OK, and it matters how exactly? The gyro forces at low decreasing RPM get weaker and weaker in comparison to the constant toppling force, until at some point the latter dominates the former and it falls. How does this slot into the arguments that I or you have been making?

Yes exactly, "applying additional torques" is the reason a toy gyro in my hand moves at angles different from the ideal 90 degrees. Here, it's easy for you to disregard the additional torques, and consider the base mechanic by itself. To avoid muddying the waters, you carefully examine the separate elements and their relative contributions to the total behavior. Though the base 90 deg. interaction is modified by the additional torques, you see beyond them to the base mechanic itself, which is gyro precession by definition.

So when a rotor, due to "additional torques," is also modified from the ideal 90 degree behavior, why can't you disregard the additional torques and see the base behavior as readily as you do for the toy gyro? Why is the toy gyro in that case still a gyro, while the rotor is not?

Actually I see two different possibilities for our disagreement here. Maybe you can tell me which it is, or some third one I didn't think of.
1. You do see the base behavior the same as that of a toy gyro, but just don't want to apply the word due to the presence of the additional torques and modified behavior. So we recognize the same mechanics, and it's a disagreement only over the label.
2. You do not think the base behavior of a gyro is there at all (or is there, but in some hugely diminished proportion to the additional torques), so the disagreement is not merely over the label, but over the physics itself.

I'm not sure where you were going with this, but I just added little annotations here to get us on the same mental track in talking about the same thing, if you care to expand or show an inference.

I... think I solved it.

Non-precession people: are you thinking that we gyro precession proponents are saying that the normal force that accelerates the blade up, is not lift but rather a direct push-up of the blade? As if, instead of the pitch link that goes in front of or behind the blade root to rotate it in pitch to control AOA, there's a pushrod that goes up to the center of the blade root to simply push it up or down?

As long as you theorists are happy, us actual pilots can continue in our own direction, which may (or may not) be 90 degrees to where you are pointed.

Agreed AC - the idea that the rotor is a gyro just because it spins is clearly entrenched in the theorists view - I lack the science and maths depth of knowledge to counter argue beyond what I know happens in a helicopter so I'll leave them to it.

btw SFT the rotor still flaps at very low rotational speed in exactly the same way as it does at normal operating speed - very unlike a gyro. You can see this on rotor start in anything but flat calm conditions.

Very low rotational speed is only a problem if there is another force present that will dominate the gyro behavior - such as, on a toy gyro, the toppling torque that any object standing on one point has. There is no equivalent torque on a horizontally mounted helicopter rotor, so the gyro behavior is unimpeded at low RPM's. It's also not a problem for a spacecraft orbiting at 1/90 RPM in LEO, or even far slower in higher orbits.

This is actually very easy to understand with no math and a bare minimum of science. You just have to extend your reasoning past rote associations, into applying general concepts (this is the science part). If you understand a gyro only as a toy gyro or flight instrument, you'll always be at a dead end. But if you understand a gyro as matter going in a circle, now you're on the path.

And what does matter do when going in a circle and pushed crossways? This:


https://cimg9.ibsrv.net/gimg/pprune....b5d498b865.png

If it still doesn't click, mentally zoom in to the node so the curvature isn't visible any more. Now you can just pretend the object was going in a straight line, and all that happened was it got forced crossways to the line, so it deflected at the point where the force happened. The simplest possible physical interaction you can imagine! Now zoom back out, and see the new path it follows.

It is your belief that because it spins it is a gyro - it is my belief that it is not - I fly a rotor system that is subject to constantly changing aerodynamic forces - if everyone of those produced a precession, the aircraft would be uncontrollable.

Up to you what you believe from a theoretical standpoint - far more learned people with aeons of helicopter design and construction experience don't believe the rotor is a gyro, I'll stick with them thanks

So, if you and your expert friends are saying that a spinning rotor has nothing in common with a gyroscope, and therefore the texts should stop comparing them as such, then you really should send a letter to the FAA.

Its too late for me, but don't let future pilots suffer the same fate of flying around with falsehoods running between their ears.

I couldn't care less what the FAA do or say, doesn't affect me in any way shape or form:):ok:

Vessbot, SFT, Meddlmoe et al. The problem here is that the real protagonist is not here to help square the circle. Here's what Nick Lappos had to say in 2005
And then a couple of posts later
I suspect you would all enjoy a great debate using those hairy equations to work out how much of the behaviour of the rotor is 'gyro dna' and how much is something else.

Vessbot give it up!

they are not going to admit that they are wrong, and they won't engage in a physics based discussion.

They will ignore the phsics parts of your arguments the same way they ignored my arguments entirely.

They will warp their personal definition of what a "gyro" is and isn't to a point that they can feel like being right.

There is no point in continuing the discussion.

This has turned out to be quite the thread for showing that if I believe principle A, then principle B can never apply, instead of applying some conflatory logic.

If you were able to magically remove a main rotor head and blades spinning at full speed and then even more magically transport it into a perfect vacuum, if you instantly applied a force to the spin axis, the apparent movement would indeed appear somewhere around 90 degrees later, just like a gyro. But if you then evacuated the vacuum surrounded the spinning assembly with normal atmosphere and tried the same experiment, you would get a different result because of the aerodynamic interaction. If then instead of a mysterious and magical force being applied at the spin axis, you tried to replicate the movement by cyclically changing the pitch of the blades, you would see that same 78 degree lag (or whatever!) before the effect of the input becomes apparent, because the aerodynamic effect is the predominant force.

Gyroscopic principles assume a solid disc, which is clearly not the case with an MRH and blades, but that doesn't mean gyroscopic effects are not felt. When you lift a tail wheel aircraft during take off, part of that yawing action you have to counteract is gyroscopic effect (along with engine torque, slipstream and P-factor). Just because the predominant force is aerodynamically derived, it doesn't mean gyroscopic forces aren't present. Instructional text books often pick on a common and easily accepted view, even if it's not the complete story. There's nothing wrong with that, as long as the critical principles of flight are understood. Gyroscopic principles are a massively complex area in themselves, so no need to complicate the explanation of what is the predominant force is with another far more complex area which is a secondary effect.

Things don't work in nature and science through picking only the simplest explanation, but it's fine to use that to illustrate a general principle. Strangely enough, thanks to some very smart design engineers, I never once had to think "better start banking early here, just in case that 78 degree lag doesn't work. Wherever I moved the cyclic, the aircraft magically followed. Good job design team!

Don't bother, the FAA still believes in LTE.

Gee, and I though the old and wise liked imparting their wisdome and knowledge to the young,...oh well. :(

Oh ****, so do I,...how can I live with myself now?! :{

Tough bickies, Robbiee, it's part of the problem where the instructor barely has 100 hours more experience than the student, "fake news" gets passed from one junior to the next, and like Forrest Trump, if you say it often enough, a lot of people will believe it.

Hmm, so a 200 hour fresh off the assembly line guy wrote the textbook? Oh well. :(

Can you detail what you mean by this phrase? Predominant over what? And don't just say "gyroscopic," I know that. But what specifically do you mean by gyroscopic; what is the source of this force that is overcome by the aerodynamic? Put another way, if there was no aerodynamic force to do the overcoming, what would still be there by default, and behaving how?

Or, put yet another way, what is the difference between aerodynamic effect and gyroscopic effect?

Quote:

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Gyroscopic principles assume a solid disc

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That's not true, they only assume matter going in a circle. See orbiting spacecraft doing a plane change.

All rotating bodies exhibit gyroscopic properties. I look at it as does Shytorque post #44 https://www.pprune.org/rotorheads/19...cession-3.htmlRather than the precession be caused by an external force, such as blowing on the rim of the spinning disc, the force is being generated from within the spinning body itself, by means of aerodynamic effects, https://www.faasafety.gov/gslac/alc/...ID=104&sID=449

Now, does this have any applicability to helicopters, after all they have lots of rotating bits?

No, for LTE, the Bell Spin Doctors concocted it to cover up the poor design of the original Kiowa small tail rotor, to avoid the wrath of the Army, after losing a few machines. The LTE story convinced people, including the FAA, that every helicopter is subject to its whims, and when you crash from running out of tail rotor, it ain't your fault. And this myth has been passed on from junior instructor to brand-new student, who then became a 100-hour instructor, and so on.
Only old B206 with the small tail rotor seemed to be affected, and a lot by the way that Fort Rucker was doing its teaching.

A Robinson will never be affected by "LTE" because Frank was a T/R specialist and his design is very efficient.

Hmm, never heard that LTE wouldn't be my fault,..dammit nobody tells me anything!

yes but only when they are willing to listen:)

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:ok:

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. ;-)

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. ;-)

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.

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 ?

I disagree with this statement :p 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?

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.

No we can't. Just start another discussion on "Settling with Power", if you don't believe me. :ugh:

From the Naval Air Training Command again.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.

and I always thought it was just white mans magic....

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?

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?:ok:

Isn't that exactly what a gyro would do if it experienced an upward force on the into wind side ?

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.

Quote:

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Originally Posted by Robbo Jock (Post 10951312)

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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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.

Quote:

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Originally Posted by [email protected] (Post 10951577)

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.

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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.

​​​​

Quote:

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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?

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​​​​​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!

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

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.

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.)

Completely untrue, consider the spacecraft in orbit. 90 degree phase lag.

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.

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?

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.