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

[vanesch]

Is there any restriction on the speed of the centre of mass of the windmill and the car? (Sorry, I should be able to work this out myself, but I'm too lazy to think.)

No, of course not. The windmill could almost be massless (made out of neutrinonium ?) and the car could go as fast as it goes (call it "lightbullet").

You could consider having two extremely light windmills, planting one down (connected with a rope to your car) that will generate electricity for a few seconds, then fold it up and take it in (with the rope, almost no effort as it is essentially massless), and plant at the same time the second windmill, having it produce electricity for a few seconds, fold it up and take it in while planting the first one again, etc...

A kind of "walking on windmills". Very clumsy, but as a proof of principle, I don't see what stops it.

Of course, there is conservation of momentum, and hence what must remain at the same velocity is the center of gravity of the air (consider a big, but limited amount) and the car, which should move at a velocity slightly smaller than the wind speed (as the car is initially at rest). So we have to "slow down" enough wind to compensate for the increase in speed of the car ; but as there is no limit as to the amount of wind we slow down (or even reverse direction), this doesn't put a hard limit on the speed of the car. This will come out of the energy balance of the whole thing I guess.

 

[Phrak]

Jeff, your definition of the 'cart to propeller advancement ratio' greatly simplified the mental picture. None of this has been obvious--witness a locked thread and, so far, the lack of a mathematically formulated proof.

There is an equivalent mechanical arrangment, to good approximation, where the propeller and wind is replaced by a second wheel on a moving surface. In swerdna's case, this surface is stationary with the room. The second wheel is smaller in diameter to obtain an avancement ratio of less than one, as you say. This arrangment should be more intuitive to grasp, without the complications of variable angle of attack.

Just as with swerdna's direct drive wheel and propeller system, there are two mechanical force couplings. These are 1) the rotating shaft, and 2) the rigid member that connects the two wheel hubs. The coupling concept seems to be the useful thing to do to generate vector diagrams.

 

[schroder]

No, the ground is also part of the power source. If the ground were frictionless, then the cart would just slide along at the same speed as the wind.

Amazing statement! You admit that without a wheel touching down, the cart can go a Maximum velocity equal to the wind speed. Now, you touch down a wheel, introducing friction with the ground, and you claim the cart goes faster! The wheel requires friction with the ground to turn. The force to turn it and overcome that friction, comes from the cart, and ultimately from the wind. That would have to slow the cart down, not speed it up! Unless you truly believe that a wheel can be both pushed and pulled at the same time! Imagine that, an over unity wheel! Simply amazing.

 

[Phrak]

Amazing statement! You admit that without a wheel touching down, the cart can go a Maximum velocity equal to the wind speed. Now, you touch down a wheel, introducing friction with the ground, and you claim the cart goes faster! The wheel requires friction with the ground to turn. The force to turn it and overcome that friction, comes from the cart, and ultimately from the wind. That would have to slow the cart down, not speed it up! Unless you truly believe that a wheel can be both pushed and pulled at the same time! Imagine that, an over unity wheel! Simply amazing.

Whoa guys! Your definitions are simply different. Jeff means the wheel doesn't slide on the surface. You are talking about rolling friction. Jeff is talking about sliding friction that ensures the wheel spins as it moves over the ground.

 

[schroder]

Whoa guys! Your definitions are simply different. Jeff means the wheel doesn't slide on the surface. You are talking about rolling friction. Jeff is talking about sliding friction that ensures the wheel spins as it moves over the ground.

What is the difference?

 

[zoobyshoe]

All wind powered devices have to "slow" down the wind in order to extract power from it. The DDWFTTW cart is designed to be able to slow down the wind with it's propeller, even when the cart itself is traveling somewhat faster than the wind.

If the cart is traveling faster than the wind, how can the wind catch up to it so the propeller can slow that wind down?

 

[rcgldr]

Jeff, your definition of the 'cart to propeller advancement ratio' greatly simplified the mental picture.

It was mentioned before, but perhaps more in the wiki thread than the previous ones here. The advance ratio has to be < 1 for downwind carts, and > 1 (with prop pitch reversed) for upwind carts.

There is a nearly equivalent mechanical arrangment where the propeller and wind may be replaced by a second wheel and second surface.

A similar analogy has been made using a yo-yo and string. The string is wound around the axis exiting forwards at the bottom, while the "wheels" of the yo-yo rest on the ground. If you pull on the string, and there's no slippage, the yo-yo will move forwards faster than the string, by the rate of the speed that the string is wound around the axis of the yo-yo. The string could be replaced by a thin rod that moved along the bottom of the axis of the yo-yo with the same result if there was no slippage. Note that the larger the axis, the faster the yo-yo moves with respect to the string with a similar advance ratio formula, yo-yo speed = string speed / (1 - (axle diameter / wheel diameter)). The speed increases as the ratio approaches 1 (at > 1, such as a thick axis with smaller hubs resting on a pair of rails, the yo-yo goes in the opposite direction, similar to an upwind cart requiring and advance ratio > 1.

I think the issue here is that using the air as a power source is more "lossy" than using a solid.

 

[schroder]

It was mentioned before, but perhaps more in the wiki thread than the previous ones here. The advance ratio has to be < 1 for downwind carts, and > 1 (with prop pitch reversed) for upwind carts.

A similar analogy has been made using a yo-yo and string. The string is winds around a relatively small axis while the "wheels" of the yo-yo rest on the ground. If you pull on the string, and there's no slippage, the yo-yo will move forwards faster than the string, by the rate of the speed that the string is wound around the axis of the yo-yo. The string could be replaced by a thin rod that moved along the bottom of the axis of the yo-yo with the same result if there was no slippage.

Which is an example of gearing and pulleys and has nothing to do with the present topic.

 

[zoobyshoe]

A similar analogy has been made using a yo-yo and string. The string is winds around a relatively small axis while the "wheels" of the yo-yo rest on the ground. If you pull on the string, and there's no slippage, the yo-yo will move forwards faster than the string, by the rate of the speed that the string is wound around the axis of the yo-yo. The string could be replaced by a thin rod that moved along the bottom of the axis of the yo-yo with the same result if there was no slippage.

Are you the power source here, or is it you AND the string?

 

[schroder]

If the cart is traveling faster than the wind, how can the wind catch up to it so the propeller can slow that wind down?

Not only that, but the cart would need to push into the wind ahead of it, which is a part of the same wind which is pushing it. Sort of like a rock caught in a glacier, moving faster than the glacier. No, I am not buying any of this!

 

[zoobyshoe]

Not only that, but the cart would need to push into the wind ahead of it, which is a part of the same wind which is pushing it. Sort of like a rock caught in a glacier, moving faster than the glacier. No, I am not buying any of this!

Hey, I just learned that friction is a power source. I think the street out front probably has a high coefficient of friction. I'm going to plug an extension cord into the asphalt and get free energy.

 

[schroder]

Hey, I just learned that friction is a power source. I think the street out front probably has a high coefficient of friction. I'm going to plug an extension cord into the asphalt and get free energy.

Yeah, I am going to stop carrying my spare tire in the trunk. From now on I will drag it behind the car, and let it power the car! Think of the gas money I will save!

 

[schroder]

I will drop out now, before they close another thread. There are PF "Mentors" here who are completely convinced thar DDWFTTFW is possible, and they do not allow for dissent! Truly amazing!

 

[Phrak]

What is the difference?

I was posting hastily, to stave off unnecessary contention. Jeff refers to static friction rather than sliding friction. In any case, Lock the tire to the hub. Pull on the wheel until it slides on the ground. Before it breaks loose and starts sliding, the opposing force to your pulling is called static friction.

Sliding friction, by the way is the opposing force while it is sliding. The two values are a bit different. Sliding friction will be less under the same load on the wheel (hence antilock breaking systems).

Rolling friction is what you think it is.

 

[schroder]

I was posting hastily, to stave off unnecessary contention. Jeff refers to static friction. In any case, Lock the tire to the hub. Pull on the wheel until it slids on the ground. Before it slides the opposing force to your pulling is called static friction.

Sliding friction, by the way is the opposing force while it is sliding. The two values are a bit different. Sliding friction will be less under the same load on the wheel (hence antilock breaking systems).

I know exactly what sliding friction and static friction is. In any case, I was referring to rolling friction. It requires friction to make the wheel roll on the ground. Friction is a force. It requires force to overcome that friction force. The force to overcome the force of rolling friction, to make the wheel roll, comes from the cart. The source of power for the cart is the force of the wind. By introducing the wheel with the ground, you are draining some of the wind power which was pushing the cart. The cart slows down, not speeds up. The wheel is draining power, not adding power. Can anyone argue with that? I mean seriously argue with that?

 

[swerdna]

Not only that, but the cart would need to push into the wind ahead of it, which is a part of the same wind which is pushing it. Sort of like a rock caught in a glacier, moving faster than the glacier. No, I am not buying any of this!

A part of me remains sceptical as it doesn’t make sense to me that the thrust of the prop can exceed the rolling resistance. I can’t deny what my tests have shown however and they seem to me to show very clearly that it can.

How do you explain what’s going on in this video? -

To begin with the turntable and cart are traveling at exactly the same speed. When the cart is released from the turntable there is no inertia to get the cart moving as in the first video (in fact the cart slows) and only the wind created by movement through calm air slows the cart relative the surface which turns the prop via the wheel. As the wind is being created by the speed of the turntable and the cart is going faster than the turntable in the opposite direction. Doesn’t this mean the cart is going faster than the wind?

You seem to be the only one prepared to give a definitive answer to my original question (thanks).

 

[rcgldr]

Amazing statement! You admit that without a wheel touching down, the cart can go a maximum velocity equal to the wind speed. Now, you touch down a wheel, introducing friction with the ground, and you claim the cart goes faster!

Without a ground interface, such as a lighter than air balloon, the vehicle simply travels at the same speed as the wind. Including the ground interface allows the vehicle to utilize the difference in between wind speed and ground speed. Note that the difference between wind speed and ground speed is constant in these examples and independent of vehicle speed. You just need some clever way to utilize the ground interface to produce some amount of thrust against the wind in order to go DDWFTTW.

Unless you truly believe that a wheel can be both pushed and pulled at the same time!

The driving wheels are pushed and pulled at the same time, or more correctly, by opposing and unaligned forces that generate a torque on the driving wheels. The driving wheels are pushed forwards at the axis by the thrust from the propeller. The driving wheels are pushed backwards at the contact patch by the grounds reaction force. These unaligned forces create a torque on the driving wheels, which is used to drive the propeller. The propeller operates at a lower speed but higher force than what occurs at the contact patch of the driving wheels. The result is forward force from the prop is higher than backwards force from the ground, but at a lower speed. The tailwind comes into play here, ground speed is much higher than apparent wind speed. For example, when the cart is moving at the same speed as the wind, the ground speed is the wind speed, but the apparent wind is zero. This allows the greater thrust force at lower relative air speed to be used to create enough thrust to overcome the other drag factors on the cart.

If the cart is traveling faster than the wind, how can the wind catch up to it so the propeller can slow that wind down?

The wind can't catch up, instead the propeller accelerates the apparent headwind in the upwind direction so that the air flow aft of the propeller is slower than the carts forward speed, thus slowing down the wind. The air flow through the prop is moving downwind slower than the wind.

Note that sailcraft face the same issue. In order for a land sail or ice boat to tack downwind faster than the wind, the heading and angle of attack of the sail has to divert the apparent wind upwind so that the wind flow aft of the sail is slower than the wind.

Both the DDWFTTW cart and the sailcraft slow the wind down, even though the vehicles themselves are traveling faster than the wind. In both cases the air is accelerated upwind enough so that the net effect is that the wind is slowed down as these vehicles pass through a volume of air.

 

[zoobyshoe]

A part of me remains sceptical as it doesn’t make sense to me that the thrust of the prop can exceed the rolling resistance. I can’t deny what my tests have shown however and they seem to me to show very clearly that it can.

You didn't answer my question, but I'm assuming from other's remarks that the idea here is to use only the power of the wind: no motors or engines. Your model has a clear motor.

 

[schroder]

A part of me remains sceptical as it doesn’t make sense to me that the thrust of the prop can exceed the rolling resistance. I can’t deny what my tests have shown however and they seem to me to show very clearly that it can.

How do you explain what’s going on in this video? -

To begin with the turntable and cart are traveling at exactly the same speed. When the cart is released from the turntable there is no inertia to get the cart moving as in the first video (in fact the cart slows) and only the wind created by movement through calm air slows the cart relative the surface which turns the prop via the wheel. As the wind is being created by the speed of the turntable and the cart is going faster than the turntable in the opposite direction. Doesn’t this mean the cart is going faster than the wind?

You seem to be the only one prepared to give a definitive answer to my original question (thanks).

I will try to write up a more detailed answer for you, but also try to give you a short answer here. Yes, the relative wind created by the cart moving with the table slows the cart. It is not primarily the relative wind on the cart in this case, but the relative wind working against that rather large center arm as well. (you might consider a more aerodynamic design for that). But it is NOT the relative wind which is turning the prop! The energy for that is coming from the wheel. As the cart gets slowed, the relative velocity between the cart and the table increases, turning the wheel and the prop. The prop finally gets enough pizazz to reverse the direction of the cart. Try what I suggested earlier, disconnect the drive shaft between the wheel and the prop. Now run everything exactly the same. When the relative wind does turn the prop, it will turn in the opposite direction, because of the pitch. This proves the relative wind is not turning the prop in the original run. More later...

 

[schroder]

Without a ground interface, such as a lighter than air balloon, the vehicle simply travels at the same speed as the wind. Including the ground interface allows the vehicle to utilize the difference in between wind speed and ground speed. Note that the difference between wind speed and ground speed is constant in these examples and independent of vehicle speed. You just need some clever way to utilize the ground interface to produce some amount of thrust against the wind in order to go DDWFTTW.

Some "clever way" is going to have to be something other than a wheel dragging on the ground! The required force to turn a wheel against the force of friction has been known since at least Roman times. It is NOT adding any thrust, it is in fact reducing available thrust. IT CANNOT provide force over and above the force it is using to make it roll! Can you provide any math to back up your claim? It is a nice fantasy, but not true.

 

[rcgldr]

It doesn’t make sense to me that the thrust of the prop can exceed the rolling resistance. I can’t deny what my tests have shown however and they seem to me to show very clearly that it can.

This is due to what we've been calling advance ratio. Effectively you've geared down the prop, decreasing it's speed, but increasing it's force. This wouldn't do any good without a tailwind to lower the apparent wind speed of a cart that is moving downwind. When the cart is moving at wind speed, the apparent wind is zero. To go barely faster than the wind would allow for a huge advance ratio, as you need very little air speed through the prop to go only slightly faster than the wind. So even after all the losses and drag factors, the multiplied force at the prop at the divided speed is sufficient to go DDWFTTW as long as there's a difference between wind speed and ground speed (a tailwind).

 

[swerdna]

You didn't answer my question, but I'm assuming from other's remarks that the idea here is to use only the power of the wind: no motors or engines. Your model has a clear motor.

Sorry but I can’t find the question you refer to. Yes there is obviously a motor involved to create the wind. They also use a motor to create wind in wind tunnel testing. Are you saying all wind tunnel testing is wrong and not valid? I don’t see how what creates the wind is important. You could say that the Sun is the motor of outside winds. Does that mean any testing done with any wind isn't valid?

 

[zoobyshoe]

The wind can't catch up, instead the propeller accelerates the apparent headwind in the upwind direction so that the air flow aft of the propeller is slower than the carts forward speed, thus slowing down the wind. The air flow through the prop is moving downwind slower than the wind.

Since this would mean that an "apparent" headwind can power the cart, why couldn't an authentic headwind? Can we make a cart with a prop geared to its wheels that will travel directly into the wind powered only by that wind?

 

[zoobyshoe]

Sorry but I can’t find the question you refer to. Yes there is obviously a motor involved to create the wind. They also use a motor to create wind in wind tunnel testing. Are you saying all wind tunnel testing is wrong and not valid? I don’t see how what creates the wind is important. You could say that the Sun is the motor of outside winds. Does that mean any testing done with any wind isn't valid?

You have a motor driving your propeller via the wheel.

 

[rcgldr]

It cannot provide force over and above the force it is using to make it roll!

Again it's a gearing factor here. Take the case of an automobile. Say the engine produces 200 lb ft of torque at some rpm and throttle position. Say the overall gear ratio from engine to driven wheel is 1:10 (typical first gear for a car). The wheel rotates at 1/10th the speed of the engine, but has 10 times the torque (minus losses).

Clearly the same gearing concept could be used to divide the angular velocity and multiply the torque from the driving wheels to the propeller. However in this case, the effective gearing can be achieved if the prop pitch (advance distance per revolution) is less than the wheel circumference (advance distance per revolution). In this case, the geometric pitch of the prop is 6" per revolution, while the wheels have a circumference of 10.5", this is a geometric gear ratio of 6:10.5, and the effective ratio would be less. Using the geometric ratio of 6/10.5, then the torque or force multiplication factor is 10.5/6 or 1.75. If efficiency was 70%, then the force at the prop would be .7 x 1.75 = 1.225, still enough to allow the cart to go DDWFTTW as evidenced by the various videos.

 

[A.T.]

Would this also work if the propeller was closer to the turntable axis than the wheel?

 

[rcgldr]

Since this would mean that an "apparent" headwind can power the cart, why couldn't an authentic headwind? Can we make a cart with a prop geared to its wheels that will travel directly into the wind powered only by that wind?

Yes, the prop pitch is reversed, and the advance ratio has to be > 1. For example, imagine the advance ratio is 4:1. Prop speed is 4 times ground speed. With a 10 mph headwind, if the cart was moving at 2 mph forwards, then the apparent wind would be -12 mph, and the prop speed would be -8 mph, slowing down the wind by 4 mph. If the cart was moving at 3 mph forwards, then the apparent wind speed is -13 mph, and the prop speed is -12 mph slowing down the wind by just 1 mph. In this case the advance ratio needs to be large enough that the wind is slowed down less when the cart is moving forwards and the prop is spinning faster. Think of it as a worm (corkscrew) gear operating in the air.

 

[swerdna]

I will try to write up a more detailed answer for you, but also try to give you a short answer here. Yes, the relative wind created by the cart moving with the table slows the cart. It is not primarily the relative wind on the cart in this case, but the relative wind working against that rather large center arm as well. (you might consider a more aerodynamic design for that). But it is NOT the relative wind which is turning the prop! The energy for that is coming from the wheel. As the cart gets slowed, the relative velocity between the cart and the table increases, turning the wheel and the prop. The prop finally gets enough pizazz to reverse the direction of the cart. Try what I suggested earlier, disconnect the drive shaft between the wheel and the prop. Now run everything exactly the same. When the relative wind does turn the prop, it will turn in the opposite direction, because of the pitch. This proves the relative wind is not turning the prop in the original run. More later...

But the cart only gets slowed because of the relative wind, which in turn causes the wheel to rotate against the turntable and the prop to turn. I don’t see how you can say its not the wind that is ultimately turning the prop. If the test was done in a vacuum or there was no wind the cart wouldn’t slow compared to the turntable and the prop wouldn’t turn.

I don’t see that it matters how much drag is created by the tether arm. In fact In a way I believe the more drag the better. I have tried a system where there is a flat vertical sail area on the tether arm to initially help the cart up to the speed of the wind. The sail then falls over to horizontal so it doesn’t restrict forward movement into a virtual headwind. It works quite well.

When the cart “gets enough pizazz to reverse the direction of the cart” it is going faster than the turntable. Given the speed of the turntable is the speed of the wind, doesn’t this mean that the cart is going faster than the wind?

I’m not trying to be argumentative, just to understand what the truth is. I appreciate your contributions and hope you stay with this thread even if it increases the chance that it will be closed.

 

[zoobyshoe]

Yes, the prop pitch is reversed, and the advance ratio has to be > 1. For example, imagine the advance ratio is 4:1. Prop speed is 4 times ground speed. With a 10 mph headwind, if the cart was moving at 2 mph forwards, then the apparent wind would be -12 mph, and the prop speed would be -8 mph. In this case the advance ratio needs to be large enough that the wind is slowed down less when the cart is moving forwards and the prop is spinning faster. Think of it as a worm (corkscrew) gear operating in the air.

If this is the case, then you have the makings of a wind powered boat superior to all others in that it can utilize any point of sail, especially the confounded directly upwind direction.

 

[swerdna]

You have a motor driving your propeller via the wheel.

The motor creates the wind, that moves the wheel ,that spins the prop, that moves the cart faster than the wind (apparently).

The sun creates the wind, that spins the prop, that generates electricity, that powers the house that Jack built.

So what?