Remember the debate about downwind vehicles

[Creator]

Nice work...now, how fast can it go AGAINST the wind??

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[spork]

Nice work...now, how fast can it go AGAINST the wind??

...

To go against the wind efficiently we have to swap our prop out for a turbine. I expect we might build one over the winter and try it next season. From 20' away you won't notice any difference at all, except I expect it will go directly upwind faster than the wind.

 

[sophiecentaur]

"You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground. "

"Wheels" are not an energy source. Something must be pushing them along the ground, making them rotate, for power to be available. What is that 'something'? You need a Force X a Speed for power to be developed. Where does the Force come from? If the action of the prop were just like a conventional 'driven' propellor, you could do at least as well by driving the wheels - and that would be nonsensical, you must agree.

"The airflow around it will be very much like the airflow around any propeller. As the cart moves, it will leave a cylinder of air behind it which is moving slower (relative to the ground) than the surrounding air."

I think you would have to agree that, if your cart were inside a tunnel (tube) with the same CSA as the cart, then it wouldn't go any faster than the airflow in the tunnel (how could it do better than a well fitting piston). Therefore there must be another source of momentum to transfer to the cart to explain how it goes faster than the air flow. The cylinder to which you refer must be much larger than the cylinder immediately around the cart or where is the momentum / velocity difference / power coming from? You can't transfer momentum if there is no (positive) velocity difference so the system has to produce one in some way.
Just because you have a system that works (and very impressive it is too!) it doesn't necessarily mean that your explanation is correct. I can find nothing in your explanation that goes further than a 'by its own bootstraps' argument and that won't do, will it? I am trying to look at the system in more depth and to explain what's happening in terms of possible energy transfers.
I can't really accept that your use of the term "ground frame" is relevant. I feel that it could probably also work for an airship, using some sort of drogue, possibly.

[quotes fixed - russ]

 

[sophiecentaur]

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels.
That would be interesting, would it not? You could even infer it from still pictures of the thing at work. Just look at the relative angles / slack in each side of the belt.
You could prove me wrong or right. I dare you.

 

[cjl]

QUOTE "You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground. "

"Wheels" are not an energy source. Something must be pushing them along the ground, making them rotate, for power to be available. What is that 'something'? You need a Force X a Speed for power to be developed. Where does the Force come from? If the action of the prop were just like a conventional 'driven' propellor, you could do at least as well by driving the wheels - and that would be nonsensical, you must agree.

The force to drive the wheels comes from the propeller. The propeller is pushing the cart (and the wheels) forward along the ground. That's the force. The reason that the propeller can generate enough force to drive the wheels even despite losses is because of the speed differential between the ground and the air.

Effectively, the energy the wheels generate is equal to the force required to turn them multiplied by the distance they move along the ground (this is a simple statement of work). The energy the propeller puts into the air is equal to the force it exerts on the air multiplied by the distance it moves through the air (again, a simple statement of work). When the cart is in steady state operation, the force generated by the propeller and the force on the wheels is the same (ignoring friction losses), so there is enough energy to drive the prop so long as the distance that the prop moves through the air is shorter than the distance the wheels move along the ground.

This condition is satisfied so long as there is a tailwind (and the way I've worded it, it makes the most sense if you consider the cart already moving faster than the wind). If the cart is moving at twice the windspeed, then the wheels will actually generate twice as much energy as the prop would require (if it were 100% efficient), since the force is the same and the wheels move twice as far along the ground as the prop does through the air. If the cart is moving at triple the wind speed, then the wheels generate 150% of the energy the prop requires, again ignoring losses.

Note that this only works because the prop moves a shorter distance through the air than the wheels do along the ground. This is why your assertion about being able to do at least as well by driving the wheels is wrong.

QUOTE "The airflow around it will be very much like the airflow around any propeller. As the cart moves, it will leave a cylinder of air behind it which is moving slower (relative to the ground) than the surrounding air."
I think you would have to agree that, if your cart were inside a tunnel (tube) with the same CSA as the cart, then it wouldn't go any faster than the airflow in the tunnel (how could it do better than a well fitting piston).

Wrong. In a tunnel, the cart would do significantly better than a well fitting piston, and (perhaps more surprisingly), the cart would do better in a tunnel with a relatively close fit between the prop blade and the wall of the tunnel than it would in free air. This is because ducted propellers are more efficient than free ones.

Therefore there must be another source of momentum to transfer to the cart to explain how it goes faster than the air flow. The cylinder to which you refer must be much larger than the cylinder immediately around the cart or where is the momentum / velocity difference / power coming from? You can't transfer momentum if there is no (positive) velocity difference so the system has to produce one in some way.

The cylinder to which I refer will be roughly the size of the propeller, though airflow through a prop disk is quite complicated. You're overthinking things here - the airflow around the cart behaves exactly as if it were a simple, propeller driven cart with an electric (or other) motor driving the propeller. Relative to the cart, the air flows smoothly around it with some velocity, with the air moving faster behind the propeller due to the propwash. Do you agree that a propeller is capable of transferring (rearwards) momentum to air even if the propeller is moving forwards through the air? If so, then you agree to the basic principle that allows this cart to work.

Just because you have a system that works (and very impressive it is too!) it doesn't necessarily mean that your explanation is correct. I can find nothing in your explanation that goes further than a 'by its own bootstraps' argument and that won't do, will it? I am trying to look at the system in more depth and to explain what's happening in terms of possible energy transfers.

The energy transfers are fairly straightforward, so long as you understand that the propeller is doing work on the air and the wheels are having work done on them by the ground (and therefore, they have different effective velocities). Once you see that, everything else basically falls into place.

I can't really accept that your use of the term "ground frame" is relevant. I feel that it could probably also work for an airship, using some sort of drogue, possibly.

An airship could not go directly downwind faster than the wind unless it were dragging something along the ground (or in some other medium with a different velocity than the air). The whole reason the cart works is because the airspeed of the cart is slower than the groundspeed. Without it being in contact with both the air and the ground, it could not work.

Oh, and it also might help to visualize the opposite case. The cart as shown here is effectively extracting energy from its groundspeed and using it to drive the vehicle by pushing against the air. Many people find it easier to visualize a cart extracting energy from its airspeed, and using it to drive the vehicle by pushing against the ground. This would be a cart driving directly upwind powered by a wind turbine. With appropriate gearing, the turbine can extract more energy from the air than is required to drive the wheels, and the cart could go directly upwind. It's exactly the same principle, but easier to visualize.

In the case of the downwind cart, the correct "gearing" has to do with the prop pitch. With the correct prop pitch, the cart can generate more force than is required at the wheels to turn the propeller, since the propeller is acting as both the method of pushing against the air and the method of altering the force/velocity balance (like a gearbox would in a car driven by the wheels).

 

[cjl]

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels.
That would be interesting, would it not? You could even infer it from still pictures of the thing at work. Just look at the relative angles / slack in each side of the belt.
You could prove me wrong or right. I dare you.

There's no doubt at all that the cart is transferring energy from the wheels to the prop. The prop pitch is in the wrong direction for energy transfer in the other direction - the wind is actually trying to spin the prop in the opposite direction from the way it turns, but the torque from the wheels is larger than this windmilling effect.

 

[Curl]

LOL I find it hilarious that there is even a debate about this issue.

Its so easy to understand with about 14-15 seconds of thinking (with the imagination part of your brain involved).

Trying to apply your book formulas to this will get you confused. Instead, just try to simulate a car like this in your head. Your brain's physics simulator will say it works. Mine did.

 

[cjl]

LOL I find it hilarious that there is even a debate about this issue.

Its so easy to understand with about 14-15 seconds of thinking (with the imagination part of your brain involved).

Trying to apply your book formulas to this will get you confused. Instead, just try to simulate a car like this in your head. Your brain's physics simulator will say it works. Mine did.

I disagree.

It might be intuitive to some, but not to everyone (It seemed quite unintuitive to me until I worked through it in my head, but now it makes perfect sense).

 

[mender]

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels.
That would be interesting, would it not? You could even infer it from still pictures of the thing at work. Just look at the relative angles / slack in each side of the belt.
You could prove me wrong or right. I dare you.

http://www.nalsa.org/:

Look at the side of the chain that has tension (the left side as we face the cart). Then look at the propeller pitch. The wheels are driving the propeller, not the other way around. You are proven wrong; thanks for daring!

Now that it has been established that you are wrong, go back and reread the careful explanations that were provided. Ask questions about things you aren't clear on; likely that's the point that you're going wrong.

I suspect that you're having trouble with the wheels being the power source. While that is true in the frame of reference that has the cart stationary and the ground moving, most people can't let go of the ground-based reference frame and that confuses them until they understand how the cart works.

In the ground frame, the evidence of the cart's ability to harvest energy from the wind while moving faster than the wind is in the speed of the air after the cart passes. If the air is moving slower relative to the ground, the cart has extracted energy from the wind despite the fact that it is moving faster than the wind.

The wheels provide the link to the ground; the prop provides the link to the air. As long as the cart has both a ground reference and an air reference, it isn't limited to the speed of either and can, with varying gearing and sufficient wind (or ground) energy, travel slower than the wind with a large amount of force or travel faster than the wind with a small amount of force.

You can't transfer momentum if there is no (positive) velocity difference so the system has to produce one in some way.

As long as the wind is blowing, there is a positive velocity difference for the cart (system) to extract energy from. The velocity difference is between the air and the ground, commonly known as wind.

 

[Pythagorean]

There's an element of truth to that though. The real problem is that many people mistake denialism for skepticism.

It's semantics about the definition of skepticism, but I agree.

Congratulations, btw.

 

[russ_watters]

Impressive creation/effort, spork.

 

[russ_watters]

As someone who'se knee-jerk reaction was also skepticism (despite being a sailor!), maybe I can help:

You seem to misunderstand the method of operation. The prop is not acting like a wind turbine - it is never extracting energy from the air, so to speak. It is acting as a propeller, pulling the cart forwards against the air. The energy source is the wheels - they are rolling against the ground.

Both wind turbines and sailboats move faster than the true wind and when I learned to sail as a kid I was very surprised to learn that the top speed of a sailboat was not directly downwind but was at an angle roughly perpendicular to the wind. And I didn't at first understand how a sailboat could travel nearly upwind in seeming violation of the "direction" from which the energy was provided. But this concept has an added twist:

A wind turbine is fixed to the ground and a sailboat moves against the water (though big ones also have relative wind indicators). But a DWFTTW device finds a way to operate with the wind being the stationary frame of reference and the ground being the moving frame. This is made clear by the fact that most, iirc, have to be pushed to get them started. They start from "stationary" in the frame of reference of the wind.

Most people on these forums know the rules: they know that kinetic energy is frame of reference dependent. But having never seen a device that selects the wind as the stationary frame of reference, it is very tough to visualize and the knee-jerk reaction is to reject the concept.

...it may actually be easier to visualize if you consider the examples where these have been run stationary on a treadmill.

 

[spork]

If the action of the prop were just like a conventional 'driven' propellor, you could do at least as well by driving the wheels - and that would be nonsensical, you must agree.

I don't agree. The prop and the wheels are acting on two different media - which happen to have a relative velocity to one another. That is key. The energy needed by the prop to produce a specified thrust is LESS than the energy that can be harvested by the wheels when the cart is pushed with that thrust.

Just because you have a system that works (and very impressive it is too!) it doesn't necessarily mean that your explanation is correct.

True enough - but it happens that their explanations ARE correct. You'd do well to ask questions about them and try to understand them, rather than to explain why they must be wrong.

I can find nothing in your explanation that goes further than a 'by its own bootstraps' argument and that won't do, will it?

Then you're not looking hard enough. Their explanations are correct.

I am trying to look at the system in more depth

It seems more like you're trying to explain away the explanations that correctly predict the cart's behavior.

You could resolve the matter once and for all by measuring the tension on both legs of the drive belt at the same time. The side with more tension in it would be the side that is transferring the power - i.e either from wheels to prop or prop to wheels...
You could prove me wrong or right. I dare you.

Done.

Impressive creation/effort, spork.

Thanks.

most, iirc, have to be pushed to get them started.

Every one we built will self-start in a tail wind.

 

[sophiecentaur]

@spork
What an interesting thread!
Thanks for your patience. I now 'have it'.
The points that I hadn't taken from initial descriptions were:
1. Without the prop moving, there is still windage', which would push the cart froward anyway.
2. The prop direction, when the wheels are going forwards, is such as to push air backwards.

If the prop had zero pitch, then forward motion could make it rotate because of the forward force on the chassis and the backward force of the wheels on the ground. Introducing some pitch into the prop blades will cause some net backward force on the surrounding air and a resulting reaction force.
When you, so competently and fully, answered my point about the force times speed I realized the significance of the gearing (and, presumably, the pitch of the prop) which gives a power difference between prop and wheel. It is this power difference that provides the acceleration to enable acceleration at speeds faster than the wind. Clearly, the more efficient / aerodynamic the cart design, the faster the cart can go before losses account for all this power difference.

How many different turbines have you tried? I should imagine that the pitch / rotation rate could be chosen for a really optimum performance. I also take your point that it should be even better in a tube.

I still have an idea that an airship could work with, rather than a drogue, a second turbine with different pitch. Again you should be able to get a force times speed imbalance between the two turbines but there would be 'slippage' on the 'retarding' turbine which is not a problem when wheels are used. Possibly a lighter than air machine with a paddlewheel dipping into water could do the job and break yet another speed record.

As for the last comment in the last post (#48) - if the cart wouldn't start from rest, I can see that it should never work (apart from a small factor of static friction).

 

[spork]

How many different turbines have you tried?

None. We've only operated this as a downwind vehicle to date. Thus we've only ever used the one propeller. We're considering building a turbine to demonstrate that it can go UP wind faster than the wind. The prop we have has been operated fixed pitch and was later converted to variable pitch.

I still have an idea that an airship could work with, rather than a drogue, a second turbine with different pitch.

I think you mean a prop and a turbine. You must have one of each. AND... you must have them in different air-masses. So you could put one up high and the other down low for example.

Again you should be able to get a force times speed imbalance between the two turbines but there would be 'slippage' on the 'retarding' turbine which is not a problem when wheels are used.

But we still get "slippage" with our prop - right?

if the cart wouldn't start from rest, I can see that it should never work (apart from a small factor of static friction).

Not necessarily true. We can set the prop to a pitch such that the cart won't self-start, but it works great once underway. Keep in mind that the entire prop is stalled while at rest (or worse yet, producing significant lift in the wrong direction at the roots). As we begin to move, the tips become unstalled first. As we move faster, the unstalled region grows toward the roots.

 

[PaulS1950]

The cart shown in the picture - is it a wind sail cart?
I have seen boats that use airfoils instead of sails - this looks like it could be the same thing.

 

[sophiecentaur]

None. We've only operated this as a downwind vehicle to date. Thus we've only ever used the one propeller. We're considering building a turbine to demonstrate that it can go UP wind faster than the wind. The prop we have has been operated fixed pitch and was later converted to variable pitch.

I have a feeling that the drive to an upwind vehicle would have to be reversed but, judging from my previous intuitions (!) I could be wrong. A 'force times speed' argument is more difficult for me to apply here.

But we still get "slippage" with our prop - right?

I can see that but i suppose that would just limit the power available.

Not necessarily true. We can set the prop to a pitch such that the cart won't self-start, but it works great once underway. Keep in mind that the entire prop is stalled while at rest (or worse yet, producing significant lift in the wrong direction at the roots). As we begin to move, the tips become unstalled first. As we move faster, the unstalled region grows toward the roots.

So a variable pitch prop would / could be a good thing. If it were feathered at rest then the wind would always push it forwards (?).

 

[A.T.]

But a DWFTTW device finds a way to operate with the wind being the stationary frame of reference and the ground being the moving frame. This is made clear by the fact that most, iirc, have to be pushed to get them started. They start from "stationary" in the frame of reference of the wind.

As spork said: They can start from rest relative to the ground:



The key of the physics is: Wind power always comes from reducing the true wind (velocity difference between air & ground). If there is true wind, then no matter how you move relative to airmass & ground, you always see the velocity difference and can potentially reduce it (harvest wind energy).

Here an animation showing the different reference frames and force vectors:

 

[kjl]

Nicely done video. I got a little kick out of the notion that you think it's unfakable. You try and make something foolproof, and I can bring you a better fool *every* time.

That's not my video - I do not think it is "unfakable" in that there is always somebody willing to believe a bigger trick or conspiracy. ( ) Where I think that video could help people to understand is that the wind->propeller->gear->wheels system is non-intuitive, especially as used in your cart. The little lego car vehicle, on the other hand, is quite easy to look at and visualize, and it has the same "wtf" brain bend. If you can understand why the lego car works, it is then easy to see why your prop cart works. (Replace the top gear with a worm gear with exactly the same gearing and now it is rotating the same direction as a propeller, and now it's an easy visualization jump from a worm gear to a prop).

As an added bonus, anybody with a basic technic set of Legos in their closet (which, frankly, any physics nerd should have :) ) can go make it, while not everybody can put together a version of even the little cart you used in the treadmill video.

Physics, and why even some very highly educated people substitute their own faulty intuition and call it physics.

To be fair, I think you'd have to admit that 1) intuition plays a very important part, even in formal science, to skip past all the stuff you've already proved to yourself to a reasonable degree (otherwise, you'd be stuck debunking perpetual motion machines all day long) and 2) the gearing in your cart is not very intuitive, and also triggers all the same red flags that perpetual motion machines do, though in this case they are false positives :) . What was most surprising to me, I think, was the vitriol and ridicule you were subjected to...


I am curious, though; when you first thought of the problem, did you initially intuitively believe downwind faster than the wind to be impossible or not?

As an aside, I hear you met my friend Garrett Lisi the other day - he coincidentally brought up your cart in conversation just this morning!

 

[rcgldr]

To go against the wind efficiently we have to swap our prop out for a turbine. I expect we might build one over the winter and try it next season. From 20' away you won't notice any difference at all, except I expect it will go directly upwind faster than the wind.

Depending on the range of pitch you can adjust on the prop, you may only need to increase the pitch to convert the prop into a turbine.

For an upwind vehicle, the pitch of the prop just needs to be increased so that the effective gear ratio is > 1, so that prop "wash" speed is greater than wheel speed. This will change the situation so the prop acts as an upwind turbine, driving the wheels, although I'm not sure how efficient the current prop will be if used in turbine mode. My guess is that a prop "wash" to ground speed ratio between 1.5x to 3.0x would be a good starting point. (The closer to 1.0, the faster the cart goes upwind, limited by the efficiency of the cart). Using the 3.0x as an example, say wind speed is 10mph, at a cart speed of 5 mph, the prop speed would be 15 mph, which would be the upper limit on speed with the 3.0x ratio with a 10 mph wind, for 0.5x upwind speed. With a 2.0x ratio and 10mph wind, upper limit would be cart speed of 10 mph, prop speed of 20 mph for a 1.0x upwind speed. If 1.5x worked, then upper limit would be cart speed 20 mph, prop speed 30 mph, a 2x upwind case. You need a 1.0 < ratio < 2.0 to achieve greater than wind speed upwind.

 

[robinson]

As we begin to move, the tips become unstalled first. As we move faster, the unstalled region grows toward the roots.

Have you thought about writing a book? Or at least a documentary? I would contribute to the cause. This is nerd heaven to me.

 

[Mercstein]

For an upwind vehicle, the pitch of the prop just needs to be increased so that the effective gear ratio is > 1, so that prop "wash" speed is greater than wheel speed. This will change the situation so the prop acts as an upwind turbine, driving the wheels,

Just to be clear, this cart, in its record breaking configuration (effective ratio < 1) will not travel forward when presented only with a direct headwind from standstill.

 

[mender]

That's because as configured it is a downwind vehicle. If placed in an "upwind" (in other words pointing the wrong direction), it will do what it was designed to do and move in the same direction as the wind, though not as effectively as when facing the proper way.

Just to be clear, this cart, in its record breaking configuration (effective ratio < 1) will not travel forward when presented only with a direct headwind from standstill.

And to be perfectly clear, the cart as configured will not travel forward into a headwind whether from a standstill or otherwise. It will always try to move with the wind; if towed into a headwind, it will promptly slow to a standstill and then go with the wind.