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#1060
by
Rodal
on 28 Sep, 2014 17:36
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An
Italian:
A physical model of the so-called 'Ziegler column', a two-degree-of-freedom system, exhibiting flutter instability as induced by dry friction.
Ziegler was at the Aeroelastic and Structures Research Laboratory at MIT during two different periods at the time of the Apollo program. (
http://www.hansziegler.com/obituaries_en.html)
(See video for divergence instability at 5:40 and at 6:19).
****Observe that in these Italian experiments we also have an inverted "pendulum" fixed at one end and experiencing an axial load (due to friction in this case) at the free end. There is no lateral load responsible for the instability.
There is no dark energy, quantum vacuum or Woodward transient mass effect for the lateral motion. It is an axial load dynamic instability where only classical mechanics and friction are present.
The case of the magnetically damped inverted pendulum may experience a different kind of dynamic instability: there is a mechanical non-conservative nonlinearity due to the
magnetic damping and there are nonlinear terms coupling the swinging motion to the torsional motion. Add to that the fact that they acknowledge coupling of the electric power and the magnetic dampening affecting the baseline***
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#1061
by
Rodal
on 28 Sep, 2014 17:42
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It is apparent that some people that dismissed my questioning of the magnetically damped inverted pendulum don't understand
self-excited motion (see:
http://forum.nasaspaceflight.com/index.php?topic=29276.msg1263257#msg1263257). Such motion is
not at all the same thing as the forced harmonic vibration of a linear dynamic system due to a natural frequency.
It used to be the case (1930's to 1960's) that Aerospace Engineers were cognizant of flutter and divergence
instabilities. As flutter problems have been successfully addressed in aerospace, perhaps dynamic instability has fallen in popularity. That would be too bad, because one cannot blindly use a Finite Element package to analyze these problems, as modeling the nonlinearities and coupling in the equations of motion is of paramount importance.
No linear analysis can solve the
transient motion of a dynamic
instability problem. A nonlinear dynamic finite element package doesn't automatically "know" what kind of nonlinearity and coupling is involved in a given problem. The engineer has to model such nonlinearities and coupling in order to solve the problem.
At MIT there was an "Aeroelasticity and Structures Research Laboratory" started by Bisplinghoff (before he became one of the top people at NASA at the beginning of the Apollo program) (
http://www.amazon.com/Aeroelasticity-Dover-Books-Aeronautical-Engineering/dp/0486691896/ref=sr_1_sc_1?ie=UTF8&qid=1411926883&sr=8-1-spell&keywords=aerolasticity+bispinghoff ). They had people like Ashley (who went to Stanford) , Halfman, Witmer, Pian, Mar, Dugundji and others. During the late 1980's MIT closed the "Aeroelasticity and Structures Research Laboratory" and has been concentrating in other areas.
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#1062
by
Rodal
on 28 Sep, 2014 18:08
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What's most worrisome for assessing
testing results of EM drives is that apparently some people (
http://forum.nasaspaceflight.com/index.php?topic=29276.msg1263257#msg1263257) don't understand
divergence instability: where the motion is impulsively towards one side, due to self-excitation. (See video by the Italian above for a very simple demonstration at 5:40 and at 6:19).
The assumption that the testing device, particularly when measuring microNewtons, is not to be questioned, is really worrisome ! And the assumption that simplistic linear analyses of the testing device suffice is also worrisome. And compound this by the fact that instead of using classical, better understood, testing devices (as the Cavendish hanging pendulum damped by oil, as used by famous physicists for testing the inverse square law of gravitation and testing the Casimir force, and as also used by Brito, Marini and Galian to nullify the EM drive results), people are using magnetically damped inverted pendulums and other non-analyzed nonlinear testing devices.
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#1063
by
Vultur
on 28 Sep, 2014 19:32
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I have recently noticed that this type of effect appears to go back to the 1920's, at least:
<<During the 1920s, Thomas Townsend Brown was experimenting with an x-ray tube known as a "Coolidge tube," which was invented in 1913 by the American physical chemist William D. Coolidge. Brown found that the Coolidge tube exhibited a net force (a thrust) when it was turned on.
Very interesting.
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#1064
by
aceshigh
on 28 Sep, 2014 20:16
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So far my feeling is that the "propellantless propulsion proponents" are doing a really great job at NOT convincing an (admittedly already reluctant) mainstream science community that there is any effect at all. ...
I agree, that is my reaction as of late, with many exceptions. Notably, Paul March
coincidentally, the only one who posts here who directly works on propellantless propulsion and the only one who can address the questions raised. I wonder how Frobnicat expects propellantless propulsion proponents here to answer the questions raised when only people directly working with those experiments can answer such questions.
Itīs a bit unfair of Frobnicat to demand specific answers from us that only a few people WORKING on those experiments can effectively give, and then say we are not helping the cause.
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#1065
by
Rodal
on 28 Sep, 2014 20:26
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....
@aceshigh,
That's not my reading of @frobnicat. My reading of @frobnicat is that according to the scientific method, the expectation is that the complete testing protocol be made available for other scientists to independently be able to reproduce such experiments, and that until those complete testing protocols are made available such tests are not completely following the scientific method.
@frobnicat also stated that @frobnicat << think(s) that they (the experimenters) are trying too hard to see something, and not hard enough to see nothing. >>
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#1066
by
JohnFornaro
on 28 Sep, 2014 20:29
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I was given this windup toy by a Swiss friend ...
Yahbut: This has nothing to do, that I can tell, with the whirly bird device. When we were kids, my brother and I had a football game. There was a vibrating table, and the football players had 1/4"x3/4" bases with short 1/16" directionally bent fibers. You'd set up the play, turn the table on, and the players would vibrate themselves forward, with the subtle differences in their bases giving or retarding advantage. A slightly different concept from your vibrating spindly legged wing ding device.
I mention the childhood game since it produced forward motion by vibration, and your wing ding thing vibrates and produces forward motion.
(Sorry for having introduced non-vetted terminology.)
An Italian:
First, if my italian were as good as his English, I'd be pleased to speak Italian with a southern drawl.
Second, this is not the problem of the whirlybird device. There is no force along the axis of the axle, which is constrained by the track. Or are you saying that the whirlybird motion, which does have a horizontal component, is exactly the cause of "flutter instability", which, constrained by the tracks, manifests in forward roling motion?
The frictional rolling of a driven wheel against a surface always comprises a "stick" region at the leading side of rolling motion and a sliding region behind it.
I knew that, but would have phrased it differently. Still the whirly bird device in the video above purports to have freely spinning wheels, not driven wheels.
where the motion is ... due to self-excitation.
Please remember that this is a family site.
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#1067
by
Rodal
on 28 Sep, 2014 20:38
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I was given this windup toy by a Swiss friend ...
Yahbut: This has nothing to do, that I can tell, with the whirly bird device. ....
Well, if you are going to argue that the analogies I presented are not perfect analogies, I concede that specific point: you are right: they are not
perfect analogies. However they contain the essential points: stick-slip friction and motion of the center of gravity.
But, I strongly disagree with the statement that it has nothing to do.
If you won't accept an imperfect analogy to understand what's going on, what would convince you? A solution of the equations of motion derived from a Lagrangian + the nonconservative force (friction)? That would practically qualify for a paper (just like the one that the Italian published in the prestigious Journal of the Mechanics and Physics of Solids), which at the moment I don't have the inclination to do
However, I will put further effort to further clarify the NASA Eagleworks test results, as I get the time....
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#1068
by
JohnFornaro
on 28 Sep, 2014 20:39
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Itīs a bit unfair of Frobnicat to demand specific answers from us that only a few people WORKING on those experiments can effectively give, and then say we are not helping the cause.
First, it is not a political "cause". Either it is a real effect, or it is not.
Second, the "unfairness" is the deliberate abandonment of input from the experimentors on the apparent basis that they seem to be in a snit because they give every appearance of confusing questioning the apparatus with questioning the expermentor's moral motives.
Third, it is a great disappointment to hear ad hominem attacks rather than physical descriptions of the actual mass and acceleration included in, just to take the most recent example, the case of "buld acceleration".
Fourth, if only the few "WORKING" people can answer specific questions, then there can be no broad agreement with their work, by definition, since only they have the "answers".
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#1069
by
JohnFornaro
on 28 Sep, 2014 20:47
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I was given this windup toy by a Swiss friend ...
Yahbut: This has nothing to do, that I can tell, with the whirly bird device. ....
Well, if you are going to argue that the analogies I presented are not perfect analogies, I concede that point: ...
Not at all my point, my imperfect interlocutor. In my intemperate
Boetian approach, I'm just not seeing the windup toy nor the
sticky friction stick slip [terminology edit.] explanation... [More editing...] as pertaining to the wheeled whirly bird device.
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#1070
by
Rodal
on 28 Sep, 2014 21:03
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Not "sticky friction", but stick slip.
The coefficient of friction is lower during sliding motion than with incipient sliding.
Think of a block acted by a normal force N, against a surface, with a static coefficient of friction mu in between them, a (lower) sliding coefficient mus, and a lateral force P.
For P<mu * N, there is no sliding, and we can have 0 < P < mu * N
For any load greater than Ps= P = mu * N we get sliding.
During sliding P = mus * N, where mus < mu.
Hence during sliding P < Ps
This is a nonlinear relation with an instability occurring at P=Ps. Think about the consequences.
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#1071
by
JohnFornaro
on 28 Sep, 2014 21:05
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Exsqeeze me on my ineptitude, but I am not following. We're talking about a wheel. Break it down for me, willya?
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#1072
by
Rodal
on 28 Sep, 2014 21:10
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Exsqeeze me on my ineptitude, but I am not following. We're talking about a wheel. Break it down for me, willya?
The wheel has a (leading) stick zone and (following) sliding zone.
You should not think of the wheel as perfectly rolling.
When you state that the wheel is "not driven" , it is not driven by a torque, but it is driven by changes in CG position producing inertial forces along the track because of coupling. There is coupling of the 3D degrees of freedom. You cannot analyze the problem as a linear problem. You have to take into account the coupling of the DOF.
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#1073
by
JohnFornaro
on 28 Sep, 2014 21:15
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I think I see.
I can get on a railroad flat car, and jerk backwards. The car will move forward. If I coast with it, gradually bringing my body forward at a rate of less than the "sticky friction", I will get to a point where I can jerk backwards again, and I can thus "propel" the railroad flat car forward.
Is this correct?
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#1074
by
Rodal
on 28 Sep, 2014 21:19
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I think I see.
I can get on a railroad flat car, and jerk backwards. The car will move forward. If I coast with it, gradually bringing my body forward at a rate of less than the "sticky friction", I will get to a point where I can jerk backwards again, and I can thus "propel" the railroad flat car forward.
Is this correct?
I have not done that in a railroad flat car, so I cannot say yes.
You
also have to do the side to side rocking.But it occurs to me that perhaps I was not communicating well that "sliding" does not mean sliding with no friction.
You slide with FRICTION , you still slide but you TRANSMIT forces (sorry about the caps to emphasize and type fast)
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#1075
by
Rodal
on 28 Sep, 2014 21:21
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And when you slide a wheel with friction you can transmit forces in both directions: along the rolling motion and perpendicular to the rolling motion (and any direction in between).
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#1076
by
Rodal
on 28 Sep, 2014 21:22
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Even with the wheel sliding everywhere (all sliding, no sticking) you still transmit forces
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#1077
by
Rodal
on 28 Sep, 2014 21:23
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So, if you would have this contraction working on rolling balls (rather than wheels), able to spin in all directions. perhaps it would not work or would not work as well.
Like the ball on old fashioned mouse.
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#1078
by
Rodal
on 28 Sep, 2014 21:27
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That would be an interesting experiment, to put this device on rolling balls instead of wheels like the ball on an old fashioned mouse.
On a flat granite surface instead of on a track
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#1079
by
Rodal
on 28 Sep, 2014 21:29
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If the balls would be able to freely spin in ALL directions, on a flat surface, I don't think the device would work.
However even ball bearings do not perfectly roll. But I would be willing to bet that on balls on a flat surface it would work much less effectively than on wheels on a track
