chaggle wrote:

spork wrote:Four things to keep in mind:
1) The prop never turns the wheels. The wheels turn the prop
2) Inertia doesn't factor into it. Even exactly at wind speed we can tow a trailer all day long.
3) When the vehicle is going directly downwind at wind speed, it feels no wind over it. The "prop" is also going downwind at wind speed. The blades of the prop however are not going directly downwind. The blades are following a spiraling path so they will always feel relative wind even when the cart is going downwind at wind speed - remember their tangential velocity.
4) When the cart is going the speed of the wind - it feels no wind. But that doesn't mean there is no wind. It simply means there is no relative wind. There's still wind moving over the ground, and that's precisely the energy this vehicle exploits. It leaves a path of slower moving air in its wake - having taken some of its energy.

So the vehicle is moving faster than the wind? In that case the wind "in its wake" hasn't reached it yet. How can it have taken some of its energy?

It helps me to imagine the same scenario, but on a windless day (a negative example). With the rig moving at wind speed (zero) there is no energy from the wind - there is no wind. The 'sails' have nothing to tack against. You can't slow the air down because it is not moving.

Then back to the rig moving with the wind on a windy day; I can now see the blades (sails) tacking across the direction of the wind, and the lift pushing the rig forward, turning the wheels that then rotate the blades (sails) that tack across the direction of the wind, air passing over the sails being slowed down (in relation to the ground)... as long as you have wind and are travelling downwind.

If you are travelling faster that the wind, think of the wind ahead of you as having energy waiting to be taken.

Maybe I am dumb, but you just said that the blades rotate the wheels which rotate the blades.

Hey Davian. Wanted to thank you for starting this thread. Some really thoughtful comments. As to your point about the apparent headwind being a possible source of energy, I asked about this literally years ago and was assured it was not. That would smack a bit of perpetual motion if you think about it. Like the cartoon of the guy using a leaf blower to run a turbine which is powering the leaf blower.

Onyx8 wrote:But there is no wind at the propellor, if there were then the ribbons would not be hanging down. It seems to me that it is advisable to keep the problem simple at first, so lets leave tacking out of the issue for now. If there is no air movement at the propellor, from where does it find the energy to drive the wheels? You can't tack if there is no wind.

With my very limited comprehension of the relevant physics involved, this seems to be a common error among those who don’t accept that DDWFTTW is possible. A man swimming downstream may go faster than the river but he will never outrun the water. Consider how the wind effects the airspeed of planes travelling at many times it’s velocity. If you can explain this I would be quite grateful.

reksio wrote:The vehicle should be able to accelerate forward in still air, on ground that moves backwards. And it is, just like those small models.

But is this really what is happening on the treadmill? If someone conducted a test on a long treadmill, such as those found in some airports and warehouses, and put the cart at the starting end with the conveyor turned off and then turned it on, we could see if the cart could begin to advance as a result of the apparent tailwind. Otherwise, could it not be argued that it is the KE created when you hold the cart down against the belt that drives it forward for a short time? Even if you restrain the cart’s forward motion with strings-as has been done-wouldn’t this tethering be similar to holding the cart down?

Another test I would love to see is putting a cart at the finish end of a long, moving belt while pressing it down-as in the videos you posted-and see if it would go all the way to the other end. I won’t hold my breath waiting though, as this would require remote control steering to prevent veering from one side to another. And yes, to whoever it was that asked about the cart veering; it does, and that’s why you would need to be able to steer it.

Basically I accept spork has proven DWFTTW is possible. Was just reading some of his more brilliant retorts back in the early days at RDF. That was all pre-Blackbird and El Mirage, of course. Given the margin of the results there I’d like to read Ony’s explanation of how/why he would argue this wasn’t sufficient proof of the basic claim. With recorded speeds nearly 3 times faster wouldn't it seem probable that say 1.3 times at least had been achieved?

It should be noted for newbies to this epic that about 2 years ago Nobel Laureate physicist Dan Kammen sent an email to a former student at Berkeley telling her that the cart going directly downwind, steady state, would violate COE. He had presumably seen the video of the cart on the treadmill, along with some of spork’s arguments that were sent to him by the student for comment.

Also, spork was recently in contact with a physics professor in Georgia-one Dr. Camp-who had told a science writer chum at Wired (?) same thing as Kammen said; conservation of energy problem. Rick got Camp to back up a bit on that assessment via a phone call, and to make an arguably equivocal statement that DFTTW was perhaps possible. (His family was released right after. ) However, like macdoc he considered the treadmill irrelevant. Then there’s Mark Drela, finally stating publicly that the treadmill alone was proof Rick was right. Just sayin… brain tormentor might be a better tag for this topic.

I'll propose something simple, just to see if everyone can accept it.

"There is no violation of any physical laws, in principle, for a wind-powered vehicle to have an average downwind-speed which is greater than the wind speed."

I'll give a simple (and very impractical) vehicle design to show why this is the case.

THE ALMOND SILLY VEHICLE MK 1

The vehicle has a set of retractable wind turbines. When retracted, the wind turbines occupy negligible space, so do not cause any drag. When deployed, the wind turbines provide electrical power, which is used to charge a battery. The wind turbines can be retracted and deployed in a negligible amount of time, and the retraction and deployment processes use negligible energy. The wind turbines and the system used to retract or deploy them have negligible weight. The vehicle has wheel brakes, and if necessary can be assumed to have any other devices required to hold it stationary on the ground against any applied force. (e.g. assume it can drive spikes into the ground if you need to.)

The vehicle starts stationary with its wheel brakes on, together with anything else needed to hold it in place. The wind turbines are deployed. For some short time, TC, the wind turbines are used to charge the battery. The vehicle then retracts its wind turbines and accelerates very quickly, travelling at a high speed, V, for time TM, after which the battery is depleted. The vehicle uses its wheel brakes to stop quickly, deploys its turbines to charge the battery and so on.. The cycle repeats.

Now, one objection is going to be: "But you have to stop and charge the battery!" Yes, but the time taken to recharge the battery, for any speed V and time travelling at that speed, TM, can be made as small as you like by adding more wind turbines. Let's take this to an extreme. Suppose you want the vehicle to be stopped for only 1 second to charge the battery, and then to travel for one hour at a speed of 1,000mph under electrical power, before stopping for its next charge. In that 1 second you have to get enough energy from the wind turbines to power the vehicle for one hour of moving at 1,000mph. Suppose this amount of energy is E. If one wind turbine provides Q, then you merely need E/Q wind turbines. You might imagine a vehicle which deploys a billion wind turbines, for one second, to get its recharging done very quickly.

This means that, even if you include the time that the vehicle is stationary and recharging, the average speed of a wind powered vehicle over a long course can, in principle, be as high as you like (ignoring Einstein's relativity).

but do we need to stop? I will now propose a slightly different design.

THE ALMOND SILLY VEHICLE MK 2

This is similar to the MK 1 vehicle (above), except that the vehicle does not stop completely during the recharging process: it merely slows down so that its speed is slightly less than the wind speed. To recharge, the braking system is used, and the vehicle slows down so that its speed is slightly less than the wind speed. The wind turbines are deployed. The wind turbines can generate electricity because there is a very slight wind speed relative to the vehicle. The battery is charged while the braking system prevents the vehicle from accelerating. When recharging is complete, the braking system is turned off, the wind turbines are retracted, and the vehicle accelerates under battery power, travelling at high speed until its battery is used up, when the cycle repeats.

The only difference here is that more wind turbines will be needed. For example, say that we want a speed of 1,000mph for one hour between charges, the wind speed is 15mph and we want the vehicle to travel at 14mph during recharging, with recharging taking one second. We will only have a wind speed relative to the vehicle of 1mph during battery charging, and we only have 1 second to get enough energy for 1 hour of travel at 1,000mph - so we will need a lot of wind turbines. The point is that any issues here are just practical ones: I don't see any violations of conservation of energy, for example, here.

This means that the average speed of a wind powered vehicle over a long course can, in principle, be as high as you like (ignoring Einstein's relativity), if the vehicle spends at least some time, which can be arbitrarily short, travelling below the wind speed - though this can be as close to the wind speed as you want. This does not mean that this is a requirement - merely that it should be trivially apparent that it is possible to do this.

Is any of the above contentious?

recursive prophet wrote:Hey Davian. Wanted to thank you for starting this thread. Some really thoughtful comments. As to your point about the apparent headwind being a possible source of energy, I asked about this literally years ago and was assured it was not. That would smack a bit of perpetual motion if you think about it. Like the cartoon of the guy using a leaf blower to run a turbine which is powering the leaf blower. <snip>

You're welcome. I was surprised that there wasn't one already in progress.

I did not mention the apparent headwind - the wind that passing over the rig due to it travelling faster than the air surrounding it - but was referring to the air(wind) travelling ahead of the rig in the downwind direction (of course) that carries the kinetic energy needed to tack against in the very near future, as the rig moves forward faster than the air is travelling over the ground.

Although the apparent headwind could turn a fan attached to a generator that could power your MP3 player....

Oldskeptic wrote:Every part of my common sense tells me that this can't happen, but my skepticism runs into a wall when reality shows my common sense to be somewhat unreliable.

Good man! It's important to be skeptical - but it's just as important to be open-minded and consider real-world evidence. The job of science is not to make things behave as we think they should, but to try and explain what we observe.

I get the feeling I'm being sold a vacuum cleaner that I don't really need, but I'm going to buy it anyway, because it looks really neat and cool and stuff, and it works pretty well — and the salesman seems like such a nice fellow. Oh, and I love how he did that thing with the floating ball!

Paul Almond wrote:THE ALMOND SILLY VEHICLE MK 1

That works fine.

THE ALMOND SILLY VEHICLE MK 2

So does that. But what I've proposed before to explain this might be called the Almond Silly Vehicle MK 3.

In this case I only have two wind turbines. I plant one turbine in the ground in at the front of my vehicle and get power from it while I drive past it at 2X wind speed. When it reaches the back of my vehicle, I yank it out of the ground and shuttle it back to the front of my vehicle (feathered), while the other turbine does the same thing. In this way I can run steady-state faster than the wind.

Onyx8 wrote:That's fine, I'm skeptical and could be wrong, it's happened before.

That's good, it's a big step. Your posts up until now have suggested that you could not be wrong. As I said before, it's good to be skeptical. But it's not good to deny basic analysis and physical evidence. If you don't buy this, you can make your own. That's right, for about $40 you can make a working model based on the build videos I posted on YouTube. Many people have done just that. I can't prove to you that I haven't tampered with the video and the data, but I can prove I haven't tampered with the cart you build with your own hands.

It just seems to me that honest questions get avoided, misinterpreted, or not answered, often things seem to be intentionally complicated in an apparent attempt to obfuscate.

It may seem that way, but it's not the case. I can tell you what is the case - people that are sure it can't work simply don't accept the answers given. They're too entrenched in their intuitive (mis)understanding of this thing. So it's not the questions that are misinterpreted, but the answers. In fact I'll make you a deal... I'll answer all your question. If you don't like any of the answers, just say why. But keep in mind "Bullshit!" is not a question.

When the ribbons are hanging straight down where does the energy come from to accelerate the vehicle?

Well, there's sort of an easy answer and a hard answer to that. You need to understand that this vehicle exploits the energy available at the ground/wind interface. Unlike a sailboat running straight downwind, it doesn't require a relative wind over itself (although it does need a relative wind over it's airfoils - which happen to be spinning). This vehicle in many ways acts as a very simple lever between ground and air. It trades a little force over a large distance (the distance it moves over the ground) for a larger force over a smaller distance (the distance it moves through the air).

Now the harder answer. Energy is a weird thing. It's not something tangible. An object does not have some amount of energy in the same way it has some amount of mass. The energy a body has is entirely dependent on the frame from which you choose to analyse it. When the cart is going downwind at exactly wind speed, the ground is moving under its wheels at wind speed. In this case (in fact in all cases as far as I'm concerned) the best frame to work from is the frame of the cart itself. In that frame it gets it's energy from the ground. Yes, I know it may sound wrong. We can go into that in more detail if you like. I assume you have no trouble believing the model cart gets its energy from the treadmill belt. Same thing exactly.

If this is a natural thing for a spinning propellor then why do we not see this in everyday life, for instance if I manually spin a propellor why doesn't it access this energy source and continue spinning indefinitely?

In our case the propeller is geared to the wheels in such a way that it can exploit the energy available at the interface between two media moving relative to one another. I freely spinning propeller can't do that - although a propeller spinning on a threaded shaft can.

I'm satisfied that the idea being discussed here is viable, that the people declaring it to be impossible are making a mistake, and that these people have almost certainly made a vehicle that does what they say it does. Having considered the situation, and read about the design, it just seems obvious that it does not present any real issues.

EDIT - I was actually going to suggest some analogy like the MK 3 vehicle that you mentioned, spork, to show that it is trivially possible in principle to do something like this.

and that's it for me. I'm withdrawing from this discussion now, so won't be justifying my position here - nothing bad that anyone has done - simply that it will take a long time and will be about a matter that I find rather trivial - and it really is one of cognitive bias that people have: it will be a bit like spending three days discussing something like the Monty Hall problem. I've no intention of getting into further discussions on this - it will just be a huge waste of time.

People insisting that it is impossible are getting caught in cognitive traps and will look silly later, though. My last words are to urge such people to reflect deeply on this matter before it is too late. Remember that you were warned! By one of the finalists in the 2010 Orson awards!

Aca wrote:not this again:D

There are more than 20 threads on talkrational about this and if i recall there was a thread on RDF too

There is a thread on just about every forum on the web on this. The people who do this are nothing if not shameless self publicists.

That said it is possible to do this but it in no way does it constitute anything more than ordinary physics where extra energy comes from the set up of the propeller/nature of the angle of incident wind and or method of set up not from some magical land.

CdesignProponentsist wrote:
From the frame of reference of the vehicle the wind behind it is moving faster. But from the ground frame of reference it has slowed the wind behind it.

Davian wrote:
If you are travelling faster that the wind, think of the wind ahead of you as having energy waiting to be taken.

Two important observations, that might help others with the understanding. Although I would replace the word "wind" with the word "air" in both.

EdwardTeach wrote:The people who do this are nothing if not shameless self publicists.

Yup. You're onto us. It's all about the fame, money, and downwind groupie chicks that throw themselves at us. It's been a HUGE wind for us in all those ways.

The_Metatron wrote:Bullshit.

I've been attempting a discussion with Metatron by PM, but I've gotten no more than the same level of scientific rigor that he displays here. More disturbingly to be honest is his astonishingly poor reading comprehension.

I'm still happy to answer genuine questions from people that are actually interested in how this thing works.

And it's NOT perpetual motion. It's a wind powered craft. So it's kind of a losing position when people assure me I haven't discovered a way to get around the laws of physics.

spork wrote:

EdwardTeach wrote:The people who do this are nothing if not shameless self publicists.

Yup. You're onto us. It's all about the fame, money, and downwind groupie chicks that throw themselves at us. It's been a HUGE wind for us in all those ways.

When is the MythBusters appearance due?

spork wrote:It's all about the fame, money, and downwind groupie chicks that throw themselves at us.

Look spork, some other guys who cannot stand it when someone says "It can't be done":
http://www.mymodernmet.com/profiles/blo ... g-house-14

spork wrote:<snip>

I'm still happy to answer genuine questions from people that are actually interested in how this thing works.

And it's NOT perpetual motion. It's a wind powered craft. So it's kind of a losing position when people assure me I haven't discovered a way to get around the laws of physics.

How well does this concept work with watercraft? has it been tried? I can see trouble there.

Has anyone built a hybrid vehicle, sort of combining Greenbird and Blackbird, so you could go fast with and across the direction of the wind?

And as mentioned in the Wired Article, the next thing is to see how fast it can be made to drive into the wind. How goes that? I read the wiki page on this and there was no mention of it.

Onyx8 wrote:Maybe I am dumb, but you just said that the blades rotate the wheels which rotate the blades.

You're not dumb, just stuck in a certain way of thinking (linear cause-effect-reasoning) that simply fails at feedback loops. It might help you to consider simpler examples of such mechanical loops:

[youtube]http://www.youtube.com/watch?v=k-trDF8Yldc[/youtube]

As you see: the ruler pushes the cart forward, that turns the small reels, they turn the blue wheel which drives along the ruler, which pushes the cart forward even more, and that turns the small reels... an so on.

The result is that the cart goes faster than the ruler that is pushing it.

Davian wrote:
How well does this concept work with watercraft? has it been tried? I can see trouble there.

You replace the wheels with an underwater turbine, which is less efficient. Also more difficult due to hull drag. Maybe with hydro foils.

Davian wrote:
And as mentioned in the Wired Article, the next thing is to see how fast it can be made to drive into the wind. How goes that?

See this post:
http://www.rationalskepticism.org/gener ... ml#p747722

And this paper:
http://orbit.dtu.dk/getResource?recordI ... ersionId=1

reksio wrote:

Onyx8 wrote:ignoring observations like chaggle just made.

chaggle's "observation" is just a misconception about what "wind" is. Wind means that the entire air mass in the area is moving relative to the ground. The wind doesn't have to "reach the vehicle" as chaggle puts it, because the vehicle is always immersed in air that moves relative to the ground (a.k.a. wind).

reksio wrote:

CdesignProponentsist wrote:
From the frame of reference of the vehicle the wind behind it is moving faster. But from the ground frame of reference it has slowed the wind behind it.

Davian wrote:
If you are travelling faster that the wind, think of the wind ahead of you as having energy waiting to be taken.

Two important observations, that might help others with the understanding. Although I would replace the word "wind" with the word "air" in both.

Why not be more specific and avoid using such confusing terms altogether?

The airmass has a speed and direction relative to the ground W
The vehicle has a speed and direction relative to the ground Vg
The vehicle will also have an speed and direction relative to the airmass Va

For any given W and Vg you should always be able to determine Va.

Unless Vg is exactly the same as W, the vehicle will be moving within the airmass, so Va will have a speed and direction, and so, in the same way that aircraft can be observed flying towards or away from cumulus clouds (which move with the airmass), the vehicle will either 'reach' or 'outpace' parcels of air in the airmass.

You should then be able to explain more simply, how Va can be used to provide energy to move the vehicle at a speed and direction where the downwind speed component of Vg exceeds the speed component of W.