Can a cart powered by wind travel faster than the wind itself?

[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 ?

 

[ThinAirDesign]

Are you afraid of the test?

Apparently you are the one afraid of the sail cart vs prop cart test schroder.

Why? Got a bit of doubt going there, or is it like you said previously ...

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

One thing for sure, no one is going to be calling you an "eccentric genius" after the test ... just plain old "eccentric" as the sail cart get's its butt kicked.

C'mon schroder ... the test was your suggestion. Let's work out the protocol together.

JB

 

[ThinAirDesign]

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

Ok, fair enough. Convince me that the outcome of a "Bernoulli vs Newton" argument somehow alters the outcome of a DDWFTTW argument and I will engage.

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

Would that somehow make you more comfortable?

JB

 

[vanesch]

Just once more, an illustration of what I wrote up in post 762 ( https://www.physicsforums.com/showpost.php?p=2052782&postcount=762 )

of the basic kinematics of a wheel rolling without slipping on a surface.

I realize that my orientations are the opposite of all videos here, sorry about that. The principle should remain, though.

In as much I can find references on the web, here is one:
http://dept.physics.upenn.edu/courses/gladney/mathphys/java/sect4/subsubsection4_1_4_3.html

(although it only treats rolling without slipping on a stationary surface, not a moving one).

There is an educational paper here:
http://www.iop.org/EJ/article/0143-0807/24/6/001/ejp3_6_001.pdf?request-id=4c8428fa-8110-4545-9d19-1b1284d162f3

(although it centers more on the study of acceleration)

Another reference is this, where you see the superposition of rotation and translation:
http://cnx.org/content/m14311/latest/

(note especially equation (3) )In any case, these sources indicate that the motion of individual points of a rigid body (such as a wheel) is given by the vectorial superposition of the translational motion of its center, and the rotational motion around that center.

I try to illustrate that in the picture (it was the calculation done in post 762).

From that, we see that the point of contact (wheel side) has the velocity of w . R + v_x which must - rolling without slipping - have exactly the same velocity as the point of contact (surface side), which is the linear velocity v_tread.

So we have that R.w + v_x = v_tread.

From which: w = (v_tread - v_x)/R

Note, btw, that the TOP point of the wheel has a velocity of v_x - R . w.

Depending on whether v_x is larger or smaller than R.w, this point moves forward or backwards.

 

[ThinAirDesign]

schroder, when viewed from the outside of the TT/cart combo, other than when the device is at rest or held to the TT through non-standard means, swernda's drawing is perfectly correct 100% of the time:

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

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

The TT is *always* moving from right to left relative to the cart wheel, thus the cart wheel is *always* turning CW.

His drawing is correct during both startup phase and operation phase. Initally, as the TT is accelerated, the cart wheel moves right relative to the TT simply because if its inertial resistance and a bit of bluff drag. Once in steady state, it moves right relative to the TT because of the thrust of the prop.

JB

 

[schroder]

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.

OK. I agree we need some consistency here to avoid confusion and unnecessary argumentation. From now on, we use the front view of the TT and consider the cart to be moving on the front edge, closest to the viewer.
The TT is turning CW, so the surface is moving Right to Left.
The wheel is turning CW.
The direction of the cart is dependent on whether we are looking at startup, beginning of translation, or steady state. But in all these cases, the wheel is rotating CW.

During startup, the TT, moving from Right to Left gives an initial transient shock to the cart which was sitting stationary. This may actually cause the cart to be momentarily pushed ahead of the TT for an instant. It is of little or no consequence, unless you are studying transient response (an interesting field btw).
After the initial transient, the cart will move along with the TT in the same direction for a brief time. It begins by moving at exactly the same speed as the TT and the linear velocity on the edge of the wheel is the same as the linear velocity on the surface of the tread. Inertia, and rolling resistance are in play here but this condition does not last long. Important to note here that the wheel is rolling on the TT surface, it is NOT glued down or flat or any other silly supposition as Vanesch has offered up!
Air resistance to the cart, mainly the propeller as well as the crossarm will cause the cart to slow down in the direction of the TT. This is the CRITICAL point! As the cart’s motion from Right to Left slows, it’s RPM on the TT slows also! Some of the rotational motion is being exchanged for translational motion! This is the classic heterodyne.
Now, we enter the final stage, where the cart has slowed down enough to a steady RPM which is less than it had when it was moving Right to Left. The cart has slowed enough that the translational motion causes it to move from Left to Right. This is where everyone believes the cart has outrun the TT and you want to add the velocities as relative velocities to arrive at a cart velocity which is greater than the TT velocity. This is precisely where you are all going wrong! This is a heterodyne, the velocities are mixing and the translation velocity is the DIFFERENCE between the cart velocity and the TT velocity. If the TT is running at 10 m/sec and the translation is 2 m/sec the cart’s velocity (linear velocity at the edge of the wheel) is 8 m/sec. It is that simple and it is also undeniable and verifiable!

Put a tach on the wheel and a tach on the TT and do your tests!

I wish I knew how to do computer animations and I do not really have the time to learn right now, but a computer animation of a heterodyne would come in handy right now. I will try to make some drawings to demonstrate what I am saying. I am hoping that someone reading this and understands a heterodyne will chime in and help me with this explanation.