DDWFTTW Turntable Test: 5 Min Video - Is It Conclusive?

[rcgldr]

I do not understand why zoobyshoe thinks that DDWFTTW could only work if pressure fore of the propeller is reduced.

Probably an efficiency issue. The propeller has to draw air from somewhere to accelerate the air behind the prop. In order to draw air the propeller has to reduce pressure. If the source of the air came from a diverted flow perpendicular to the prop, the net thrust would be less even though the pressure differential at the prop would be the same, because there'd be a second intake diverting surface in front of the prop with low pressure on the prop side and ambient pressure on the forward side, opposing the thrust. Ultimately the thrust is related to the acceleration of air affected by a prop times the mass of the affected air (the intergral sum of the component parts). The Newton 3rd law pair here is the force exerted by the prop onto the air and the coexistant force exerted by the air onto the prop.

 

[zoobyshoe]

You seem to argue (and I rejected that) that these are two different effects that add up, and both need to be taken into account / considered as separate contributions:

I'm not sure what you're objecting to. Are you objecting to that fact there are two separate effects (which there are) or are you objecting to an implied need to add them separately? I am aware that if you know the pressure difference, that's all you need. My complaint is that Jeff doesn't ever seem to figure in the low pressure created simply by virtue of the air passing over the front surface of the prop, air which is accelerated, not slowed, and which is shed as vortices at the wing tips, as opposed to being compacted behind the propeller. This effect increases the pressure difference. It's favorable to the cart. The "pull" it creates does, indeed, make the "push" far more effective. Jeff explains thrust from the prop on the cart according to how much air is slowed: his own, idiosyncratic way of apprehending the situation.

 

[swerdna]

(1). In your video, this does not quite happen. The closest the cart comes to this is right at the very beginning, T = 7 seconds, when the TT starts up and the cart momentarily moves at nearly the same speed, both moving CW. The wheel is obviously turning because the propeller is turning. They are linked by a flexishaft, remember?

I didn’t say it was, I said IF it was. The TT and the cart have to move at different speeds for the wheel to revolve. I hope we agree on that.

(2). Yes. At T = 8 seconds both the cart and the TT are still accelerating and moving in the same direction, CW and the TT linear velocity is greater than the cart’s linear velocity on the wheel circumference.

So we agree that it’s the TT and cart moving at different speeds which causes the wheel to revolve.

(3) This is true for as long as they are going in the same direction only! At T = 12 seconds, the cart is moving at it’s Max speed because it is still moving CW with the TT. If you had a tach on the wheel, HERE is where you would record the highest RPM.

I disagree. The TT and cart are traveling at different speeds when they are moving in the same direction, when the cart “stops”, and when it travels against the motion of the TT. During this whole process the difference in speed between the TT and cart is steadily increasing and the speed of the revolutions of the wheel are also increasing. The highest RPM would be recorded when the cart reaches terminal speed against the motion of the TT.

(4). Yes the Difference increases because the cart starts to slow down! The TT does not accelerate continuously does it? At T = 13 seconds, the cart is seen to be stationary for a moment on the TT. At this point, the linear velocity on the edge of the wheel is exactly the same as the linear velocity on the surface of the TT. This is a critical point.

Do you agree that the differential speed increase caused as the cart slows down means that the revolutions of the wheel speed up?

(5) NO. Again, the Difference increases because the cart is slowing down and the TT is no longer speeding up. At T = 17 seconds the cart has reached its terminal linear velocity, which is less than at T = 12 seconds. From this point onwards, the TT and the cart are in a heterodyne. The translation of the cart in the CCW direction (measured in m/sec) is the Difference between the linear velocity of the TT surface in m/sec MINUS the linear velocity at the wheel in m/sec.

What you call “slowing down” could equally (and probably more correctly) be described as “moving in that direction less“. When the cart is moving against the motion of the TT it is “moving in that direction less” every bit as much as it is when it is “slowing down“ (more so in fact).

If you can give me very accurate dimensions of the TT diameter, wheel diameter, and a very accurate measurement from center of TT to center of the track of the wheel, and the RPM of the TT, I can calculate the ballpark figures for the RPM of the wheel at T = 12 seconds (High RPM) and T = 13 seconds (RPM where wheel linear velocity equals TT linear velocity) and at T = 17 seconds (when the wheel linear velocity is LESS then the TT linear velocity and it is translating) But it is much preferable that you put a tach on the wheel and measure this directly.
I can give you a calibration method if and when you are ready to do this test. Thanks!

As I explained to another member earlier I no longer have an operating cart for my turntable as I’ve used some pieces for another cart. To do your experiment I would have to reconstruct the cart and also purchase and fit a tachometer. I’m not prepared to spend the time and money on a test that I can’t see any sense in or need for. I don’t need to fit a tachometer to the cart to know that the wheel increases it’s revolutions until it reaches it’s terminal speed. That you can’t see this I find really amazing, especially for an engineer. I will continue to give serious consideration to any argument you wish to present that you’re right and I’m wrong.

 

[vanesch]

My complaint is that Jeff doesn't ever seem to figure in the low pressure created simply by virtue of the air passing over the front surface of the prop, air which is accelerated, not slowed, and which is shed as vortices at the wing tips, as opposed to being compacted behind the propeller.

You can try to figure all this out, but it is extremely complicated. It is much easier to trust Newton's 3rd law, which says that action = reaction, together with Newton's second law (F = m a), but here in the form: F = d p / d t.

In other words, the trick is first to calculate the entire force that is exerted (by the propeller) on the air, and to find that out, we make the momentum balance: we see what was the momentum of the air "before" and then the momentum of the air "after".

Remember that momentum of something = mass of something x velocity.

Now, here we make an approximation: we consider that there is air that is left "untouched" by the propeller, and an amount of air that is "uniformly displaced" by the propeller. In reality, there will be different categories of air, those that are untouched, there are some that are a bit displaced, there are others that are more displaced etc... so we should sum all these different contributions, but we don't, we simply say that some amount of air is affected in the same way by the propeller, and the rest not at all.

The momentum balance of the unaffected air will of course be 0: it will have the same momentum before and after. So we don't need to take it into account ; what counts is only what is displaced by the propeller. Per unit of time, a certain mass of air is affected by the propeller, and we call that "m". That means that the mass in an amount of time dt is M = m dt. That amount of mass had a velocity v_in before it was affected by the propeller, and is then accelerated and ends up at a velocity v_out (it is the uniformity of v_out which is the approximation we make: not all bits of air are accelerated to the same velocity).

That means that the momentum "after" is M x v_out while the momentum "before" is M x v_in. (for the non-affected air, the momentum in was MMM v_vin and the momentum out is also MMM v_in, where MMM is the big mass of the unaffected air ; so the balance doesn't matter).

So the momentum gained, by the air, in a time dt is given by M x v_out - M x v_in.
That momentum was gained in an amount of time dt, so the force (Newton's first law) that must have been exerted on that air must have been: F = ( M x v_out - M x v_in ) / dt.

Or: F = m x (v_out - v_in). (as M / dt = m).

So, no matter what or how, this must be the force exerted on the air. And as it is the propeller that does this, the opposite force must be the one exerted on the propeller.

So, if we know that a certain amount of air changes velocity, all the rest doesn't actually matter: it is this momentum balance which counts, and it will be the final sum of all the little bits of pressure that work on all the pieces of surface of the propeller. We don't have to go into that detailed description: we have the end result of the balance.

 

[vanesch]

If you can give me very accurate dimensions of the TT diameter, wheel diameter, and a very accurate measurement from center of TT to center of the track of the wheel, and the RPM of the TT, I can calculate the ballpark figures for the RPM of the wheel at T = 12 seconds (High RPM) and T = 13 seconds (RPM where wheel linear velocity equals TT linear velocity) and at T = 17 seconds (when the wheel linear velocity is LESS then the TT linear velocity and it is translating) But it is much preferable that you put a tach on the wheel and measure this directly.
I can give you a calibration method if and when you are ready to do this test. Thanks!

Ok, let us take the example: radius of the turntable 1 m (take it that this is the track of the wheel, and that the table itself is 1.2 m), angular rotation 6.28 rad/second of the table, diameter of the little wheel 5 cm.
So, show us your calculation for the angular rotation of the axle of the wheel
1) when the cart is perfectly in sync with the TT
2) when the cart is momentaneously stationary wrt the outside ground (when it flips from going CW to CCW on the TT)
3) at the end when it is moving CCW. BTW, it would be fun if you could calculate the axle rotation velocity in the hypothetical case that the arm of the cart were now turning CCW at also 6.28 rad / s.
Don't forget to tune your heterodyne

 

[swerdna]

I’ve built an air cart version of the Brennan Torpedo but I’m having a problem with it. The string is being pulled from a spool on the prop shaft and the cart is being held against a backstop until it gets up to thrust speed. It works fine except when the cart leaves the stop it surges slightly which makes the spool overrun and that tangles the string up. It was worst when I tried it with nylon fishing line. That horrible stuff is just designed to tangle. If I put enough friction on the spool to stop it overrunning it puts too much friction on the system and it doesn’t work. Any ideas? I may just concentrate on a water model as I think it would be more constant and stable.

 

[schroder]

I disagree. The TT and cart are traveling at different speeds when they are moving in the same direction, when the cart “stops”, and when it travels against the motion of the TT. During this whole process the difference in speed between the TT and cart is steadily increasing and the speed of the revolutions of the wheel are also increasing. The highest RPM would be recorded when the cart reaches terminal speed against the motion of the TT.





What you call “slowing down” could equally (and probably more correctly) be described as “moving in that direction less“. When the cart is moving against the motion of the TT it is “moving in that direction less” every bit as much as it is when it is “slowing down“ (more so in fact).



As I explained to another member earlier I no longer have an operating cart for my turntable as I’ve used some pieces for another cart. To do your experiment I would have to reconstruct the cart and also purchase and fit a tachometer. I’m not prepared to spend the time and money on a test that I can’t see any sense in or need for. I don’t need to fit a tachometer to the cart to know that the wheel increases it’s revolutions until it reaches it’s terminal speed. That you can’t see this I find really amazing, especially for an engineer. I will continue to give serious consideration to any argument you wish to present that you’re right and I’m wrong.

If you put the TT and cart back together, with a tachometer on the TT and a tachometer on the wheel, you will have built a very valuable teaching aid for rotational and translational motion. In particular, a very nice demonstration of a mechanical heterodyne which every mechanical engineering department and physics department will be happy to have. That is where you will get the most benefit out of this, not by chasing after some DDWFTTW myth. You have something valuable there if only you recognize it! You find it amazing that I believe the cart slows down to reach the steady state. I find it amazing that you believe in DDWFTTW. One of us is wrong! The best way to determine that is to do the test with the tachometer and I guarantee you will be amazed to see the cart is actually slowing down. There is nothing magical or mystifying about that. Translational motion is replacing rotational motion while energy is being conserved. What I am explaining is according to the laws of physics while DDWFTTW is not! I find it amazing that you choose to believe the stranger of the two claims rather than believe in a heterodyne. I will try to find some more examples of heterodynes in mechanical machinery. They are rare, which is why your TT and cart is a valuable asset. I believe heterodynes also happen on large rollers in paper mills and in some other machinery used in the lumber industry, as well as inside gas turbines. Once you understand the principle, the cart on the TT makes perfect sense. I think you are wasting your time chasing the wind when your TT is far more valuable and certainly worth the cost of at least one digital tachometer and preferably two. But that is entirely up to you. I will try to make some drawings to explain how translation takes the place of rotation, but I am not good at computer animations so it may be a bit crude. Give what I say some consideration as you will benefit if I am right.

 

[Subductionzon]

schroder, I am pretty sure which one of us is wrong! No matter how many times you use the word I am pretty sure that everone but you agrees that there are no radio frequencies involved here. So unless you have some unique definition for the word "heterodyne" I would appreciate it if you would use mechanical terms here. You still have not answered my claim that the relative velocity that you should be looking at is the velocity of the cart relative to the turntable. Two questions. First what are you measuring velocity relative to? Second regardless of what you are measuring velocity relative to, what is the velocity of the cart measured to relative to the turntable? ie, if you put a speedometer on the wheels of the cart what value would you get? And if you measured the speed of the outer edge of the turntable in m/s what value would you get?

 

[vanesch]

If you put the TT and cart back together, with a tachometer on the TT and a tachometer on the wheel, you will have built a very valuable teaching aid for rotational and translational motion.

THAT's for sure !

You find it amazing that I believe the cart slows down to reach the steady state. I find it amazing that you believe in DDWFTTW. One of us is wrong!

THAT's for sure too !

Translational motion is replacing rotational motion while energy is being conserved. What I am explaining is according to the laws of physics while DDWFTTW is not! I find it amazing that you choose to believe the stranger of the two claims rather than believe in a heterodyne.

You have clearly not understood what is the mathematics behind a heterodyne, nor what is the mathematics behind the concept of velocities, nor the most basic notions of kinematics. However, you are somehow *convinced* that you know something about it, you even think you have some exceptional expertise on it.

I've seen other people self-deluded like that, but I have never seen people who do this on an elementary level like this - that's what fascinates me with you. As I said, even without a formal training, I don't know many people who would make the claims about the rolling wheel you are making (especially that the wheel is turning when the cart goes in sync with the table). For most people this would even intuitively be obvious - even though they don't have the formal tools to confirm it. You have distorted your view entirely by thinking that you have a superior understanding of the thing (which is nothing else but a complete confusion between two totally unrelated phenomena, which are frequency mixing in a non-linear component of a scalar signal and vector addition in a velocity field) to a point which is downright fascinating.

But that is entirely up to you. I will try to make some drawings to explain how translation takes the place of rotation, but I am not good at computer animations so it may be a bit crude. Give what I say some consideration as you will benefit if I am right.

Will it benefit to you if you finally will see that you are wrong on this ? If so, swerdna's test might have some use. But the outcome is evident.

 

[vanesch]

You still have not answered my claim that the relative velocity that you should be looking at is the velocity of the cart relative to the turntable.

The problem is that even the notion "relative velocity" is distorted by schroder. Some posts back, when I asked him if he disputed the claim that if a cart was going 2 m/s to the left, and a tread was going 10 m/s to the right, the velocity of the cart wrt the tread was 12 m/s, and for sure he disputed that (see post 755). He claimed that the relative velocity was 8 m/s. (ok, it might have been mph instead of m/s). At that point, I still thought that there might be a misunderstanding about the actual setup - or that he was just trolling.

But it becomes more and more clear that it was not a misunderstanding concerning the setup.

Schroder doesn't understand what it means "relative velocity", or how to obtain it when you have the individual velocities, as he confuses this with mixing frequencies of 2 and 10 (in other words, he gives himself the liberty to alter signs at will in the velocity composition).

I have never met anyone who has such a profound confusion. It is a remarkable phenomenon, and a true challenge to find out how to tackle it, pedagogically. Have you EVER met anybody who disputed such an elementary claim with such vehemence ?
(especially somebody who claims to be an engineer ?)

 

[swerdna]

If you put the TT and cart back together, with a tachometer on the TT and a tachometer on the wheel, you will have built a very valuable teaching aid for rotational and translational motion. In particular, a very nice demonstration of a mechanical heterodyne which every mechanical engineering department and physics department will be happy to have. That is where you will get the most benefit out of this, not by chasing after some DDWFTTW myth. You have something valuable there if only you recognize it! You find it amazing that I believe the cart slows down to reach the steady state. I find it amazing that you believe in DDWFTTW. One of us is wrong! The best way to determine that is to do the test with the tachometer and I guarantee you will be amazed to see the cart is actually slowing down. There is nothing magical or mystifying about that. Translational motion is replacing rotational motion while energy is being conserved. What I am explaining is according to the laws of physics while DDWFTTW is not! I find it amazing that you choose to believe the stranger of the two claims rather than believe in a heterodyne. I will try to find some more examples of heterodynes in mechanical machinery. They are rare, which is why your TT and cart is a valuable asset. I believe heterodynes also happen on large rollers in paper mills and in some other machinery used in the lumber industry, as well as inside gas turbines. Once you understand the principle, the cart on the TT makes perfect sense. I think you are wasting your time chasing the wind when your TT is far more valuable and certainly worth the cost of at least one digital tachometer and preferably two. But that is entirely up to you. I will try to make some drawings to explain how translation takes the place of rotation, but I am not good at computer animations so it may be a bit crude. Give what I say some consideration as you will benefit if I am right.

Don’t think of the cart as “slowing down”, think of it as “moving less in that direction“.

I can clearly see that the cart is “slowing down” in relation to the turntable if I use myself as the stationary reference. I can also see that it then “speeds up“. But what has the speed of the cart got to do with the speed of the wheel? The tachometer is measuring the speed (revolutions) of the wheel isn’t it? I’m not amazed that you think the cart is slowing down, I’m amazed that you think the speed (revolutions) of the wheel is slowing down. You seem to be talking about the speed of the cart and the speed of the wheel revolving as if they are the same thing (they aren‘t).

When the cart is “hovering (stopped) the speed of the running surface (circumference) of the wheel is the same as the speed of the particular part of the TT surface it runs on. Let's say this is 10kph. If the cart moves against the motion of the TT by 2kph then the speed of the running surface of the wheel is 12kph. It absolutely has to be, there is no other option (assuming it isn‘t slipping).

I have also Googled heterodyne and can’t find any mechanical reference for the term and don’t see how it applies to the TT/cart. Any mechanical examples would be appreciated.

I believe all this is irrelevant anyway as I really don’t see that what happens before the cart reaches terminal speed is of any importance, as long as no effect like stored energy is carried over. Tests I‘ve done on my equipment have conclusively shown me that stored energy at least is not carried over. If there is some other effect called heterodyne then it should be able to be clearly explained and proven by the person that claims the effect exists. If the cart was held against the motion of the TT until the prop thrust could do it, would a heterodyne effect still be applicable? I know enough about basic mechanics to know that the revolutions of the wheel (nothing to do with the speed of the cart) do not slow down as the cart goes throught the process of reaching terminal speed.

Give what I say some consideration as you will benefit if I am right.

ETA - If the turntable and cart are such a valuable asset why don’t you build one of your own? It should be very easy for a professional engineer to do. You could then do your own tests and prove conclusively that you are correct.

 

[ThinAirDesign]

I’ve built an air cart version of the Brennan Torpedo but I’m having a problem with it. The string is being pulled from a spool on the prop shaft and the cart is being held against a backstop until it gets up to thrust speed. It works fine except when the cart leaves the stop it surges slightly which makes the spool overrun and that tangles the string up. It was worst when I tried it with nylon fishing line. That horrible stuff is just designed to tangle. If I put enough friction on the spool to stop it overrunning it puts too much friction on the system and it doesn’t work. Any ideas? I may just concentrate on a water model as I think it would be more constant and stable.

In an attempt to tune out the nasty behavior, I would first simply add weight to the "torpedo". Some combination of weight and different property string should eliminate the feedback, but admittedly the needed weight might sink the project on other grounds.

Water will certainly give more damping, but with careful tuning air would work just fine.

JB

 

[The Dagda]

The problem is that even the notion "relative velocity" is distorted by schroder. Some posts back, when I asked him if he disputed the claim that if a cart was going 2 m/s to the left, and a tread was going 10 m/s to the right, the velocity of the cart wrt the tread was 12 m/s, and for sure he disputed that (see post 755). He claimed that the relative velocity was 8 m/s. (ok, it might have been mph instead of m/s). At that point, I still thought that there might be a misunderstanding about the actual setup - or that he was just trolling.

But it becomes more and more clear that it was not a misunderstanding concerning the setup.

Schroder doesn't understand what it means "relative velocity", or how to obtain it when you have the individual velocities, as he confuses this with mixing frequencies of 2 and 10 (in other words, he gives himself the liberty to alter signs at will in the velocity composition).

I have never met anyone who has such a profound confusion. It is a remarkable phenomenon, and a true challenge to find out how to tackle it, pedagogically. Have you EVER met anybody who disputed such an elementary claim with such vehemence ?
(especially somebody who claims to be an engineer ?)

I've seen this hundreds of times on many forums, he's now locked into the claim by pride, so that even if he did have a glimmer of understanding he's unable to admit he was wrong. When he says things like people consider me a genius, then I can assume he has a high opinion of himself. That doesn't help, the only conclusion is for him to stop posting or to admit he's wrong. Science does not rest on this, only his feelings.

Interestingly I've seen people go on to deny your claims on several threads, even though they are obviously wrong and their arguments have been destroyed. One example is finding WMD's in Iraq, I've seen run long after it was confirmed there were none. It's amazing really.

 

[A.T.]

I am aware that if you know the pressure difference, that's all you need.

That's good then.

 

[rcgldr]

Jeff doesn't ever seem to figure in the low pressure created simply by virtue of the air passing over the front surface of the prop

I've always stated that the prop works the same as any conventional prop. Lower pressure in front, pressure increasing through the prop swept area so higher pressure at the aft edge of the prop "disk".

air which is accelerated, not slowed

Accelerated from the cart's and prop's frame of reference, slowed from the air's (wind) frame of reference. In either frame of reference the net result is some amount of air is accelerated upwind per unit time and this is related to the thrust at the prop. I include the fact that the wind is slowed down by the upwind acceleration from the prop because all wind powered (wind speed different than ground speed) devices extract power by slowing down the wind.

 

[uart]

Wow this has been a pretty amazing thread. Personally I chose not to get involved in this one because I has already grown too tired of trying to convince the doubters in previous threads. I have to admit though that when I saw the first dwfttw video posted here (many long DWFTTW threads ago) that I first thought it was probably a hoax, but only because I thought the fan was operating as a turbine driving the wheels rather than a propeller driven by the wheels. As soon this design aspect was pointed out then it took me about 5 minutes to realize it was easily feasible (physically), and given all the independent verifications, there were just no doubts about the reality of it.

I know the basic maths of this has been posted dozens of times (and probably either ignored or misunderstood by the doubters) but now that this thread has quietened down a bit let's recap why this design is totally feasible and violates no physical laws.

Taking the case of the cart moving at the wind speed "v", show that the wheels can easily provide the power to turn the prop to provide enough trust to both overcome the extra resistance at the wheels (due to the requirement that they drive the prop) and to supply excess thrust to acelerate the cart to faster than "v" (v = wind speed).

Given that the trust of the propeller (rate of change of momentum) is :

F = r u

Where r is the mass per second thrown by the prop and u the velocity (m/s) at which the air is thrown.

And that the power an ideal propeller needs to throw this air (KE per second) is :

P_{\mbox{req}}= \frac{1}{2} r u^2

Then if we use say just half of the available thrust to generate the power required to turn the prop (that is allowing the component of drag at the wheels, due to the requirement that they turn the prop, to be just one half of the available thrust of the prop) then we have power available of :

P_{\mbox{avail}} = \frac{1}{2} F v = \frac{1}{2} r u v

So even with only half the available thrust used for generating power at the wheels we still have :

\frac{P_{avail}}{ P_{req}} = \frac{v}{u}

Clearly this can be made greater than unity when the wind speed “v” is sufficiently large, so more power is available at the wheels than required to turn the prop! Now given that a good http://en.wikipedia.org/wiki/Propeller" is about 80% efficient (P_out over P_in) at moving air then this thing is just a no brainer. I simply cannot believe that anyone with even one year of university level Physics could fail to understand this. It’s just mid boggling to me that some of these silly arguments could have gone on this long in a Physics forum.

 

[rcgldr]

I’ve built an air cart version of the Brennan Torpedo but I’m having a problem with it. The string is being pulled from a spool on the prop shaft and the cart is being held against a backstop until it gets up to thrust speed. It works fine except when the cart leaves the stop it surges slightly which makes the spool overrun and that tangles the string up.

You need something to take up the slack on the string. The simplest method would be mount the string to a pole about 2 or 3 feet tall, and then just under 2 or 3 feet away from the pole on the string, attach a small weight (like ones used for fishing.). Note that this weight setup shouldn't effect the efficiency of the system. Using heavier monofilament fishing line, like 60 lb test fishing line, would be much lighter than string but less likely to tangle.

 

[spork]

I have never met anyone who has such a profound confusion. It is a remarkable phenomenon, and a true challenge to find out how to tackle it, pedagogically. Have you EVER met anybody who disputed such an elementary claim with such vehemence ?

Sure have! humber on the JREF forum, a few physics professors, and an instructor of mechanical engineering at my own alma mater (GA Tech), to name a few.

 

[vanesch]

Sure have! humber on the JREF forum, a few physics professors, and an instructor of mechanical engineering at my own alma mater (GA Tech), to name a few.

I wasn't talking about DDTWFTTW (although it isn't that complicated either), I was talking about the confusion concerning relative motion and non-slipping wheels. I never met anyone who made such claims (that the wheel is spinning when the cart turns in sync with the table, that the wheel will slow down when the cart starts turning "backwards", and that the relative velocity of a thing going 2 m/s to the left wrt another thing going 10 m/s to the right, is not 12 m/s but rather 8 m/s).

What you write about physics professors and mechanical engineering instructors is worrisome, though...
That a business professor or a sailing instructor wouldn't catch on, I can understand. But people who are supposed to teach the tools of which this is an elementary application, that's worrisome.

 

[spork]

I wasn't talking about DDTWFTTW (although it isn't that complicated either), I was talking about the confusion concerning relative motion and non-slipping wheels...

Understood. The folks I listed are stuck on physics principles just that basic. We're up to 80 pages specifically dedicated to humber's extraordinary misunderstanding of the most basic physics principles on the JREF forum. He and the GA Tech instructor share the astonishing quality of being unbelievably wrong (on some VERY basic stuff) while being unbelievably arrogant.

 

[ThinAirDesign]

A couple schroder observations:

A: Clearly the guy has thought the 'sailcart vs. propcart' race over and determined that it's a loser for him. At this point he has got to just be desperately hoping the whole idea will just get dropped. Can't think of any other reason for him to suggest it and then just go silent on the whole idea in spite of repeated requests.

Not going to drop it -- he can get on board and contribute to the setup of the test that he suggested or we'll just aim them DDW and turn them loose. The results aren't going to reflect well on his position no matter what.

B: the funny part about his "heterodyne" obfuscation? -- who cares what the wheels are doing as long as the cart is going DDWFTTW? In swerdna's video, once the cart starts turning CCW, it's going DDWFTTW -- even if schroder is right and the wheels are slowing down (He's not. They're not).

The only possible exception to this would be if the TT itself were slowing down and there were a variable tranmission on the cart -- this would allow momentary use of stored energy to power the cart against the TT. Problem is, the TT isn't slowing down (time the stripe on the TT) and there is no variable transmission between the wheel and the prop.

With a fixed (or increasing) CW TT speed, once the cart turns CCW, schroder can argue anything he wishes regarding the rotation speed of the wheels ... the cart has passed through windspeed and is now beating the wind. If the wheels are slipping and slowing down (they're not), then the cart is dragging a slowing, slipping wheel along for the DDWFTTW ride.

I would still love for schroder, with his "academy" credentials, to make himself available to be the "expert" on a MythBuster's episode -- but by his silence on that suggestion, he's no more interested in being put in that losing position than the sailcart vs propcart race.

JB

 

[swerdna]

Let’s not turn this into a “beat up Schroder” thread. The truth will eventually “beat up” anyone who continues to deny it.

Schroder has said (post #847”) . . .

“I will try to find some more examples of heterodynes in mechanical machinery.”

and - “I will try to make some drawings to explain how translation takes the place of rotation, but I am not good at computer animations so it may be a bit crude.”

Let’s see how hard he tries and what he comes up with. If he expects me to disassemble one cart to remake another, and also go to the time and expense of purchasing and fitting tachometers, then it’s not unreasonable for me to expect him to provide some credible evidence of his claims that are to be tested first.

As far as I can understand them his claims are . .

(1) That the revolutions of wheel of the cart are slowing down as the cart moves against the motion of the TT. He has asked me to fit a tachometer to the wheel to prove this. I don’t know why he keeps saying “the cart is slowing down” when he wants me to measure the revolutions of the wheel. At the very least he should clarify whether he is talking about the revolutions of the wheel or the relative movement of the cart and TT and stop talking about the two things as if they are the same.

(2) That there is a “mechanical heterodyne” effect that is somehow making the cart demonstrate that it’s traveling DDWFTTW when it isn’t. I have no knowledge of any such mechanical heterodyne effect and would like him to provide credible evidence that such a thing even exists. I don’t see how it’s possible for a cart to very clearly demonstrate that it’s traveling DDWFTTW and not atually be doing so.

The ball’s in your court Schroder.

 

[schroder]

Let’s not turn this into a “beat up Schroder” thread. The truth will eventually “beat up” anyone who continues to deny it.

Schroder has said (post #847”) . . .

“I will try to find some more examples of heterodynes in mechanical machinery.”

and - “I will try to make some drawings to explain how translation takes the place of rotation, but I am not good at computer animations so it may be a bit crude.”

Let’s see how hard he tries and what he comes up with. If he expects me to disassemble one cart to remake another, and also go to the time and expense of purchasing and fitting tachometers, then it’s not unreasonable for me to expect him to provide some credible evidence of his claims that are to be tested first.

As far as I can understand them his claims are . .

(1) That the revolutions of wheel of the cart are slowing down as the cart moves against the motion of the TT. He has asked me to fit a tachometer to the wheel to prove this. I don’t know why he keeps saying “the cart is slowing down” when he wants me to measure the revolutions of the wheel. At the very least he should clarify whether he is talking about the revolutions of the wheel or the relative movement of the cart and TT and stop talking about the two things as if they are the same.

(2) That there is a “mechanical heterodyne” effect that is somehow making the cart demonstrate that it’s traveling DDWFTTW when it isn’t. I have no knowledge of any such mechanical heterodyne effect and would like him to provide credible evidence that such a thing even exists. I don’t see how it’s possible for a cart to very clearly demonstrate that it’s traveling DDWFTTW and not atually be doing so.

The ball’s in your court Schroder.

Swerdna, thank you for this post. You are one of the more reasonable people here and I am convinced you are interested in the truth of this matter. It is for that reason that I will continue to reply to you.
I am working on the a better presentation of my analysis of the cart on the Turntable but I cannot devote very much time to this at the moment. I admit that some of my earlier statements were confusing and even wrong. But I am more convinced than ever that the cart is in a heterodyne with the TT. If you examine your video carefully, you will see that after the cart reverses direction, it will cover one revolution CCW for every 2.4 revolution CW of the TT. It just so happens that 2.4 is the modulus for a first Bessel null. That confirms the heterodyne beyond any reasonable doubt.
It is the revolutions of the cart on the TT that has slowed, not the wheel rpm. Sorry for the confusion on that. What is important to note here is this: The TT is the more massive and directly powered by the motor, so in any heterodyne contest the cart will lose and it will be forced to drop out in a Bessel null. That is exactly what is happening right in front of our eyes in the video. When the cart is on the road, being pushed by the wind, the planet Earth is the more massive (by far!) and the little cart cannot ever force a heterodyne with the earth. Consequently, no heterodyne occurs and the cart just continues along, being pushed by the wind at sub-wind speed with nothing remarkable happening as on the TT.
The TT is clearly a rotating reference frame while the Earth is an inertial frame. Previously, I did not think this was important to the question but I now realize it is all important. What is happening on the TT is due to the heterodyne of two things that are rotating and beating against one another. The wheels of the cart do go faster than the tread. However, as I pointed out, this cannot happen with the earth.
I repeat, what you have, the TT, is far more interesting than chasing the wind. I am now considering building one myself and also looking for any practical applications but going faster than the wind is not one of them!

 

[spork]

I am more convinced than ever that the cart is in a heterodyne with the TT. If you examine your video carefully, you will see that after the cart reverses direction, it will cover one revolution CCW for every 2.4 revolution CW of the TT.

How can this be? The cart gradually accelerates. At what point do you choose to make this relative speed calculation?

It just so happens that 2.4 is the modulus for a first Bessel null. That confirms the heterodyne beyond any reasonable doubt.

Even if it did just happen to be moving at a speed ratio of 2.4 it would be wildly innaccurate to say "That confirms the heterodyne beyond any reasonable doubt". The first thing you'd have to do to prove the heterodyne is to give us even a hint as to what this physical system has to do with a heterodyne. Are you talking about two physical frequencies combining? Is the cart hopping or vibrating? If so, what does that have to do with anything? If so wouldn't this simply be the mechanism causing it to go DDWFTTW? Afterall, we can SEE that it is going DDWFTTW.

...in any heterodyne contest the cart will lose and it will be forced to drop out in a Bessel null. That is exactly what is happening right in front of our eyes in the video.

It most definitely is not what's happening before our eyes. I am forced to declare a moratorium on the terms heterodyne and bessel null until you're prepared to offer even a vague idea of how they apply to this physical system.

The wheels of the cart do go faster than the tread. However, as I pointed out, this cannot happen with the earth.

And yet despite your considered analysis... it does.

I am now considering building one myself and also looking for any practical applications

I generally recommend not telling a joke you don't get.

 

[vanesch]

I am working on the a better presentation of my analysis of the cart on the Turntable but I cannot devote very much time to this at the moment. I admit that some of my earlier statements were confusing and even wrong. But I am more convinced than ever that the cart is in a heterodyne with the TT. If you examine your video carefully, you will see that after the cart reverses direction, it will cover one revolution CCW for every 2.4 revolution CW of the TT. It just so happens that 2.4 is the modulus for a first Bessel null. That confirms the heterodyne beyond any reasonable doubt.

Ok, this goes beyond the level of tolerance we can accept here at PF. As you know, the rules here specify that for any unusual claim, you need a peer-reviewed source. I will be more relax, and ask you to show us a "generally accepted" (say, popular textbook, or university website or similar) source where your "heterodyne" thing is put in relationship with a wheel on a table or similar, where a clear definition of the term is given and where examples of its application are shown that make it clear that it is a concept that is "known and useful" in this particular problem. Several times now, posters (including, but not only, me) have indicated that the only known definition of the term heterodyne was in radio receiver technology, and have indicated not to know anything about any relationship with a simple mechanical problem, and its relationship with the zeros of Bessel functions (no, pointing to FM will not be sufficient). This makes your claim (to say the least), "unusual", and hence needs backing up.

This must be in the next post, or sanctions will follow for disinformation and lack of sources.

It is the revolutions of the cart on the TT that has slowed, not the wheel rpm. Sorry for the confusion on that.

Your behaviour concerning that "confusion" was unacceptable. Insulting people trying to point out your "confusion" was not the correct way of handling this. Next time, think before you write.

 

[schroder]

Ok, this goes beyond the level of tolerance we can accept here at PF. As you know, the rules here specify that for any unusual claim, you need a peer-reviewed source. I will be more relax, and ask you to show us a "generally accepted" (say, popular textbook, or university website or similar) source where your "heterodyne" thing is put in relationship with a wheel on a table or similar, where a clear definition of the term is given and where examples of its application are shown that make it clear that it is a concept that is "known and useful" in this particular problem. Several times now, posters (including, but not only, me) have indicated that the only known definition of the term heterodyne was in radio receiver technology, and have indicated not to know anything about any relationship with a simple mechanical problem, and its relationship with the zeros of Bessel functions (no, pointing to FM will not be sufficient). This makes your claim (to say the least), "unusual", and hence needs backing up.

This must be in the next post, or sanctions will follow for disinformation and lack of sources.



Your behaviour concerning that "confusion" was unacceptable. Insulting people trying to point out your "confusion" was not the correct way of handling this. Next time, think before you write.

If anyone should be "sanctioned" for wild unsubstantiated claims, it is YOU, for your wild unsubstantiated claim of directly downwind faster than the wind, which has NO basis in fact and NO peer review.

A heterodyne is most clearly understood when considering radio or audio frequencies but it most definitely is applicable to any mechanical frequencies as well. Simply because YOU do not understand it is no basis for denouncing it. I suggest you study up on it!


YOUR behaviour regarding slinging of insults is the worst I have ever seen on any forum, anywhere and your being a "PF mentor" is unacceptable. You should just slink away in shame!

 

[spork]

A heterodyne is most clearly understood when considering radio or audio frequencies but it most definitely is applicable to any mechanical frequencies as well.

And yet you can't offer a single example of a mechanical heterodyne. hmmmm...

Simply because YOU do not understand it is no basis for denouncing it. I suggest you study up on it!

Ah yes, the ever classic "I don't have to defend my ludicrous claims - you should study up on my ludicrous claims yourself".

 

[A.T.]

 

[vanesch]

 

[vanesch]

If anyone should be "sanctioned" for wild unsubstantiated claims, it is YOU, for your wild unsubstantiated claim of directly downwind faster than the wind, which has NO basis in fact and NO peer review.

I would like to point out that I did not do so. I only pointed out, that, as an elementary exercise in classical mechanics, a sufficiently ideal device modeled according to the descriptions in the videos, is perfectly capable of functioning as the videos show, without claiming that the videos themselves are valid: whether they are true videos or even fake videos, they show a phenomenon which is capable of being explained perfectly well with an elementary mechanical model. The model shows that one has a certain margin for non-idealities and dissipative losses that do not affect the main behaviour. In doing so, I used very standard concepts which are normally known to any first-year mechanics student.

I also pointed out the Galilean equivalence (and its limitation) between the experiments shown in the video (whether these are true videos, or fake ones doesn't matter) and a "true" DWFTTW setup.

None of these "claims" is unusual, but if you need any reference for a particular step, I can provide them easily. Do you need a source for the claim that:
- work is force x distance ?
- power is work per unit of time ?
- kinetic energy of a mass of air is 1/2 m v^2 ?
- momentum of a mass of air is m x v ?

Concerning the kinematics of a non-slipping wheel on a surface, I *did* provide some sources in the end.

I asked you to provide me a source for your extravagant claim that the relative motion between a cart going to the left at 2 m/s and a tread going to the right at 10 m/s is not 12, but rather 8 m/s, based upon an incomprehensible reasoning with a "heterodyne", and especially what the ratio of the rotation rate of the cart versus the rotation rate of the table has necessarily to do with a zero of a Bessel function.
Contrary to generally accepted concepts such as kinetic energy, power, momentum etc.. in a mechanical problem, your claims seem to be totally unfounded. Hence the valid request for a source.

As I said, your NEXT post will have to contain so (or a retraction of all your claims with a "heterodyne").