Helicopter Dynamics: Gyroscopic Precession
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Perhaps one way to clarify the way that the disk is controlled by aerodynamic forces, rather than precession is to remember that some Kaman helicopters had trailing edge control surfaces, rather than push/pull rods at the blade cuff.
The blades fly to their new position, as required by the pilot. The rest of the helicopter follows them.
I regularly argued this with the late Lu Z. He was convinced that the forces put in by the pilot precessed "the gyro" of the rotor disk, ignoring aerodynamics.
The blades fly to their new position, as required by the pilot. The rest of the helicopter follows them.
I regularly argued this with the late Lu Z. He was convinced that the forces put in by the pilot precessed "the gyro" of the rotor disk, ignoring aerodynamics.
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Yes. That makes a great deal of sense.
Those aerodynamic forces do still "precess a gyro" though, right?
There's still a lot of angular momentum to redirect.
The torque from the changed lift on the blades acts about 90 degrees out of phase from the input, perpendicular to the intended direction of motion, to shift the angular momentum of the disk in the direction intended, pulling the hanging part along as desired, with a response speed that is still limited by the size of the torque that can act. I guess the lift and angular momentum both scale with disk area and with rotation speed squared, so it naturally coordinates.
It's very impressive to think how the pioneers got it all to work.
Those aerodynamic forces do still "precess a gyro" though, right?
There's still a lot of angular momentum to redirect.
The torque from the changed lift on the blades acts about 90 degrees out of phase from the input, perpendicular to the intended direction of motion, to shift the angular momentum of the disk in the direction intended, pulling the hanging part along as desired, with a response speed that is still limited by the size of the torque that can act. I guess the lift and angular momentum both scale with disk area and with rotation speed squared, so it naturally coordinates.
It's very impressive to think how the pioneers got it all to work.
Those aerodynamic forces do still "precess a gyro" though, right?
There's still a lot of angular momentum to redirect.
There's still a lot of angular momentum to redirect.
Do you precess a gyro, or does a gyro precess?
Angular momentum is a red herring.
Sounds like we are going to embed ourselves in the "centrifugal is a real force because the FAA ground school says so" argument, also a Lu Z favourite cherished from the past.....
Gyros require rigidity....that is the underlying difference. When you apply a force to any part of the gyro to tilt it, you are applying it to the entire rigid shape, but you are not doing this to the (conventional) rotor system we are considering......
The blades...as individuals.... fly differently when their own pitch is changed. They do not react as a solid system, rather each blade flies it's own path, it is just for ease of understanding that we can think of it as a "disc" and thus we entertain simple explanations like gyroscopic precession. But it is not a "disc" it is a set of fast moving individually free blades following a very similar path that looks like a solid disc to an observer.
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A gyro precesses if acted on by a couple. Not otherwise.
Angular momentum is a red herring?
Try stopping a spinning bike wheel with your tongue!
The angular momentum of the disk is what it is, whether it's a solid disk or a series of spokes. To change its direction needs a couple applied normal to the angular momentum and to desired direction of motion. If you swing a bolas around your head, it's not easy to change its plane of motion.
For a helicopter, I'm quite happy that this couple comes from aerodynamic forces on the individual blades and not torque from the control rods, but in order to redirect that vector a couple must applied, and there is going to be a reaction force at the hub, as well as the standard transmission of the life and thrust forces to the body of the helicopter.
Angular momentum is a red herring?
Try stopping a spinning bike wheel with your tongue!
The angular momentum of the disk is what it is, whether it's a solid disk or a series of spokes. To change its direction needs a couple applied normal to the angular momentum and to desired direction of motion. If you swing a bolas around your head, it's not easy to change its plane of motion.
For a helicopter, I'm quite happy that this couple comes from aerodynamic forces on the individual blades and not torque from the control rods, but in order to redirect that vector a couple must applied, and there is going to be a reaction force at the hub, as well as the standard transmission of the life and thrust forces to the body of the helicopter.
Angular momentum is a red herring....that means it is not relevant to the discussion at hand, it doesn't mean I should stick my tongue on a bicycle wheel/gyroscope to see if it is real.
Yes, it is real.
Maybe I should try stopping a elephant charging in a very straight line with no angular momentum using only my tongue so that I can truly grasp the concepts.
Yes, it is real.
Maybe I should try stopping a elephant charging in a very straight line with no angular momentum using only my tongue so that I can truly grasp the concepts.
awblain, how does a puff of wind apply a couple to the disc? Remember that the forces coming from the swash plate are being applied through 360 degrees of rotation, not just at one spot at 90 degrees.
For your theory to work, one blade receives an input ONLY at 90 degrees to the pilot's right, no more, no less, and that makes it precess down. Something pushes down on the tip of the blade for a poofteenth of a second, or pushes on the left side of the mast, and the disc tilts. Horsefeathers.
Precession and couples at 90 degrees are not the reason a rotor system behaves the way it does.
For your theory to work, one blade receives an input ONLY at 90 degrees to the pilot's right, no more, no less, and that makes it precess down. Something pushes down on the tip of the blade for a poofteenth of a second, or pushes on the left side of the mast, and the disc tilts. Horsefeathers.
Precession and couples at 90 degrees are not the reason a rotor system behaves the way it does.
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Ascend,
It's not a "puff of wind", it's the bulk airflow over the blades. How does this puff of wind lift the helicopter off the ground?
The control input modifies the blade flight and airflow, and in a hugely amplified way, the lift is changed - you directed me right on that, so follow through.
It's a question of how the dynamics of the blades are changed, not that they are well-defined and that they change.
Then there is something special about a rotor system that makes it different from other objects in the rest of classical mechanics, unless by "precession and couples", you mean something different from angular momentum and forces. Summarize what it is, and I will believe you.
Helmet,
Stopping the elephant with your tongue would be equally illuminating if we concerned about linear momentum. If you were changing the flightpath of a fixed wing aircraft it would be important, and if you're barreling along in a helicopter it matters too. I disagree that it's a red herring.
It's not a "puff of wind", it's the bulk airflow over the blades. How does this puff of wind lift the helicopter off the ground?
The control input modifies the blade flight and airflow, and in a hugely amplified way, the lift is changed - you directed me right on that, so follow through.
It's a question of how the dynamics of the blades are changed, not that they are well-defined and that they change.
Precession and couples at 90 degrees are not the reason a rotor system behaves the way it does.
Helmet,
Stopping the elephant with your tongue would be equally illuminating if we concerned about linear momentum. If you were changing the flightpath of a fixed wing aircraft it would be important, and if you're barreling along in a helicopter it matters too. I disagree that it's a red herring.
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Think of one individual blade. Like all mass, it has inertia and so is actually trying to travel in a straight line relative to the universe. However, the hinged "tie" to the rotor mast means that it rotates around that, instead (this generates tensile forces along the blade and at the blade root, but they can be ignored for the purpose of the discussion).
To get the blade to fly to the new position required by the pilot, inputs are made to change the blade pitch angle. The resulting changes in airflow then alter the aerodynamic force, which overcomes the inertia of the blade. It's position relative to the vertical axis of the mast changes as its inertia is overcome.
As the blade changes its path. it applies forces to the rotor mast, so the helicopter is dragged along behind.....
BTW, gyro theory became more logical to me when I considered what an individual molecule of its mass was doing. It's all about inertia!
To get the blade to fly to the new position required by the pilot, inputs are made to change the blade pitch angle. The resulting changes in airflow then alter the aerodynamic force, which overcomes the inertia of the blade. It's position relative to the vertical axis of the mast changes as its inertia is overcome.
As the blade changes its path. it applies forces to the rotor mast, so the helicopter is dragged along behind.....
BTW, gyro theory became more logical to me when I considered what an individual molecule of its mass was doing. It's all about inertia!
Then there is something special about a rotor system that makes it different from other objects in the rest of classical mechanics
The rotor system is not a special system in terms of classical mechanics, it is a different system because it is not a gyroscope. Thus the laws of gyroscope mechanics do not apply to the rotor system because the rotor system does not qualify as a gyroscope - at least the one we are talking about here.
Angular, or linear, bike wheel, or elephant momentum is still real, and it is still part of what is happening in the rotor system and the gyro....but it remains irrelevant to our discussion on precession.
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The property of angular momentum is due to rotation symmetry of our Universe.
If has no concern about hinges or lack thereof.
Just drop the "gyroscope" and "precession" labels. A gyroscope is a particular type of device that has angular momentum. Angular momentum is changed by the action of a couple in the same way, whether it's in a gyroscope, an elephant or a fruitbat.
If has no concern about hinges or lack thereof.
Just drop the "gyroscope" and "precession" labels. A gyroscope is a particular type of device that has angular momentum. Angular momentum is changed by the action of a couple in the same way, whether it's in a gyroscope, an elephant or a fruitbat.
Awblain - if your gyro precession theory was correct, we wouldn't need to change the pitch of the blades and could do without feathering hinges altogether.
All we would need is a hydraulic ram to exert a linear force 90 degrees before the desired blade position, give it a poke or pull and let precession do the rest.
It doesn't work like that, as you can tell by looking at pretty much any helicopter in the world - we need the feathering hinges (or similar aerodynamic device like tabs) to change the pitch on the blades because it is the aerodynamic forces that dictate where and how the blades move, not gyroscopic precession.
All we would need is a hydraulic ram to exert a linear force 90 degrees before the desired blade position, give it a poke or pull and let precession do the rest.
It doesn't work like that, as you can tell by looking at pretty much any helicopter in the world - we need the feathering hinges (or similar aerodynamic device like tabs) to change the pitch on the blades because it is the aerodynamic forces that dictate where and how the blades move, not gyroscopic precession.
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[email protected],
I don't have a gyroscopic precession theory, but I am standing on the shoulders of the giants from the collective work of over 300 years of classical mechanics. If that's not holding true in helicopters, then we need to find an Einstein to put us straight about what's actually happening.
I agree entirely that aerodynamic forces impose the couple, and they need the hinges; saying change the direction of angular momentum is just a complementary way of saying what you're saying.
That's exactly what you have in the controls - a mechanism to ensure that the blade flies in a way that alters its motion in the desired direction. That comes from an aerodynamic force, but where does it act? To tilt the disk down at the front and up at the back, where are the maximum and minimum lifting sections of the disk? To the left and right or to fore and aft?
If you were to do that you would get just such a motion. Twist a portable fan or a large circular saw in your hands to watch it at work. But the moment you are required to impose for rapid motion is large. In a helicopter that large force comes from the airflow on the blades not from the hub, but it's still there.
We should drop "gyroscope", as it too vividly conjures pictures of a spinning top toy, but I claim that the angular momentum of the rotor remains an important, defining property. If it wasn't, then you could roll or pitch the machine as fast in rpm as you can yaw it, and I don't believe that's nearly the case.
I don't have a gyroscopic precession theory, but I am standing on the shoulders of the giants from the collective work of over 300 years of classical mechanics. If that's not holding true in helicopters, then we need to find an Einstein to put us straight about what's actually happening.
I agree entirely that aerodynamic forces impose the couple, and they need the hinges; saying change the direction of angular momentum is just a complementary way of saying what you're saying.
That's exactly what you have in the controls - a mechanism to ensure that the blade flies in a way that alters its motion in the desired direction. That comes from an aerodynamic force, but where does it act? To tilt the disk down at the front and up at the back, where are the maximum and minimum lifting sections of the disk? To the left and right or to fore and aft?
All we would need is a hydraulic ram to exert a linear force 90 degrees before the desired blade position, give it a poke or pull and let precession do the rest.
We should drop "gyroscope", as it too vividly conjures pictures of a spinning top toy, but I claim that the angular momentum of the rotor remains an important, defining property. If it wasn't, then you could roll or pitch the machine as fast in rpm as you can yaw it, and I don't believe that's nearly the case.
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All we would need is a hydraulic ram to exert a linear force 90 degrees before the desired blade position, give it a poke or pull and let precession do the rest.
Awblain, You're on the right (blade) track. A lighter blade system does react quicker to pilot inputs iaw Newton's first and second laws.
The pilot only has control of the pitch angle of the blades, nothing else. The pitch change rods rotate the blades in the sense of blade pitch; they don't directly push/pull the blade roots. If they did, the feedback forces would be huge and the aircraft would tend to move, whilst the blades carried on in rigidity, iaw gyroscopic properties.
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Thank you, Shy, I believe that is all entirely correct.
I just found a few model helicopter forums where people are worried about why the roll rate of their toys is (claimed inexplicably) limited, and why when they swap to using heavier/longer blades it is even more limited.
I just found a few model helicopter forums where people are worried about why the roll rate of their toys is (claimed inexplicably) limited, and why when they swap to using heavier/longer blades it is even more limited.
We're resorting to MODELS now?
And you wonder why they don't scale up?
70 years ago they were making models fly with bamboo blades, and "proving" their theories of flight, but when they built a real aircraft it was uncontrollable. Until they put hinges on the blades and used a swash plate. Because the toy models relied on gyroscopic precession, and real machines needed aerodynamics. Toss in some Reynolds numbers and see why.
Of course angular momentum is in there. The bigger the blade inertia, the more there is. Nobody is denying that. But it isn't relying on precession to fly the disc to a new attitude.
Dunno why we bother replying to your posts, as you are impervious to reality. Ask Nick.
And you wonder why they don't scale up?
70 years ago they were making models fly with bamboo blades, and "proving" their theories of flight, but when they built a real aircraft it was uncontrollable. Until they put hinges on the blades and used a swash plate. Because the toy models relied on gyroscopic precession, and real machines needed aerodynamics. Toss in some Reynolds numbers and see why.
Of course angular momentum is in there. The bigger the blade inertia, the more there is. Nobody is denying that. But it isn't relying on precession to fly the disc to a new attitude.
Dunno why we bother replying to your posts, as you are impervious to reality. Ask Nick.
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We're not resorting to anything, we're trying to avoid incomplete or misleading statements - like yours about a "puff of wind" and "scaling" which miss the point.
The impression that I get from you is that there is no place for mentioning that the control inputs sculpt much more powerful aerodynamic forces to shift the orientation of rotation of the disk, whether rigid, flexible, flapping or whatever, which is odd, since that's what's happening.
It's a rotating system whose angular momentum is being redirected, so it is precessing. That might not help to illustrate some point about stability, and using the terms "precess" and "gyroscope" probably isn't helpful as it seems to imply rigidity to you, but it is a complementary explanation of how the disk arrives at a new attitude, and it would affect how, and how quickly, stability is lost. You get precession from a torque at an angle that's not 90 degrees away from a rotation axis, it's just different, and more complex, as the motion doesn't stay in a plane.
Where does the differential lift induced by the controlled blade pitch changes point? Which way does it try to twist the disk, and which way does the disk move? That's a question for you, and one which I think you answered correctly on 3rd March.
The impression that I get from you is that there is no place for mentioning that the control inputs sculpt much more powerful aerodynamic forces to shift the orientation of rotation of the disk, whether rigid, flexible, flapping or whatever, which is odd, since that's what's happening.
It's a rotating system whose angular momentum is being redirected, so it is precessing. That might not help to illustrate some point about stability, and using the terms "precess" and "gyroscope" probably isn't helpful as it seems to imply rigidity to you, but it is a complementary explanation of how the disk arrives at a new attitude, and it would affect how, and how quickly, stability is lost. You get precession from a torque at an angle that's not 90 degrees away from a rotation axis, it's just different, and more complex, as the motion doesn't stay in a plane.
Where does the differential lift induced by the controlled blade pitch changes point? Which way does it try to twist the disk, and which way does the disk move? That's a question for you, and one which I think you answered correctly on 3rd March.
There is no disk!!!!!!!! There is only a set of free to flap and feather blades.
The blade is merely a wing. It reacts to pitch and velocity.
When pitch or velocity changes, the lift forces change. No different to the wing of a plane. When alpha changes cyclically like a helicopter, then blade flies up and blade flies down, when it changes on a plane, wing flies up or wing flies down. Plane does not roll or rotate 90 degrees every time the pitch (therefore lift and drag, etc) changes..... Must be the lack of angular momentum I guess.....
So the blade is clever enough to know that it should change it's behaviour based not on cyclical pitch change, but because it must act as a gyroscope? And it knows this because when it was a normal aerodynamic wing, it was getting the airflow at roughly the same velocity across it's whole body, but now it is a wing of a helicopter it can tell that the airflow is slow on one end of it, and fast on the other....... Therefore, if your airflow is slow at the root and fast at the tip, you should behave gyroscopically as a wing. Is that the idea?
Or....does a helicopter wing (blade) behave as a wing and react to airflow velocity and pitch changes like all other wings?
The blade is merely a wing. It reacts to pitch and velocity.
When pitch or velocity changes, the lift forces change. No different to the wing of a plane. When alpha changes cyclically like a helicopter, then blade flies up and blade flies down, when it changes on a plane, wing flies up or wing flies down. Plane does not roll or rotate 90 degrees every time the pitch (therefore lift and drag, etc) changes..... Must be the lack of angular momentum I guess.....
So the blade is clever enough to know that it should change it's behaviour based not on cyclical pitch change, but because it must act as a gyroscope? And it knows this because when it was a normal aerodynamic wing, it was getting the airflow at roughly the same velocity across it's whole body, but now it is a wing of a helicopter it can tell that the airflow is slow on one end of it, and fast on the other....... Therefore, if your airflow is slow at the root and fast at the tip, you should behave gyroscopically as a wing. Is that the idea?
Or....does a helicopter wing (blade) behave as a wing and react to airflow velocity and pitch changes like all other wings?
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Models follow the same laws of physics as manned helicopters, so don't automatically dismiss an argument just because the argument mentions them.
Regarding angular momentum, the path of each rotor blade can be characterized just as accurately using the concept of angular momentum as it can without it. It's just apples and oranges... 6 of one, half dozen of another. Angular momentum is just an extension of F=MA.
I write rotor dynamics code for full flight sims (several of you have probably even flown them), and the results are identical regardless of whether they are based on angular momentum or not.
Regarding angular momentum, the path of each rotor blade can be characterized just as accurately using the concept of angular momentum as it can without it. It's just apples and oranges... 6 of one, half dozen of another. Angular momentum is just an extension of F=MA.
I write rotor dynamics code for full flight sims (several of you have probably even flown them), and the results are identical regardless of whether they are based on angular momentum or not.
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The individual blades are rotating wing. You need to consider its winginess and its rotating nature to get a picture of what it's doing.
Helmetfire, a bolas or a slingshot is effectively a single/double/triple blade; try not paying attention to their angular momentum when playing with them and you'll get hurt.
Helmetfire, a bolas or a slingshot is effectively a single/double/triple blade; try not paying attention to their angular momentum when playing with them and you'll get hurt.
We should drop "gyroscope", as it too vividly conjures pictures of a spinning top toy, but I claim that the angular momentum of the rotor remains an important, defining property. If it wasn't, then you could roll or pitch the machine as fast in rpm as you can yaw it, and I don't believe that's nearly the case.
The distance between the flapping hinge and the rotor hub dictates the control power (which can be loosely defined as how much you have to move the cyclic to achieve a specified change in attitude of the fuselage). Where there is no mechanical hinge (Lynx for example which uses the flexing of the titanium rotor head) an 'effective hinge offset' is given as a percentage of the distance between hub and blade tip and represents where a mechanical hinge would have to be to achieve the same control power.
So the pitch and roll rate of a helicopter is not limited by precession, it is limited by its rotor hub design and its control power.
On a teetering head the fuselage follows the rotors under the influence of gravity (hence why low G is so dangerous as you have little control over the fuselage position), with mechanical or composite hinges you create a lever whereby moving the blades provides an equal and opposite reaction on the fuselage.
Awblain, perhaps you should look at how the pitch change mechanism works - a pitch change rod following a swash plate produces a sinusoidal vertical movement akin to a piston in a cylinder in a car engine. The movement of the pitch change rod is not constant, it has two stationary points (like top dead centre and bottom dead centre) and accelerates and decelerates between the two. Hence the rate of pitch change (and therefore its resulting aerodynamic effects) are at maximums 90 degrees from the point where the initial change is made (ie from the TDC or BDC).