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Coriolis vs Conservation of Angular momentum

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Coriolis vs Conservation of Angular momentum

Old 14th Dec 2020, 22:35
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Coriolis vs Conservation of Angular momentum

As I understand it, conservation of angular momentum is what explains the phenomenon of rotor blade acceleration/ deceleration during a corresponding CG change in relation to its axis of rotation. To me, this has nothing to do with Coriolis force, which as I understand it is really just a kinematics term to describe a fictitious force that results in a changing vector on moving reference frame. I am failing to see how this relates to a rotor system. Please enlighten me, am I missing part of the puzzle or are helicopter text books using incorrect terminology?
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Old 15th Dec 2020, 03:05
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Things gotta be Trump-ised, or dumbed down, to be understood by the lowest common denominator, the student pilot.

Projecting a 3-D object onto a 2-D horizontal plane produces some funny-looking results - a rotor system, coned upwards with power, and tilted forward for flight, gives the image of the front blade becoming longer and the rear blade becoming shorter. Bring in Kenny Coriolis, and Bob is your mother's brother, it is understandable. You are probably looking at the FAA books online, and last time I looked, they were full of errors of interpretation and of statement. Bring a pinch of salt, and keep your mind open. You seem to have the hang of it.
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Old 15th Dec 2020, 06:36
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Originally Posted by Ascend Charlie View Post
Things gotta be Trump-ised, or dumbed down, to be understood by the lowest common denominator, the student pilot.

Projecting a 3-D object onto a 2-D horizontal plane produces some funny-looking results - a rotor system, coned upwards with power, and tilted forward for flight, gives the image of the front blade becoming longer and the rear blade becoming shorter. Bring in Kenny Coriolis, and Bob is your mother's brother, it is understandable. You are probably looking at the FAA books online, and last time I looked, they were full of errors of interpretation and of statement. Bring a pinch of salt, and keep your mind open. You seem to have the hang of it.
........absolutely solid gold AC.
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Old 15th Dec 2020, 08:35
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Originally Posted by heliman500 View Post
As I understand it, conservation of angular momentum is what explains the phenomenon of rotor blade acceleration/ deceleration during a corresponding CG change in relation to its axis of rotation. To me, this has nothing to do with Coriolis force, which as I understand it is really just a kinematics term to describe a fictitious force that results in a changing vector on moving reference frame. I am failing to see how this relates to a rotor system. Please enlighten me, am I missing part of the puzzle or are helicopter text books using incorrect terminology?
The rotor blades are often analyzed in a coordinates system that rotates with the rotor blade, has its origin in the rotor center, and a primary axis parallel to the (coned) pitch axis. In this system you get a coriolis force, propeller moment, etc.
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Old 15th Dec 2020, 12:48
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If you actually understand what Coriolis Affect is, you will also understand that it has no affect in a reference space as small as the diameter of a helicopter rotor system.
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Old 15th Dec 2020, 13:28
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Originally Posted by RMK View Post
If you actually understand what Coriolis Affect is, you will also understand that it has no affect in a reference space as small as the diameter of a helicopter rotor system.
RMK is correct. The two are related, as the Coriolis is "caused" by movement N/S being closer(or further away) to the perpendicular of Earths axis of rotation, thus being closer to the 'centre of the roundabout'. But over a rotor disc dimension, the effect of Earth's Curvature is beyond negligible. Blade coning, however, will cause an increase in Nr as the diameter will be (slightly) reduced, and therefore angular velocity will increase in order to conserve angular momentum.

Same reason it's funny to get kids to sit at the edge of roundabout as you start pushing it, then get them to try and touch the middle. Hilarious.
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Old 15th Dec 2020, 15:29
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Originally Posted by RMK View Post
If you actually understand what Coriolis Affect is, you will also understand that it has no affect in a reference space as small as the diameter of a helicopter rotor system.
if you (and DeltaNg) actually understand what Coriolis Force/Effect is, you will also understand that it applies to all rotating frames of reference, and the example using the earth (typically long range artillery and weather systems) is but one example.

Last edited by 212man; 15th Dec 2020 at 15:52.
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Old 15th Dec 2020, 15:44
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Originally Posted by RMK View Post
If you actually understand what Coriolis Affect is, you will also understand that it has no affect in a reference space as small as the diameter of a helicopter rotor system.
,...but what about Coriolis "Effect"?
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Old 15th Dec 2020, 17:29
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If a bolt detaches itself from a blade root whilst the rotor is turning, does it end up along the same blade at the tip? Nope - Coriolis Effect is an apparent deviation in a rotating system, even a rotor disc.
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Old 15th Dec 2020, 17:37
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Originally Posted by DeltaNg View Post
RMK is correct. The two are related, as the Coriolis is "caused" by movement N/S being closer(or further away) to the perpendicular of Earths axis of rotation, thus being closer to the 'centre of the roundabout'. But over a rotor disc dimension, the effect of Earth's Curvature is beyond negligible. Blade coning, however, will cause an increase in Nr as the diameter will be (slightly) reduced, and therefore angular velocity will increase in order to conserve angular momentum.

Same reason it's funny to get kids to sit at the edge of roundabout as you start pushing it, then get them to try and touch the middle. Hilarious.
Of course I did not consider the roation of the earth (indeed negledible for a helicopter rotor), but the rotation of the rotor.

The Coriolis force appears in a rotating coordinate system when moving an object in a straight line. This movement in a straigth line (in the rotating system) requires rotational accelerations and decelerations when analyzed in a non-rotating system. The force necessary for these changes in rotational speed, and therfore changes in rotational momentum, is the coriolis force.

Of course this difference in speed changes relative to the two coordinate systems also applies to trajectories other than straight lines, but this is the easiest excample.

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Old 15th Dec 2020, 18:06
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As crab said, Coriolis Effect is an apparent deviation. The bolt in his example departs in a straight line as there is no force acting on it to change its path; it is on a radial path to the tip plane; the blade tip was in line with it initially. However, by the time it reaches the tip plane the tip has moved so it does not arrive at the tip of the rotor blade but where it was at the moment of departure. A static observer watching from above will see the bolt on this straight, undeviating path. An observer at the rotor hub looking along the blade and rotating with it will see the bolt apparently lag behind the blade and seem to follow a curved route. CE is not a force and no acceleration occurs.

Conservation of Angular Momentum is the element that applies to a coning rotor disc and there is a mathematical explanation of it that I am not competent to give. The FAA manual is incorrect; I had some interesting discussions with the trainer on my gyro instructors course as he had based his teaching on that book.



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Old 15th Dec 2020, 18:36
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Originally Posted by dickmct View Post
As crab said, Coriolis Effect is an apparent deviation. The bolt in his example departs in a straight line as there is no force acting on it to change its path; it is on a radial path to the tip plane; the blade tip was in line with it initially. However, by the time it reaches the tip plane the tip has moved so it does not arrive at the tip of the rotor blade but where it was at the moment of departure. A static observer watching from above will see the bolt on this straight, undeviating path. An observer at the rotor hub looking along the blade and rotating with it will see the bolt apparently lag behind the blade and seem to follow a curved route. CE is not a force and no acceleration occurs.

Conservation of Angular Momentum is the element that applies to a coning rotor disc and there is a mathematical explanation of it that I am not competent to give. The FAA manual is incorrect; I had some interesting discussions with the trainer on my gyro instructors course as he had based his teaching on that book.
I don't know (or care) enough about physics to know if what the FAA is telling me is correct or not. However, if you are correct in that the FAA is not, you should let them know so as to spare future generations of misinformation.
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Old 15th Dec 2020, 19:00
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I don't know what the FAA manual says, but in reality a rotor is not demonstrating conservation of angular momentum at all! It can hardly be described as being in a zero torque, frictionless, environment!
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Old 15th Dec 2020, 19:25
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Originally Posted by 212man View Post
I don't know what the FAA manual says, but in reality a rotor is not demonstrating conservation of angular momentum at all! It can hardly be described as being in a zero torque, frictionless, environment!
Its basically; When a blade flaps up, its center of mass moves inward towards the hub, causing it to accelerate. When the blade flaps down, the center of mass moves outward towards the tip, causing it to decelerate.

Fully-articulated rotors absorb this acceleration and deceleration through the lead/lag hinges. Semi-ridged rotors don't experience it to the extent that fully-articulated rotors do, as the center of mass remains virtually unchanged as the system teeters, due to the underslinging of the blades.

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Old 15th Dec 2020, 19:27
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Originally Posted by [email protected] View Post
If a bolt detaches itself from a blade root whilst the rotor is turning, does it end up along the same blade at the tip? Nope - Coriolis Effect is an apparent deviation in a rotating system, even a rotor disc.
[email protected] Totally agree, however, a departing component from a rotor system is not a normal condition, my point of concern is specific to CG movement during blade coning and the consequential Rrpm change.

Last edited by heliman500; 15th Dec 2020 at 20:24.
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Old 15th Dec 2020, 19:32
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Robbiee, the FAA have been miss-describing information for many years. They are not perfect, however, I have noticed they are receptive to change. VRS terminology being a great example, but i don't want to go down that rabbit hole.
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Old 15th Dec 2020, 19:52
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Originally Posted by 212man View Post
I don't know what the FAA manual says, but in reality a rotor is not demonstrating conservation of angular momentum at all! It can hardly be described as being in a zero torque, frictionless, environment!
212man Correct, Angular momentum is not applicable when there is a torque applied, however, consider the rotor system in autorotation or rapid descent (power on or off), this is when conservation of angular momentum is prevalent.

dickmct couldn't agree more

Originally Posted by Robbiee View Post
Its basically; When a blade flaps up, its center of mass moves inward towards the hub, causing it to accelerate. When the blade flaps down, the center of mass moves outward towards the tip, causing it to decelerate.

Fully-articulated rotors absorb this acceleration and deceleration through the lead/lag hinges. Semi-ridged rotors don't experience it to the extent that fully-articulated rotors do, as the center of mass remains virtually unchanged as the system teeters, due to the underslinging of the blades.
Robbiee correct, however the principle you are describing is conservation of angular momentum, and not Coriolis effect. Conservation of angular momentum is just one part of the formula that describes the theoretical / fictitious Corilis effect

Last edited by heliman500; 15th Dec 2020 at 20:12.
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Old 15th Dec 2020, 19:58
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Originally Posted by Ascend Charlie View Post
Projecting a 3-D object onto a 2-D horizontal plane produces some funny-looking results - a rotor system, coned upwards with power, and tilted forward for flight, gives the image of the front blade becoming longer and the rear blade becoming shorter. Bring in Kenny Coriolis, and Bob is your mother's brother, it is understandable. [...] Bring a pinch of salt, and keep your mind open. You seem to have the hang of it.
Just checked my Dr. Henry Velkoff text, and the phenomenon is just as AC described it! 😂

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Old 15th Dec 2020, 20:33
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Originally Posted by heliman500 View Post

Robbiee correct, however the principle you are describing is conservation of angular momentum, and not Coriolis effect. Conservation of angular momentum is just one part of the formula that describes the theoretical / fictitious Corilis effect
Thing is, the FAA gave me a license to fly helicopters, not to design them, so what they want me to know about the physics behind that design is good enough for me.

However, if someone can prove to me that the FAA's definition of Coriolis Effect will cause me to crash my helicopter, then I'd be more than happy to discover the "truth".

Until then, I'll just sit Corilois Effect on the shelf next to Settling with Power, Loss of Tail Rotor Effectiveness,...and any other titles that we give to things that the rest of you don't like,...that don't effect how I fly, or my odds of crashing anyway.
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Old 15th Dec 2020, 22:02
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Originally Posted by dickmct View Post
As crab said, Coriolis Effect is an apparent deviation. The bolt in his example departs in a straight line as there is no force acting on it to change its path; it is on a radial path to the tip plane; the blade tip was in line with it initially. However, by the time it reaches the tip plane the tip has moved so it does not arrive at the tip of the rotor blade but where it was at the moment of departure. A static observer watching from above will see the bolt on this straight, undeviating path. An observer at the rotor hub looking along the blade and rotating with it will see the bolt apparently lag behind the blade and seem to follow a curved route. CE is not a force and no acceleration occurs.
The opposite would happen if an inward-shooting bolt cannon at the blade tip, shoots a bolt initially toward the hub. An observer at the cannon would see the bolt curve forward and to the right, while an outside observer would see it go in a straight line (yet still forward of the blade). This is Coriolis force, and it is a direct result of the conservation of angular momentum. They're not two different things. Check out this video, skip to the 7 minute mark:

Conservation of Angular Momentum is the element that applies to a coning rotor disc and there is a mathematical explanation of it that I am not competent to give. The FAA manual is incorrect; I had some interesting discussions with the trainer on my gyro instructors course as he had based his teaching on that book.
The difference between the free particle that is our inward-shot bolt (or a water droplet in the experiment in the video, or an air parcel on a large scale wind on a rotating planet) vs. a piece of rotor blade in the middle of coning in, is that it's not free but rather rigidly mounted to the rest of the blade. So it pulls the blade forward and RPM goes up.

How does this "apparent" deviation result in an actual force/torque? Seems like a something-for-nothing deal, which we know isn't possible. From an energy budget perspective, we know that angular momentum was conserved: for a free particle, decreased radius converts to a higher angular speed. This can also be put in terms of straight-ahead plain old momentum: Actual speed stays the same, which means that the distance per time stays the same. Which means that the same distance (per time) wrapped around a smaller circle, covers more angle (per time). So the "apparent" path deviation to the right (to an observer rotating to the left with the system) is still a very real angular speed deviation, pulling the rotor forward.
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