Jump to content

Does anyone really want to talk about time travel?


Recommended Posts

Re: Does anyone really want to talk about time tra

 

Because my field of expertise is closed-loop control systems, and there is a characteristic of such systems that defies explanation as to where its energy comes from: It is called divergent instability. It is most often experienced as the screeeeeeech of positive feedback through a closed-loop microphone and amplifier. In my business, a closed-loop airplane control system that has a divergent oscillatory mode, when stimulated with a very small input, will continue to oscillate in larger and larger amplitudes, until the airplane tears apart. THAT IS AN AWFUL LOT OF ENERGY!!

Granted, I don't know much/anything about airplane dynamics. But the closed loop you described with the microphone and speaker does not 'defy' explanation. The 'loop' in question is not 'closed'. In a typical microphone-speaker system there is also an amplifier. This amplifier has an external power source which exists out the loop. This power is used to amplify the sound coming into the microphone and out of the speakers. This happens so fast that the entire sound cannot escape the speaker before being picked up by the microphone. The sound becomes a high pitched tone constantly receiving energy from the energy supply of the amp.

 

Can you tell me more about this airplane example or does it also eventually come down to the input of an energy source? Does the same phenomenom occur in gliders?

 

James

 

 

"What will be, has already happened"

 

 

Link to post
Share on other sites
  • Replies 55
  • Created
  • Last Reply

Top Posters In This Topic

Re: Does anyone really want to talk about time tra

 

Granted, I don't know much/anything about airplane dynamics.

And my knowledge of power electronics is somewhat limited...I can read/analyze schematics, but don't ask me to design a circuit! :)

 

The 'loop' in question is not 'closed'. In a typical microphone-speaker system there is also an amplifier.

Hmmm. Are you sure the amp is not in the loop? The reason I ask is that the op amps we use to drive electro-hydraulic servovalves on airplanes usually do contain internal feedback, as this helps us shape hi-frequency gain performance. Plus, I thought use of feedback resistors (albeit high impedance ones) was common to limit the voltage gain thru the amp? (See here )

 

This amplifier has an external power source which exists out the loop.

True enough. But don't the power supply inputs to the amp also (typically) contain current limiting resistors? As I understand it, even with feedback resistors to limit voltage gain, these current limiting resistors are employed in the forward path to prevent the amp from drawing so much power that it hits the voltage rails of the power supply...? Correct me if I am wrong.

 

The sound becomes a high pitched tone constantly receiving energy from the energy supply of the amp.

Yes, and I believe the only reason it "stops" at that tone is because we artificially limit the frequency response. We do the same thing in airplane controls...we compensate the heck out of the control laws to keep things in the linear range, for no other practical reason than linear controls are much more predictable and easy to analyze.

 

Can you tell me more about this airplane example or does it also eventually come down to the input of an energy source? Does the same phenomenom occur in gliders?

Yes, it can also happen in a glider if the glider's aerodynamic design has an underdamped "short period" longitudinal (pitch) response. The input energy (thrust and speed) at the onset of the oscillation is the forcing function; however, the throttle can remain fixed and the pitch oscillations will continue to increase in amplitude until structural G-limits are exceeded, and that is when wings start to shear off! :eek: One flight technique taught to pilots who get into "pilot-induced oscillation" (e.g. mismatch of pilot's freq response to airplane dynamics) is to pull thrust to idle, let go of the stick, and allow the natural damping of the airplane to quiet things down.... that is, if the airplane's design is statically stable. If you are in an unstable airframe (F-16 and most other new fighters), you might as well pull the ejection lever! :)

 

The point I am getting at here (and guess I should state clearly) is: "What happens BEYOND the linear range that we go to great lengths to force our human devices to obey?" Chaos Theory provides some interesting answers, for when we look beyond the "period doubling route to chaos" (which is equivalent to the screech and the airplane breaking up), what we find is that the chaos eventually goes away, and the system settles into another stable attractor basin...at a HIGHER energy level than the stable attractor basin that it all started at...??? The stock market is another classic example of a chaotic feedback system. After each market crash, we have seen it rebound, and achieve stable ("DC") levels above the previous stable levels.

 

There's something strange and wonderful about transitioning through a chaotic region. Nature seems to allow higher levels of order (energy) on the other side. And let's not forget the "minor footnote" with regard to the 2nd law of thermo (entropy): This "law" is defined as valid for a CLOSED system (know of any that meet this strict definition) that is far from thermal equilibrium. The very concept of the "dipole" of "far from thermal equilibrium" probably has some interesting implications to advanced energy research.

 

Kind Regards,

 

RainmanTime

 

 

corruptissima re publica plurimae leges

Link to post
Share on other sites

Re: Does anyone really want to talk about time tra

 

Can you tell me more about this airplane example or does it also eventually come down to the input of an energy source?

Oh yeah...another good example is the American Airlines flight 587 Airbus crash that happened in New York one month after 9/11. My theory (which other controls engineers support) is that the flight data shows a clearly undamped, stop-to-stop rudder oscillation. It was initiated by the relatively small yaw inputs of passing thru the wake turbulence of a JAL 747. A known synchronization problem (peculiar to this Airbus design) of the dual hydraulic systems that drive the rudder caused a large loss of phase margin. When you couple the yaw disturbance with a degraded (negative) phase margin system, the rudder bangs back and forth until the tail breaks off.

 

The "official" NTSB position is that the pilot was dancing on the rudder pedals. Not likely, unless the person piloting the vehicle had no experience in flying and use of the rudder. Quite simply, my theory could be shot down if Airbus would publish frequency response data for the rudder control system when subjected to the hydraulic synchronization problem. Not only is the NTSB not asking for such data (although I have put a bug in their ear), but Airbus is not willfully offering it up.

 

Conspiracy theory, anyone? :(

 

Kind Regards,

 

RainmanTime

 

 

corruptissima re publica plurimae leges

Link to post
Share on other sites

Re: Does anyone really want to talk about time tra

 

It is called divergent instability. It is most often experienced as the screeeeeeech of positive feedback through a closed-loop microphone and amplifier.

I put some thought to this and:

 

I don't understand how you can have a divergent instability. I understand how you can have a CONVERGENT instability. I don't understand how in the 'screech' there is a deviation causing the instability. RT, can you help me to understand?

 

 

Teamwork! Adding value to our collective laziness is all we ask.

 

 

Link to post
Share on other sites

Re: Does anyone really want to talk about time tra

 

Hi Leot:

 

I don't understand how you can have a divergent instability. I understand how you can have a CONVERGENT instability. I don't understand how in the 'screech' there is a deviation causing the instability. RT, can you help me to understand?

What you must understand about the sound example is that final "tone" is actually an oscillatory wave. The frequency of the wave determines the pitch of the tone. But I admit it is hard to think of in this example.

 

Let's try the example of the airplane. It is more physically visual so perhaps easier to see the divergence. Typically, a closed-loop control law wants the airplane's nose (pitch attitude with respect to the earth) to remain fixed for any steady-state flight condition. Thus, the control laws will have "converged" on a steady-state pitch attitude that performs the needed function (e.g. holds airplane altitude constant). If an external disturbance upsets the airplane (a vertical wind gust) AND the airplane's control system has negative phase margin (a measure of stability), the control laws will command the nose of the airplane to oscillate back and forth (above and below) the previous "convergent" pitch attitude solution. Under the condition of negative phase margin, the amplitudes of the pitch attitude excursions get larger and larger (divergence) as time goes on, until..... the airplane breaks!

 

It's tough to really get the feel with words. I always "insisted" that my students bring airplane models to class so we could visualize flight concepts together.

 

Kind Regards,

 

RainmanTime

 

 

corruptissima re publica plurimae leges

Link to post
Share on other sites

Re: Does anyone really want to talk about time tra

 

the feed back is just a change in frequency. But every thing comes back to this. Does any one want to talk about time travel. I sure do.

 

Place your self in the middle of 2 kerr black holes. Causing a vac. bubble. Add singularities and a whole lot of x-rays and bam your in another time.

 

Black holes swirling around you cause the imense gravity field you need. the singularities that phase in and out of exsistance help stable the the gravity around you and then you have x-ray bursts that are a waste product. But don't tell the x-rays that. They are a bit sensitive and have a tendency to have violent out bursts.

 

But seriously space is full of particals and light travels on molcules that are charged by the sun thus charging the next molcule in a domino effect. This keeps happening even in to a black hole. but the light molcules are streched. Then are consumed or run in to a planet changing its properties. Being scattered and refracted.

 

Are we traveling the speed of light right now. I bet we are.

 

also the atomic clocks work better in space where they have less gravity to deal with.

 

 

Link to post
Share on other sites
Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...