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Coriolis vs Conservation of Angular momentum

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Coriolis vs Conservation of Angular momentum

Old 22nd Dec 2020, 03:14
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When a rotor flaps, the actual spin axis (rotor mast) doesn't move as the blades do
Beg to differ crab, the spin axis of the rotor is perpendicular to the plane of rotation, think of the CV joints on your car and the relevant spin axis for each of the components.
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Old 22nd Dec 2020, 07:59
  #122 (permalink)  
 
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I don't understand. Why would the blades move at all? They're in a vacuum. There's no lift to cause displacement. Pitch/AOA would have no effect.
Since there is no aero force, the pitchrods will follow the swash plate uphill and down, altering the pitch of the blades but also pulling them down and pushing them up a little - no phase lag and no precession. Even with your central pushrods, the blades will rise and fall but follow the swash plate exactly.

Completely untrue, consider the spacecraft in orbit. 90 degree phase lag.
not phase lag - might well be precession though.

Now you're presenting that "the shaft axis is the rotors spin axis" as a necessary feature of gyros that separates them from rotors. But this isn't a difference either, because it's false. See the cardboard on the pencil gyro video, where the spin axis was displaced from the shaft axis. Like a rotor.
The pencil with the cardboard disc isn't attached to the disc nor is it providing the drive to keep the disc spinning - it is simply a support to stop the disc falling to the ground so that is a false argument. A gyro's spin and shaft axis remain aligned throughout its movement and precession. The apparent spin axis of a rotor is displaced from the shaft axis of a rotor when the disc flaps.

I think this is what happens when you have knowledge as a collection of facts, rather than understanding as the inference from particular facts to a general principle, and then application of the general principle to some other particular thing in question.
And you took me to task for talking down to people...........
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Old 22nd Dec 2020, 10:42
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To get to this topic, I click on "last page" but it never is the last page, there is always another para from Vessbot, who valiantly fights the fight of the Righteous. But sometimes you are the Wrongeous. But I no longer care, the glaze has descended over my eyeballs.
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Old 22nd Dec 2020, 14:38
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Originally Posted by Ascend Charlie View Post
To get to this topic, I click on "last page" but it never is the last page, there is always another para from Vessbot, who valiantly fights the fight of the Righteous. But sometimes you are the Wrongeous. But I no longer care, the glaze has descended over my eyeballs.
Roses are red
Violets are blue
How far ahead?
By π over 2
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Old 22nd Dec 2020, 15:27
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Errm. You may have glazed over, but I'm still interested.

The question was "Is phase lag not the same as gyroscopic precession". I can think of a couple of ways of making that question more accessible. First we can use the term torque induced precession (as Wikipedia does), in order to reduce the angst about comparisons between gyroscopes and rotor systems.

Second we could rephrase it as an exam question, maybe "Either (a) Describe in terms of its underlying physics any cause of control phase lag in a helicopter that is not wholly explained by torque induced precession; Or (b) choosing a suitable reference system estimate what proportion of helicopter control phase lag arises solely from torque induced precession".

I am genuinely interested in the answer. Many instructors have been told over the years that using torque induced precession as a first order explanation for helicopter control phase lag is either plain wrong, or a gross oversimplification. It would be nice to have that confirmed or squashed.

Last edited by Wide Mouth Frog; 22nd Dec 2020 at 18:23.
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Old 23rd Dec 2020, 02:52
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To put the gyroscope notion to bed I'll offer up one example. A helicopter running on the deck of a wildly pitching and rolling ship, providing there is no SAS engaged, the rotor will maintain the same path relative to the helicopter and ship.
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Old 23rd Dec 2020, 03:05
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And it is a really weird feeling to be on that pitching deck.
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Old 23rd Dec 2020, 06:18
  #128 (permalink)  
 
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To put the gyroscope notion to bed I'll offer up one example. A helicopter running on the deck of a wildly pitching and rolling ship, providing there is no SAS engaged, the rotor will maintain the same path relative to the helicopter and ship.
Nice one Megan that is an inconvenient truth for the pro-gyro clan, the lack of rigidity in space mentioned by AC a while ago and ignored by the playground bullies
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Old 23rd Dec 2020, 06:32
  #129 (permalink)  
 
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With apologies to Casabianca:

The cab sat on the pitching deck,
Blades spinning round the head;
The gyro force that would try to wreck,
Was missing feared dead.


Yet steadfast were the critics stood,
‘Precession is the norm’
They spake as if ‘twas writ in blood,
A proud, though childlike form.


The deck it rolled, the disc it flapped,
Yet tip path plane it stayed in place;
The argument, now tightly wrapped,
A rotor’s not rigid in space.
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Old 23rd Dec 2020, 11:37
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In space, no-one can hear the gyro theorists scream...
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Old 23rd Dec 2020, 11:57
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Originally Posted by Wide Mouth Frog View Post

I am genuinely interested in the answer. Many instructors have been told over the years that using torque induced precession as a first order explanation for helicopter control phase lag is either plain wrong, or a gross oversimplification. It would be nice to have that confirmed or squashed.
The off axis torque that is induced in a rotor system is the cyclic component of the aeodynamic forces. This creates extra lift on one side of the mast and reduced lift on the other side. Lets say more lift on the left side and less lift on the right side of the helicopter with a rotor turning counterclockwise viewed from above. This does not lead to a sideways tilt of the rotor disk (or rotor cone), but this leads to a forward tilt. This means that the tilt hase 90 degree phase lag. This large scale phenomenon is called the gyroscopic effect.

But the gyroscopic effect is not some magic or extra phenomenon but is an emergent effect from the normal physical behavior of the masses rotating about one center.

When looking at a smaller scale for example at the movement of an individual rotor blade (or a narrow angular section of a gyroscope disk) then it becomes apparent that this delay is only caused by the time it takes to accelerate the blade upwards and downwards. The blade does not instantly reach its highest vertical displacement at the angular position with the highest vertical force. It has its highest vertical acceleration at the point of highest vertical force.

Now this could lead to a highest displacement at any delay angle, not necessarily at 90 degrees. Just looking at the aerodynamic forces one would expect a 180 degree phase lag, because the upwards movement needs to be slowed down first by downward forces.

But the blade is still attached to a center point so the centripetal force has a downward component relative to the original plane of rotation. This downward component leads to the blade decelerating towards its highest point much earlier than from the external forces, and then moving downward. This downward component of the centripetal force is proportional to the rotation speed, the flapping displacement and the mass of the blade.

This leads to a 0 degree phase shift between external accelerating force and vertical speed. Under ideal conditions (one unmoving center point) this leads to a 90 degree phase shift of the displacements.

Real helicopter rotors are not ideal gyroscopes and the rotor blade flapps about a hinge location that is not at the center of rotation. This leads to a phase lag somewhat smaller than 90 degrees.

But this imperfection is small and the phade lag is close to 90 degrees on real helicopters, especially with articulated rotor systems. Therefore I would call a helicopter rotor a gyro.
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Old 23rd Dec 2020, 12:12
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Originally Posted by megan View Post
To put the gyroscope notion to bed I'll offer up one example. A helicopter running on the deck of a wildly pitching and rolling ship, providing there is no SAS engaged, the rotor will maintain the same path relative to the helicopter and ship.
No, this would only be the case if the flapping hinge offset was zero. The real flapping hinge offset is 2-6% for articulated rotors and 6-18% for "rigid" rotors (hingeless or flexbeam rotors)

This leads to a small tilt roughly 90 degrees relative to the ships tilt
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Old 23rd Dec 2020, 12:31
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Originally Posted by megan View Post
To put the gyroscope notion to bed I'll offer up one example. A helicopter running on the deck of a wildly pitching and rolling ship, providing there is no SAS engaged, the rotor will maintain the same path relative to the helicopter and ship.
The primary reason the rotor disk follows the ship deck is because the swashplate stays aligned to the deck, and the rotor disk follows the swashplate. Disconnect the pitch links and the rotor doesn't follow the ship deck nearly as readily (hinge offset and other extraneous forces like elastomeric damping-friction-springiness are the cause for the alignment).
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Old 23rd Dec 2020, 13:39
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Originally Posted by MeddlMoe View Post
No, this would only be the case if the flapping hinge offset was zero. The real flapping hinge offset is 2-6% for articulated rotors and 6-18% for "rigid" rotors (hingeless or flexbeam rotors)

This leads to a small tilt roughly 90 degrees relative to the ships tilt
Interesting. I thought megan's reply was a great answer to part b of my question that would lead you to a non-gyro answer. So to check I looked at wikipedia and one of the shortest helicopter carriers is the UKs RFA Argus and it is 175m long. Wiki also tells me that a 60mph wind over a long fetch will generate waves 15m tall with a period of about 15 seconds. I'm sure going up and down 15m every 15 seconds feels like wildly pitching, but it's not a huge angle change (about 5 degrees according to my trig). You'd need quite a beady eye to spot that amount of precession in that environment, especially if the hinge offset worked to reduce it.
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Old 23rd Dec 2020, 15:44
  #135 (permalink)  
 
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Just looking at the aerodynamic forces one would expect a 180 degree phase lag, because the upwards movement needs to be slowed down first by downward forces.
Or in fact by a reduction in the upward force due to the reduction in pitch angle and AoA as explained earlier and the reason it is around the 90 degree mark.

And you have completely forgotten the periodic drag changes as the blade spins round (the reason rotors have drag hinges or dampers)- something a gyro doesn't have - there are more differences than similarities between a rotor and a gyro but you can't seem to see past the spinning bit. Each blade speeds up and slows down in its journey because as CL increases due to increase in AoA, so does CD (coefficients of lift and drag respectively).

What would a gyro do if you sped it up on one side? It can't because it would speed up the other side too - unlike a rotor.

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Old 23rd Dec 2020, 16:18
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Can we please stop trying to answer the question "is a rotor a like a gyroscope", and try and answer the question posed which is "is torque induced precession the primary cause of helicopter control phase lag". The answer to the first question is a matter of opinion, the answer to the second (when we get to it) will be a matter of fact.
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Old 23rd Dec 2020, 16:48
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Originally Posted by [email protected] View Post
Or in fact by a reduction in the upward force due to the reduction in pitch angle and AoA as explained earlier and the reason it is around the 90 degree mark.
Can you explain in more detail, why you think that pure aerodynamic forces applied as a sinusoidal function of time would not lead to 180 degrees phase shift?

0 degrees - - >acceleration proportional to force
90 degrees - - > speed (integral of acceleration)
180 degrees - - > displacement (second integral of acceleration)

Originally Posted by [email protected] View Post
And you have completely forgotten the periodic drag changes as the blade spins round (the reason rotors have drag hinges or dampers)- something a gyro doesn't have - there are more differences than similarities between a rotor and a gyro but you can't seem to see past the spinning bit. Each blade speeds up and slows down in its journey because as CL increases due to increase in AoA, so does CD (coefficients of lift and drag respectively).

What would a gyro do if you sped it up on one side? It can't because it would speed up the other side too - unlike a rotor.
The drag and coriolis effect are a order of magnitude smaller than the lift effects for normal operation. They have virtually no appreciable effect on the phase lag of the tilt of the rotor cone.

After all a gyroscope is not ideally stiff. It is also subject to microscopic deformations. And forms a very minute cone from gravity. This cone also leads to coriolis effect when tilted. This will lead to tiny elastic deformations

Even sound waves lead to tiny displacements in a gyroscope.

I think we are not talking about absolute purity, are we?

A gyroscope is only a sample implementation. It does not define the gyroscopic effect.
Thats like saying "this is not true friction, if it is not exactly like a brake pad" or "this is not true bernoulli effect because it is not exactly like a venturi tube"
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Old 23rd Dec 2020, 17:14
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The primary reason the rotor disk follows the ship deck is because the swashplate stays aligned to the deck, and the rotor disk follows the swashplate. Disconnect the pitch links and the rotor doesn't follow the ship deck nearly as readily (hinge offset and other extraneous forces like elastomeric damping-friction-springiness are the cause for the alignment).
Or you could just say the rotor is not rigid in space - if you were to constrain a gyro in the way you suggest, secondary precesssion would occur but this doesn't happen either.

This does not lead to a sideways tilt of the rotor disk (or rotor cone), but this leads to a forward tilt. This means that the tilt hase 90 degree phase lag. This large scale phenomenon is called the gyroscopic effect.
No, this is called flapback.

But the blade is still attached to a center point so the centripetal force has a downward component relative to the original plane of rotation. This downward component leads to the blade decelerating towards its highest point much earlier than from the external forces, and then moving downward. This downward component of the centripetal force is proportional to the rotation speed, the flapping displacement and the mass of the blade.
No, it is aerodynamic damping - as the blade flaps up it sees a reduction in AoA and thus a reduction in CL.

an you explain in more detail, why you think that pure aerodynamic forces applied as a sinusoidal function of time would not lead to 180 degrees phase shift?
sorry, I misread your post - it is 180 degrees because the pitch change starts 90 degrees before max rate of pitch increase and hence max rate of flap up.
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Old 23rd Dec 2020, 18:45
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https://www.rotorandwing.com/2017/08...ty-rotor-gyro/
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Old 24th Dec 2020, 04:02
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Wiki also tells me that a 60mph wind over a long fetch will generate waves 15m tall with a period of about 15 seconds. I'm sure going up and down 15m every 15 seconds feels like wildly pitching, but it's not a huge angle change (about 5 degrees according to my trig). You'd need quite a beady eye to spot that amount of precession in that environment, especially if the hinge offset worked to reduce it.
Having had the experience of sliding across the ships deck and being brought to a sudden stop at the edge of the deck by the tie down that the deck crew had not had chance to tension up I can tell you from experience that the tip path plane does not change one iota relative to the ship, if it does you would need a micrometer. Substantial ship experience, as with some other posters here, teetering and fully articulated heads, both the same reaction. First thing on landing if you were SAS equipped was punch it off as it would try to maintain reference to the horizon and you could end up finding out where the blade flapping stops were, or worse.
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