Ditching a helicopter: (incl pictures)
Iconoclast
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To: 3top
I can understand the problem you outlined. We never had it. Even though the springs were removed we never had a problem with the shoes sticking to the clutch drum. It might be that your lube oil was viscous and the shoes stuck due to the adhesion between the shoe face and the drum. It may also have been a factor of humidity. Where we were even though we were operating in the Arctic Ocean the humidity level was quite low. Regarding the condition of our lube oil, we changed it every 25 hours. The Coast Guard also insisted on the use of high quality lubricants. Our rotor heads were lubed with Texaco grease and were lubed every 5 hours and our engines were lubed with Texaco aviation oil. We also rotated the rotor backwards much like they do on Hughes 500s and Bells to see if the rotor spool is stuck to the casing.
I can understand the problem you outlined. We never had it. Even though the springs were removed we never had a problem with the shoes sticking to the clutch drum. It might be that your lube oil was viscous and the shoes stuck due to the adhesion between the shoe face and the drum. It may also have been a factor of humidity. Where we were even though we were operating in the Arctic Ocean the humidity level was quite low. Regarding the condition of our lube oil, we changed it every 25 hours. The Coast Guard also insisted on the use of high quality lubricants. Our rotor heads were lubed with Texaco grease and were lubed every 5 hours and our engines were lubed with Texaco aviation oil. We also rotated the rotor backwards much like they do on Hughes 500s and Bells to see if the rotor spool is stuck to the casing.
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3top,
If it does not put you to too much trouble, I would be grateful if you could get hold of the details of those publications. I will try and get hold of copies.
I do not have a problem understanding WHAT happens, it is WHY!! Perhaps that is my fault for not just being able to accept it like others can?
"Some days you are statue, some days you are the pigeon!"
If it does not put you to too much trouble, I would be grateful if you could get hold of the details of those publications. I will try and get hold of copies.
I do not have a problem understanding WHAT happens, it is WHY!! Perhaps that is my fault for not just being able to accept it like others can?
"Some days you are statue, some days you are the pigeon!"
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Gyroscopic precession is a mysterious force, until you look at the fundamental physics behind it. The best way to figure it out is to look it up in a Physics text, or an Engineering Mechanics text.
The underlying principles are basic. The gyro has a fixed angular momentum due to the spinning mass. This is a set quantity, and(like linear momentum) must be preserved. If you try to rotate the gyro, you are adding to the angular momentum (because the rotation of the whole gyro in another plane induces a change to the total angular momentum). The faster the rotation you impart, the more the momentum change that you request. As a total system, the momentum must be constant, when you disturb the angular momentum balance, the extra rotation 90 degrees out results.
One way of describing it is to picture a wheel spinning free in space. Imagine it in front of you spinning with its axle pointing at you, rotating clockwise (top to the right). It is happy to spin there, undisturbed, until you push on the top rim (12 o'clock position)in a direction away from you. The wheel is rigid, of course, so each part of it cannot move relative to any other. Because you start it moving away from you, the piece of the wheel at the top has two components of its velocity, one to the right, and one directly away from you. This means that you have tried to disturb its velocity, relative to the rest of the wheel. Because the rototion you called for (top away, bottom toward you) has no rotational speed at the sides of the wheel (the 3 and 9 o'clock pieces of the wheel see no translation due to your push), the mass has the confusing need to have more energy at the top and bottom (more total speed) than it does at the sides (3 and 9 o'clock) The "precession" of the gyro is simply the motion that the gyro makes that rotates this 3 and 9 o'clock part to EXACTLY match the 12 and 6 o'clock part. The speed of rotation imparted on the wheel is proportional to the force you exert on the wheel, and its angular momentum (speed of rotation, basically). The slower it spins, the faster the precessional speed to make up for the larger disturbance you caused.
Try this site for one stab at the explanation: http://sprott.physics.wisc.edu/demobook/chapter1.htm
The underlying principles are basic. The gyro has a fixed angular momentum due to the spinning mass. This is a set quantity, and(like linear momentum) must be preserved. If you try to rotate the gyro, you are adding to the angular momentum (because the rotation of the whole gyro in another plane induces a change to the total angular momentum). The faster the rotation you impart, the more the momentum change that you request. As a total system, the momentum must be constant, when you disturb the angular momentum balance, the extra rotation 90 degrees out results.
One way of describing it is to picture a wheel spinning free in space. Imagine it in front of you spinning with its axle pointing at you, rotating clockwise (top to the right). It is happy to spin there, undisturbed, until you push on the top rim (12 o'clock position)in a direction away from you. The wheel is rigid, of course, so each part of it cannot move relative to any other. Because you start it moving away from you, the piece of the wheel at the top has two components of its velocity, one to the right, and one directly away from you. This means that you have tried to disturb its velocity, relative to the rest of the wheel. Because the rototion you called for (top away, bottom toward you) has no rotational speed at the sides of the wheel (the 3 and 9 o'clock pieces of the wheel see no translation due to your push), the mass has the confusing need to have more energy at the top and bottom (more total speed) than it does at the sides (3 and 9 o'clock) The "precession" of the gyro is simply the motion that the gyro makes that rotates this 3 and 9 o'clock part to EXACTLY match the 12 and 6 o'clock part. The speed of rotation imparted on the wheel is proportional to the force you exert on the wheel, and its angular momentum (speed of rotation, basically). The slower it spins, the faster the precessional speed to make up for the larger disturbance you caused.
Try this site for one stab at the explanation: http://sprott.physics.wisc.edu/demobook/chapter1.htm
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>You're likely to start an argument just by mentioning gyroscopic precession.,< ~ Heedm
OK!
Gyroscopic precession has *virtually* nothing to do with a helicopter's rotor.
Gyroscopic precession is little more than a simplistic means of describing the basic end result at the rotor, after the application of a cyclic input. (i.e. 90-degrees later ~ basically). It is no good for explaining the 'how'.
The simplest argument against the use of 'gyroscopic precession' is to look at its algorithm. Mass and rotational velocity are the predominant variables. The relative mass and rpm in a gyroscope are humoungeous. The relative mass and rpm in a rotor disk are negligible.
The helicopter's rotor disk 'flies' to position. The eventual demise of the sinusoidal swashplate will result in rotor blades that can fly to position in 12.3-degrees or 123-degrees.
OK!
Gyroscopic precession has *virtually* nothing to do with a helicopter's rotor.
Gyroscopic precession is little more than a simplistic means of describing the basic end result at the rotor, after the application of a cyclic input. (i.e. 90-degrees later ~ basically). It is no good for explaining the 'how'.
The simplest argument against the use of 'gyroscopic precession' is to look at its algorithm. Mass and rotational velocity are the predominant variables. The relative mass and rpm in a gyroscope are humoungeous. The relative mass and rpm in a rotor disk are negligible.
The helicopter's rotor disk 'flies' to position. The eventual demise of the sinusoidal swashplate will result in rotor blades that can fly to position in 12.3-degrees or 123-degrees.
Iconoclast
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To: Dave Jackson
As much as you and many others insist that gyroscopic precession has nothing to do with helicopter rotors I would suggest that you contact the training departments of Sikorsky, Boeing, Bell and even Robinson. I have the training text for the first three and they all address gyroscopic precession as the motivating force in changing disc position. They also address a phase angle of 90-degrees. Robinson on the other hand provides illustrations showing that the pitch horn is offset by 18-degrees from the teeter hinge and they address gyroscopic precession as being referenced in the text. The problem is the illustration they use is that of a Bell system which does not have an offset between the pitch horn and the teeter hinge and the supporting text states that there is a 90-degree phase angle still talking about the Robinson head. I personally believe that they would have a very difficult time in describing how the Robinson system can respond like a Bell, which has a 90-degree phase angle, and the Robinson has a 72-degree phase angle. According to what Nick Lappos stated the mechanics being trained would have to be a graduate engineer or physicist in order to understand the mechanics of the system. That is probably why in Frank Robinsons’ response to me he indicated that we wouldn’t understand how it works.
Now do you know where my confusion stems from?
Regarding the demise of the sinusoidal swashplate it has already happened. The Lynx does not have a swashplate but it does have an isolation bearing that separates the rotating elements of the flight control system from the stationary bits. The Lynx has an offset similar to the Robinson (15-degrees) and with out the electronic flight control system the blades would dip down to the left with forward cyclic. There could be a thousand different ways of eliminating the swashplate but those systems would be so complex as to be rife with single point catastrophic failures.
[ 19 November 2001: Message edited by: Lu Zuckerman ]
As much as you and many others insist that gyroscopic precession has nothing to do with helicopter rotors I would suggest that you contact the training departments of Sikorsky, Boeing, Bell and even Robinson. I have the training text for the first three and they all address gyroscopic precession as the motivating force in changing disc position. They also address a phase angle of 90-degrees. Robinson on the other hand provides illustrations showing that the pitch horn is offset by 18-degrees from the teeter hinge and they address gyroscopic precession as being referenced in the text. The problem is the illustration they use is that of a Bell system which does not have an offset between the pitch horn and the teeter hinge and the supporting text states that there is a 90-degree phase angle still talking about the Robinson head. I personally believe that they would have a very difficult time in describing how the Robinson system can respond like a Bell, which has a 90-degree phase angle, and the Robinson has a 72-degree phase angle. According to what Nick Lappos stated the mechanics being trained would have to be a graduate engineer or physicist in order to understand the mechanics of the system. That is probably why in Frank Robinsons’ response to me he indicated that we wouldn’t understand how it works.
Now do you know where my confusion stems from?
Regarding the demise of the sinusoidal swashplate it has already happened. The Lynx does not have a swashplate but it does have an isolation bearing that separates the rotating elements of the flight control system from the stationary bits. The Lynx has an offset similar to the Robinson (15-degrees) and with out the electronic flight control system the blades would dip down to the left with forward cyclic. There could be a thousand different ways of eliminating the swashplate but those systems would be so complex as to be rife with single point catastrophic failures.
[ 19 November 2001: Message edited by: Lu Zuckerman ]
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Lu
The results of gyroscopic precession and aerodynamic precession are the same, but the result of gyroscopic precession is easier to see. It therefore is a quick way to explain precession to mechanics and pilots who have to work with it but do not necessarily have to know its 'innards'. If a pilot or mechanic want to fully understand precession then to continue to explain it in terms of gyroscope precession becomes a problem.
I am not saying "that gyroscopic precession has nothing to do with helicopter rotors". It plays a role, but its direct roll is a very small one and its indirect role is exactly that ~ 'indirect'.
A gyroscope (gyroscopic precession) functions because of high mass and high rpm. Aerodynamic plays no significant role.
A rotor (aerodynamic precession) functions because of aerodynamic forces. Mass and high rpm play no significant role
Direct role:
I believe the magnitude of gyroscopic precession's direct role in aircraft is dependent upon the rigidity, mass and speed of the device. A propeller will have a reasonable amount of gyroscopic precession, whereas a teetering rotor will have very little.
Indirect role:
Nick mentioned "The wheel is rigid, of course, so each part of it cannot move relative to any other.". What he is saying [I think
]is that a force at one location on the rigid wheel is creating forces at all locations on the rigid wheel. If a force at 90-degrees azimuth results in an upward force of 5 lbs at that location then there will be a force of -5 lbs at 270-degrees. There will be a sine(45) * 5-lb force at 45 and 135-degrees. There will be a 0-lb force at 0 and 180-degrees, etc,etc. The culmination of all the upward forces is at 180-degrees, and this is the high point. . The culmination of all the downward forces is at 0-degrees, and this is the low point.
Nick is talking about a singular force being distributed around the circumference due to the rigidity of the circumference. As he said, he is describing gyroscopic precession.
The helicopter rotor does not have this rigid ring at its circumference. It also does not have a singular force at one location. The pitch of its blades is providing a force directly to all the locations.
For these reasons, I believe that aerodynamic precession is a cleaner description.
_______________
In addition
Gyroscopic precession and aerodynamic precession only give the same results because the swashplate and gyroscopic precession produces a sinusoidal curve. When the swashplate (pure sinusoidal curve) is replaced, gyroscopic precession must fly out the window. This is because the new pitch controllers will be able to put into the blade any pitch at any azimuth they want to. I.e. no longer pure sinusoidal.
In the case of an intermeshing or coaxial helicopter, it may be possible to add a little additional short-term pitch to the lower blade as it passes through the downwash of the upper blade.
_______________
Phase lag, Gamma, offset and delta-3 are subjects for other future threads.
The results of gyroscopic precession and aerodynamic precession are the same, but the result of gyroscopic precession is easier to see. It therefore is a quick way to explain precession to mechanics and pilots who have to work with it but do not necessarily have to know its 'innards'. If a pilot or mechanic want to fully understand precession then to continue to explain it in terms of gyroscope precession becomes a problem.
I am not saying "that gyroscopic precession has nothing to do with helicopter rotors". It plays a role, but its direct roll is a very small one and its indirect role is exactly that ~ 'indirect'.
A gyroscope (gyroscopic precession) functions because of high mass and high rpm. Aerodynamic plays no significant role.
A rotor (aerodynamic precession) functions because of aerodynamic forces. Mass and high rpm play no significant role
Direct role:
I believe the magnitude of gyroscopic precession's direct role in aircraft is dependent upon the rigidity, mass and speed of the device. A propeller will have a reasonable amount of gyroscopic precession, whereas a teetering rotor will have very little.
Indirect role:
Nick mentioned "The wheel is rigid, of course, so each part of it cannot move relative to any other.". What he is saying [I think
]is that a force at one location on the rigid wheel is creating forces at all locations on the rigid wheel. If a force at 90-degrees azimuth results in an upward force of 5 lbs at that location then there will be a force of -5 lbs at 270-degrees. There will be a sine(45) * 5-lb force at 45 and 135-degrees. There will be a 0-lb force at 0 and 180-degrees, etc,etc. The culmination of all the upward forces is at 180-degrees, and this is the high point. . The culmination of all the downward forces is at 0-degrees, and this is the low point.Nick is talking about a singular force being distributed around the circumference due to the rigidity of the circumference. As he said, he is describing gyroscopic precession.
The helicopter rotor does not have this rigid ring at its circumference. It also does not have a singular force at one location. The pitch of its blades is providing a force directly to all the locations.
For these reasons, I believe that aerodynamic precession is a cleaner description.
_______________
In addition
Gyroscopic precession and aerodynamic precession only give the same results because the swashplate and gyroscopic precession produces a sinusoidal curve. When the swashplate (pure sinusoidal curve) is replaced, gyroscopic precession must fly out the window. This is because the new pitch controllers will be able to put into the blade any pitch at any azimuth they want to. I.e. no longer pure sinusoidal.
In the case of an intermeshing or coaxial helicopter, it may be possible to add a little additional short-term pitch to the lower blade as it passes through the downwash of the upper blade.
_______________
Phase lag, Gamma, offset and delta-3 are subjects for other future threads.
Iconoclast
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To: Dave Jackson
Not being a graduate engineer I find it difficult to follow Nicks' example. Did he or did he not say that when the force was applied to the upper portion of the spinning wheel that the wheel reacted 90-degrees later. If he did not say there was a reaction then the axis of the wheel had to be rigidized so that no movement is allowed. The normal explanation using the bicycle wheel is to have it mounted on an axis that allowed the person conducting the test to hold the spinning wheel in space. Under this condition when a force was applied to the axis the wheel would move in a direction 90-degrees after the input. Another experiment is for the individual to sit on a chair that is free to rotate. Holding the spinning wheel in the same manner and applying a lateral force the spinning wheel (gyro rotor) will cause the individual to rotate on the chair.
I don’t understand your comparison of a spinning gyroscope rotor to a helicopter rotor by saying that the gyro spins at a high rate and therefor has more “angular momentum” (if that’s the correct term) than a helicopter rotor which weighs ten thousand times more than the gyro rotor but is spinning much slower speed. Some time back I described the Cheyenne rotor system. It is controlled by a gyroscope (control gyro) and is installed above the rotorhead, and is positioned in relation to the rotor by applied force to the swashplate. Unlike any other helicopter there is no direct linkage from the pilots controls to the rotorhead. The movement of the control gyro initiates all pitch change. Now this control gyro weighs considerably less than the rotor system that it controls but never the less, it is a gyro. It is not traveling at a great speed but for its’ size it can generate a great deal of nutating forces. Gyros and rotors can be of any size and can rotate at any speed. If they both are properly designed, speed of rotation and mass make no difference.
Not being a graduate engineer I find it difficult to follow Nicks' example. Did he or did he not say that when the force was applied to the upper portion of the spinning wheel that the wheel reacted 90-degrees later. If he did not say there was a reaction then the axis of the wheel had to be rigidized so that no movement is allowed. The normal explanation using the bicycle wheel is to have it mounted on an axis that allowed the person conducting the test to hold the spinning wheel in space. Under this condition when a force was applied to the axis the wheel would move in a direction 90-degrees after the input. Another experiment is for the individual to sit on a chair that is free to rotate. Holding the spinning wheel in the same manner and applying a lateral force the spinning wheel (gyro rotor) will cause the individual to rotate on the chair.
I don’t understand your comparison of a spinning gyroscope rotor to a helicopter rotor by saying that the gyro spins at a high rate and therefor has more “angular momentum” (if that’s the correct term) than a helicopter rotor which weighs ten thousand times more than the gyro rotor but is spinning much slower speed. Some time back I described the Cheyenne rotor system. It is controlled by a gyroscope (control gyro) and is installed above the rotorhead, and is positioned in relation to the rotor by applied force to the swashplate. Unlike any other helicopter there is no direct linkage from the pilots controls to the rotorhead. The movement of the control gyro initiates all pitch change. Now this control gyro weighs considerably less than the rotor system that it controls but never the less, it is a gyro. It is not traveling at a great speed but for its’ size it can generate a great deal of nutating forces. Gyros and rotors can be of any size and can rotate at any speed. If they both are properly designed, speed of rotation and mass make no difference.
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The question is, "does gyroscopic precession have a direct role in the workings of a rotorhead".
In my current aircraft, if I wish to move forward, I of course apply forward cyclic. However, if one looks at how the swash-plate moves when I apply forward cyclic, they will see something quite different to what they might expect. The swash-plate will not tilt forward, but rather, will tilt to the right. But because the designers know it will take 90 degrees in the direction of rotation before a result of this control input is realized, the aircraft moves forward.
I'm not an engineer or a physics professor, but I certainly believe gyroscopic precession plays a direct role in how a helicopter rotor head works.
In my current aircraft, if I wish to move forward, I of course apply forward cyclic. However, if one looks at how the swash-plate moves when I apply forward cyclic, they will see something quite different to what they might expect. The swash-plate will not tilt forward, but rather, will tilt to the right. But because the designers know it will take 90 degrees in the direction of rotation before a result of this control input is realized, the aircraft moves forward.
I'm not an engineer or a physics professor, but I certainly believe gyroscopic precession plays a direct role in how a helicopter rotor head works.
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"This is the thread that never ends, Yes it goes on and on my friend..." 
It seems that the same group of people join in this subject whenever it starts. We mostly get closer to agreement each time, and the understanding of the underlying physics increases.
Nick's right when he talks about summing the spins. The energy argument is a little mixed up, and it actually applies to special cases only. One important point, the angular momentum is not constant, it is conserved. If you apply a moment to a spinning object you change the angular momentum in magnitude, direction, or both.
In the past I've found that many don't accept some of the jumps in a rotational dynamics based description of gyroscopic precession, or they don't have the background to understand it. If you want a fairly complete attempt at explaining gyroscopic precession, I put one on page two of the Helicopter Dynamics: Gyroscopic Precession thread. I warn you, it's long.
Don't believe what Dave says about a gyroscope requiring high mass and high rpm. Gyroscopic effects happen on all rotating systems. Read my previous message in this thread.
The biggest problem with gyroscopic precession is that the term is used completely inaccurately when describing helicopter flight. It is not a force. It is not an explanation of why a force applied to a spinning object appears to manifest itself 90 degrees later. It is an example of that apparent 90 degree lag.
When you spin a top, if the surface it's contacting offers little friction, then the top spins about one axis until that axis leans from the vertical. Once it leans from the vertical, gravity tries to pull it over. Instead of falling over, the top continues to spin about it's axis, but the axis rotates in space, or precesses.
That's what gyroscopic precession is.
The top precesses rather than falling over because the force of gravity causes a moment about the contacting point of the top. Since the top is spinning, that moment adds more spin to it, but about a different axis. When these spins are added, the result is the top wants to spin about it's axis while leaning 90 degrees away from the direction that the force was applied. Gravity continues to act on the top, so the top continues to "fall" 90 degrees away from the direction the force was applied. The result is the precession.
Lu. I think you're right about the 18 degree left roll in a Robinson. I was watching one the other day, and he couldn't fly straight, always banked left. He probably understands physics, unlike the few other pilots who have flown Robinson helicopters.
Matthew.
It seems that the same group of people join in this subject whenever it starts. We mostly get closer to agreement each time, and the understanding of the underlying physics increases.
Nick's right when he talks about summing the spins. The energy argument is a little mixed up, and it actually applies to special cases only. One important point, the angular momentum is not constant, it is conserved. If you apply a moment to a spinning object you change the angular momentum in magnitude, direction, or both.
In the past I've found that many don't accept some of the jumps in a rotational dynamics based description of gyroscopic precession, or they don't have the background to understand it. If you want a fairly complete attempt at explaining gyroscopic precession, I put one on page two of the Helicopter Dynamics: Gyroscopic Precession thread. I warn you, it's long.
Don't believe what Dave says about a gyroscope requiring high mass and high rpm. Gyroscopic effects happen on all rotating systems. Read my previous message in this thread.
The biggest problem with gyroscopic precession is that the term is used completely inaccurately when describing helicopter flight. It is not a force. It is not an explanation of why a force applied to a spinning object appears to manifest itself 90 degrees later. It is an example of that apparent 90 degree lag.
When you spin a top, if the surface it's contacting offers little friction, then the top spins about one axis until that axis leans from the vertical. Once it leans from the vertical, gravity tries to pull it over. Instead of falling over, the top continues to spin about it's axis, but the axis rotates in space, or precesses.
That's what gyroscopic precession is.
The top precesses rather than falling over because the force of gravity causes a moment about the contacting point of the top. Since the top is spinning, that moment adds more spin to it, but about a different axis. When these spins are added, the result is the top wants to spin about it's axis while leaning 90 degrees away from the direction that the force was applied. Gravity continues to act on the top, so the top continues to "fall" 90 degrees away from the direction the force was applied. The result is the precession.
Lu. I think you're right about the 18 degree left roll in a Robinson. I was watching one the other day, and he couldn't fly straight, always banked left. He probably understands physics, unlike the few other pilots who have flown Robinson helicopters.
Matthew.
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Heedm
>"Don't believe what Dave says about a gyroscope requiring high mass and high rpm. Gyroscopic effects happen on all rotating systems."<
I agree with the above.
A clarification of the comment to which you refer is;
Do we agree on this?
[ 20 November 2001: Message edited by: Dave Jackson ]
>"Don't believe what Dave says about a gyroscope requiring high mass and high rpm. Gyroscopic effects happen on all rotating systems."<
I agree with the above.
A clarification of the comment to which you refer is;
- The relative mass and rpm of a gyroscopic is high visa-vie the relative mass and rpm of a helicopter rotor.
- The relative aerodynamic activity of a gyroscopic is low visa-vie the relative aerodynamic activity of a helicopter rotor.
Do we agree on this?
[ 20 November 2001: Message edited by: Dave Jackson ]
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Dave said,
The rpm of a gyroscope is higher, but the mass of a typical helicopter rotor is higher. What really matters is the magnitude of the angular momentum. Angular momentum is a function of rpm and the distribution of mass about the center of rotation (moment of inertia). Without being given specific examples, it's hard to say which has a higher angular momentum, but the one with the highest will be the hardest to stop.
Try stop a gyro with your hand. Now try stop a helicopter rotor with your hand. Which one was easiest? That one had less angular momentum.
Higher angular momentum only means that when you apply a force, it has less of an effect than if you applied that same force to something with lower angular momentum. That is why a gust of wind hitting a rotor that is slow during startup can result in much more blade motion.
When I think of a gyroscope, I think of something that has been designed to not interact with air. However helicopter rotors are designed specifically to interact with the air. Therefore, I agree, but I don't see how this is significant.
I've heard you mention your aerodynamic precession theory before. I don't think you're wrong, I just think you're giving another name to something we already understand. If you follow through my very long post where I describe why an apparent 90 degree lag takes place, I think you'll find that the theory behind it is the same for gyroscopic precession and aerodynamic precession.
I say we stop confusing people. Don't use the term precession. It is absolutely wrong. Don't use the term Gyroscopic Precession, it's not theory, it's just an example of something that behaves similarly. From now on to avoid confusion, let's talk about non-relativistic rotational kinematics.
Matthew.
[ 20 November 2001: Message edited by: heedm ]
- The relative mass and rpm of a gyroscopic is high visa-vie the relative mass and rpm of a helicopter rotor.
The rpm of a gyroscope is higher, but the mass of a typical helicopter rotor is higher. What really matters is the magnitude of the angular momentum. Angular momentum is a function of rpm and the distribution of mass about the center of rotation (moment of inertia). Without being given specific examples, it's hard to say which has a higher angular momentum, but the one with the highest will be the hardest to stop.
Try stop a gyro with your hand. Now try stop a helicopter rotor with your hand. Which one was easiest? That one had less angular momentum.
Higher angular momentum only means that when you apply a force, it has less of an effect than if you applied that same force to something with lower angular momentum. That is why a gust of wind hitting a rotor that is slow during startup can result in much more blade motion.
- The relative aerodynamic activity of a gyroscopic is low visa-vie the relative aerodynamic activity of a helicopter rotor.
When I think of a gyroscope, I think of something that has been designed to not interact with air. However helicopter rotors are designed specifically to interact with the air. Therefore, I agree, but I don't see how this is significant.
I've heard you mention your aerodynamic precession theory before. I don't think you're wrong, I just think you're giving another name to something we already understand. If you follow through my very long post where I describe why an apparent 90 degree lag takes place, I think you'll find that the theory behind it is the same for gyroscopic precession and aerodynamic precession.
I say we stop confusing people. Don't use the term precession. It is absolutely wrong. Don't use the term Gyroscopic Precession, it's not theory, it's just an example of something that behaves similarly. From now on to avoid confusion, let's talk about non-relativistic rotational kinematics.
Matthew.
[ 20 November 2001: Message edited by: heedm ]
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Heedm
>"...the mass of a typical helicopter rotor is higher."<
Respectfully, I think that your are loosing sight of the word 'relative'. If a helicopter had a rotor, where the weight was proportional to that of a gyroscope, the helicopter would never lift off the tarmac. Heck, it probably would have sunk into the tarmac.
_________________
>"I think you'll find that the theory behind it is the same for gyroscopic precession and aerodynamic precession ."<
I have always agreed with you on this. In fact, during the previous set of postings I did the math to verify it.
________________
The following is another reason for using 'Aerodynamic non-relativistic rotational kinematics' in preference to 'Gyroscopic non-relativistic rotational kinematics'.
The swashplate and also the basic Bell, 2-blade, teetering (normal to span) rotor exhibit exactly the same characteristic as a gyroscope. I.e. 90-phase offset.
All rotorheads with a flapping hinge offset have a phase offset that is less than 90-degrees. 'Gyroscopic non-relativistic rotational kinematics' cannot represent this (at least not easily). 'Aerodynamic non-relativistic rotational kinematics' (blade flying to position) can.
Would you agree to this?
_______________
We can't be having Lu's ~ England/USA/Australia/& all point between ~ communication problem. We are probably next door neighbors.
[ 20 November 2001: Message edited by: Dave Jackson ]
>"...the mass of a typical helicopter rotor is higher."<
Respectfully, I think that your are loosing sight of the word 'relative'. If a helicopter had a rotor, where the weight was proportional to that of a gyroscope, the helicopter would never lift off the tarmac. Heck, it probably would have sunk into the tarmac.
_________________
>"I think you'll find that the theory behind it is the same for gyroscopic precession and aerodynamic precession ."<
I have always agreed with you on this. In fact, during the previous set of postings I did the math to verify it.
________________
The following is another reason for using 'Aerodynamic non-relativistic rotational kinematics' in preference to 'Gyroscopic non-relativistic rotational kinematics'.
The swashplate and also the basic Bell, 2-blade, teetering (normal to span) rotor exhibit exactly the same characteristic as a gyroscope. I.e. 90-phase offset.
All rotorheads with a flapping hinge offset have a phase offset that is less than 90-degrees. 'Gyroscopic non-relativistic rotational kinematics' cannot represent this (at least not easily). 'Aerodynamic non-relativistic rotational kinematics' (blade flying to position) can.
Would you agree to this?
_______________
We can't be having Lu's ~ England/USA/Australia/& all point between ~ communication problem. We are probably next door neighbors.
[ 20 November 2001: Message edited by: Dave Jackson ]
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Dave,
Perhaps you need a quantitative way to describe your definitioin of 'relative'. How about the Lock number (gamma = rho*c*cla*R^4/Ib where rho is density, c is chord, cla is the lift curve slope, R is rotor radius and Ib is the blade moment of inertia)? So what you're arguing is that gamma is much larger than 1 (and I think a reasonable number for argument's sake would be 8--more for most articulated and teetering rotors, and less for rigid rotors).
Perhaps you need a quantitative way to describe your definitioin of 'relative'. How about the Lock number (gamma = rho*c*cla*R^4/Ib where rho is density, c is chord, cla is the lift curve slope, R is rotor radius and Ib is the blade moment of inertia)? So what you're arguing is that gamma is much larger than 1 (and I think a reasonable number for argument's sake would be 8--more for most articulated and teetering rotors, and less for rigid rotors).
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Edited stuff in italics
Dave, I was a bit confused about what you meant by the relative mass and rpm. The only significant comparison between the two of which I'm aware is angular momentum. I think helicopter rotors have more angular momentum than the average gyroscope.
There are many other relationships that include mass and rpm. Rotational Kinetic Energy is one of the obvious ones. I didn't consider including more concepts to be useful.
As far as what type of non-relativistic rotational kinematics to use, drop the terms "gyroscopic" and "aerodynamic" and I'll agree with you.
The flapping hinge offset is described by basic physics quite easily. This is a very small part of the explanation. There are aerodynamic and control geometry effects that have a much larger effect on gamma. I alluded to this in my long post on gyroscopic precession. The natural frequency of the blade becomes less than the 1/rev of a blade with the flapping hinge coincident with the center of rotation. Because of this, the blade wants to flap in a full cycle in less time it turns through a complete revolution, so when it's finished a 1/4 cycle (max displacement) then blade has rotated less than 90 degrees (gamma less than 90).
Were it not for a driving force that is mechanically in phase to rotation of the blade (via swashplate) then the blade's motion could be quite erratic.
I live in Comox. I was in Vancouver two weeks ago, wondered if I should look you up. Two kids screaming in the back and then being rear ended by some greaseball who figured smoking cigarettes was more important than stopping at red lights reaffirmed my belief that although Vancouver is a fun, beautiful city, the best view of it is the one in the rear view mirror.
Matthew.
[ 20 November 2001: Message edited by: heedm ]
Dave, I was a bit confused about what you meant by the relative mass and rpm. The only significant comparison between the two of which I'm aware is angular momentum. I think helicopter rotors have more angular momentum than the average gyroscope.
There are many other relationships that include mass and rpm. Rotational Kinetic Energy is one of the obvious ones. I didn't consider including more concepts to be useful.
As far as what type of non-relativistic rotational kinematics to use, drop the terms "gyroscopic" and "aerodynamic" and I'll agree with you.
The flapping hinge offset is described by basic physics quite easily. This is a very small part of the explanation. There are aerodynamic and control geometry effects that have a much larger effect on gamma. I alluded to this in my long post on gyroscopic precession. The natural frequency of the blade becomes less than the 1/rev of a blade with the flapping hinge coincident with the center of rotation. Because of this, the blade wants to flap in a full cycle in less time it turns through a complete revolution, so when it's finished a 1/4 cycle (max displacement) then blade has rotated less than 90 degrees (gamma less than 90).
Were it not for a driving force that is mechanically in phase to rotation of the blade (via swashplate) then the blade's motion could be quite erratic.
I live in Comox. I was in Vancouver two weeks ago, wondered if I should look you up. Two kids screaming in the back and then being rear ended by some greaseball who figured smoking cigarettes was more important than stopping at red lights reaffirmed my belief that although Vancouver is a fun, beautiful city, the best view of it is the one in the rear view mirror.
Matthew.
[ 20 November 2001: Message edited by: heedm ]
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To: Dave Jackson
“The swashplate and also the basic Bell, 2-blade, teetering (normal to span) rotor exhibit exactly the same characteristic as a gyroscope. I.e. 90-phase offset”.
“All rotorheads with a flapping hinge offset have a phase offset that is less than 90-degrees. 'Gyroscopic non-relativistic rotational kinematics cannot represent this (at least not easily). 'Aerodynamic non-relativistic rotational kinematics' can.
On the typical Sikorsky with the possible exception of the S-76 the pitch horn leads the blade by 45-degrees. The control servos are offset from their direction of control by 45-degrees. When the pilot pushes forward cyclic the fore and aft servo will move down. This causes the swashplate to dip 45-degrees ahead of the longitudinal centerline. With the swashplate in this position, the advancing blade will have maximum control input and will (pick one), fly as an individual blade down over the nose or, it will dip down over the nose as a part of a collective assembly called a rotor system, with this movement, caused by gyroscopic precession. In either case, the opposite blade will be doing exactly that, move in the opposite direction. Other helicopters have different relationships between the pitch horn and the positioning of the servos but in any case when you add up the servo offset and the pitch horn lead it adds up to 90-degrees.
“The swashplate and also the basic Bell, 2-blade, teetering (normal to span) rotor exhibit exactly the same characteristic as a gyroscope. I.e. 90-phase offset”.
“All rotorheads with a flapping hinge offset have a phase offset that is less than 90-degrees. 'Gyroscopic non-relativistic rotational kinematics cannot represent this (at least not easily). 'Aerodynamic non-relativistic rotational kinematics' can.
On the typical Sikorsky with the possible exception of the S-76 the pitch horn leads the blade by 45-degrees. The control servos are offset from their direction of control by 45-degrees. When the pilot pushes forward cyclic the fore and aft servo will move down. This causes the swashplate to dip 45-degrees ahead of the longitudinal centerline. With the swashplate in this position, the advancing blade will have maximum control input and will (pick one), fly as an individual blade down over the nose or, it will dip down over the nose as a part of a collective assembly called a rotor system, with this movement, caused by gyroscopic precession. In either case, the opposite blade will be doing exactly that, move in the opposite direction. Other helicopters have different relationships between the pitch horn and the positioning of the servos but in any case when you add up the servo offset and the pitch horn lead it adds up to 90-degrees.
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Thanks Kyrilian
The Lock number could be a perfect way to scale the rotor.
It might also be usable for scaling the gyroscope, by using '1' as a constant for 'density x lift curve slope'
______________
Heedm
>"As far as what type of non-relativistic rotational kinematics to use, drop the terms "gyroscopic" and "aerodynamic" and I'll agree with you.
I'm happy to drop the term 'gyroscopic'. It is a good one for holding a spinning bicycle wheel in your hand and saying "Wow". It is also a valid analogy for describing a basic teetering rotor head, which is used in conjunction with a 90-degree offset pitch horn and a swashplate.
I believe that the analogy with the gyroscope starts to lose its validity as delta-3, flapping hinge offset and rigid rotors are introduced. It will probably lose more of its validity as 'smart materials' are incorporated into the rotor blades. An expert in the field has stated that the swashplate is no longer totally compatible with many current rotorhead designs.
The helicopter rotor is an aerodynamic device. I believe that the best way to describe its operation, is aerodynamically (I.e. the blade flies to position), now and even more so in the future.
So what have you got to say to that?
______________
Lu
Why introduce the control system for 'warp drive' into the argument when we are still try to put the simple gyroscope to rest.
The Lock number could be a perfect way to scale the rotor.
It might also be usable for scaling the gyroscope, by using '1' as a constant for 'density x lift curve slope'
______________
Heedm
>"As far as what type of non-relativistic rotational kinematics to use, drop the terms "gyroscopic" and "aerodynamic" and I'll agree with you.
I'm happy to drop the term 'gyroscopic'. It is a good one for holding a spinning bicycle wheel in your hand and saying "Wow". It is also a valid analogy for describing a basic teetering rotor head, which is used in conjunction with a 90-degree offset pitch horn and a swashplate.
I believe that the analogy with the gyroscope starts to lose its validity as delta-3, flapping hinge offset and rigid rotors are introduced. It will probably lose more of its validity as 'smart materials' are incorporated into the rotor blades. An expert in the field has stated that the swashplate is no longer totally compatible with many current rotorhead designs.
The helicopter rotor is an aerodynamic device. I believe that the best way to describe its operation, is aerodynamically (I.e. the blade flies to position), now and even more so in the future.
So what have you got to say to that?
______________
Lu
Why introduce the control system for 'warp drive' into the argument when we are still try to put the simple gyroscope to rest.
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To: Dave Jackson
“I'm happy to drop the term 'gyroscopic'. It is a good one for holding a spinning bicycle wheel in your hand and saying "Wow". It is also a valid analogy for describing a basic teetering rotor head, which is used in conjunction with a 90-degree offset pitch horn and a swashplate”.
Response:
Why do you feel that a Bell rotor system emulates the qualities of a gyroscope just because it has a 90-degree pitch horn? Yet you totally discount the Sikorsky and all of the other multi blade helicopters which have a precession angle of 90-degrees. The Bell is a two-blade system and it is designed with a 90-degree lead on the pitch horn. The Robinson has a 72-degree lead on the pitch horn and it too is a two-blade system. The reason it has a 72-degree lead is because the pitch horn can’t cross the cone hinge. If it did, it would have severe pitch flap coupling to such an extent that it would be uncontrollable. That is why I feel so strongly about the R-44 as on it the pitch horn has crossed the cone hinge causing a reversal to the normal pitch flap coupling.
If you remember in past posts I acknowledged aerodynamic precession as an alternate theory to gyroscopic precession but whether you think aerodynamic or gyroscopic precession the precession or phase angle is 90-degrees. How in engineering or plane talk do you prove aerodynamics over gyroscopic precession when the resultant to the pilots input takes place 90-degrees later in the direction? What law in aerodynamics or blade theory dictates that from an aerodynamic standpoint the blade will respond in 90-degrees?
[ 21 November 2001: Message edited by: Lu Zuckerman ]
“I'm happy to drop the term 'gyroscopic'. It is a good one for holding a spinning bicycle wheel in your hand and saying "Wow". It is also a valid analogy for describing a basic teetering rotor head, which is used in conjunction with a 90-degree offset pitch horn and a swashplate”.
Response:
Why do you feel that a Bell rotor system emulates the qualities of a gyroscope just because it has a 90-degree pitch horn? Yet you totally discount the Sikorsky and all of the other multi blade helicopters which have a precession angle of 90-degrees. The Bell is a two-blade system and it is designed with a 90-degree lead on the pitch horn. The Robinson has a 72-degree lead on the pitch horn and it too is a two-blade system. The reason it has a 72-degree lead is because the pitch horn can’t cross the cone hinge. If it did, it would have severe pitch flap coupling to such an extent that it would be uncontrollable. That is why I feel so strongly about the R-44 as on it the pitch horn has crossed the cone hinge causing a reversal to the normal pitch flap coupling.
If you remember in past posts I acknowledged aerodynamic precession as an alternate theory to gyroscopic precession but whether you think aerodynamic or gyroscopic precession the precession or phase angle is 90-degrees. How in engineering or plane talk do you prove aerodynamics over gyroscopic precession when the resultant to the pilots input takes place 90-degrees later in the direction? What law in aerodynamics or blade theory dictates that from an aerodynamic standpoint the blade will respond in 90-degrees?
[ 21 November 2001: Message edited by: Lu Zuckerman ]
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Dave said, "I believe that the analogy with the gyroscope starts to lose its validity as delta-3, flapping hinge offset and rigid rotors are introduced. It will probably lose more of its validity as 'smart materials' are incorporated into the rotor blades."
If you apply a moment to a rotating body, the result must be a vector sum of the original angular momentum and the impressed angular momentum. Doesn't matter if there is an unusual control geometry, aerodynamic effects, etc. Gyroscopic precession is actually an illustration of that effect. Many basic helicopter texts use gyroscopic precession to mean that effect. If you use this latter definition, then it's valid as long as the rotors are turning.
"The helicopter rotor is an aerodynamic device. I believe that the best way to describe its operation...."
I try to talk only about the motion of the rotors due to a force being impressed upon them. Yes, some of those forces are generated aerodynamically. How they are generated does not change the effect they have on the system.
Rotors are also accelerated by an internal combustion engine. Should that be in a theory on why rotors lag by 90 degrees?
I really don't care what you want to call it. To me, rotational dynamics says it all. So does many other terms. I don't see that specifying the origin of these forces creates any deeper understanding of the concepts, rather it may confuse.
I'm sure given the budget we could build a model of a rotor system that generates "lift" through magnetism or something other than aerodynamics. It would exhibit a 90 degree lag as well.
Matthew.
[ 21 November 2001: Message edited by: heedm ]
If you apply a moment to a rotating body, the result must be a vector sum of the original angular momentum and the impressed angular momentum. Doesn't matter if there is an unusual control geometry, aerodynamic effects, etc. Gyroscopic precession is actually an illustration of that effect. Many basic helicopter texts use gyroscopic precession to mean that effect. If you use this latter definition, then it's valid as long as the rotors are turning.
"The helicopter rotor is an aerodynamic device. I believe that the best way to describe its operation...."
I try to talk only about the motion of the rotors due to a force being impressed upon them. Yes, some of those forces are generated aerodynamically. How they are generated does not change the effect they have on the system.
Rotors are also accelerated by an internal combustion engine. Should that be in a theory on why rotors lag by 90 degrees?
I really don't care what you want to call it. To me, rotational dynamics says it all. So does many other terms. I don't see that specifying the origin of these forces creates any deeper understanding of the concepts, rather it may confuse.
I'm sure given the budget we could build a model of a rotor system that generates "lift" through magnetism or something other than aerodynamics. It would exhibit a 90 degree lag as well.
Matthew.
[ 21 November 2001: Message edited by: heedm ]
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Lu
>"Why do you feel that a Bell rotor system emulates the qualities of a gyroscope just because it has a 90-degree pitch horn?"<
I don't.
The following statement is from an aerodynamist, who's name I've forgotten. "For blades freely articulated at the center of rotation, or teetering rotors, the response is lagged by exactly 90-degrees in hover".
The pitch horn does not make the rotor emulate the gyroscope. The pitch horn only aligns the cyclic control stick with the rotor.
____________________
>"Yet you totally discount the Sikorsky and all of the other multi blade helicopters. "<
I don't discount the Sikorsky. Its only that I, you and perhaps a few others are having difficulty understanding the basics, so it is probably a little premature to introduce additional complexities.
____________________
Re the Robinson helicopters:
Lu, we are currently having trouble with one attribute of the rotor. The rotor has dozens of attributes. It is therefore a little premature to come to conclusions on the Robinson's rotors. The best we can do, at this point in time, is speculate.
_____________
Re non-relativistic rotational kinematics
Nick explained gyroscopic precession a couple of days ago. Heedm went to even greater lengths in explaining it a couple of months ago. How can you keep asking for an explanation when it would appear that you are not attempting to delve into what they are saying?
I feel that this has to be understood before phase lag and delta-3 can be considered. You may wish to re-digest what they have said.
[ 21 November 2001: Message edited by: Dave Jackson ]
>"Why do you feel that a Bell rotor system emulates the qualities of a gyroscope just because it has a 90-degree pitch horn?"<
I don't.
The following statement is from an aerodynamist, who's name I've forgotten. "For blades freely articulated at the center of rotation, or teetering rotors, the response is lagged by exactly 90-degrees in hover".
The pitch horn does not make the rotor emulate the gyroscope. The pitch horn only aligns the cyclic control stick with the rotor.
____________________
>"Yet you totally discount the Sikorsky and all of the other multi blade helicopters. "<
I don't discount the Sikorsky. Its only that I, you and perhaps a few others are having difficulty understanding the basics, so it is probably a little premature to introduce additional complexities.
____________________
Re the Robinson helicopters:
Lu, we are currently having trouble with one attribute of the rotor. The rotor has dozens of attributes. It is therefore a little premature to come to conclusions on the Robinson's rotors. The best we can do, at this point in time, is speculate.
_____________
Re non-relativistic rotational kinematics
Nick explained gyroscopic precession a couple of days ago. Heedm went to even greater lengths in explaining it a couple of months ago. How can you keep asking for an explanation when it would appear that you are not attempting to delve into what they are saying?
I feel that this has to be understood before phase lag and delta-3 can be considered. You may wish to re-digest what they have said.
[ 21 November 2001: Message edited by: Dave Jackson ]
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Heedm
This is getting perversely pleasurable.
>"If you apply a moment to a rotating body, the result must be a vector sum of the original angular momentum and the impressed angular momentum."<
>"I don't see that specifying the origin of these forces creates any deeper understanding of the concepts, rather it may confuse."<
Why talk about "impressed angular momentum' when one can eliminate the middlemen, go right to the source and just say 'thrust'?
________________
>"Yes, some of those forces are generated aerodynamically. "<
>"Rotors are also accelerated by an internal combustion engine. Should that be in a theory on why rotors lag by 90 degrees? "<
There is only one force that is of interest. It is the only one that is variable and it is aerodynamic. The "original angular momentum" is an uninteresting constant. Or at least the RRPM better be a constant or there are serious problems.
__________
As previously mentioned, we are talking about the same thing. I prefer looking at it aerodynamically and use aerodynamic algorithms, where as you prefer to look at it dynamically and use dynamic algorithms. Are these fair statements?
This is getting perversely pleasurable.
>"If you apply a moment to a rotating body, the result must be a vector sum of the original angular momentum and the impressed angular momentum."<
>"I don't see that specifying the origin of these forces creates any deeper understanding of the concepts, rather it may confuse."<
Why talk about "impressed angular momentum' when one can eliminate the middlemen, go right to the source and just say 'thrust'?
________________
>"Yes, some of those forces are generated aerodynamically. "<
>"Rotors are also accelerated by an internal combustion engine. Should that be in a theory on why rotors lag by 90 degrees? "<
There is only one force that is of interest. It is the only one that is variable and it is aerodynamic. The "original angular momentum" is an uninteresting constant. Or at least the RRPM better be a constant or there are serious problems.
__________
As previously mentioned, we are talking about the same thing. I prefer looking at it aerodynamically and use aerodynamic algorithms, where as you prefer to look at it dynamically and use dynamic algorithms. Are these fair statements?