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Coriolis vs Conservation of Angular momentum

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Coriolis vs Conservation of Angular momentum

Old 19th Dec 2020, 00:18
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Originally Posted by [email protected] View Post
If a toy gyro isn't rotating at high speed it falls over so it does matter.
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?

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

On a rotor, the mechanical input is the start of the process, the next stage is the aerodynamic forces [normal force] that are a result of the mechanical change in pitch to the blades - these accelerate the blades [beginning at the point where the normal force was applied], 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 [other normal force is applied later] and the reducing mechanical input.
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.
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Old 19th Dec 2020, 00:53
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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?
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Old 19th Dec 2020, 02:14
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I... think I solved it.
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.
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Old 19th Dec 2020, 13:01
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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.
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Old 19th Dec 2020, 13:48
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Originally Posted by [email protected] View Post
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:




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.

Last edited by Vessbot; 19th Dec 2020 at 14:07.
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Old 19th Dec 2020, 13:57
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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
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Old 19th Dec 2020, 16:36
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Originally Posted by [email protected] View Post
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.
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Old 19th Dec 2020, 17:44
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I couldn't care less what the FAA do or say, doesn't affect me in any way shape or form
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Old 19th Dec 2020, 18:31
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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
In fact, many helicopters have control phase angles of less or more than 90 degrees, based on the hinge angle (delta 3) and the blade inertia vs its flap damping. I know of one helo that had a lead angle of almost 180 degrees, and the S-76 has a lead angle of 56 degrees, both not at all 90 degrees!

The way the blades flap as they whirl around, and the way the cyclic makes them flap is not gyroscopic at all, but a gyro and a rotor share the same need to make fundamental physics happy. The conservation of angular momentum is the key, so there is a bit of gyro DNA in a rotor, but not enough to make the lead angle precisely 90 degrees. In fact, it s almost never exactly 90 degrees in any helicopter. In fact, it is not even the same angle in one helicopter. let me explain:

The real phase angle of a helo can be easily found, just tilt the swash plate to the north, and watch where the rotor plane dips lowest after all settles out. The typical rotor will dip lowest somewhere around west, so we call that one a 90 degree phase angle (engineers call it gamma, the swash plate phase angle).

If you take a given helo and do that test at three different speeds, you will find three different gammas, because the airspeed has a strong effect. In other words, the "gyro precession" angle of a rotor varies with airspeed!

For a helo with delta 3, the gamma shifts for each degree of delta 3, so the Robbie, with 18 degrees of delta 3 has a gamma of 72 degrees. This has lost Lu a lot of sleep over the last few years, as he ponders the universal significance of that horrid 18 degree shortfall, all the while thousands of Robbies have flown millions of miles.

This has been beaten to death here on PPRuNe over the last few years.

Suffice it to say, the typical rigging angle of 90 degrees for most helos is an approximation, it works, it can be different for any model helos, and it is not due to gyroscopic precession.
And then a couple of posts later
It is how the rotor behaves as it flaps, it is approximated in some pretty hairy equations, and it is not generally 90 degrees.

Just because our helos are rigged that way does not make their gama exactly 90 degrees, but they are close enough so that it works. I would bet that a sizable percentage of production helos have gammas that are 10 degrees different than their controls are rigged, but it is so hard to notice, nobody cares.

When we build helos, we actually look to see what it s. I personally did it on the S-76 and Comanche. It is also not a constant for an aircraft, as slowrotor has observed, it changes with rpm (because it is a factor of the blade flapping inertia, centripital field, aerodynamic damping, hinge offset and several other things that escape me (phase of the moon?) Gamma even changes short term vs long term. If the rotor is rapidly flexed with cyclic, it dips in a different place than where it ends up long term. Boelkow drivers know this, and automatically compensate.
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.
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Old 19th Dec 2020, 19:07
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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.
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Old 19th Dec 2020, 19:41
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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!

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Old 19th Dec 2020, 21:07
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then you really should send a letter to the FAA.
Don't bother, the FAA still believes in LTE.
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Old 19th Dec 2020, 21:14
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Originally Posted by [email protected] View Post
I couldn't care less what the FAA do or say, doesn't affect me in any way shape or form
Gee, and I though the old and wise liked imparting their wisdome and knowledge to the young,...oh well.
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Old 19th Dec 2020, 21:16
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Originally Posted by Ascend Charlie View Post
Don't bother, the FAA still believes in LTE.
Oh shit, so do I,...how can I live with myself now?!
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Old 20th Dec 2020, 00:07
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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.
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Old 20th Dec 2020, 00:45
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Originally Posted by Ascend Charlie View Post
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.
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Old 20th Dec 2020, 01:44
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Originally Posted by Two's in View Post
because the aerodynamic effect is the predominant force.
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?

Gyroscopic principles assume a solid disc


That's not true, they only assume matter going in a circle. See orbiting spacecraft doing a plane change.
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Old 20th Dec 2020, 05:21
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All rotating bodies exhibit gyroscopic properties. I look at it as does Shytorque post #44 Helicopter Dynamics: Gyroscopic Precession
Gyroscopic precession in the case of helicopter rotors is a result of aerodynamic effects, not the cause of them
Rather 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,
Naval Air Training Command, Introduction to Helicopter Aerodynamics TH-57, page 4-5

406. PHASE LAG VERSUS GYROSCOPIC PRECESSION


The rotor system is not a gyro; however it sometimes behaves in a way that may be likened to a gyro. Just as an analogy is a comparison based on similarities in some respects between things that are otherwise dissimilar, phase lag in a non-rigid system can be compared to the effects of precession that occur in a rigidly mounted gyroscope. A gyro exhibits gyroscopic precession in response to an applied force, while the rotor system responds ‘similarly’ using the principle of phase lag discussed in the next section. The phenomenon of precession occurs in rigid rotating bodies that manifest an applied force 90 degrees after the application in the direction of rotation. The force is actually described as causing the rigid body to rotate as if acted upon by a different force 90 degrees later. Although precession is not a dominant force in rotary-wing aerodynamics, aviators must consider it because rotating rigid components may exhibit some of the characteristics of a gyro.

Figure 4-2 illustrates the effects of phase lag on a typical rotor disk when force is applied at a given point. A downward force applied to the disk at point A results in maximum downward movement (displacement) of the disk at point B. The association of a rotor’s movement with phase lag and precession stems from similarity to the effects of a force on a gyro rather than its similarity to an actual gyro. Both a gyro and a rotor are circular systems and respond to applied forces somewhat similarly, but through completely separate mechanisms. However, numerous writings and pilots use the terms interchangeably even though they are not the same thing.
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?

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Old 20th Dec 2020, 05:58
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Hmm, so a 200 hour fresh off the assembly line guy wrote the textbook? Oh well.
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.
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Old 20th Dec 2020, 06:03
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Originally Posted by Ascend Charlie View Post
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!
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