retreating side of an S-61, I would advise you to look at the tip path change when you move the cyclic. The disc will tip down in the direction of cyclic movement and, although you can’t see the rear of the disc I will assure you if one part goes down the opposite side of the disc will go up. Even on a 7 bladed CH-53.
In one post I mentioned Hookes joint effect in agreeing with a post made previously. On a hooks joint there is not a constant velocity in the rotation of the input Vs the output and that is when I mentioned a constant velocity joint being employed on a front drive automobile as opposed to using a Hookes joint in the drive system. The leading and lagging is a function of the law of conservation of angular momentum or as it is known as Corriolis forces. To minimize the effects of Corriolis forces on a single rotor helicopter they underselling the rotor system to lower the center of mass relative to the drive point on the rotor shaft. Speaking of center of mass, the center of mass on the blades does not move in or out relative to the root or tip of the blade. However it can move up or down relative to the flapping hinge and this is what causes the Coriollis force to kick in. The mass of the blades increases due to centrifugal force and does two things it increases the stiffness of the blade and it increases the moment of inertia of the disc system providing the forces necessary to cause precession when a perturbing force is appplied.
Now lets discuss the exceptions to the statements above. I previously stated that with a soft inplane rotor system the leading and lagging forces are not transmitted to the rotorhead and then into the drive system. A major exception to this is the Robinson head. The Robinson head has cone hinges, which are in effect, offset hinges that allow flapping in relation to the rotorhead itself. As I stated previously when a blade flaps it leads and lags. The Robinson head does not provide for leading and lagging brought about by flapping. Nevertheless, the tendency to lead and lag is there. These forces are so great that any lead lag motion is reacted by the cone hinges and then into the teeter hinge and then into the drive shaft and transmission. This fact is borne out by the high replacement frequencies for the respective hinges/bushings as they are worn into an egg shape. The Robinson head like the Bell head is underslung to minimize the tendency to lead and lag but the Robinson head unlike the bell head has the ability to flap and the Bell doesn’t.
Hopefully, I have covered all of the points in your respective posts. If not let me know as I am not going anywhere.
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The Cat
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Concur with offshoreigor. To test this a droop plot can be taken, plotting Torque against RRPM. It is generally accepted to be normal for the RRPM to drop as TQ increases, but in these days of FADEC is is quite possible to have no change (or even an increase) in RRPM with TQ increase, should the designer wish.
One could argue that a drop in RRPM will increase coning angle as will an increase in TQ, which may not be totally desirable, especially on a rigid head - answers on a postcard to .........
One could argue that a drop in RRPM will increase coning angle as will an increase in TQ, which may not be totally desirable, especially on a rigid head - answers on a postcard to .........
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Grey Area,
I rather thought your first post was echoing my sentiments?
I would think that any tendency for Nr to rise with increased coning from droop would be masked by the droop itself. ie the droop ing would not be as great as it otherwise would be. (that's how I explain it to my wife, anyway).
Lu, please avoid using expressions like "the blade mass will increase".
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Another day in paradise
I rather thought your first post was echoing my sentiments?
I would think that any tendency for Nr to rise with increased coning from droop would be masked by the droop itself. ie the droop ing would not be as great as it otherwise would be. (that's how I explain it to my wife, anyway).
Lu, please avoid using expressions like "the blade mass will increase".
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Another day in paradise
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To: 212man
I used the term mass as it applies to the definition of a Newton. In this case mass = weight. What I was trying to imply is that the reflected weight of a rotor blade that weighs 234 pounds would due to centrifugal force be equal to a weight of 72,000 pounds measured at the offset hinge. This weight combined with the other blades creates a disc that has a polar moment of inertia of 360,000 pounds, which provides three things. 1) Stiffness of the blade and the disc. 2) Gyroscopic rigidity of the disc and, 3) the inertial energy to cause the blade disc to move when perturbed by an external force (control input).
Hopefully in this description I didn’t misuse some words such as polar moment…..
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The Cat
I used the term mass as it applies to the definition of a Newton. In this case mass = weight. What I was trying to imply is that the reflected weight of a rotor blade that weighs 234 pounds would due to centrifugal force be equal to a weight of 72,000 pounds measured at the offset hinge. This weight combined with the other blades creates a disc that has a polar moment of inertia of 360,000 pounds, which provides three things. 1) Stiffness of the blade and the disc. 2) Gyroscopic rigidity of the disc and, 3) the inertial energy to cause the blade disc to move when perturbed by an external force (control input).
Hopefully in this description I didn’t misuse some words such as polar moment…..
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The Cat
Guest
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Well, whatever, the mass remains constant; it's the force that changes. As polar moment of inertia (I) is equal to a half mass times radius(squared) I suppose it rather depends on the diameter and number of blades of the system you quote figures for.
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Another day in paradise
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Another day in paradise
Guest
Posts: n/a
Only read halfway through this thread and started to get confused myself.
Just so I can clarify my understanding of the different topics, conservation of angular momentum surely does not relate to lead and lag, it applies to flare effects with regards increase of NR during the flare.
I was under the impression that lead and lag was as a result of the differing drag produced around the disc given changes in relative rotational velocity as the disc moves through the air.
As for static droop, it is caused by increasing collective pitch from MPOG to hover power where it is not compensated for in a governor. Any further changes in NR in flight caused by manouevering would be termed transient droop.
One way to explain conservation of angular momentum when related to flare effects is as follows:-
Imagine a flat rotor disc with an arbitary rotor blabe length which would produce a set figure for the circumference of the disc area. If you apply 'G' loading to the disc it will increase the coning angle of the disc i.e. in the flare. As the blade cones it slightly reduces the radius of the disc or the distance of the tip path plane from the shaft axis. This therefore gives a slight reduction in the circumference of the disc area as the radius is decreased. The blades momentum means it wants to travel the same distance round the disc in the same time but has a smaller disc round which to travel therefore it will travel further in terms of rotation whilst in the flare. The result is an increase of NR.
Feel free to educate me if I am too far from the mark, I'm always learning too!
Just so I can clarify my understanding of the different topics, conservation of angular momentum surely does not relate to lead and lag, it applies to flare effects with regards increase of NR during the flare.
I was under the impression that lead and lag was as a result of the differing drag produced around the disc given changes in relative rotational velocity as the disc moves through the air.
As for static droop, it is caused by increasing collective pitch from MPOG to hover power where it is not compensated for in a governor. Any further changes in NR in flight caused by manouevering would be termed transient droop.
One way to explain conservation of angular momentum when related to flare effects is as follows:-
Imagine a flat rotor disc with an arbitary rotor blabe length which would produce a set figure for the circumference of the disc area. If you apply 'G' loading to the disc it will increase the coning angle of the disc i.e. in the flare. As the blade cones it slightly reduces the radius of the disc or the distance of the tip path plane from the shaft axis. This therefore gives a slight reduction in the circumference of the disc area as the radius is decreased. The blades momentum means it wants to travel the same distance round the disc in the same time but has a smaller disc round which to travel therefore it will travel further in terms of rotation whilst in the flare. The result is an increase of NR.
Feel free to educate me if I am too far from the mark, I'm always learning too!