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Coriolis vs Conservation of Angular momentum

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Coriolis vs Conservation of Angular momentum

Old 20th Dec 2020, 10:44
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Gee, and I though the old and wise liked imparting their wisdome and knowledge to the young,...oh well.
yes but only when they are willing to listen
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Old 20th Dec 2020, 10:46
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So, after many years of helicopter design, construction and operation it turns out the whole industry has been getting it wrong because they didn't realise it is all about precession - thank goodness Vessbot and MeddlMoe are here to correct our outdated and erroneous thinking
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Old 20th Dec 2020, 11:29
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Originally Posted by megan View Post
Naval Air Training Command, Introduction to Helicopter Aerodynamics TH-57, page 4-5
406. PHASE LAG VERSUS GYROSCOPIC PRECESSION
That paragraph you quote looks very similar to a note I wrote for the local beefers in1977 to try to persude them to stop calling phase lag 'gyroscopic precession'.

My teaching materials did/do not mention gyroscopes at all, but I would accept 'like a gyroscope' as a quick one off.

I was taught 'gyroscopic precession', but I am a physicist and knew it was not right. When I was tasked to teach it I went to as many sources as I could find. The ones coming from designers and engineers did not mention gyroscopes, but the ones tracing their history back to the early days of helicopters in the US did, and that included the CFS ones.

Needless to say, I was treated with suspicion and 'gyroscopic precession' continued to be the accepted cannon.

In later years I have realised that my own 'explanation' was possibly too simple, but at least it came with the rider that many other things could affect the phase angle. ;-)
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Old 20th Dec 2020, 11:54
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I think a lot of the discord here comes from different understandings of 'gyroscope'.

To me a gyroscope is a symmetrical solid object rotating fast enough to exhibit rigidity in space.

The motion of such an object can be analysed using the same mathematics of forces, masses and accelerations as any other object, but when that is done there is a set of relatively simple gyroscope equations to use to predict the precession of the gyroscope at 90 degrees to a torque applied to the axis.

If an object is not solid or is not symmetrical or is not spinning fast enough it is not a gyroscope.

Its motion can still be analysed using mathematics developed to derive the motion of a gyroscope, but in the case of a helicopter rotor there are very many other factors to take into account and some more qualified than me say these dominate the motion of the blades. I follow them.

If you do not accept my definition of a gyroscope then we will never agree. ;-)
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Old 20th Dec 2020, 14:15
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Originally Posted by [email protected] View Post
So, after many years of helicopter design, construction and operation it turns out the whole industry has been getting it wrong because they didn't realise it is all about precession - thank goodness Vessbot and MeddlMoe are here to correct our outdated and erroneous thinking
It just so happens that I used to work on rotor analysis for many years. I don't know what kind of engineers you have been talking to, but they seem to be not very good, or more likely you have misunderstood them.

What is your alternative physically coherent explanation for the phase lag? Just saying "coriolis is wrong" is not enough.
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Old 20th Dec 2020, 18:58
  #106 (permalink)  
 
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This thread was doing fine until Vessbot asked a simple question "is phase lag not the same as gyroscopic precession". Quite a lot of things went off the rails after that.

I've got a suggestion that, if it's right, may help answer that question and then hopefully bypass some of the controversy that followed. Would it be fair to say that helicopter control phase lag is the sum of all gyroscopic precessions caused by perturbations (forces ?) applied to the rotor ?
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Old 20th Dec 2020, 19:47
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Originally Posted by HissingSyd View Post
If you do not accept my definition of a gyroscope then we will never agree. ;-)
I disagree with this statement We may not agree on the label, but we can bypass the label entirely in talking about the real subject matter: the underlying physics; and agree or disagree on that.

A single point mass exhibits 90 degree precession due to the only possible travel path around the new circle after a normal acceleration. Together so far?

If there are enough of those point masses to weld together into a solid disk, and a force on one particle gets passed to all of them, then does anything different happen? Or is the resulting path the same as that of the single point, and for the same reason?
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Old 20th Dec 2020, 21:47
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If there are enough of those point masses to weld together into a solid disk, and a force on one particle gets passed to all of them, then does anything different happen?
You've just hit the nail on the head. That is exactly why a gyroscope (or a spinning top or the Earth) behaves the way it does; it is a solid object, spinning - the force exerted on one particle is passed to all of them because they are all welded together. Apply a force at point x to a spinning disc/sphere/cylinder/whatever and it will be applied to all of the point masses and hence the resultant felt at point x plus 90 degrees because they're all welded together. Whirl a singular mass on a string, or your spacecraft in orbit, or a rotor blade and apply a force to it, it will be felt at the point that force is applied - there is nothing else to affect that force application. The plane of rotation may or may not change (depending on the force, the mass, the rotational speed) but the applied force will be felt at the point of application. That is the difference.
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Old 20th Dec 2020, 22:15
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Originally Posted by Vessbot View Post
I disagree with this statement We may not agree on the label, but we can bypass the label entirely in talking about the real subject matter: the underlying physics; and agree or disagree on that.
No we can't. Just start another discussion on "Settling with Power", if you don't believe me.
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Old 21st Dec 2020, 02:10
  #110 (permalink)  
 
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From the Naval Air Training Command again.
In part B of Figure 4-1 the maximum displacements are shown to occur 90 degrees after the applied force. This delay in maximum displacement is called phase lag, and is a property of all rotating systems acted on by a periodic force. For a system that is hinged at the axis of rotation the phase lag is 90 degrees. An applied force causes maximum displacement 90 degrees later in the cycle, in the direction of rotation. A system that is hinged at some distance from the axis of rotation (like a fully articulated rotor head) has a phase lag of slightly less.
I wonder if reference to the gyroscopic comes from the high lighted portion of text, both gyro and rotor are rotating systems being acted upon by a periodic force.
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Old 21st Dec 2020, 02:51
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and I always thought it was just white mans magic....
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Old 21st Dec 2020, 09:29
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When a gyro precesses, it takes its spin axis with it and, if you tried to restrain that spin axis somehow, it would create strong torques that could induce secondary precession effects.

When a rotor flaps, the actual spin axis (rotor mast) doesn't move as the blades do - look at introducing a 20 kt head wind gust to a stationary helicopter on the ground, the disc flaps back but nothing else changes - can a gyro do this?
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Old 21st Dec 2020, 11:31
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MeddleMoe - I realise I didn't answer your question:

As a blade moves round the rotor disc, the pitch change rods follow the swash plate, changing the pitch of the blades and thus their Angle of Attack (AoA). As the AoA changes, so does the amount of lift the blade produces causing it to flap upwards or downwards.

If the uncorrected swash plate is titled forwards, the forward half of the swash plate is creating a reduced pitch command and the rear half is producing an increased pitch demand with neutral pitch at the 3 and 9 o'clock positions.

So, as the blade passes the 3 o'clock (US rotation) it sees reducing pitch and AoA and begins to flap down - at the 12 o'clock position it sees minimum pitch and minimum AoA and is at its maximum rate of flapping down.

From 12 o'clock to 9 o'clock, the pitch down demand is reducing (as the pitch change rod starts to climb uphill again) but the pitch and AoA are still less than at the 3 and 9 so the blade continues to flap down but at a reducing rate until it gets to its lowest point at 9 o'clock.

This is why the corrected swash plate is tilted at 90 degrees so that forward cyclic equals nose low blade and therefore forward movement of the helicopter.

There are different combinations of jack positioning and pitch change rod placement on different helicopters but they all achieve the same end result.

The blades flapping is a result of aerodynamic forces induced by the change in pitch and AoA.

Is that coherent enough?
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Old 21st Dec 2020, 15:32
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look at introducing a 20 kt head wind gust to a stationary helicopter on the ground, the disc flaps back but nothing else changes - can a gyro do this?
Isn't that exactly what a gyro would do if it experienced an upward force on the into wind side ?
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Old 21st Dec 2020, 16:58
  #115 (permalink)  
 
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Sorry Crab I missed the point you were making about rigidity, but I think Vessbot has already dealt with that with the cardboard disc on a pencil video, where there is almost no link to the spin axis, and the orbiting rocket where there is absolutely no link to the spin axis.
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Old 21st Dec 2020, 17:51
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Originally Posted by Robbo Jock View Post
You've just hit the nail on the head. That is exactly why a gyroscope (or a spinning top or the Earth) behaves the way it does; it is a solid object, spinning - the force exerted on one particle is passed to all of them because they are all welded together. Apply a force at point x to a spinning disc/sphere/cylinder/whatever and it will be applied to all of the point masses and hence the resultant felt at point x plus 90 degrees because they're all welded together. Whirl a singular mass on a string, or your spacecraft in orbit, or a rotor blade and apply a force to it, it will be felt at the point that force is applied - there is nothing else to affect that force application. The plane of rotation may or may not change (depending on the force, the mass, the rotational speed) but the applied force will be felt at the point of application. That is the difference.


Careful you don't mix up two separate steps, when you say "is felt," for the two setups. In both single mass and solid disk, the force applied TO the body is at X. The force applied BY it (after travel time, for the single mass), is at X+90.

Originally Posted by [email protected] View Post
When a gyro precesses, it takes its spin axis with it and, if you tried to restrain that spin axis somehow, it would create strong torques that could induce secondary precession effects.
Originally Posted by [email protected] View Post

When a rotor flaps, the actual spin axis (rotor mast) doesn't move as the blades do - look at introducing a 20 kt head wind gust to a stationary helicopter on the ground, the disc flaps back but nothing else changes - can a gyro do this?


A rotor also takes the spin axis with it. ("Virtual axis," "tip path axis," "along disk axis"...) it is no longer aligned with the shaft axis. So I'm not seeing this as a difference.

​​​​
Originally Posted by [/color
[email protected];10951693]

The blades flapping is a result of aerodynamic forces induced by the change in pitch and AoA.

Is that coherent enough?


​​​​​All is coherent and true, but it doesn't answer the question for an "alternative" explanation. How is this not gyroscopic precession?

Also, when you find yourself explaining the most rudimentary thing that you'd tell to a day-one student, to someone who identified himself as having "worked on rotor analysis for many years" (that the pitch link controls AOA and lift, which is a force that moves the blade… really? Hold the phone while I write this down!) it should serve as a strong clue that there might be a miscommunication somewhere. A few posts ago (#83, "I think I solved it") I asked a question trying to suss this out, but it fell by the wayside. Again, do you think our position is that the force applied to the blade is something other than aerodynamic lift? Because NO ONE is saying this in the slightest!


Last edited by Vessbot; 21st Dec 2020 at 18:18.
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Old 21st Dec 2020, 19:19
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But take that rotor and put it in space - or other convenient vacuum - and leave the swashplate 'uncorrected', the blades will follow the pitch change rods faithfully but without the phase lag.

No aerodynamics = no flapping

A rotor also takes the spin axis with it. ("Virtual axis," "tip path axis," "along disk axis"...) it is no longer aligned with the shaft axis. So I'm not seeing this as a difference.
because a gyro doesn't do this - the shaft axis is the rotors spin axis, the other terms are apparent in that they are used to describe the apparent shift in the axis to explain things like Hookes joint effect.

BTW - if you took self-stated credentials at face value on the internet you would make far more assumptions and mistakes than explaining basics to check understanding - 'working on rotor analysis' covers a multitude of disciplines such as vibration analysis which looks more at mass and dynamic balancing than aero issues. It wasn't meant as a snub to MeddleMoe.
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Old 21st Dec 2020, 21:05
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Originally Posted by [email protected] View Post
But take that rotor and put it in space - or other convenient vacuum - and leave the swashplate 'uncorrected', the blades will follow the pitch change rods faithfully but without the phase lag.
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.

Maybe you're talking about my imaginary setup of pushrods that go to the center of the blade root (instead of in front or behind) so as to push it up or pull it down directly, instead of controlling pitch/AOA. This would displace them in a vacuum, and there would be a 90 degree phase lag force.

(How much of this lagged force results in lag of displacement, depends on the elasticity of the blades, the mounting, etc., just like the toy gyro in your fingers with varying strength of grasp. (Except the former is more likely to rip out of the mounting and cause catastrophic damage to the vacuum chamber and building.) But put a U-joint in the shaft, and no problem you're golden. Unimpeded 90 degree precession.)


No aerodynamics = no flapping
Completely untrue, consider the spacecraft in orbit. 90 degree phase lag.

because a gyro doesn't do this - the shaft axis is the rotors spin axis, the other terms are apparent in that they are used to describe the apparent shift in the axis to explain things like Hookes joint effect.
You presented that "a gyro [...] takes its spin axis with it" as a difference between gyros and rotors. I pointed out that they both do this, so this difference doesn't exist.

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.

BTW - if you took self-stated credentials at face value on the internet you would make far more assumptions and mistakes than explaining basics to check understanding - 'working on rotor analysis' covers a multitude of disciplines such as vibration analysis which looks more at mass and dynamic balancing than aero issues. It wasn't meant as a snub to MeddleMoe.
Actually you're right. At the slightest gray area of interpretation, it's better to err toward including more detail rather than less. No snub.

But, again, do you think we're saying that any force other than lift is applied to the rotor?

Last edited by Vessbot; 22nd Dec 2020 at 02:47.
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Old 21st Dec 2020, 21:50
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Originally Posted by [email protected] View Post
MeddleMoe - I realise I didn't answer your question:

As a blade moves round the rotor disc, the pitch change rods follow the swash plate, changing the pitch of the blades and thus their Angle of Attack (AoA). As the AoA changes, so does the amount of lift the blade produces causing it to flap upwards or downwards.

If the uncorrected swash plate is titled forwards, the forward half of the swash plate is creating a reduced pitch command and the rear half is producing an increased pitch demand with neutral pitch at the 3 and 9 o'clock positions.

So, as the blade passes the 3 o'clock (US rotation) it sees reducing pitch and AoA and begins to flap down - at the 12 o'clock position it sees minimum pitch and minimum AoA and is at its maximum rate of flapping down.

From 12 o'clock to 9 o'clock, the pitch down demand is reducing (as the pitch change rod starts to climb uphill again) but the pitch and AoA are still less than at the 3 and 9 so the blade continues to flap down but at a reducing rate until it gets to its lowest point at 9 o'clock.

This is why the corrected swash plate is tilted at 90 degrees so that forward cyclic equals nose low blade and therefore forward movement of the helicopter.

There are different combinations of jack positioning and pitch change rod placement on different helicopters but they all achieve the same end result.

The blades flapping is a result of aerodynamic forces induced by the change in pitch and AoA.

Is that coherent enough?
Congratulations, you have hereby explained how the gyroscopic effect works on a rotor blade.

What is next? There is no such thing as friction, it is just collisions between micro-structures and van der vaals forces dissipating energy.
There is no such thing as the bernoulli effect but air molecules accelerating from high pressure to low pressure following a gradient in collision probability.

Last edited by MeddlMoe; 21st Dec 2020 at 23:19.
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Old 22nd Dec 2020, 02:46
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Originally Posted by MeddlMoe View Post
Congratulations, you have hereby explained how the gyroscopic effect works on a rotor blade.

What is next? There is no such thing as friction, it is just collisions between micro-structures and van der vaals forces dissipating energy.
There is no such thing as the bernoulli effect but air molecules accelerating from high pressure to low pressure following a gradient in collision probability.
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.

Everybody knows the common-sense and intuitive behavior of a mass traveling in a straight line, and what happens if you push it in a perpendicular direction when it’s moving slowly vs. at high speed. At high speed, the normal acceleration is still the same (it only depends on the force) but is only deflected a tiny fraction of the angle due to the original velocity component. No problem here.

But when you wrap this big velocity into a circle, and give the brass disk a high quality polish so that it appears still, it engages the part of the brain that intuitively recognizes still objects. This decouples the still-looking gyro experience from the easy and intuitive fast-moving object concept, and some people can’t bridge that gap. So instead of the fast-moving object’s resistance to normal acceleration, the different “rigidity in space” (ooooooohh) label is applied, and is referenced in the brain to some other mysterious behavior.

Same with precession, which is even more mysterious and exotic seeming. I would guess most people don’t understand its nature in a gyro, but do understand the behavior of a blade since it’s kind of a necessity in the helicopter game. But the fancy label that the brass disk’s behavior is filed behind, I think acts as somewhat of a block from examining that behavior in detail, and definitely a block from applying that behavior to a helicopter blade.

When you try to apply the label to the helicopter rotor behavior, they point to the understood behavior as the real one that makes it work, so there’s no room for the other fictitious behavior that has somehow into supposed existence. They can’t see that there is no other behavior, it’s the one they already understand! This is the danger of labels. “It can’t be B because it’s already A,” not seeing that A = B even after asking them what the difference is many times. (Of course you can’t do away with them, because then how could you refer to things? They’re a double-edged tool, and best used wisely.)

Edit: To give credit where it’s due, in the last few posts he did point out some potential differences, but I showed why I don’t think they work out as such.

Last edited by Vessbot; 22nd Dec 2020 at 03:03.
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