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

[ThinAirDesign]

The thrust of the prop alone can never exceed the “thrust” (rolling resistance) of the wheel that gives the prop the thrust energy to begin with.

I disagree -- the thrust of the prop ALONE can and obviously does "exceed the rolling resistance of the wheels that gives the prop the thrust to begin with"

Now in truth, swernd and I likely don't disagree all that much -- perhaps it's even just a matter of semantics: The prop is able to do this because it's operating in such a favorable environment -- a tailwind.

Let's take a well known type certified combo (Cessna 172, Lycoming engine, Sensenich prop) and add only one variable ... tailwind. Tie the plane down and instrument with load cells for thrust and you'll find that of course with a tailwind there's a lot more thrust for a given horsepower than in still air. No mystery there.

Same with the cart prop -- it's working in a very favorable environment and thus doesn't have the same horsepower requirement to generate the needed thrust to move the cart forward as it would in still air.

Again, I'm pretty sure swerdna and I don't disagree on this point and are just wording it differently.

JB

 

[ThinAirDesign]

However, I must ask you where you stand in the Bernoulli vs Newton debate on the force most responsible for lift on airplane wings.

Oh, I'm all for it.

JB

 

[OmCheeto]

Here is where I don't believe you, or more correctly, I do not believe DDWFTTW because it is all based upon a misintrepretation of what is happening on the treadmill.

I AM saying that the cart with respect to the tread, is going SLOWER than the tread with respect to the air! Is that clear enough for you to understand?

Whilst looking at this post, my acquaintance mentioned the term "luftmensch".
I'd never heard the term, so I queried him for a definition.
Either he is not a good communicator, or I am not a good interpreter.
We spent some time laughing after doing some surfing the net on the origin of the word.

Anyways, which video are we referring to at the moment? There are many treadmill videos.

 

[rcgldr]

In my estimation it also all boils down to the propeller. If you'll allow me to erroneously refer to the force the ground exerts on the wheel linked to the propeller as "thrust", then: can the propeller generate more thrust on the cart from its interaction with the air than the thrust it takes to turn the wheel that turns the propeller?

Yes, the propeller generates more thrust, but at a much lower speed. Since power = force x speed, the force is higher, but the power is less because of the slower speed speed of the air through the prop versus the speed of the ground at the wheels.

Similarly, the torque at the driven tires of a car is greater than the torque at the engine's crankshaft because of gearing, but the power is reduced because of drivetrain losses.

However, I must ask you where you stand in the Bernoulli vs Newton debate on the force most responsible for lift on airplane wings.

Start another thread for this one please.

 

[rcgldr]

Anyways, which video are we referring to at the moment?

swerdna started this thread with this video:

http://www.youtube.com/watch?v=MCB1Jczysrk&fmt=18

 

[OmCheeto]

Yes, the propeller generates more thrust, but at a much lower speed. Since power = force x speed, the force is higher, but the power is less because of the slower speed speed of the air through the prop versus the speed of the ground at the wheels.

Similarly, the torque at the driven tires of a car is greater than the torque at the engine's crankshaft because of gearing, but the power is reduced because of drivetrain losses.

Start another thread for this one please.

Hmmmm... More thrust at a lower speed...

= more push at less than the speed of the wind...

Where's that lever that makes the "cha-ching" noise?

Actually. I'm interested in reducing swerdna's device to a purely mathematical model based on observational data. swerdna, can you recreat your original video, with the camera placed directly above the platform, with the video starting from nothing moving, and ending with the DDWFTTW sprite moving at maximum speed? Thanks!

 

[swerdna]

Hmmmm... More thrust at a lower speed...

= more push at less than the speed of the wind...

Where's that lever that makes the "cha-ching" noise?

Actually. I'm interested in reducing swerdna's device to a purely mathematical model based on observational data. swerdna, can you recreat your original video, with the camera placed directly above the platform, with the video starting from nothing moving, and ending with the DDWFTTW sprite moving at maximum speed? Thanks!

Sorry but I canabalised the turntable cart to use some parts to build a small cart for outdoor wind testing. Unfortunately there hasn’t been a decent wind where I live for about three weeks (it‘s the “calm“ season) so I haven‘t been able to test it yet. Also I don’t have much spare time at present. I have a DDStreamFTTStream device I want to build and test as well. Besides, what information you would get from an overhead movie that you can’t get from the existing one?

 

[schroder]

I'm always doubting that you are in fact trolling, as the things you write are so elementary and so terribly wrong that I'm torn between "this guy is just trying to tickle my b**ls" and "this is the most funny cocksure ignoramus I've ever met".


NEXT, let us add a translation movement to the wheel. We give the center of the wheel, on top of its rotation movement, a translation movement with a velocity (vx, vy).


We see that the velocity of the bottom point of the wheel is given by (w R - vl, 0).




( + w R + vx, vy) = (v_tread,0)

from which:
w R + vx = v_tread and vy = 0




Taaaa tadam !
e) if the wheel... is moving to the LEFT (vx < 0) and the tread is moving to the right, then...

w = (v_tread - vx) / R is a bigger number than when the wheel were not moving (vx = 0).

It is spinning FASTER counterclockwise than if it were stationary...




Nope. It accelerates.




Tell me, are you tickling them (my b**ls) or are you really that misguided ?

Your calculation is incorrect because you are treating this the same as a wheel rolling on the road and ignoring the fact that this is clearly a heterodyne.
You are saying w R – vx = v tread That is Wrong!
The Correct expression is : w R – v tread = vx In a heterodyne the Difference is ALWAYS produced!
Look, see if you can follow along here:
The wheel is rotating CCW and stationary on the tread such that the linear velocity at the rim of the wheel is 10 m/sec. That is from Left to Right at the base of the wheel.
The tread is moving from Left to Right at 10 m/sec so there is no relative motion between the tread and the wheel. The wheel appears to be stationary on the tread.
NOW use your powers of visualization! Magnify the point where the wheel is in contact with the tread until you can see tow horizontal surfaces which are in contact and both are moving to the Right at 10 msec.
Can you do that? You see the two surfaces. Place a dot on each one, one over the other so that you can draw a vertical line between the two dots. This shows they are not moving with respect to one another.
OK. Now let the top dot, which is on the wheel (the bottom of the wheel) translate to the LEFT.
What does it take to allow that to happen? Only two ways that can happen: Either the lower dot speeds up to the Right which represents an accelerating tread, OR THE UPPER DOT SLOWS DOWN AND MOVES TO THE LEFT!
In order for the wheel to “advance” in the opposite direction the tread is moving, it must SLOW down!
As I said before, this makes perfect sense. The wheel is initially operating idly on the tread, without working into a load, so it goes to the RIGHT, and it is turning Faster than the tread initially. As the propeller starts working, the wheel is now working into a load and it SLOWS DOWN which results in it moving to the LEFT.
You do not even need to understand a heterodyne to see that what I am saying is correct.
Think it over.
Now, about your comments: I have been called a (somewhat) eccentric genius, but nobody has ever publicly called me a “cocksure ignoramus” and I will not stand for it now! I expect an apology or you should be removed from your position as a “PF mentor”. I am also not “tickling your b**ls” but would instead prefer to administer a swift kick there! After you realize how wrong you are about this cart, you will no doubt feel as if such a kick has indeed been delivered.

 

[vanesch]

Y
You are saying w R – vx = v tread That is Wrong!
The Correct expression is : w R – v tread = vx

That's the same expression !

But it is not the correct one, in its two forms. See my other post for a detailed explanation. I can't get more explicit than that.

Look, see if you can follow along here:
The wheel is rotating CCW and stationary on the tread such that the linear velocity at the rim of the wheel is 10 m/sec. That is from Left to Right at the base of the wheel.

Yes.

The tread is moving from Left to Right at 10 m/sec so there is no relative motion between the tread and the wheel. The wheel appears to be stationary on the tread.

That's what it means: rolling without slipping, indeed. (I mean, the fact that the point of contact is stationary, not that "the wheel" - by which we mean of course the axle of the wheel - is stationary of course)

NOW use your powers of visualization! Magnify the point where the wheel is in contact with the tread until you can see tow horizontal surfaces which are in contact and both are moving to the Right at 10 msec.

Yes.

Can you do that? You see the two surfaces. Place a dot on each one, one over the other so that you can draw a vertical line between the two dots. This shows they are not moving with respect to one another.

Very, very good. That's what it is called: rolling without slipping.

OK. Now let the top dot, which is on the wheel (the bottom of the wheel) translate to the LEFT.

Ok, but it would be slipping if that were the case.

What does it take to allow that to happen? Only two ways that can happen: Either the lower dot speeds up to the Right which represents an accelerating tread, OR THE UPPER DOT SLOWS DOWN AND MOVES TO THE LEFT!

Sure. But first of all, note that when you do that, the wheel axle itself doesn't move. What you are describing is the stationary wheel, which is now SLIPPING on the tread.

But that's not what we are going to do. We are going to leave those dots IN CONTACT (non-slipping), and we are going to move THE AXLE of the wheel to the left. As such, the angular velocity of the line linking the axle of the wheel with the dot will increase of course.

As I said before, this makes perfect sense. The wheel is initially operating idly on the tread, without working into a load, so it goes to the RIGHT, and it is turning Faster than the tread initially. As the propeller starts working, the wheel is now working into a load and it SLOWS DOWN which results in it moving to the LEFT.

We were talking about the situation where the cart with the wheel was moving to the left at 2 m/s (or mph take your pick) and the tread was moving at 10 m/s to the right. The wheel was rolling without slipping on the tread. Both velocities are expressed as seen from an observer on the firm ground, right. The cart is soon going to fall off the tread on its left side.

You were claiming that
1) the velocity of the cart wrt to the tread was not 12 m/s but rather 8 m/s
2) that the rotation velocity of the wheel moving to the left was slower than the rotation velocity of a wheel stationary with the ground.

These are, honestly, totally ludicrous statements, that I think even most serious high school students would recognize as erroneous (if not, one must stop teaching physics in high school because it is time lost).

Now, about your comments: I have been called a (somewhat) eccentric genius, but nobody has ever publicly called me a “cocksure ignoramus” and I will not stand for it now! I expect an apology or you should be removed from your position as a “PF mentor”. I am also not “tickling your b**ls” but would instead prefer to administer a swift kick there! After you realize how wrong you are about this cart, you will no doubt feel as if such a kick has indeed been delivered.

There's a first time for everything

Usually I do not use such language here, but honestly, the words there are really what I'm thinking. You leave me perplex. Come on, you can't be THAT ... well. So you just want to play, right ? You can't really mean what you write, right ? Or do you ?

Seriously, or we are talking about two totally different setups, or you do not have the slightest bit of understanding of kinematics - at a level that I've never seen before. Given your algebra demonstration in the beginning of this post...

 

[rcgldr]

The wheel is rotating CCW and stationary on the tread such that the linear velocity at the rim of the wheel is 10 m/sec. That is from Left to Right at the base of the wheel.

All of the videos I've seen so far have the tread moving from right to left with the cart wheels rotating CW, but it doesn't matter.

The tread is moving from Left to Right at 10 m/sec so there is no relative motion between the tread and the wheel.

Only at the contact patch.

Magnify the point where the wheel is in contact with the tread until you can see two horizontal surfaces which are in contact and both are moving to the right at 10 msec. You see the two surfaces. Place a dot on each one, one over the other so that you can draw a vertical line between the two dots. This shows they are not moving with respect to one another. Now let the top dot, which is on the wheel (the bottom of the wheel) translate to the left.

This isn't what happens. Using the floor as a frame of reference, when the wheel speed is zero relative to the floor, the wheel dot's horizontal component of speed = tread speed x cos(angle between vertical line down from axis and line to the dot on the wheel). At 0 degrees, the dot is in contact with the tread and moving left to right at 10 m / sec. At 90 and 270 degrees, the dot is moving vertically, with zero horizontal speed. At 180 degrees, the dot is at the top of the wheel moving right to left at 10 m / sec, ... The average speed of the dot is 0 m / sec if the cart is not moving with respect to the floor.

If the cart is moving right to left at 2 m / sec, then at 0 degress wheel dot speed = 10 m / s to the right. At 90 and 270 degrees,the horizontal dot speed = 2 m / sec to the left. At 180 degrees witht the dot at the top, speed = 14 m / sec to the left, ... for an average speed of 2 m / sec to the left, same as the axis. The wheel is rotating CCW and relative to the axis of the wheel the wheel surface is moving at 12 m /sec.

Another way to look at this case is to imagine a second horizontal surface in contact with the top of the wheel. The speed of the cart is the average speed of the two surfaces, 1/2 (upper surface speed + lower surface speed). If lower surface speed is -10 m / sec and upper surface speed is +10 m / sec, then wheel axis speed is 0 m / sec. If lower surface speed is -10 m /sec and upper surface speed is +14 m / sec, then wheel axis speed is +2 m / sec. Note that wheel surface speed = (upper surface speed) - (wheel axis speed) = (wheel axis speed) - (lower surface speed).

 

[A.T.]

Yes, the propeller generates more thrust, but at a much lower speed. Since power = force x speed, the force is higher, but the power is less because of the slower speed speed of the air through the prop versus the speed of the ground at the wheels.

Hmmmm... More thrust at a lower speed...

= more push at less than the speed of the wind...

No. The lower speed of the propeller is relative to the air (while the higher speed of the wheels is relative to the ground). You have to add this lower speed to windspeed in order to get the cart's speed relative to the ground. And this gives you more than the speed of the wind.

 

[schroder]

That's the same expression !



Ok, but it would be slipping if that were the case.



Usually I do not use such language here, but honestly, the words there are really what I'm thinking. You leave me perplex.

Yeah, yeah, I made a typo, there should have been a plus sign as in:
You are saying w R + vx = v tread That is Wrong
Only someone like you would jump on that to try and discredit the rest of what I am saying.
Translating IS rolling without slipping, but I now seriously doubt that you are capable of understanding that. I also seriously doubt that you want to understand what is happening with this cart on the treadmill. You are a typical member of academia who sees this as some sort of joke. You can afford to be wrong and have a laugh about it. I am a professional engineer and cannot accept being wrong, even when it is others who are wrong. Maybe someday you will grow up and understand that.
Swerdna could easily put a tachometer on the wheel and see it slowing down as it translates to the left.
TAD and Co could easily race the propeller cart against a cart with a sail and four freely rolling wheels.
This whole thing stinks to heaven for the lack of verification and the reason is it is not true.
And you, sir are a MORON!

!

 

[ThinAirDesign]

I am a professional engineer and cannot accept being wrong, ...

TAD and Co could easily race the propeller cart against a cart with a sail and four freely rolling wheels.

Really schroder? -- that's all you need to accept that you are wrong is a race between a prop cart and a sail cart?

I'm asking seriously schroder -- you haven't answered any of my previous questions regarding what sort of test you were referring to above so it makes me think that when it comes to the details of any test you are avoiding an exchange with me.

If a test makes sense to me I'll perform it (for all other tests ask swerdna). If you want to see a prop cart vs sail cart race, I'm happy to arrange one -- all I need to know is what excuse you're going to make when the sail cart gets left in the proverbial dust.

??

And you, sir are a MORON!

Tell you what ... I will include "I sir am a MORON" and a link to this thread, at the end of every one of my PF posts if the sail cart wins. You will do the same if the prop cart wins. DEAL?

JB

 

[rcgldr]

Swerdna could easily put a tachometer on the wheel and see it slowing down as it translates to the left.

Unless you're looking at a mirror image, swerdna's turntable is moving to the left (the side closest to the viewer), while the cart eventually translates to the right. The speed of the wheel and prop are constantly increasing until the cart reaches it's terminal speed.

http://www.youtube.com/watch?v=MCB1Jczysrk&fmt=18

 

[vanesch]

Yeah, yeah, I made a typo, there should have been a plus sign as in:
You are saying w R + vx = v tread That is Wrong

It is correct, if you understand the smallest bit of mechanics, if you know what a vector in 2 dimensions is, you see that it is right, and that's it. Not being able to understand the velocity field of a rotating circle, especially when the explanation is written in front of you, and when you have a picture drawn, but even disputing someone who explains it to you means that you are genuinely beyond the point of hope of ever understanding the most basic elements in mechanics - coupled with an attitude that precludes any improvement. This discussion has no sense anymore, I could just as well trying to have my cat do trigonometry.

Translating IS rolling without slipping, but I now seriously doubt that you are capable of understanding that. I also seriously doubt that you want to understand what is happening with this cart on the treadmill. You are a typical member of academia who sees this as some sort of joke. You can afford to be wrong and have a laugh about it. I am a professional engineer and cannot accept being wrong, even when it is others who are wrong. Maybe someday you will grow up and understand that.

I'm also a professional engineer, btw. I have 2 masters, one in engineering and one in physics. I have doubts you are one, but then, surprises are not excluded. If, as an engineer, you are not capable of seeing that if thing A is going 2 m/s to the left, and thing B is going 10 m/s to the right in a frame of reference, then thing A is going 12 m/s wrt to thing B, I don't know how you got your degree, or you must have forgotten all of it, or... I don't know. As I said, I've never met anybody so clueless as to the most elementary concepts of mechanics.

And you, sir are a MORON!

As I said, I've never met anybody at the same time so clueless as to the most basic notions in a field, and at the same time so terribly cocksure about his totally erroneous claims.

There's really no point anymore pursuing this discussion with you.

 

[ThinAirDesign]

There's really no point anymore pursuing this discussion with you.

Correct -- all that is left is the race between a sail cart and a prop cart.

You know how with big fights they always have some tag line to promote -- per his comment to you and my "I Sir am a MORON!" offer to schroder seen above, I proposed we call it the following:

"The race to be the MORON!"

Don't dilly dally schroder. You suggested the race. Let's work out the details and GET IT ON!

JB

 

[rcgldr]

More thrust at a lower speed ... = more push at less than the speed of the wind ...

The magnitude of the thrust from the prop is less than the wind speed (at least for the current DDWFTTW carts), but the direction is opposite. Swerdna's cart goes about 1.43 times the wind speed. If the wheel was directly under the prop, it would advance 14.07 inches per revolution, while the prop pitch advances 6 inches per revolution, for an advance ratio of .426. I guestimated the prop path radius to be about 31.6 inches (8 inches from wheel). The turntable rotates backwards at about 1.58 times per second, and the prop circles forward at about .69 times per second. Translating this into an outdoor equivalent:

wind speed +17.8 mph
cart speed +25.6 mph (1.43 wind speed)

apparent wind (relative to cart) = (wind speed - cart speed) = -7.8 mph
prop thrust speed based on prop pitch (relative to cart) = -10.9 mph

The prop attempts to accelerate an apparent headwind of 7.8 mph to 10.9 mph upwind, a 4.1 mph acceleration of air upwind. The actual speed will be less due to slip, but a sufficient amount of air is accelerated upwind against the tailwind to provide the thrust required to propel the cart.

From the ground frame of refererence, the prop is attempting to slow the 17.8 mph tailwind down to 13.7 mph with the same 4.1 mph acceleration of air upwind, even though the prop itself and the cart are moving downwind at 25.6 mph, 7.8 mph faster than the wind.

 

[swerdna]

Swerdna could easily put a tachometer on the wheel and see it slowing down as it translates to the left.

Assuming we are talking about this video - http://nz.youtube.com/watch?v=MCB1Jczysrk&fmt=18

When you say “put a tachometer on the wheel” I’m guessing that you don’t mean to fit the tachometer to the axle of the wheel of the cart to measure the revolutions of the wheel as it’s clearly obvious that the wheel speeds up from being stationary to a constantly sustainable speed. The revolutions of the wheel never slow down. I’m guessing you mean to fit the tachometer to the stationary axle of the turntable to measure the revolutions of the cart and tether around that axle. Is my guess correct?

If so then a tachometer is not required to see what occurs. The cart starts being stationary wrt the turntable axle. It then starts to move (clockwise) wrt the turntable axle and for a period this movement at first speeds up then slows down and briefly stops. It then starts to move again wrt the turntable axle (anti-clockwise) and progressively speeds up to constantly sustainable speed. The reason the cart slows and briefly stops moving clockwise to then move anti-clockwise is because the increasing thrust of the prop makes it do so. The net effect of the cart wrt the turntable axle is that it speeds up from being stationary to a constantly sustainable speed. It only slows down and briefly stops to change direction.

ETA - The period when the cart moves clockwise has nothing to do with the DDWFTTW claim. The claim isn’t that a cart can go from stationary to DDWFTTW, It’s that a cart can sustain DDWFTTW regardles of how it gets there.

 

[OmCheeto]

Sorry but I canabalised the turntable cart to use some parts to build a small cart for outdoor wind testing. Unfortunately there hasn’t been a decent wind where I live for about three weeks (it‘s the “calm“ season) so I haven‘t been able to test it yet. Also I don’t have much spare time at present. I have a DDStreamFTTStream device I want to build and test as well. Besides, what information you would get from an overhead movie that you can’t get from the existing one?

I don't know how to do frame by frame scrolling in these video's, and there is no clock, so my guestimates on velocity wouldn't be very accurate. But that's ok. I've nearly everything needed to build one on my back porch. Might be as soon as this weekend, and I'll have my very own DDWSTTW device built.

Btw, I loved your http://www.youtube.com/watch?v=VgaXpHOxtQg&feature=related" video.

hmmm... a new design just popped into my head: a giant hamster cage DDWFTTW device.

at least there'd be no gyroscopic torsional forces, which I believe is what pitted vanesch and myself against each other.

 

[zoobyshoe]

Oh, I'm all for it.

I said "in" not "on".

 

[swerdna]

I don't know how to do frame by frame scrolling in these video's, and there is no clock, so my guestimates on velocity wouldn't be very accurate. But that's ok. I've nearly everything needed to build one on my back porch. Might be as soon as this weekend, and I'll have my very own DDWSTTW device built.

Btw, I loved your http://www.youtube.com/watch?v=VgaXpHOxtQg&feature=related" video.

hmmm... a new design just popped into my head: a giant hamster cage DDWFTTW device.

at least there'd be no gyroscopic torsional forces, which I believe is what pitted vanesch and myself against each other.

Good luck with building your turntable and cart. Can’t wait to see the movie, read the book, wear the tee shirt . . .

 

[ThinAirDesign]

OmCheeto:

I'll have my very own DDWSTTW device built.

Well, there certainly are many ways to do exactly that -- the wrong ratios are a really great place to start.

JB

 

[ThinAirDesign]

I said "in" not "on".

Humor zoo. Humor.

JB

 

[OmCheeto]

Good luck with building your turntable and cart. Can’t wait to see the movie, read the book, wear the tee shirt . . .

Someone is selling T-shirts? I'll take 10! xxx Large.

Sewn together, they might make a good sail for my device, in the "I am a Moron" race.

 

[zoobyshoe]

Humor zoo. Humor.

JB

Answer thi. Answer.

Also: I think you can stop including your initials at the end of each post; no one gives a hoot, so you're just contributing to the heat death of the universe.

 

[ThinAirDesign]

Answer thi. Answer.

Off topic. New thread.

Also: I think you can stop including your initials at the end of each post; no one gives a hoot, so you're just contributing to the heat death of the universe.

Sure thing.

JB

 

[zoobyshoe]

Off topic. New thread.

No. Goes to the question of how the prop creates thrust.

Sure thing.

JB

Why not include a list of your memberships and college degrees?

 

[schroder]

When you say “put a tachometer on the wheel” I’m guessing that you don’t mean to fit the tachometer to the axle of the wheel of the cart to measure the revolutions of the wheel as it’s clearly obvious that the wheel speeds up from being stationary to a constantly sustainable speed. The revolutions of the wheel never slow down. I’m guessing you mean to fit the tachometer to the stationary axle of the turntable to measure the revolutions of the cart and tether around that axle. Is my guess correct?

If you are serious about doing this test, then this is my suggestion:

Fix a flexible cable to the unused end of the wheel’s axle (one end is presently used to power the propeller) Mount a small digital tachometer on top of the cross arm which supports the wheel and connect the new flexi shaft to the tach. You really should have another tach on the turntable, to ensure that it is not accelerating during the test as that will cause a false reading on the wheel tachometer.

When the test starts, everything is at rest. When the turntable starts running CW, and the cart initially moves along with it and the wheel is turning CCW. There may even be a very short spike when the cart is pushed ahead of the TT by the shock of turning it on, but that is a short transient response and may be impossible to capture. It is of no significance anyway. During the time that the cart is moving along with the TT in the CW direction, the linear velocity of the edge of the wheel rim is exactly the same as the linear velocity of the TT and they are both accelerating in the same direction at the same time. Similar to when they are both standing still 0 = 0 in the stationary case. In the run up case maybe 5 = 5 whatever the velocities are, they need to be either measured or computed.

The next stage is when the air resistance to the prop and the crossarm causes the CW motion of the cart to slow down and then it will reverse to CCW. Corresponding to this slowing down in the CW direction, there will be a slowing down in the CCW rpm of the wheel. It is THIS that you should look for with the tachometer! You WILL see the rpm reduce as the cart starts to translate in the CCW direction, opposite to the direction the TT is going. I GUARANTEE that you will see this reduction in RPM as it is the ONLY explanation which is within the laws of physics and mechanics.

The reason for the reduction in RPM is simple, the propeller is starting to work against the air and it is extracting energy from the wheel which slows the wheel down. The loss in rpm of the wheel is compensated for by the translational movement of the cart, in accordance with the conservation of energy. If you have an electric fan handy, turn it on and let it get up to speed. Next, place a plastic bag over it so it has little or no air to work against. Amazingly, the fan speeds up! This is like the wheel which has nothing to work against. Lift off the bag and the fan slows down, this is like the wheel which is now driving the propeller to do work against the air.

Everyone here assumes the wheel is speeding up, because that is what your eyes and your mind conditioning is telling you from everyday experience of watching wheels rolling on a road. But that is NOT the case here and if you do this test you will prove it beyond all doubt.

Finally, Swerdna, once you have this demo perfected, with tachs on the TT and the wheel and a nice readout on a PC, go and get a patent on it FAST! It does not demonstrated DDWFTTW (it debunks it) but it serves as an excellent teaching aid for rolling and translating motion. In particular, it teaches the heterodyning of two rotating wheels and how that produces a translational motion. Every mechanical engineering school and physics department should have one of these. This is the silver lining which will come out of this DDWFTTW FARCE!

 

[vanesch]

When the test starts, everything is at rest. When the turntable starts running CW, and the cart initially moves along with it and the wheel is turning CCW.

What if the cart is GLUED to the TT ? Is the wheel then also turning CCW ?? Is it turning at all ?
Do you realize what you are saying here ? What if the wheel is a square wheel ? Not a wheel at all ? A bolt ?

The next stage is when the air resistance to the prop and the crossarm causes the CW motion of the cart to slow down and then it will reverse to CCW.

What you describe would be correct if the wheel were touching a stationary surface (a ring outside of the turntable, that is fixed to the ground or something). But it is running on the turntable itself, you are aware of that, no ? (otherwise - as I suggested earlier - we are in fact talking about two totally different setups). The wheel is ON THE TURNTABLE, not on a fixed support next to it.

 

[schroder]

What if the cart is GLUED to the TT ? Is the wheel then also turning CCW ?? Is it turning at all ?
Do you realize what you are saying here ? What if the wheel is a square wheel ? Not a wheel at all ? A bolt ?




What you describe would be correct if the wheel were touching a stationary surface (a ring outside of the turntable, that is fixed to the ground or something). But it is running on the turntable itself, you are aware of that, no ? (otherwise - as I suggested earlier - we are in fact talking about two totally different setups). The wheel is ON THE TURNTABLE, not on a fixed support next to it.

The wheel is NOT glued, square or bolted. It is a wheel and it is turning. Care to argue that point?
What I describe IS correct. Are you afraid of the test? Why not just let the test be done and keep your opinions to yourself in the meanwhile?

 

[rcgldr]

When the turntable starts running CW, and the cart initially moves along with it and the wheel is turning CCW.

The cart only moves along with the TT when it's forward motion was prevented by a block. In this case the cart wheel isn't turning at all.

The next stage is when the air resistance to the prop and the crossarm causes the CW motion of the cart to slow down and then it will reverse to CCW.

Also inertia during the time the TT is accelerating.

Corresponding to this slowing down in the CW direction, there will be a slowing down in the CCW rpm of the wheel.

The wheel on the cart is turning CW.

The reason for the reduction in RPM is simple, the propeller is starting to work against the air and it is extracting energy from the wheel which slows the wheel down.

The prop exerts an CCW torque on the wheel, but the force between tread and wheel result in a greater CW torque on the wheel, so the wheel accelerates in the CW direction until the cart reaches it's terminal speed.

 

[swerdna]

If you are serious about doing this test, then this is my suggestion:

Fix a flexible cable to the unused end of the wheel’s axle (one end is presently used to power the propeller) Mount a small digital tachometer on top of the cross arm which supports the wheel and connect the new flexi shaft to the tach. You really should have another tach on the turntable, to ensure that it is not accelerating during the test as that will cause a false reading on the wheel tachometer.

When the test starts, everything is at rest. When the turntable starts running CW, and the cart initially moves along with it and the wheel is turning CCW. There may even be a very short spike when the cart is pushed ahead of the TT by the shock of turning it on, but that is a short transient response and may be impossible to capture. It is of no significance anyway. During the time that the cart is moving along with the TT in the CW direction, the linear velocity of the edge of the wheel rim is exactly the same as the linear velocity of the TT and they are both accelerating in the same direction at the same time. Similar to when they are both standing still 0 = 0 in the stationary case. In the run up case maybe 5 = 5 whatever the velocities are, they need to be either measured or computed.

The next stage is when the air resistance to the prop and the crossarm causes the CW motion of the cart to slow down and then it will reverse to CCW. Corresponding to this slowing down in the CW direction, there will be a slowing down in the CCW rpm of the wheel. It is THIS that you should look for with the tachometer! You WILL see the rpm reduce as the cart starts to translate in the CCW direction, opposite to the direction the TT is going. I GUARANTEE that you will see this reduction in RPM as it is the ONLY explanation which is within the laws of physics and mechanics.

The reason for the reduction in RPM is simple, the propeller is starting to work against the air and it is extracting energy from the wheel which slows the wheel down. The loss in rpm of the wheel is compensated for by the translational movement of the cart, in accordance with the conservation of energy. If you have an electric fan handy, turn it on and let it get up to speed. Next, place a plastic bag over it so it has little or no air to work against. Amazingly, the fan speeds up! This is like the wheel which has nothing to work against. Lift off the bag and the fan slows down, this is like the wheel which is now driving the propeller to do work against the air.

Everyone here assumes the wheel is speeding up, because that is what your eyes and your mind conditioning is telling you from everyday experience of watching wheels rolling on a road. But that is NOT the case here and if you do this test you will prove it beyond all doubt.

Finally, Swerdna, once you have this demo perfected, with tachs on the TT and the wheel and a nice readout on a PC, go and get a patent on it FAST! It does not demonstrated DDWFTTW (it debunks it) but it serves as an excellent teaching aid for rolling and translating motion. In particular, it teaches the heterodyning of two rotating wheels and how that produces a translational motion. Every mechanical engineering school and physics department should have one of these. This is the silver lining which will come out of this DDWFTTW FARCE!

Sorry but I can’t agree with this bit - “When the turntable starts running CW, and the cart initially moves along with it and the wheel is turning CCW.”

The turntable moves CW and the cart initially also moves CW at an increasingly slower speed, but the wheel always turns (revolves) CW (never CCW). To revolve CCW the wheel would have to travel faster then the motion of the turntable in the same direction and this never happens. The direction of the motion of the wheel as a whole changes but the direction of the wheel revolving doesn't. Either you are wrong or I’m completely misunderstanding you. Please explain where and when the wheel ever revolves CCW. Here’s a pic to help explain what I mean.

http://www.accommodationz.co.nz/images/directions.bmp

ETA - The shorter arrow from the axle indicates it's traveling slower than the TT.

 

[schroder]

Sorry but I can’t agree with this bit - “When the turntable starts running CW, and the cart initially moves along with it and the wheel is turning CCW.”

The turntable moves CW and the cart initially also moves CW at an increasingly slower speed, but the wheel always turns (revolves) CW (never CCW). To revolve CCW the wheel would have to travel faster then the motion of the turntable in the same direction and this never happens. The direction of the motion of the wheel as a whole changes but the direction of the wheel revolving doesn't. Either you are wrong or I’m completely misunderstanding you. Please explain where and when the wheel ever revolves CCW. Here’s a pic to help explain what I mean.

http://www.accommodationz.co.nz/images/directions.bmp

ETA - The shorter arrow from the axle indicates it's traveling slower than the TT.

Since the TT is a circular device, I suppose there is room for confusion. Your drawing is correct in a frontal view of the TT and the cart is running on the front edge closest to the viewer. In the video I saw, the cart began translating while it was running on the back edge, farthest from the viewer. In that frame of reference, the TT is turning CW, the edge of the TT is moving from Left to Right, the cart is moving from Right to Left and the wheel is turning CCW. Correct?

 

[rcgldr]

Since the TT is a circular device, I suppose there is room for confusion. Your drawing is correct in a frontal view of the TT and the cart is running on the front edge closest to the viewer. Left to Right, the cart is moving from Right to Left and the wheel is turning CCW. Correct?

Yes, but for consitency in this thread, let's keep the view of the cart as seen from outside the TT looking in, where the wheel turns CW. Most of the straight line cart videos also have the tread moving right to left and/or the cart moving left to right with the cart wheels turning CW as viewed from the camera.

I'm still confused why you don't think that the carts speed relative to the tread isn't 12 mph ("left to right") when the tread is going "right to left" at 10 mph and the cart is going "left to right" at 2 mph relative to the floor.

 

[vanesch]

The wheel is NOT glued, square or bolted. It is a wheel and it is turning. Care to argue that point?

Yes, but IF it were bolted or glued, it would move all the same with the TT, no ? The motion of the cart would be exactly the same as initially in the actual test, no ? And then the wheel wouldn't turn, would it ? So how come you think it is turning when the cart is moving all the same ?

What I describe IS correct.

I don't know anymore what you describe. But it is certainly not the setup on the video of the turntable.

Are you afraid of the test? Why not just let the test be done and keep your opinions to yourself in the meanwhile?

Hell no I'm not "afraid of the test". Your test comes down to:
"the bike's wheels, of a bike placed upon a truck, are spinning when the truck advances, look at the speedometer of the bike". No need to look at the speedometer of the bike to see that it reads 0 mph, and that the wheels are not turning, even if the truck is moving.
What I'm saying is: "the wheels of the bike are of course not turning when the truck advances, because imagine that the bike is glued or bolted to the truck". And you tell me that I'm wrong, because the bike is not glued on it, but just *placed* on it.

I'll ask you this in a different way: imagine that the cart had a remote-controlled brake on the wheel. Imagine that you apply the brake when the TT starts up. The cart would move with the TT, as in fact it does in the beginning (and we're talking about that phase, right) ? Imagine those brakes ultra-strong. Still convinced that the wheels are turning ?