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

[schroder]

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.

I have no problem with anyone being argumentative, this is an argumentative issue! Have you tried my suggestion yet? Run the table with no cable between the prop and the wheel. If the prop runs in the opposite direction, then it shows conclusively that the relative wind could not have been driving the prop in your first run. That will be a good place to start. Please give it a try, if it is not too much trouble to disconnect the cable. Thanks!

 

[schroder]

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

Obviously the placement on the end affords more leverage. But, in principle, it would still work with a closer-in placement. The problem will be prop clearance with the table. Probably need to elevate the prop higher. But the principle would be the same. Yes, it would still work.

 

[schroder]

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.

Just one small problem Jeff! This is not a driving wheel. It is, in actual fact, a driven wheel! It is a source of drag, not thrust and it cannot be a source of thrust at the same time it is a source of drag! It is that simple!

 

[rcgldr]

Wind that is ultimately turning the prop.

Ultimately yes, but it goes through a process. The tailwind increases the thrust from the prop at any given rpm (igorning "cavitation" type effects here). The thrust from the prop results in a forwards force at the axle of the wheel. The turntable exerts a backwards force at the contact point of the wheel. Unlike my previous posts explanations, since the forward force is applied at the axle there is no torque generated by that force. The backwards force exerted by the turntable to the contact patch of the wheel creates a torque force that is used to drive the propeller. Since the propeller pitch is less than the wheel circumference, the thrust speed is reduced, but the thrust force is increased by the effective reduction in gearing. In this case the prop geometrically advances 6" per revolution, while the wheel advances 10.5" per revolution, a geometric advance ratio of 6/10.5.

Using the cart as the frame of reference, when the turntable spins up, the cart experiences a negative ground speed and a positive air speed. The thrust from the prop resists the positive air speed and the cart accelerates forwards. Eventually the cart reaches the point where there is a negative ground speed and zero relative air speed, but at this point the prop is generating a large amount of thrust at a very slow speed, enough to keep the cart accelerating forwards. Eventually, the apparent wind becomes a headwind, negative air speed, and this reduces the thrust from the prop at a given rpm. As the cart speeds up, the thrust speed increases at a lower rate than the ground and apparent wind speed and eventually stops accelerating when the thrust equals the overall drag.

 

[rcgldr]

This is not a driving wheel. It is, in actual fact, a driven wheel. It is a source of drag, not thrust and it cannot be a source of thrust at the same time it is a source of drag.

The wheel is indeed a source of drag or better stated, the ground exerts a backwards force at the point of contact of the wheel, opposing forward motion. However, this backwards force times the radius of the wheel equals the torque applied to the wheel. The wheel in turn transfers this torque to the propeller, and it's the propeller that generates the thrust. Because of the effective gearing (advance ratio), the torque is multiplied and the speed divided. The propeller generates more thrust than the force from the wheel, but at a much lower speed. The power output (thust times air speed) is less than power input (drag force times ground speed), but the force from the prop is higher than the force from the ground with enough left over to compensate for the other drag factors.

Again, a realtive tailwind is required for this to work, since the reduction in speed would render the prop usless unless the apparent wind is significantly slower than the ground speed. As stated before when the cart is moving at the same speed as the wind, then the apparent wind is zero, the prop is generating all of the induced wind and thrust, and the ground is opposing this thrust, most of it going to the driven wheel, creating a torque on that wheel, which transfers the torque to the propeller, which generates a higher thrust at a lower speed because of the advance ratio.

As an example of the effective gearing, imagine someone standing on the bottom pedal on a 10 speed bicycle in high gear. How much force would it take another person to push the bike forwards, which would raise the riders center of mass by the diameter of the pedals? It would only take a fraction of the persons weight to do this. The force from the person would be opposed by a backwards force from the ground, creating a torque on the driven wheel, and this torque would be multiplied while the angular speed was divided at the pedals. Even if the gearing was 1:1, the fact that the pedals have a smaller diameter than the wheel would still result in a higher force at the pedals than at the wheel because of the smaller radius of the pedals versus the wheel (even though the torque would be the same). It's the angular equivalent of a lever.

 

[vanesch]

Just one small problem Jeff! This is not a driving wheel. It is, in actual fact, a driven wheel! It is a source of drag, not thrust and it cannot be a source of thrust at the same time it is a source of drag! It is that simple!

I think what you are confusing is the word "friction" which means somehow "power dissipation" to you. But the friction here is just a mechanical binding, it is (ideally) not dissipative. As such, you could, instead of a "friction wheel" consider a dented wheel on a dented "floor" and then you see maybe more the mechanical side of it.

This is not a "free energy" machine or anything. Consider the following: consider, instead of having air in the original experiment of the OP, a static "turntable" above the turning turntable, all that in a vacuum. So we now have a spinning disk, and above that, a fixed disk. The fixed disk is a "replacement" for the static air. It is perfectly possible to design a cart between both wheels (the fixed one and the turning one) such that it does exactly the same as the propeller-driven cart, by having a wheel on the top (static) disk, another wheel on the bottom (turning) disk, and gears linking both. Indeed, it is sufficient that the gears impose a certain ratio of speed between the top wheel and the bottom wheel and you can make it go any speed. If the ratio is -1, (that is, the top wheel will run just as fast as the bottom wheel, but in the opposite direction), then the cart will run at half the speed of the turning disk. If the ratio is 1, the system will be blocked (there's no possible motion from the cart for which the top wheel and the bottom wheel will have exactly the same speed in the reference frame of the cart). If the ratio is 0, then the cart will remain fixed in space. If the ratio is 1/2, however, the cart will run at the same speed as the lower wheel, but in the opposite direction (indeed, from the cart's PoV, both wheels go forward, but the bottom one twice as fast as the top one, satisfying its gearing ratio).
By playing with the gearing ratio, you can obtain about any motion you want.

Now, think of the static air mass as playing more or less the role of the static wheel. There's no reason why things now become impossible.

This is comparable to the way you can roll up a wire on a wheel by pulling the wire and the wheel towards you. If the diameter on which you wind up the wire is smaller than the diameter of the wheel which rolls on the floor, then the wheel will come faster towards you than that you are pulling the wire towards you, and the wheel will actually wind up the wire around it - see attachment.

Edit: I also added a rough sketch of the cart between two turntables...

 

[schroder]

Again, a realtive tailwind is required for this to work, since the reduction in speed would render the prop usless unless the apparent wind is significantly slower than the ground speed. As stated before when the cart is moving at the same speed as the wind, then the apparent wind is zero, the prop is generating all of the induced wind and thrust, and the ground is opposing this thrust, most of it going to the driven wheel, creating a torque on that wheel, which transfers the torque to the propeller, which generates a higher thrust at a lower speed because of the advance ratio.

So, an apparent tailwind is required for this to work, your words, not mine. And when the cart is moving at the same speed as the wind, the apparent wind is zero. Now you say the prop is inducing all the wind and thrust, but it is the torque on the wheel, transferred to the propeller, which powers the prop. There is no wind pushing the cart, it is zero. And yet something is still turning that wheel, which is turning that propeller which is powering the cart. That is perpetual motion Jeff. There is no other way to describe it. I can't believe you really believe in this.

 

[Phrak]

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?

If you are inclined to argue some stance that is beyond my recognition, please give a clear one. This requires a clear statement of what you think is fact, and some sound motivation for it. I'll not participate in any more bickering beyond this.

 

[schroder]

I think what you are confusing is the word "friction" which means somehow "power dissipation" to you. But the friction here is just a mechanical binding, it is (ideally) not dissipative. As such, you could, instead of a "friction wheel" consider a dented wheel on a dented "floor" and then you see maybe more the mechanical side of it.

Are we talking about "ideal" mathematical abstractions or about a real mechanical device on a real turntable? Friction is always dissipative and while it is conserved (as heat) it is not mechanically conserved. It will slow the cart down, never speed it up!

 

[schroder]

If you are inclined to argue this stance, please give a clear one. I'll not participate in any more bickering beyond this. Take your time. Please, take your time.

I believe my stance is very clear and is based upon established physics. Your stance is not at all clear.

 

[A.T.]

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

Obviously the placement on the end affords more leverage. But, in principle, it would still work with a closer-in placement. The problem will be prop clearance with the table. Probably need to elevate the prop higher. But the principle would be the same. Yes, it would still work.

I was just wondering if this would work for linear movement with linear wind. The the only difference seems to be, that here the simulated wind at the propeller is much faster then at the wheel.

 

[vanesch]

Are we talking about "ideal" mathematical abstractions or about a real mechanical device on a real turntable? Friction is always dissipative and while it is conserved (as heat) it is not mechanically conserved. It will slow the cart down, never speed it up!

Binding friction is not dissipative. As I said, if this confuses you, consider it to be a dented wheel on a dented turntable.

 

[vanesch]

So, an apparent tailwind is required for this to work, your words, not mine. And when the cart is moving at the same speed as the wind, the apparent wind is zero. Now you say the prop is inducing all the wind and thrust, but it is the torque on the wheel, transferred to the propeller, which powers the prop. There is no wind pushing the cart, it is zero. And yet something is still turning that wheel, which is turning that propeller which is powering the cart. That is perpetual motion Jeff. There is no other way to describe it. I can't believe you really believe in this.

This is absolutely not perpetual motion, not any more than the little car between a static and a turning wheel is (see my previous post).
What you have is that there is a mechanical binding between the cart and the turntable on one hand (given by the wheel on the turntable), and there's another "binding" between the propeller and the air mass, and there is a mechanical link between both. For a certain RPM of the wheel on the table, a certain RPM of the propeller is set, and this, in turn, gives a certain force on the cart. There will be a balance between the power taken or given by the wheel on the table, and the power taken or given by the propeller. The cart will settle in that motion when both are in equilibrium, and that equilibrium is given by the equivalent of a gearing ratio (including, in this case, the efficiency of the propeller).

 

[schroder]

I was just wondering if this would work for linear movement with linear wind. The the only difference seems to be, that here the simulated wind at the propeller is much faster then at the wheel.

Yes, but in fact it is not the simulated wind at the back of the propeller which is driving this. It is the wheel turning on the table, and powering the prop via a cable. This does not demonstrate a down wind vehicle at all! If the inventor will do the test I proposed, by running the device with the cable disconnected, the wind created will turn the prop in the other direction, as a turbine, not a propeller. If the cable were to be connected in the same way, and this device were in a wind tunnel, with the table stationary, it would in fact attempt to move upwind, not downwind! A simple test with an electric fan will prove this. But it will only try to move upwind, the fan will push it back slowly. What is happening here is an upwind configured device is being powered by the wheels to try and prove DDWFTTW, which is silly. If you want to test this as a true downwind device, disconnect the cable and connect it to the other side of the wheel and run it again. It will try to go downwind, but considerably less that the table speed. We can try all these tets if the owner/inventer is willing to discover the truth of the matter.

 

[rcgldr]

When the cart is moving at the same speed as the wind, the apparent wind is zero. Now you say the prop is inducing all the wind and thrust, but it is the torque on the wheel, transferred to the propeller, which powers the prop. There is no wind pushing the cart, it is zero.

I didn't state that. The prop is generating the wind and thrust that pushes the cart. The torque required to drive the prop coexists with a torque on the driven wheel, and the wheel exerting a forwards force on the ground, which reacts with an equal and opposite force on to the wheel. However at the point of application of force from the ground the speed involved is much higher than the induced wash from the prop, and this effectively multiplies the power (force times speed), which is the "clever way" that the DDWFTTW cart takes advantage of the difference between wind speed and ground speed.

So ultimately, the difference between wind speed and ground speed create a situation where opposing forces correspond to differing amounts of power, more power obtained from the wheel + ground interface than consumed by prop + air interface, providing enough excess power to enable the cart to go DDWFTTW.

 

[zoobyshoe]

This is absolutely not perpetual motion, not any more than the little car between a static and a turning wheel is (see my previous post).
What you have is that there is a mechanical binding between the cart and the turntable on one hand (given by the wheel on the turntable), and there's another "binding" between the propeller and the air mass, and there is a mechanical link between both. For a certain RPM of the wheel on the table, a certain RPM of the propeller is set, and this, in turn, gives a certain force on the cart. There will be a balance between the power taken or given by the wheel on the table, and the power taken or given by the propeller. The cart will settle in that motion when both are in equilibrium, and that equilibrium is given by the equivalent of a gearing ratio (including, in this case, the efficiency of the propeller).

Here is my problem with this DDW notion: power available from the wind is proportional to V^3

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind. The more it manages to accelerate you the weaker it becomes relative to you. If it accelerates you to 15 mph you now experience it as a 5 mph wind which is only 1/64 as strong as a 20 mph wind. At some point before you ever reach wind speed its power to accelerate you will be held in check by your friction over the ground.

 

[schroder]

Here is my problem with this DDW notion: power available from the wind is proportional to V^3

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind. The more it manages to accelerate you the weaker it becomes relative to you. If it accelerates you to 15 mph you now experience it as a 5 mph wind which is only 1/64 as strong as a 20 mph wind. At some point before you ever reach wind speed its power to accelerate you will be held in check by your friction over the ground.

Exactly! But the claim is that a wheel rolling on the ground, which must overcome friction in order to roll, and must be powered by the cart, will now somehow add torque and thrust to that propeller and make the cart go even faster. I guess if I lower an anchor from my 18 ft. skiff, it should make it go faster, by the same reasoning.

 

[vanesch]

Here is my problem with this DDW notion: power available from the wind is proportional to V^3

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind. The more it manages to accelerate you the weaker it becomes relative to you.

True, but the faster you go over the floor, and the more energy you can pump out of that. Do you agree with me that if you'd have an infinitely big sail, then you would be static wrt the wind, move at windspeed, and be able to extract a lot of energy from the wheels on the floor ? Now, imagine you charge a battery with that for 10 minutes (while you are going exactly at downwind speed). After 10 minutes, you take down your sail, and you switch on your electrical motor and drive 10 times faster than the wind. After, say, 30 minutes, your batteries are discharged, so you put up your sail again, go at downwind speed again, and recharge your batteries. 10 minutes later, again, you switch to you motor.

Your average speed will be (1 x 10 + 10 x 30 )/40 = 7.75 times the downwind speed.

If it accelerates you to 15 mph you now experience it as a 5 mph wind which is only 1/64 as strong as a 20 mph wind. At some point before you ever reach wind speed its power to accelerate you will be held in check by your friction over the ground.

This is like Zeno's paradox :-)

Look at my attachment in one of the previous postings, with the gearboxed two-wheel cart between a fixed disk and a turntable, which can go at any speed. Think of the air as the fixed disk. Or the other example of the winding wire: disk moving faster than the wire downwire.

 

[vanesch]

Exactly! But the claim is that a wheel rolling on the ground, which must overcome friction in order to roll, and must be powered by the cart, will now somehow add torque and thrust to that propeller and make the cart go even faster. I guess if I lower an anchor from my 18 ft. skiff, it should make it go faster, by the same reasoning.

A wheel must not "overcome friction in order to roll". If it overcame friction, it would slide! The wheel serves simply as a mechanical binding, from which one can extract more mechanical energy from the difference in velocity between the air and the turntable. There is no problem in energy conservation here, because there is no strict limit on how much energy is extracted from the turntable.

 

[schroder]

True, but the faster you go over the floor, and the more energy you can pump out of that. Do you agree with me that if you'd have an infinitely big sail, then you would be static wrt the wind, move at windspeed, and be able to extract a lot of energy from the wheels on the floor ? Now, imagine you charge a battery with that for 10 minutes (while you are going exactly at downwind speed). After 10 minutes, you take down your sail, and you switch on your electrical motor and drive 10 times faster than the wind. After, say, 30 minutes, your batteries are discharged, so you put up your sail again, go at downwind speed again, and recharge your batteries. 10 minutes later, again, you switch to you motor.

Your average speed will be (1 x 10 + 10 x 30 )/40 = 7.75 times the downwind speed.



This is like Zeno's paradox :-)




Look at my attachment in one of the previous postings, with the gearboxed two-wheel cart between a fixed disk and a turntable, which can go at any speed. Think of the air as the fixed disk.

Sure, an infinitely big and massless sail. In other words, a free source of boundless energy! Very nice but that is not what we are talking about here at all.

 

[schroder]

A wheel must not "overcome friction in order to roll". If it overcame friction, it would slide!

It must overcome the static friction and only have kinetic and rolling friction. I thought that was understood but thank you for the correction.

 

[atyy]

OK, taking my cue from one of vanesch's posts, I am thinking along these lines. If I push an object so that it attains an initial velocity above that of the wind, then it will continue downwind faster than the wind (assume no air resistance). Now, can I use the wind to get it to that point rather than me pushing it? I attach a sail (assume that's the only thing that has air resistance) and wheels which roll without slipping. The sail brings it up to wind speed. After that, withdraw the wheels and sail, and convert the rotational energy in the wheels into translational energy (am I violating angular momentum conservation?). From that point on, the object moves downwind faster than the wind since we have assumed no air resistance (I don't quite know how to prevent it from touching the ground, since I have assumed ground friction for rolling without slipping).

 

[rcgldr]

If a 20 mph wind presses on your sail or propeller and accelerates you directly downwind to 10mph, it has reduced its own speed relative to you by half and you now experience it as a 10 mph wind, which is only 1/8 as strong as a 20 mph wind.

Except the propeller isn't stationary, it's rotating, generating an upwind thrust component. Say the wind is 10 mph, the cart is moving at 12 mph, and the propeller is generating -5 mph of thrust. Then the air flow at the prop is 7 mph, 3 mph slower than the wind speed of 10 mph. There is a limit, but the limit is faster than the wind speed.

The same goes for a sailcraft when it's heading is not parallel to the wind. The sailcraft experiences an apparent crosswind = true wind speed x sin(angle between wind and sailcraft heading), independent of the sailcrafts forward speed (since apparent crosswind is perpendicular to sailcraft's forward speed). If the ground drag factors are very low, such as an ice boat, then the limiting factor is how much apparent headwind can be achieved for a given apparent crosswind. The iceboat downwind component of speed can be more than double the true wind speed, and it's faster to tack at an angle and back than it is to go directly downwind. Again, in order for this to work, the sail has to divert the apparent wind upwind so that the net air flow off the sail is slower than the true wind.

So for both the DDWFTTW cart and the sailcraft, the wind is slowed down in order to obtain power from the wind, but the wind is able to be slowed down even though these craft are moving downwind faster than the wind, due to the upwind thrust created by the craft.

 

[schroder]

OK, taking my cue from one of vanesch's posts, I am thinking along these lines. If I push an object so that it attains an initial velocity above that of the wind, then it will continue downwind faster than the wind (assume no air resistance). Now, can I use the wind to get it to that point rather than me pushing it? I attach a sail (assume that's the only thing that has air resistance) and wheels which roll without slipping. The sail brings it up to wind speed. After that, withdraw the wheels and sail, and convert the rotational energy in the wheels into translational energy (am I violating angular momentum conservation?). From that point on, the object moves downwind faster than the wind since we have assumed no air resistance (I don't quite know how to prevent it from touching the ground, since I have assumed ground friction for rolling without slipping).

Just how do you assume no air resistance? If there is no air resistance, there is no wind! The wind is a medium that the cart is within. It is in the front of the cart and in the back of the cart. Once the sail pushes you up to wind speed (if it can) then you would be pushing against the wind in front of you and all that is pushing you is the wind behind you! If you were stuck in molasses and it was flowing in one direction, do you suppose the molasses in the back can push you faster than the molasses in the front. What if you dragged your foot, or a wheel on the ground. Would that speed you up or slow you down?

 

[rcgldr]

So you put up your sail again, go at downwind speed again, and recharge your batteries.

Shh ... you weren't supposed to give away the secret of those DDWFTTW carts shown in the videos. No one was supposed to realize that the gear boxes on those treadmill carts were really hiding a clever generator/motor/capacitor setup.

OK, taking my cue from one of vanesch's posts, I am thinking along these lines. I attach a sail (assume that's the only thing that has air resistance) and wheels which roll without slipping. The sail brings it up to wind speed. After that, withdraw the wheels and sail, and convert the rotational energy in the wheels.

Well instead of charging batteries, you could spin up a big flywheel while under sail power at near wind speed, then drop the sail and use the flywheel power to drive the cart faster than the wind for a net average speed greater than the wind.

 

[atyy]

Just how do you assume no air resistance? If there is no air resistance, there is no wind! The wind is a medium that the cart is within. It is in the front of the cart and in the back of the cart. Once the sail pushes you up to wind speed (if it can) then you would be pushing against the wind in front of you and all that is pushing you is the wind behind you! If you were stuck in molasses and it was flowing in one direction, do you suppose the molasses in the back can push you faster than the molasses in the front. What if you dragged your foot, or a wheel on the ground. Would that speed you up or slow you down?

The sail is withdrawn at the moment we convert the rotational energy in the wheels into translational energy.

 

[zoobyshoe]

True, but the faster you go over the floor, and the more energy you can pump out of that.

No, Power is proportional to V^3 means you don't go faster over the floor: the wind loses its ability to accelerate you till you reach a terminal velocity which must be < wind speed. Your gearing won't help because the wind is simply losing its power to turn the gear train.

Do you agree with me that if you'd have an infinitely big sail, then you would be static wrt the wind, move at windspeed, and be able to extract a lot of energy from the wheels on the floor ?

Of course not. Energy is conserved. However large your sail, once you start using part of the wind's force to charge batteries your speed will decrease.

This is like Zeno's paradox :-)

Quoi? It's just conservation of energy.

 

[atyy]

Well instead of charging batteries, you could spin up a big flywheel while under sail power at near wind speed, then drop the sail and use the flywheel power to drive the cart faster than the wind for a net average speed greater than the wind.

Yes, that's equivalent to what I was thinking, but why doesn't this violate angular momentum conservation?

 

[atyy]

From that point on, the object moves downwind faster than the wind since we have assumed no air resistance (I don't quite know how to prevent it from touching the ground, since I have assumed ground friction for rolling without slipping).

Am I allowed to have the thing sprout wings at the moment I withdraw sails and wheels, so that it doesn't touch the ground from that point on? It would be much more convenient without gravity, but I can't get rolling without slipping without gravity, since I'm using it to produce the friction.

 

[rcgldr]

Instead of charging batteries, you could spin up a big flywheel while under sail power at near wind speed, then drop the sail and use the flywheel power to drive the cart faster than the wind for a net average speed greater than the wind.

Yes, that's equivalent to what I was thinking, but why doesn't this violate angular momentum conservation?

Because it's not a closed system. You have an external power source, the wind versus ground speed. If you want to make it a closed system, then you'd have to include the atmosphere and the earth, then angular momentum would be conserved (ok, maybe you have to include the moon, sun, tidal effects, ...).