Down wind faster than the wind

[my_wan]

It is so, you are getting more momentum out in a way that you can not put it in, and also increasing the energy as a result.

You are accelerating faster when the only force on you is friction.

If the only force was friction why does it stop working when the propeller is disconnected. In fact any amount of friction reduces it's ability to climb the treadmill so claiming the only force is friction doesn't wash.
http://www.youtube.com/watch?v=LMgDvC5lqsY

You can see all of spork's videos here:
http://www.youtube.com/user/spork33

 

[.13.]

As for the actual DWFTTW:

I don't think the treadmill proves this concept since it is the driving force in that demonstration.

That being said I think the DWFTTW is possible though. You can get some fraction of the total wind energy to your use. Let's call it E(max). The kinetic energy of the vehicle is E(max) = 1/2mv^2. So v = sqrt[2E(max)/m]. If E(max) is sufficiently large and/or m is sufficiently small then v is greater than wind speed.

 

[my_wan]

In the same way that the sail, moving with the water, can use its speed against the air to increase its speed against the air. Please try to think about it.

Good point.

No, that is just wrong. The energy does not come from nothing - it comes from the kinetic energy of the wind. When two objects are moving with respect to each other (air vs. ground), then braking them against each other releases energy, which can be utilized - in this case, to propel you forward.

Good point.

You want a simple example how that works? Imagine yourself in a car on a highway. You are moving at the same speed as the cars in your direction. You take a ball and throw it at a car going in the opposite direction. It bounces from it, then it bounces from a car behind you in your direction, and then you recatch it. Guess what? The ball, when you recatch it, will be travelling forward faster than when you threw it, giving you net increase in momentum and kinetic energy (do the calculation if you don't believe me). The momentum and energy did not come from nowhere - it came from the two cars that the ball hit, slowing them with respect to each other. Your speed is now higher than the speed of other cars in your direction - but you needed the cars going in the opposite direction for this to work.

Very nice analogy.

I'm still chasing a good quantitative analysis. I'm starting with some rather dumb linear assumptions on the grounds that the elements of the basic equation can be substituted with more realistic relationships later in the derivation.

 

[Thabiguy]

Just taking the case of the vehicle reaching the air speed. Then with respect to the propellor, the air is motionless.

The air is motionless with respect to the device as a whole, but not with respect to the blades of the propeller. (It's more obvious in the blower version.) The blades are moving backwards with respect to the air, which makes the air exert forward force on them.

How can it now get any energy to supply to the wheels?

The energy comes from the air being slowed, with respect to the ground, as it pushes the propeller forward.

If the propellor is directly connected to the wheels, which seems the case in the first video, the propellor would now act to stop the wheels.

No, because the air is still pushing forward on the propeller. It's easier to see in the blower version, but it's also true in the propeller version, when f < 1.

 

[my_wan]

As for the actual DWFTTW:

I don't think the treadmill proves this concept since it is the driving force in that demonstration.

That being said I think the DWFTTW is possible though. You can get some fraction of the total wind energy to your use. Let's call it E(max). The kinetic energy of the vehicle is E(max) = 1/2mv^2. So v = sqrt[2E(max)/m]. If E(max) is sufficiently large and/or m is sufficiently small then v is greater than wind speed.

This is at the root of the debate spork and I had on physicsforums. I still consider it a reasonable proof of concept. However, with the source of energy reversed it doesn't necessarily represent the same efficiency you would get in the wind power source case. However, I will concede to spork that in his design the prop is acting more like a prop than a windmill even in the wind driven case. This allows the prop to go from a 59% theoretically perfect efficiency (Betz' law) to over 90% practical efficiency. I have no doubt that the device can actually work regardless of specific efficiency limitations because you can always just use a larger cross section of wind for more power.

Why doesn't a skeptic here take sporks lead and actually build one. He was told it wouldn't work so he went empirical. Good lead to follow. It needs independent verification as a matter of principle.

@spork
The limited slip ring I argued before will show the drive train torque coming from the direction of the wheels. This is a good thing as it does give you that ~35 to 40% extra practical efficiency in the prop alone due the the entropy arguement I made. If you forget the prop efficiency and the specifics of the aerodynamic forces it can be thought of as a single force vector on the craft frame itself. This provides the wheel feedback that determines the direction of the torque vectors on the drive shaft. So I stand by the principles of my arguement but must consede the actual numbers from it is working in favor of the efficiency of your design.

 

[spork]

The limited slip ring I argued before will show the drive train torque coming from the direction of the wheels.

That's correct. And I never argued otherwise. The wheels produce a torque on the axle, which produces a torque on the prop shaft. This is true whether we're on the treadmill or open road. Equally true is the fact that the prop produces a force on the cart through the thrust bearing which produces a force on the wheels, which cause them to spin (against the road surface). This is also true whether we're on the cart or the open road.

The two cases are absolutely indistinguishable by the cart. The equivalency of inertial frames guarantees that this is true.

This is a good thing as it does give you that ~35 to 40% extra practical efficiency in the prop alone due the the entropy arguement I made.

Well, you've gone back and forth on that issue twice now. I maintain, as I always have, that we were never missing that magical 35-40% prop efficiency.

So I stand by the principles of my arguement

And I still stand by the principle of equivalency of inertial frames given to us by Galileo, Newton, and Einstein (and spork).

 

[Modified]

Lets make a simple example of a vehicle that can travel downwind faster than the wind, that we can all agree on. Assume an iceboat with zero drag and blade friction in the forward direction with the sail down. We know iceboats can go several times the speed of the wind at an angle, even if you don't believe the downwind component can be faster than the wind. Get the boat up to speed, then drop the sail and turn downwind, and with zero friction and air resistance it will continue downwind faster than the wind indefinitely. After some time the average speed in the downwind direction will be faster than the wind. With very little friction and drag, the boat will eventually slow down, but there are some non-zero values of friction and drag below which it is possible to average faster than the wind in the downwind direction, by re-accelerating occasionally. So I think we can all agree that in theory traveling downwind faster than the wind is possible.

 

[humber]

J13, I think we are almostin agreement, but not that the actual device will work.
I would like to stick to the demonstrations for the moment if I may, because here lies a source of confusion.

Spork, try this (thought) experiment. Place the cart so that is runs in the same direction as the belt. Well, it travels at the speed of the belt or perhaps a little backwards. Now hold it with your finger, and allow the propeller to turn, and then release it. It will now move faster than the belt. This has nothing to do with the propeller, because in this direction, the same effect can be achieved with a mass as with the propeller. If you impede the cart briefly, so that it has less opportunity to pick up energy, it will come to a stop before the end of the belt, when the mass (or propeller) stops. So would it the other way, given a chance.

In the other direction, it travels at (almost) the speed of the belt because the propeller itself creates drag. Just like you need a finger to spin the propeller in the first place. Drag 'sticks' the cart to the belt. The propeller doesn't drive the cart, it couples it to the air, so that the belt can drive it. If a momentum-less propeller were possible, the cart would drift backwards, because 100% transfer is impossible, but the initially stored momentum makes up that loss and some, propelling it forward.
Only the stored momentum does work in both cases. Even the sliding will be enough for just a little bit more energy be gained to keep it running. Sorry, but it's an illusion.

In your demonstrations, you are seeing the movie, but not the beginning and the end. Momentum is always conserved. The system must includes all sources of energy, including your input, and run continuously. Many over-energy devices (I know this is not your claim) appear to work when you see only some of the inputs, or for a short time.
The real cart is also a matter of momentum but for another reason. I could try to explain...

My_wan,
It's not skepticism, but physics. This problem is merely the result of intuitive thinking. No shame in that, it led Da Vinci to make many errors.

 

[my_wan]

That's correct. And I never argued otherwise. The wheels produce a torque on the axle, which produces a torque on the prop shaft. This is true whether we're on the treadmill or open road. Equally true is the fact that the prop produces a force on the cart through the thrust bearing which produces a force on the wheels, which cause them to spin (against the road surface). This is also true whether we're on the cart or the open road.

The two cases are absolutely indistinguishable by the cart. The equivalency of inertial frames guarantees that this is true.

Not completely equivalent for the reasons I originally specified on physicsforums. The vectorial components have different values in the wind driven and treadmill driven cases. In the treadmill case the initial force vector comes directly from the treadmill to the wheels. In the wind driven case the force vector comes through the frame of the craft itself over and above the prop force vector alone and then feeds back through the wheels. Yes the direction of the vector through the drive shaft is the same so it still doesn't suffer the efficiency limitation from Betz' law but it is still a mistake to assume the vectors are identical. You could easily reinvoke Betz' law by something as simple as changing the gear ratio.

Well, you've gone back and forth on that issue twice now. I maintain, as I always have, that we were never missing that magical 35-40% prop efficiency.

Not exactly. When Jeff Reid questioned the relevancy of Betz' law I readily admitted that I would have to rethink it before our debate ever started. When I did respond I specifically stated that it was "with qualifications". You went further than simply claiming the the craft wasn't missing the 35-40% efficiency, you claimed via Galilean relativity that the source of power couldn't make a difference. My argument was and still is that it does for the same reason an airplane propeller has an effective efficiency of over 90% while a windmill has a theoretically perfect efficiency of 59%. If Galilean relativity alone is enough to prove equivalency then you can't have two different efficiencies depending on whether the wind is blowing the prop or the prop is blowing the wind as Betz' law states. As I pointed out this is the result of how entropy is dissipated through the system. My argument stands regardless of the fact that the vectors can lined up in such a way to prevent the craft from suffering inefficiencies due to Betz' law.

And I still stand by the principle of equivalency of inertial frames given to us by Galileo, Newton, and Einstein (and spork).

Here is the claim of equivalency again. You have to be real careful assuming relativistic equivalency of reversal when dealing with dissipative systems. Even though the craft propeller system was spared from Betz' law by the proper direction of the force vector through the drive shaft the force on the frame in the wind driven case is still over twice that of the treadmill driven case. It also means that you need a wind speed greater than the treadmill speed to be equivalent to the power imparted by the treadmill. This is due to only being able to take advantage of a portion of ground wind speed and all of the treadmill speed. The wheels don't slip on the treadmill the way they do in air.

So no I don't take my argument back. More analysis showing the prop alone doesn't suffer the limitations of Betz' law in no way changes that argument.

Point of fact: The "principle of equivalency" has nothing to do with the equivalency of coordinate systems or motion vs at rest. The equivalence principle states only that gravity and acceleration is equivalent or that gravitational and inertial mass are indistinguishable. Velocity is relative, acceleration is not.

 

[my_wan]

Spork, try this (thought) experiment. Place the cart so that is runs in the same direction as the belt. Well, it travels at the speed of the belt or perhaps a little backwards.

No. It would it would do exactly the same thing. If you reversed the wheel to prop gear it would zoom off the belt faster than the speed of the belt. LOL Why a (thought) experiment when all he has to to is do it?

Spork, this would be an easy new video to make.

My_wan,
It's not skepticism, but physics. This problem is merely the result of intuitive thinking. No shame in that, it led Da Vinci to make many errors.

Physics is defined empirically. You are the one assuming you know the physics without actually doing the experiment (empirical test). You are therefore mistaking your intuition for actual physics. That's why this thread is interesting in the first place because the actual physics is counterintuitive, and you fell for your intuition. You even called your experiment a (thought) experiment when all spork has to do is set the craft on the treadmill backwards and no thought is required. Perhaps you should add that skepticism back into the equation, self skepticism.

 

[ponderingturtle]

In the same way that the sail, moving with the water, can use its speed against the air to increase its speed against the air. Please try to think about it.

But it is moving against both and useing that force to increase its speed against both.

You want a simple example how that works? Imagine yourself in a car on a highway. You are moving at the same speed as the cars in your direction. You take a ball and throw it at a car going in the opposite direction. It bounces from it, then it bounces from a car behind you in your direction, and then you recatch it. Guess what? The ball, when you recatch it, will be travelling forward faster than when you threw it, giving you net increase in momentum and kinetic energy (do the calculation if you don't believe me). The momentum and energy did not come from nowhere - it came from the two cars that the ball hit, slowing them with respect to each other. Your speed is now higher than the speed of other cars in your direction - but you needed the cars going in the opposite direction for this to work.

This is not some issue of redefining coordinate systems.



The claim here is that if the you are on a treadmill going some speed and have wind blowing in the same dirrection of motion at a lower speed it will accelerate at the sources.

 

[ponderingturtle]

Just connect two sailboats, iceboats, or land yachts with a long sliding bar with a seat in the middle, and let them tack back and forth in opposite directions. The seat and the center of mass of the system could move straight downwind faster than the wind, or move straight upwind.

The nature of vector addition might well kill this. The boats might be moving faster than the wind, but is the down wind component of their velocity faster than the wind?

 

[JWideman]

I don't think this will be resolved until you've posted the plans as promised, spork.

 

[ponderingturtle]

If the only force was friction why does it stop working when the propeller is disconnected. In fact any amount of friction reduces it's ability to climb the treadmill so claiming the only force is friction doesn't wash.
http://www.youtube.com/watch?v=LMgDvC5lqsY

You can see all of spork's videos here:
http://www.youtube.com/user/spork33

Does it accelerate to be faster than the wind or start there? It would seem to lose power as it approached wind speed, and be limited to under the speed of the wind.

 

[my_wan]

But it is moving against both and useing that force to increase its speed against both.

Are you not going against both when the OP craft gets faster than the wind?

This is not some issue of redefining coordinate systems.

If the ball started with you and came back to you then the coordinate system was never redefined. The one and only coordinate system was yours and you never even accelerated. Yet the ball came back to you faster than the ball left you.

The claim here is that if the you are on a treadmill going some speed and have wind blowing in the same dirrection of motion at a lower speed it will accelerate at the sources.

Are you talking propeller created wind or just wind? What is "the sources"? I can't even see a claim here much less admit it's the OP claim.

 

[my_wan]

Does it accelerate to be faster than the wind or start there? It would seem to lose power as it approached wind speed, and be limited to under the speed of the wind.

Watch the first video, without the treadmill. That's what it really does.

 

[technoextreme]

Think again. The headwind at above wind speed turns the prop in the same direction it takes to increase the speed of that wind in the same headwind direction.

Yeh gods you have no idea how a screw works do you...... What you are describing is a violation of basic mechanics. Headwind-One direction. Tailwind-Other direction. It's always going to stop because it's going in the wrong direction.

Physics is defined empirically. You are the one assuming you know the physics without actually doing the experiment (empirical test). You are therefore mistaking your intuition for actual physics. That's why this thread is interesting in the first place because the actual physics is counterintuitive, and you fell for your intuition. You even called your experiment a (thought) experiment when all spork has to do is set the craft on the treadmill backwards and no thought is required. Perhaps you should add that skepticism back into the equation, self skepticism.

Please don't delude yourself into thinking you are doing any better. The fact that you replied to my draw the free body diagram suggestion with a assinine remark means that you just as guilty of gullibility than anyone else.

Humber, think of the treadmill as an engine that drives the propeller.

Airplane on a treadmill! Airplane on a treadmill! Airplane on a treadmill! Im going to scream if I hear one more person say that this has nothing to do with airplane on a treadmill I will scream. I SAID THAT ALL THE WHILE. THE LITTLE CART EXPERIMENT PROVES NOTHING BECAUSE IT RELIES ON THE PHYSICS BEHIND AIRPLANE ON A TREADMILL AND NOTHING ELSE. The cart experiment is invalid. Wrong. Faulty assumption. Your not getting power from the ground so why even make such an experiment in the first place.

 

[my_wan]

Yeh gods you have no idea how a screw works do you...... What you are describing is a violation of basic mechanics. Headwind-One direction. Tailwind-Other direction. It's always going to stop because it's going in the wrong direction.

Yet you still don't seem to get the fact that the wheel and prop are directly tied to each other. If you turn it around the wheels turn the other way but that means so does the prop because they are tied together. The craft alway always blows air in the opposite direction that it moves, forward or backward. Move it one direction the prop blows one way, move it the other direction the prop blows the other direction. Turned the other way the craft would go faster than the wind backward. Think an airplane can do that?

 

[.13.]

Airplane on a treadmill! Airplane on a treadmill! Airplane on a treadmill! Im going to scream if I hear one more person say that this has nothing to do with airplane on a treadmill I will scream. I SAID THAT ALL THE WHILE. THE LITTLE CART EXPERIMENT PROVES NOTHING BECAUSE IT RELIES ON THE PHYSICS BEHIND AIRPLANE ON A TREADMILL AND NOTHING ELSE. The cart experiment is invalid. Wrong. Faulty assumption. Your not getting power from the ground so why even make such an experiment in the first place.

I'm not sure what you are trying to say here. Wether you agree with my post (or rather a single sentence) or not. The sentence you quoted was from a post that was specifically written in an attempt to explain why the vehicle accelerates forward on those treadmill videos.

 

[technoextreme]

I'm not sure what you are trying to say here. Wether you agree with my post (or rather a single sentence) or not. The sentence you quoted was from a post that was specifically written in an attempt to explain why the vehicle accelerates forward on those treadmill videos.

I'm agreeing with you. Im just pointing out you gave the same explanation as airplane on a treadmill.