Coriolis vs Conservation of Angular momentum
Thread Starter
Joined: Jan 2006
Posts: 24
Likes: 0
From: USA
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?
Joined: Sep 2002
Aviation Qualifications: CPL
Posts: 4,721
Likes: 636
From: Great South East, tired and retired
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.
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.
Joined: Jan 2002
Posts: 129
Likes: 0
From: Yendys
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.
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.
Joined: May 2016
Posts: 35
Likes: 0
From: Europe
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?
Guest
Joined: Feb 2003
Posts: 210
Likes: 0
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.
Joined: Oct 1999
Aviation Qualifications: ATPL
Posts: 7,371
Likes: 926
From: Den Haag
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 December 2020 at 14:52.
Joined: Sep 2018
Aviation Qualifications: CPL
Posts: 888
Likes: 65
From: California
Joined: Apr 2000
Aviation Qualifications: ATP+Mil
Posts: 10,959
Likes: 1,814
From: EGDC
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.
Joined: May 2016
Posts: 35
Likes: 0
From: Europe
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.
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.
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.
Joined: Jul 2012
Posts: 23
Likes: 17
From: scotland
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.
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.
Joined: Sep 2018
Aviation Qualifications: CPL
Posts: 888
Likes: 65
From: California
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.
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.
Joined: Oct 1999
Aviation Qualifications: ATPL
Posts: 7,371
Likes: 926
From: Den Haag
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!
Joined: Sep 2018
Aviation Qualifications: CPL
Posts: 888
Likes: 65
From: California
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.
Thread Starter
Joined: Jan 2006
Posts: 24
Likes: 0
From: USA
Originally Posted by [email protected]
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.
Last edited by heliman500; 15th December 2020 at 19:24.
Thread Starter
Joined: Jan 2006
Posts: 24
Likes: 0
From: USA
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.
Thread Starter
Joined: Jan 2006
Posts: 24
Likes: 0
From: USA
dickmct couldn't agree more
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.
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.
Last edited by heliman500; 15th December 2020 at 19:12.
Joined: Feb 2015
Posts: 258
Likes: 58
From: USA
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.
Joined: Sep 2018
Aviation Qualifications: CPL
Posts: 888
Likes: 65
From: California
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
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.
Joined: Sep 2016
Posts: 936
Likes: 67
From: USA
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.
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.