Damn, I haven't heard any of those myths. I feel so left out. :(

I'd bet that if asked to elaborate beyond a simple off the cuff one liner, to where some nuance is involved in considering separately multiple simultaneous effects and their relative contributions, he'd say something a lot closer to what I'm saying.

Also you didn't engage in of the reasoning I laid out, so it seems it's not really "thoughts" that you're following, but rather statements.

I'll agree to that. Fly safe! :8

Let's reduce it to a scale you might understand.

You are standing on flat ground, with a bucket of water in your right hand, it is heavy. You start to turn on the spot - you might even get up to 10 RPM. Are you a gyroscope? No? OK, continue.

As you pass a reference point straight ahead, you decide to raise the bucket shoulder high, with a stiff arm. Keep turning. The bucket comes up, but it isn't at its highest point when you are looking at your reference, it is some many degrees afterwards. This is a form of phase lag. The force exerted on the bucket took time to move the bucket upwards, and during that time, you kept turning. Are you a gyroscope? Hmm? Was it precession that made the bucket come up?

Let's stop right here actually. Like all rotating matter, yes, the bucket constitutes a gyroscope.

Yes, this is gyroscopic precession.

Is it your position that if the weight was very light and I was very strong (like, even near-zero light and near-infinitely strong) I could have instantly shifted the point at which the bucket is traveling? (Not just made it move in a new direction, but actually displaced its line of travel?)

Never said "instantly". But you keep bringing it into the conversation.

F=ma. For a given force, the acceleration will depend on the mass. An empty bucket will get to shoulder level in fewer degrees of turn than a full bucket. If it were a gyroscope, it would be exactly 90 degrees every time, no variation permitted. An R22 blade, being light, will get to its max displacement in 72 degrees of turn. Not a gyroscope.

I do, because even with an instant impulse and normal acceleration only at the 0 degree point, with no normal acceleration afterward, in a simple system the max displacement will take 90 degrees regardless of the mass. You're championing normal acceleration as the cause of the phase lag. But since the phase lag happens with no normal acceleration, the normal acceleration cannot be its cause.

The problem with comparing the person holding buckets to a helicopter rotor blade is that in the first case, the person applies all of the force to change the “flight path” of the bucket. The bucket doesn’t naturally want to change.

The force to change the flight path of a helicopter rotor comes via lift, from changing the angle of attack of the blade itself, so it “wants” to fly up.

What is the significance of the source of the force? The normal force, no matter its nature, causes a deflection of the path of the object. Whether it comes from lift, or anything else. I don't see what difference it would make.

The force isn't an instant thing. It starts from zero, builds to a max upwards, then transitions to a max downwards, and back again - a force of some kind is always there, up and down.

Can you explain, then, why an R22 only needs 72 degrees of advance? If it was a gyroscope, it has to be 90 degrees flat.

The R-22 main rotor head has delta-3, which affects the phase angle between control input and rotor response. There are other factors that affect that angle, but if there is delta-3 involved, that has to be accounted for. I don’t have a drawing of the head to calculate delta-3 from, but looking at photos and the lack of a hub spring, I’d guess 18 degrees.

Yes, this is true in a rotor. It's constantly transitioning through 1 sinusoidal cycle of force per rev, and not instant.

You've been contending that a continuous force is required for the path to continue diverging over the course of 90 degrees from the original path (what would have been without the force.) And, an instant-force situation shows that there is no such requirement, since the same divergence would happen. If it's not required in the instant-force case, why would it be required in any case? How does it fit in as an explanation, for a motion that happens just as merrily with or without it?

I also keep trying to bring up the instant-force case since it's the simpler setup, and any time like this it's crucial that the simple setup be understood and agreed on first before complications are added on top. If there's error and confusion on the ground floor, nothing meaningful can be said if you add more pieces on top of that.

It takes only one moment to deflect the flight path, and from then on, with no more normal force, that deflected path continues to diverge for 90 degrees, and then starts coming back. (This is like a spacecraft doing a plane change, or this piece of spinning cardboard, 6:30 into this video: )

https://cimg1.ibsrv.net/gimg/pprune....80427d131a.png


Once this is understood, then you can modify it to considering a wave of force peaking at the 0 degree point, instead of an instant impulse there. And just like the instant impulse that had its effect 90 degrees later, the wave does too. The max force peaks 90 degrees later. The slightly reduced force on either side of the peak at -10 and +10 degrees, is displaced to 80 and 100, etc. (this is like a classic toy gyro where tilting the axis applies the appropriate normal forces all the way around... or a helicopter rotor with an imaginary simple ball pivot in the middle)

Not specifically. But like I said before, everything I've been saying is perfectly compatible with complications in the hub design adding more forces to those described above, that modify the basic behavior. The question for you, if you're contending that there is no gyroscopic behavior in the mix, is twofold: first, how is it that this rotating body is not subject to the forces/behaviors (among others) that all other rotating bodies are? What is the basis for this interruption in physical law? And second, if we grant this disappearance of the normal force and deflection, what are the forces that do cause the deflection we see, and which looks suspiciously like one that disappeared?

Why are you having such a resistance to the concept of multiple superimposed effects?

Oh boy, this is getting tedious. It is called lift.

You admit that the R22 does not behave in the manner a gyroscope is obliged to perform. It has forces acting upon it. So it is only LIKE a gyroscope, which is the basic premise of the statements by Nick Lappos et al. It is not a straightforward gyroscope. It is LIKE a gyro.

You cut out and didn’t answer the first question, which was why the rotor is not subject to the forces/behaviors of rotating bodies. It’s only after that, that you can try to substitute some other force in and explain why it’s subject to it as… a non-rotating body? Whatever that might even mean.
No, I don’t admit that. You’re again being obtuse and throwing out every bit of nuance, to set up this false all-or-nothing dichotomy where if the behavior is not exactly like a pure gyroscope, then there is no gyroscopic effect. And that is just not so. There is a gyroscopic effect, among other effects present at the same time.

Can you say what you mean by “LIKE” a gyro? Does the motion only look the same but something different causes it to look like that? Is there some part of the force arrangement that makes a gyro behave like a gyro, that is lacking in a rotor? What is the force that is lacking?

I think here is a fundamental misunderstanding by some of you:

The coriolis effect, gyroscopic effect, and centrifugal force are not some kind of "extra physics" or "magical effects" that occur on top of conservation of mass, energy and momentum. These effects are just apparent phenomena when analyzing movements in rotating reference systems and are caused by the conservation of mass, energy and momentum.

So the question "is it coriolis, or conservation of momentum" doesn't make sense, because the latter causes the first.

Eric Laithwaite waving a 40lb, 2,500rpm gyroscope around one handed and generating 'lift'.


I saw him do this during a lecture, with a complete absence of any safety arrangements. It got our attention.

+1

It is about frames of reference. Choose inertial or choose rotating, but not both at the same time.

For me the inertial frame and conservation of angular momentum is simplest in giving an insight into movement about the dragging hinges and most people can relate this to the spinning ice skater.

Indeed it is. Phase lag has almost nothing to do with gyroscopic effects nor with forces and their delayed effects. It is almost entirely an aerodynamic effect - a result of flapping to equality of blade thrust in a cyclicly varying aerodynamic environment. In an ideal scenario, with a freely articulated rotor responding to cyclic control input, is is 90 degrees.

Anything more complex will perturb that, but not grossly. I am not at all sure about teetering rotors. ;-)

Yes! There’s a huge tendency for people to reason based on labels and word association, instead of physical behavior. If the explanation for the phenomenon they’re familiar with is label A, and someone else says it’s B, they reject B because it’s not A, instead of examining what’s happening in B and how that may or may not interact (or even may be the same, or an example of) with what’s happening in A.

It’s a fairly simple part of how matter behaves, if it’s going in a straight line and a normal force acts on it, the path deflects. Now if initially it’s going in a circle because it’s attached to the center, and a normal force acts on it, it deflects the same way. Because the new post-deflection path is still wrapped around a circle, the maximum displacement before it comes back, is 90 degrees after where the force acted. May have to draw a picture to really see it, but it’s not too complicated. But now, when you give a name to the force that caused the deflection (lift), this… somehow doesn’t happen any more?

I would like for the people who reject the label (gyro precession) to instead say why they reject the mechanic behind it.

How is a “gyroscopic effect” different from an “aerodynamic effect” in this case? Doesn’t the aerodynamic effect impart a normal force on the blade, causing its path to deflect (flap) into a new circle where the maximum difference from the old circle is 90 degrees after the force change?

Typical student/instructor interaction in the States;
- Instructor: Hey Joe wanna know why, when we move the cyclic forward the rotor disk actually tilts right
- Student: Sure Tim, lay it on me.
- Instructor: Well when force is applied to a spinning object, the effect is felt 90° later in the direction of rotation. So if we didn't offset the linkage to compensate, when you move the cyclic forward, you'd end up going to the left.
- Student: Hmm,,...cool

Typical instructor/student interaction in the old world;
- Instructor: Mr. Smith, Its time for you to learn why when you move the cyclic forward, the rotor disk actually tilts left.
- Student: Yes Professor Bruno, I'm eager to learn
- Instructor: Mr. Smith, take this Theoretical Physics book home tonight and read chapters 4 and 8, also take this Applied Physics book and read chapters 7 and 18. Then, here is a 6 hour video lecture from Oxford's Physics department, I want you to watch. That should fully prepare you for the 7 hour block I have reserved for us to discuss the matter tomorrow.
- Student: Thank You Professor Bruno, I look forward to our meeting tomorrow, to discuss this riveting topic.

You guys really know how to pour on the overkill!
:p

The same topic was over-killed decades ago with a similar result:

https://www.pprune.org/rotorheads/20...binson-10.html

Vessbot - I get the impression you know a lot about physics but very little about helicopters.

Phase lag is governed by Lock number which is a ratio between blade inertia and aerodynamic damping.

A swash plate does not impart an instant or impulse force to a rotor, it builds gradually as a result of turning a rotating motion into a vertical one because you have to change the pitch of the blade to make it climb or descend.

No gyros I have seen have hinges that limit how much the movement of the mass of the gyro can be transmitted to the hub - a rotor system does.

Just because systems exhibit similar behaviours doesn't mean they have the same cause.

Yep, that note from Frank was definitely more than I needed to know,...or could even follow. Glad I don't have to be an engineer, or physicist, to wiggle the stick around.

It’s true I’m not a helicopter expert, but please give me a little more credit than this. Just a few posts ago, I wrote virtually the same thing: “It's constantly transitioning through 1 sinusoidal cycle of force per rev, and not instant.”

The reason I pointed to the instant-impulse case was to show that the continued presence of a normal force is not required to continue the divergence of the tip path over the 90 degree phase difference; this was in response to another poster who was arguing that such a presence is required.

Here’s one, at 6:30 in. The gyro is a cardboard disk with a small hole on a pencil tip, free to swivel about 45 degrees (more than most or all helicopters). Within that range, it transmits no movement to the pencil, and he does a beautiful precession demo within about half of that range.

I’m not seeing the salience of this point, as the phase lag between the input and the resulting tilt of a heli rotor is all about the behavior of the rotor itself, and not any motion that it may or may not pass to the fuselage. I think it’s clear that that step happens with no phase lag - rotor disk tilts forward, helicopter goes forward with it.

​Any way that I can see it, the cause here is the same as well. If it’s different, what’s the difference? What fundamental mechanic causes a gyro to behave like a gyro, that’s missing from a helicopter rotor that behaves like a gyro?

(This question isn’t answered by pointing to other mechanics there are present - I know there are many. Offset flapping hinges, delta 3, lead-lag hinges, oscillations of various orders both through hinges and elastic paths that interact in ways that blow minds… yes. I am asking about absence, not presence).

The missing element is Rigidity in Space. Because of all the twisting, flapping, dragging and aerodynamic blade forces, the "disk" is dynamically unstable - and gyroscopes are not. It will not hold its position or orientation, it will diverge and crash, unless the coffee-stabilised pilot stops it from doing so, by playing with the cyclic.

What RPM do most gyros rotate at?

Even with his cardboard disc, the precession demonstrated is 90 degrees - as you have already been told, phase lag is frequently not 90 degrees, with some very large variations depending on rotor design blade inertia etc etc - but a gyro is ALWAYS 90 degrees.

Does the angle of gyroscopic precession change with density altitude? Phase lag does change with that variable because of the change in aerodynamic damping - how does that fit with your theory?

The similarity between precession and phase lag is why it has been used as a simple explanation of behaviour of a rotor but many far cleverer people than you and I have said they are not the same and one is not an explanation for the other.

What is your explanation for why phase lag is frequently not 90 degrees when gyro precession is always 90 degrees?

If all gyros behave exactly the same way, from the cardboard disc in the video through a spinning bicycle wheel to a proper gyro with a number of degrees of freedom designed to do a specific job, why do helicopter rotors not all follow the same rigid ('scuse the pun) reactions if it really is precession?

AC makes a very valid point about rigidity in space - you can't cherry pick the facets of gyro behaviour - if a rotor obeys one, it should obey all - and it doesn't.

This is not correct. The Lock number has very little to do with it.

The critical parameter for the phase shift is the relative flapping hinge offset. This is the ratio between the distance of the flapping hinge from the rotor axis and the blade tip radius. (or the virtual hinge location for a flexbeam rotor).

Due to this distance you get bending moment that tilts the whole helicopter including its rotor shaft axis. This happens at an phase angle smaller than 90° (0° phase angle for a totally rigid rotor with a relative hinge offset equal to 1).
You still have the gyroscopic effect with 90° phase shift for the tilt of the roation axis of the rotor blades (which is then generally not parallel to the rotor shaft axis). The combination of these to tilting effects results in a total tilt at an intermediate phase angle.

Hmm, does it matter at all that the book says, that a spinning rotor is "like" a gyro (as opposed to "is" a gyro) and the effect is "approximately" 90° later (as opposed to "is" 90° later)?

I mean, it sounds to me like they are just making a comparison, not literally saying a spinning rotor is a gyroscope.

,...unless you expert physicists are saying that a spinning rotor has absolutely nothing in common with a gyroscope? Is that it,...'cause this thread is hard to follow for us average joes?

The reason a gyro precesses at 90 deg while some rotors exhibit phase lag less than 90 deg is because rotors with hinge offset (or effective hinge offset) inject an additional moment into the disk that alters the precession angle. This additional moment is a result of the flapped rotor blades being connected to the rotor rotational axis at a location that is not the rotor center (i.e., the offset hinge).

One can use the exact same math to predict phase lag on a rotor as they would to predict how a gyro precesses at 90 deg. We do it all the time in the simulation world. The trick is in properly accounting for all the moments/forces being applied to the disk. Conceptually, you can almost think of a gyro as an infinite number of teetering rotors all spinning around the same axis.

The reason rotor phase lag changes with density altitude is because the magnitude of aero forces from the blades change in relation to the magnitude of the inertial forces from hinge offset. Think of it this way, if you have infinite density, then the blade will perfectly follow the blade pitch, which would result in perfect 90-deg phase lag regardless of the hinge offset. As air density reduces, the aero forces reduce, and the blade can start to deviate from it's pitch angle more and more. With the reduced "stiffness" of the aero forces, the effect of the inertial forces as a result of the hinge offset becomes more dominant, and hence the phase angle increase.

The math explains it all, whether it's a gyro or a rotor. There are no secret physics at play that are peculiar to either case. The two scenarios are entirely consistent with each other.

So why does a teetering rotor not exhibit exactly 90 degree phase lag and why does that change with density altitude when there are no hinges or hinge offsets to consider?

There are subtle effects that can result in a teetering rotor from producing exactly 90 deg phase lag. For example, any of the following will result in a deviation from an exact 90-deg phase lag:
- pitch-flap coupling as a result of a delta3 angle
- friction/damping in teetering hinge
- aerodynamic effects like modifications to blade lift as a result of dynamic operating conditions (varying angle of attack, etc.)
- the rotor hub being underslung
- etc.

Of course, any phase lag should be considered in relation to the blade pitch being injected by the swashplate. There are all sorts of geometric/mechanical scenarios that can give the pilot the impression of a phase lag (i.e., flight control mixing, bellcrank geometries, etc.), when in reality the phase lag might be very small.

So, unlike a gyro, there are countless other factors that affect flapping - yet people are still adamant a rotor is a gyro?

Gyros and rotors both obey the same fundamental laws of physics that perfectly describe their behavior. F=MA applies equally to both. The differences lie in the forces that exist and where they get applied to the "disk". If you take a gyro with a mass and rotational inertia that is identical to a rotor, and apply the exact same forces to it that are applied to the rotor, then the gyro will demonstrate the exact same phase lag as the rotor.

There's nothing special about either a gyro or a rotor. They are both just a collection of interconnected masses moving in space. If you choose to define a gyro as something that must operate in a vacuum and have no forces acting on it other than gravity, then yes, a gyro is different than a rotor... but there is nothing in the physics that requires such a limited definition.

Big variance. A flight instrument rotor spins at 10K to 15K RPM, according to Google. A spacecraft in low Earth orbit is at 1 rotation every 90 minutes. It doesn't matter.

This is not true. You can wiggle a toy gyro back and forth in your fingers, and depending on the stiffness you hold it with (in other words, applying other forces), control the amount of precession that occurs. When you're going down the runway in a taildragger airplane, gyro precession will tend to yaw you to the left while raising the tail, and a lot of money, pride, personal safety, and often preservation of historical artifacats, depends on your application of other forces to control this precession to zero degrees.

It fits in by applying other forces (which vary based on circumstances) in addition to the gyroscopic ones, to the object.

Because there are other forces.

This is like asking "what is your explanation for why my car is drifting to the right while I'm holding the steering wheel straight ahead, when a car with the steering wheel straight always goes straight ahead?"

Because there are other forces.

I really hesitate to open a second rabbit hole when people are struggling so hard with the concept of more than one force acting on an object, and a resulting motion that's a combination of the motions due to the individual forces - but a rotor does have rigidity in space. When you take away rotor-fuselage drag interactions, aerodynamic forces on the blades, etc. the rotor has the same tendency as all rotating mass. Just think of a spinning rotor in a vacuum chamber.

---

Now, I have to ask you again, at a very simple level: what mechanic from a precessing gyro that causes it to precess, is absent from a seemingly precessing rotor?

On a the cardboard gyro, a normal force was applied which deflected the path in that direction, and the new circle had a max offset from the old one 90 degrees later. Yes or no?

On a helicopter rotor, a normal force is applied which deflects the blade path in that direction, and the new circle has a max offset from the old one about 90 degrees later. Yes or no?

I'm adamant that the factor that moves the new tip path 90 degrees after the input, is the same as the one that does the same thing in gyros. And it seems to be the dominant factor in a rotor, when stacked together with all the others, since the result is only modified slightly.

If a toy gyro isn't rotating at high speed it falls over so it does matter.

Of course you can but that is just applying additional torques and muddying the waters of what is discussed here

The big difference between your last two lines is that one is 90 degrees and the other is 'about' 90 degrees - can you show me a gyro that doesn't precess at 90 degrees? That would be far more beneficial to your argument.

On a rotor, the mechanical input is the start of the process, the next stage is the aerodynamic forces that are a result of the mechanical change in pitch to the blades - these accelerate the blades, assisted by the mechanical input and governed by the laws of aerodynamics - the movement eventually cancels itself out when the lift produced is negated by the braking/damping effect of the air and the reducing mechanical input.

Yes! Yess!!

I have to go so I'll be back later with a normal post, but please ruminate on this for a bit.

From a physics standpoint, a "gyro" can be defined as an object whose effects due to rotational motion dominate the behavior of the object. In this regard, a rotor is a gyro because the effects of rotation dominate. Whether or not a "gyro" is a disk, or a ring with spokes, or just spokes, or a slender rod rotating about it's mid point (like a teetering rotor) is irrelevant.

To go further, at some point even a gyro is no longer considered a gyro. Take a gyro and slow it down. Any rotational effects become less dominant. Same with a rotor. Now slow it down again... and again... and again. At some point, the effects due to rotation are negligible, and neither object behaves in a manner that is comparable to their rotating states.

It's all simply masses moving around a center point. You can define "gyro" and "rotor" any way you want... but you can not imply that the same physics do not apply to each.

The lock number has very little influence on the phase lag. (It is also not the delta 3 angle, as somebody else claimed)

The phase lag is predominantly determined by the flapping hinge offset. This is the ratio between the distance from the rotor axis to the flapping hinge and the distance from the rotor axis to the blade tip.

Then you get two effects that occur simultaneously and therefore overlap:

1. The gyroscopic effect leads to a maximum deflection at 90° phase shift from the maximum lift force. This effect does not affect the orientation of the rotor shaft. This means that the rotor blades rotate about a (slightly) different axis as the rotor shaft.

2. The non-zero flapping hinge offset leads to bending moments in the rotor shaft due to the deflection. This leads to a rolling or pitching moment of the entire helicopter, which then turns according to the magnitude of this bending moment and its inertia. This leads to a change of the orientation of the rotor shaft axis. The phase shift of this effect smaller than 90° depending on the hinge offset.

These two tilts of rotation axes combined lead to a combined phase shift, that is smaller than 90°.

Note that also flexbeam rotors have a hinge offset, which is determined for the equivalent ratio between blade deflection and bending moment at the rotor shaft.

SFT,

Thanks for the clarity, game over. I'm curious as to your background on the subject.