Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

Coriolis vs Conservation of Angular momentum

Rotorheads A haven for helicopter professionals to discuss the things that affect them

Coriolis vs Conservation of Angular momentum

Old 16th Dec 2020, 20:59
  #41 (permalink)  
 
Join Date: Sep 2018
Location: California
Posts: 605
Likes: 0
Received 5 Likes on 3 Posts
Originally Posted by Ascend Charlie View Post
Well, the chief test pilot at Sikorsky wrote these, I tend to follow his thoughts rather than yours:
Damn, I haven't heard any of those myths. I feel so left out.
Robbiee is offline  
Old 16th Dec 2020, 22:05
  #42 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Ascend Charlie View Post
Well, the chief test pilot at Sikorsky wrote these, I tend to follow his thoughts rather than yours:
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.
Vessbot is offline  
Old 16th Dec 2020, 22:08
  #43 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by ApolloHeli View Post
I think you've hit the nail on the head there mate. We all agree that saying phase lag happens due to gyroscopic precession or that helicopters behave like gyroscopes is close enough for the average student, but it's still an approximation. After all, it's getting the concept and theory across in a digestible and understandable form for the student that matters. "Lies to children" as Scott Manley would put it.
I'll agree to that. Fly safe!
Vessbot is offline  
Old 16th Dec 2020, 23:06
  #44 (permalink)  
 
Join Date: Sep 2002
Location: Great South East, tired and retired
Posts: 4,050
Likes: 0
Received 16 Likes on 12 Posts
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?
Ascend Charlie is online now  
Old 16th Dec 2020, 23:41
  #45 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Ascend Charlie View Post
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.
Let's stop right here actually. Like all rotating matter, yes, the bucket constitutes a gyroscope.

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?
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?)
Vessbot is offline  
Old 16th Dec 2020, 23:50
  #46 (permalink)  
 
Join Date: Sep 2002
Location: Great South East, tired and retired
Posts: 4,050
Likes: 0
Received 16 Likes on 12 Posts
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.
Ascend Charlie is online now  
Old 16th Dec 2020, 23:56
  #47 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Ascend Charlie View Post
Never said "instantly". But you keep bringing it into the conversation.
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.
Vessbot is offline  
Old 17th Dec 2020, 01:20
  #48 (permalink)  

Avoid imitations
 
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,074
Received 56 Likes on 28 Posts
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.
ShyTorque is offline  
Old 17th Dec 2020, 02:01
  #49 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by ShyTorque View Post
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.
Vessbot is offline  
Old 17th Dec 2020, 02:19
  #50 (permalink)  
 
Join Date: Sep 2002
Location: Great South East, tired and retired
Posts: 4,050
Likes: 0
Received 16 Likes on 12 Posts
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 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.
Ascend Charlie is online now  
Old 17th Dec 2020, 05:26
  #51 (permalink)  
 
Join Date: Feb 2015
Location: USA
Posts: 178
Likes: 0
Received 6 Likes on 3 Posts
Originally Posted by Ascend Charlie View Post
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.
SplineDrive is online now  
Old 17th Dec 2020, 05:28
  #52 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Ascend Charlie View Post
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.
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:
)




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)

Can you explain, then, why an R22 only needs 72 degrees of advance?
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?

If it was a gyroscope, it has to be 90 degrees flat.
Why are you having such a resistance to the concept of multiple superimposed effects?

Last edited by Vessbot; 17th Dec 2020 at 19:39.
Vessbot is offline  
Old 17th Dec 2020, 07:12
  #53 (permalink)  
 
Join Date: Sep 2002
Location: Great South East, tired and retired
Posts: 4,050
Likes: 0
Received 16 Likes on 12 Posts
what are the forces that do cause the deflection we see,
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.
Ascend Charlie is online now  
Old 17th Dec 2020, 08:00
  #54 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Ascend Charlie View Post
Oh boy, this is getting tedious. It is called lift.
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.
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. That is what helps the Trump-brains understand the wobbles of a rotor head.
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?
Vessbot is offline  
Old 17th Dec 2020, 09:33
  #55 (permalink)  
 
Join Date: May 2016
Location: Europe
Posts: 35
Likes: 0
Received 0 Likes on 0 Posts
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.
MeddlMoe is offline  
Old 17th Dec 2020, 10:08
  #56 (permalink)  
 
Join Date: Jan 2008
Location: LONDON
Posts: 84
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Vessbot View Post
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?
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.
netstruggler is offline  
Old 17th Dec 2020, 11:31
  #57 (permalink)  
 
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by MeddlMoe View Post
I think here is a fundamental misunderstanding by some of you:
[...]
So the question "is it coriolis, or conservation of momentum" doesn't make sense, [...]
+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.
HissingSyd is offline  
Old 17th Dec 2020, 11:52
  #58 (permalink)  
 
Join Date: May 2016
Location: Exeter
Posts: 45
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Ascend Charlie View Post
Oh boy, this is getting tedious.
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. ;-)
HissingSyd is offline  
Old 17th Dec 2020, 17:33
  #59 (permalink)  
 
Join Date: Sep 2016
Location: USA
Posts: 742
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by MeddlMoe View Post
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.
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.

Originally Posted by HissingSyd View Post
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. ;-)
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?

Last edited by Vessbot; 17th Dec 2020 at 17:45.
Vessbot is offline  
Old 17th Dec 2020, 20:58
  #60 (permalink)  
 
Join Date: Sep 2018
Location: California
Posts: 605
Likes: 0
Received 5 Likes on 3 Posts
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!
Robbiee is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.