Down wind faster than the wind

[CyCrow]

OK, simple analysis time, using real numbers. First, look at some example data for a prop: http://www.gwsus.com/english/product/powersystem/eps-dx.htm Near the bottom we have EP1470 that delivers 305g thrust with 30.24W input power. As this is electrical input through an inefficient brushed motor, efficiency is probably about 60%, much worse than mechanical gearing.

305g thrust = 3N. If we extract the power to run the prop through the wheels, geared to get the same RPM required to give 3N thrust, the required gear ratio depends on the speed over ground. The mechanical power is force * distance / time, so 30W = 30Nm/s, 10 m/s speed required. So this prop at the RPM that gives 3N thrust in still air, needs to have a ground/wind speed of 10m/s if the mechanical losses are the same as the electric motor/gearbox.

Satisfied?

// CyCrow

 

[humber]

The quote was too good to pass up. As for scalability, you are moving the goalposts. You claim the concept is impossible in general, not at any particular scale. It is harder to get good efficiency as you scale aerodynamics down, so the fact that it warks at small scale is even better.
// CyCrow

Please, for the last time. These are indirect wind devices, that can do that.
This method is not available to this machine. If you claim that this cart in someway mimics this process, the please explain how it applies to a cart with a prop driving the wheels

Own goal again. But you are right, of course, the aerodynamic behavior of a feather, can be readily equated with that of a brick.

 

[humber]

The textbooks will tell you that kinetic energy is not an absolute property. Kinetic energy is a function of velocity (1/2 mv²), and velocity is a relative property.

If the car on the floor and the car on the treadmill have the same mass, then they have the same kinetic energy relative to any inertial reference frame. Relative to the floor, both these cars have zero kinetic energy. Relative to a frame of reference going at the same speed as the treadmill, both cars have kinetic energy as defined by the familiar formula. This is equivalent to the kinetic energy of the car going at 60mph along a road when measured in the frame of reference where the road is stationary. The kinetic energy of the car travelling at 60mph is zero when measured in a frame of reference going at the same speed as the car.

Yes. You can stand on the top of the Eiffel tower, and view it from there.

You agree that when stationary, both cars have zero kinetic energy, but when the belt is on, one has the same as that of a real car.
If I turn the belt off, I should expect that the kinetic energy will be returned
to me as the motor is now driven by that energy, to becomes a generator.
Please explain.

 

[sol invictus]

The road is stationary in that it is the “constant” that the speed of the wind and vehicle are measured against in the local environment. The “natural” position of the vehicle is to be stationary relative the road and a “moving” vehicle will slow down and “stop” relative to if the road isn’t being constantly accelerated. In the scenario we are discussing the only energy source that moves the vehicle relative to the road is the wind not the road.

That's just plain wrong. Please go read about Newton's first law of motion - I'm tired of explaining this.

The only thing that makes "at rest relative to the ground" at all special for the cart is that in that case the wheels aren't spinning, and therefore there is no friction in the wheel bearings. But friction can be very small compared to all the other forces at issue here, and regardless, the claim was that this vehicle violates some law of physics. If so, friction could be zero and it still wouldn't work.

 

[sol invictus]

Caveat: those annecdotal details are based on displacement hull sailboats. I'm looking at the Skeeter graphs and wondering if the Apparent Wind shift (extreme) minus hull friction mean I may have been mistaken.

Yeah - as far as I can tell the graphs in the link I posted are actual GPS data from an iceboat race, and they clearly show the boats moving on a steady reach with a downwind velocity component much faster than the wind.

I'm pretty sure I've done the same on windsurfers, but I admit I can't be certain without measurements.

But even if the iceboat data was due to "stored momentum", it's still perfectly valid. The debate here was whether wind powered vehicles can proceed steadily downwind faster than the wind. But the minute you admit a boat of any kind can travel in any direction faster than the wind, you're done - neglecting friction that boat could simply lower its sails, turn downwind (which requires zero energy in principle), and travel forever at constant speed.

 

[sol invictus]

Yes. You can stand on the top of the Eiffel tower, and view it from there.

humber, can you answer one simple question? What's the kinetic energy of a 1000 kg car moving at 10 m/s relative to a road? Please don't forget to take into account that the road is on a planet spinning around its axis and orbiting its sun at 30,000 m/s.

 

[humber]

By the wind acting on what? It can be the propeller because that would make the vehicle go backwards. How does a forwards wind give energy to a propeller that is creating a backwards wind? Please explain exactly what part and how the vehicle is getting energy from the wind.

This is indeed confusing. You will find many interpretations in this thread. Don't worry for the moment about which way the prop. turns.

Generally, the prop drives the wheels and cart forward.
On the treadmill all of the energy comes from the belt, in practice, "the wind".

The treadmill is said to demonstrate the cart's ability to exceed downward windspeed. Please keep in mind that this is thought of to be the equivalent of a real wind test.

The argument is, that at windspeed, there is no relative velocity between the cart and the tailwind, and it is therefore in still air.
It is reasoned that this allows such a demonstration to take place in still air, as long as a "moving frame of reference" is provided, which is the belt.

The treadmill has an imaginary tailwind, moving at the speed of the belt. What you are expected to believe, is that the forward motion of the craft is seen to be forward velocity in excess of the tailwind.

What you see is the cart struggling up the treadmill, and infer "thrust" from the propeller. ( It is drag. It prevents the cart being carried back by the belt)

This "thrust" is said to "push against" the tailwind, so propelling it beyond windspeed. The wheels and gearing are said to play a part in this process.
There is a also a putative connection with tacking.

Empirical proof of equivalency is as follows:
1. Put the cart and treadmill in the back of a van
2. Run the belt to 10mph
3. Drive downwind at 10mph
4. Observe that the model behaves as expected.
5. QED


This is all possible because all things are "relative". There you go.

 

[humber]

humber, can you answer one simple question? What's the kinetic energy of a 1000 kg car moving at 10 m/s relative to a road? Please don't forget to take into account that the road is on a planet spinning around its axis and orbiting its sun at 30,000 m/s.

Where is the moon? Please be more precise.

 

[Michael C]

You agree that when stationary, both cars have zero kinetic energy, but when the belt is on, one has the same as that of a real car.

No I don't. Read what I wrote. I say that both the car on the belt and the car on the floor have the same kinetic kinetic energy relative to any given frame of reference. Relative to the frame of reference where the floor is stationary, both cars have zero kinetic energy. It doesn't matter whether the belt is on or off, or if the wheels of a car are touching it or not: if the car is stationary with respect to the floor, it has zero kinetic energy with respect to the floor. Relative to the frame of reference moving at the same speed as the surface of the treadmill, both cars have a kinetic energy of 1/2mv², where m is the mass of the car and v is the velocity of the car relative to the treadmill.

 

[sol invictus]

After 750+ posts you've almost managed to grasp the basics of this...

The argument is, that at windspeed, there is no relative velocity between the cart and the tailwind, and it is therefore in still air.
It is reasoned that this allows such a demonstration to take place in still air, as long as a "moving frame of reference" is provided, which is the belt.

You're still having trouble there. The belt isn't a "moving frame of reference" - it's just a moving belt. The correct statement is that a cart traveling to the right down a level road at wind speed is equivalent (by a change of reference frame) to a cart at rest in still air, but with the surface under its wheels moving to the left at that same speed. Since the laws of physics are invariant under changes of reference frame, so are the forces and so are the accelerations - if the cart in one situation accelerates to the right, so must the cart in the other.

Empirical proof of equivalency is as follows:
1. Put the cart and treadmill in the back of a van
2. Run the belt to 10mph
3. Drive downwind at 10mph
4. Observe that the model behaves as expected.
5. QED

That is one example. But the empirical proof of this equivalency is the last 500 years of experiments in physics, every one of which supports it.

Where is the moon? Please be more precise.

The planet in question has no moon. So - what's the answer?

 

[humber]

Either you haven't taken the time to think this through or you're an idiot.

If the traffic is travelling at 2 mph, and you're travelling at 1mph, you can go an entire 1 mph faster without ever pushing past anyone! There is no force pushing you backwards.

In fact, if you're wearing roller-skates, the bumping and jostling of the dense crowd pushing past you will speed you up until you're going the same speed as everyone else. No effort required on your part. You're effectively floating or sailing along with the crowd.

As this brings you up to the same speed of the crowd, there's no longer any bumping or jostling because no-one is passing you anymore, and you still aren't passing anyone yourself

There is no difference. Change in velicity, kinetic energy, and how far the 60 mph pedestrian 'bounces' will be identical relative to the hypothetical giant treadmill belt as it would be with a stationary pedestrian relative to the road.

Brian-M,
There is no simple pushing and pulling. Change the people to a stream of small table-tennis balls. Do you think that there will always be a "gap" in front of you, or that that some of the balls to the left and right will not get knocked into your path?
The balls from behind, do not "push" you. They momentarily strike you, and bounce off, transferring some of their momentum to you. (They now lose momentum, which they must pick up from somewhere else.)

You will be accelerated, little by little, and therefore must pass by others, but some will be much slower than you, (having lost almost all their momentum, in a previous collision) and you will lose some of yours if you hit them. Collision are not "equivalant" or you would not make progress at all.

If you want to see what I mean, take a look at something that describes the movement of electron through a conductor. The electron stream has an average velocity, but individual particles (charges) may take a longer path, or move backwards. They rattle through the wire as if in a pinball machine. This causes heat to be generated, through the wires "resistance".

Similarly, in air, this is drag. As you are progress, you are constantly gaining and losing momentum, but your average momentum will increase with time.] The distance that you will travel over the journey is your free mean path, which will be longer than a direct line.
This happens at the molecular level, which in bulk, looks like an approximately one-directional force when applied to a larger body. (There are separate turbulent effects as well).
[/QUOTE]

In fact, if you're wearing roller-skates, the bumping and jostling of the dense crowd pushing past you will speed you up until you're going the same speed as everyone else. No effort required on your part. You're effectively floating or sailing along with the crowd.
In fact, if you're wearing roller-skates, the bumping and jostling of the dense crowd pushing past you will speed you up until you're going the same speed as everyone else. No effort required on your part. You're effectively floating or sailing along with the crowd.

In some idealised world, perhaps, but the metaphor I used indicates that this is not possible. You will only be able to accumulate a certain amount of momentum, before you begin to lose more than you gain.
You can move to a velocity, but the losses increase with velocity, so you reach a maximum before windspeed. There is certainly drag when travelling downwind.
On those roller skates, you can feel the wind dragging you along, but also in your face, and flapping your clothes.

There is no difference. Change in velicity, kinetic energy, and how far the 60 mph pedestrian 'bounces' will be identical relative to the hypothetical giant treadmill belt as it would be with a stationary pedestrian relative to the road.

I meant not on a treadmill, but in reality, assuming the person to be a simple mass?

 

[humber]

After 750+ posts you've almost managed to grasp the basics of this..

No, I have always understood, I just don't agree.

You're still having trouble there. The belt isn't a "moving frame of reference" - it's just a moving belt. The correct statement is that a cart traveling to the right down a level road at wind speed is equivalent (by a change of reference frame) to a cart at rest in still air, but with the surface under its wheels moving to the left at that same speed. Since the laws of physics are invariant under changes of reference frame, so are the forces and so are the accelerations - if the cart in one situation accelerates to the right, so must the cart in the other.

OK, but I was tying-in the practical with the concept, and common mis/usage of some of the terms. I fully comprehend the concept, as given in this thread.

That is one example. But the empirical proof of this equivalency is the last 500 years of experiments in physics, every one of which supports it.

But you do agree, that the given example is valid?
I see this as an example of misunderstanding of what that concept means, and implies.

The planet in question has no moon. So - what's the answer?

I was being flippant. Your question is loaded as in "when did you stop beating your wife"
You know that I will give you an answer where I use the "ground" as a reference. This will give you the opportunity to tell me I am wrong because I have not considered the velocity of the planet itself. But you already know I disagree with that interpretation. Isolating one example, in support a general case.
Yes, I know. Aircraft watches loose or gain time. There is the Coriolis effect. I am familiar with Foucault.

I should also add that I made a prop-driven cart when I was around 10 years old. Balsa-wood frame, prop, belt drive to the wheels ( you can make a figure-of-8 to easily reverse the drive), all probably taken from other broken toys. Didn't work then, but now I know more, I know why.

 

[humber]

No I don't. Read what I wrote. I say that both the car on the belt and the car on the floor have the same kinetic kinetic energy relative to any given frame of reference. Relative to the frame of reference where the floor is stationary, both cars have zero kinetic energy. It doesn't matter whether the belt is on or off, or if the wheels of a car are touching it or not: if the car is stationary with respect to the floor, it has zero kinetic energy with respect to the floor. Relative to the frame of reference moving at the same speed as the surface of the treadmill, both cars have a kinetic energy of 1/2mv², where m is the mass of the car and v is the velocity of the car relative to the treadmill.

Please understand that I can do the arithmetic that allows me to agree with you. I get the idea. To be of use, such an interpretation must be manifest in some way, and not a merely a crackerjack calculation.

You are evading the question.
If the "kinetic" energy of the vehicle can be said to be "there" but not accessible, is it real? If so, then it should be capable of running the treadmill as a generator. A real car could certainly do this. So why not?

 

[Michael C]

You are evading the question.
If the "kinetic" energy of the vehicle can be said to be "there" but not accessible, is it real? If so, then it should be capable of running the treadmill as a generator. A real car could certainly do this. So why not?

No, I'm not avoiding the question. Once again:

Kinetic Energy is a relative property

It's not a question of the kinetic energy being "there" or "real". The kinetic energy of any object will have a different value, depending on which inertial frame you use to measure it. It's nonsense to say that the kinetic energy of a car is equal to a certain value without defining the frame of reference. For most situations, it is tacitly assumed that the frame of reference is that of the surface of the earth. When we get to comparing cars on treadmills with cars running on roads, however, it becomes important, to avoid confusion, to define the frame of reference. A car running at 60 km/h along a flat road has a certain kinetic energy relative to the frame of reference containing the road. It has zero kinetic energy relative to a frame of reference moving at the same speed as itself.

As for making a car run a treadmill by holding it in place and turning the wheels against the surface of the treadmill, it's not the "kinetic energy" of the car that's making the treadmill turn. The car is simply acting as a motor for the treadmill, imparting energy from its turning wheels. When the treadmill is being turned by its own motor, is it the kinetic energy of the motor that makes it turn?

 

[GreyICE]

The kinetic energy of a vehicle alone can’t be geared up to make the vehicle travel faster (that would be perpetual motion/free energy).

Whatever help the vehicle gets from a tailwind is completely lost when the vehicle reaches the speed of the wind.

At the speed of the wind the vehicle only has it’s own kinetic energy to accelerate faster than the wind (impossible)

When the vehicle travels faster than the speed of the wind it loses the advantage of a tailwind and gains the disadvantage of a headwind.

If I can demonstrate something that you will 100% agree makes a vehicle move faster than the prevailing wind speed with no additional energy added, will you admit the above is completely false?

Oh and you've obviously never sailed, as you can easily sail against a headwind, and it is hardly impossible as you imply.

 

[shuttlt]

Isn't there some way of introducing zero-point energy into this discussion?

 

[sol invictus]

No, I have always understood, I just don't agree.

You can challenge the validity of Galilean relativity, but to do so you must account for the huge body of evidence for it. You haven't done so. You could argue that the principle has been misapplied in this case, but you haven't done so (repeating something over and over is not an argument).

But you do agree, that the given example is valid?
I see this as an example of misunderstanding of what that concept means, and implies.

I have no idea what example you're referring to.

You know that I will give you an answer where I use the "ground" as a reference. This will give you the opportunity to tell me I am wrong because I have not considered the velocity of the planet itself. But you already know I disagree with that interpretation.

Then you're a religious person and you deny the fundamentals of physics that have developed over the last 500 years and all the evidence for them.

Isolating one example, in support a general case.
Yes, I know. Aircraft watches loose or gain time. There is the Coriolis effect. I am familiar with Foucault.

You're deeply, deeply confused. The relativity of simultaneity is totally irrelevant to this. Everything Einstein did is irrelevant. The Coriolis force is totally irrelevant. We are talking about GALILEAN RELATIVITY, as in Galileo and Newton.

The fact that you still have not understood that - after a thread this long - all but proves you never will. You're obviously too stubborn to even consider that you might not understand something - plainly you haven't read my, or other's posts, or you've simply immediately forgotten them.

You even admitted you're wrong by welching on the bet. You're just a troll.

 

[spork]

This Spork demonstration is like jumping off the ground and claiming you can levitate as it only considers a “successful” part of a total sequence.

So if someone holds me off the ground and then carefully lets me go, and I continue to levitate indefinitely, you won't be impressed. You really are a tough audience.

If the person didn’t hold the vehicle stationary on the belt until it gained enough propeller speed to move against the direction of the belt it would never do so.

Wanna bet?

If the belt was accelerated from being stationary to begin with and the vehicle was not restrained, the vehicle would always travel with the belt to some degree (due to the friction between the belt and wheels) and the vehicle would never gain enough propeller speed to move against the direction of the belt.

Wanna bet?

I the belt was long enough in the clip shown the propeller would loose power to the friction between the belt and wheels and the vehicle would always end up moving with the belt.

Wanna bet?


Didn't think so. You just want to B.S.

 

[humber]

You can challenge the validity of Galilean relativity, but to do so you must account for the huge body of evidence for it. You haven't done so. You could argue that the principle has been misapplied in this case, but you haven't done so (repeating something over and over is not an argument).

After 750+ posts you've almost managed to grasp the basics of this..

I was not presenting an argument, but an opinion. A reaction to yours, above

I have no idea what example you're referring to.

The treadmill in a van. The one I described in the post. One of the supporting
examples in this thread.

You could argue that the principle has been misapplied in this case, but you haven't done so (repeating something over and over is not an argument).

I think that considering the treadmill to be a windtunnel will do. That is falsifiable.

Then you're a religious person and you deny the fundamentals of physics that have developed over the last 500 years and all the evidence for them
You're deeply, deeply confused. The relativity of simultaneity is totally irrelevant to this. Everything Einstein did is irrelevant. The Coriolis force is totally irrelevant. We are talking about GALILEAN RELATIVITY, as in Galileo and Newton.

The fact that you still have not understood that - after a thread this long - all but proves you never will. You're obviously too stubborn to even consider that you might not understand something - plainly you haven't read my, or other's posts, or you've simply immediately forgotten them.

You even admitted you're wrong by welching on the bet. You're just a troll.

A difference that makes no difference. Call it what you will.
The treadmill is not equivalent to a windtunnel.
Show me a text that supports that idea.
I have already stated many times, that Einstein has no place in this. Too bad that your colleagues appear not to agree.

 

[Modified]

Nice. Could you post a video of it (maybe to YouTube?).

Here you go.