Ride Like the Wind (only faster)
October 11, 2011 12:13 PM   Subscribe

Three years ago, a question was posed to two Internet forums. Could you build a wind powered vehicle that could travel downwind, faster than the wind? The lines were quickly drawn and the battle was on, including here on the blue. It took nearly two years for the debate to be settled, and on July 2, 2010, what seemed impossible was achieved. The answer is yes, you can.
posted by smoothvirus (96 comments total) 17 users marked this as a favorite

 
Oh god. Not again. 50 drachma fine if you don't read every post linked before claiming you know why it won't work.
posted by Mei's lost sandal at 12:15 PM on October 11, 2011 [1 favorite]


Faster than the wind when? The wind rarely remains at a constant speed for long.

"wind-powered" how? I presume wind-powered electrical generators are ineligible... but what about wind-wound springs?
posted by LogicalDash at 12:18 PM on October 11, 2011


I should note that when this was here previously, I had the principle exactly correct, but I had the mechanism completely backwards.

Here is how my explanation should have read:

It took me a bit to realize why, but it works, and here's how:

Nobody disputes that by sailing crosswind, a sailboat can sail faster than the wind. This is because the wind is hitting the sail at an angle.

The propeller blades are angled. The wind isn't flowing over the blades straight on, it's flowing over pushing on them at an angle, just like the sailboat sailing crosswind. The extra power is transferred to the wheels thrust, et voila.


I had the sailboat analogy correct, but the power isn't transferred from the propeller to the wheels, it's the other way around. The wind is pushing on the surface of the prop, not flowing around it.
posted by smoothvirus at 12:18 PM on October 11, 2011


LogicalDash- Invoice will be forthcoming.
posted by Mei's lost sandal at 12:19 PM on October 11, 2011 [10 favorites]


Yeah, and how are we measuring "wind speed" anyway? You can set up a spinner and measure how fast it spins, but you'll get different readings depending on how far from the edge you are measuring. Probably not significant unless the spinner is the size of that damn car.
posted by LogicalDash at 12:23 PM on October 11, 2011


huh, I missed this debate the first time around. How did y'all survive without my halfassed explanations based un misremembered highschool physics lessons?
posted by Think_Long at 12:25 PM on October 11, 2011 [9 favorites]


I read this totally wrong and thought it was about making a wind-powered vehicle that could travel directly INTO the wind.
posted by EndsOfInvention at 12:26 PM on October 11, 2011 [3 favorites]


I thought sure this article had been posted previously. I can't find it though, and I'm glad you did. It's a great story, and another great example of Wired's writing talent (which has been getting a lot of MeFi play recently).
posted by Popular Ethics at 12:26 PM on October 11, 2011



I read this totally wrong and thought it was about making a wind-powered vehicle that could travel directly INTO the wind.

It will travel directly INTO the wind if you change the camber of the propeller blades.
posted by smoothvirus at 12:31 PM on October 11, 2011 [3 favorites]


Incredibly long and excruciating thread here (SailingAnarchy) between the developers and the un-believers.
posted by Mei's lost sandal at 12:31 PM on October 11, 2011


...and the mass exodus from Dubstep Dancing Guy begins.
posted by griphus at 12:32 PM on October 11, 2011 [1 favorite]


Yeah, and how are we measuring "wind speed" anyway?

You owe Mei's lost sandal 100 drachma. They're comparing it to a reference balloon, as the articles make perfectly clear.
posted by atbash at 12:32 PM on October 11, 2011 [3 favorites]


How big?
posted by LogicalDash at 12:33 PM on October 11, 2011


How fast does this thing need to get going to obtain self-sufficiency? From what I read, just putting it on the ground won't get it moving, it needs to be traveling at speed first.

Although, from what the writer said about a treadmill, perhaps it is always trying to get going. After all, if you place it on a treadmill, turn the treadmill on and the thing starts moving forward, not being on a treadmill shouldn't matter either.

In other words, I like the idea, I just wish the author of the wired piece could write at a high school level. I found the entire article disjointed and confusing.
posted by Hactar at 12:35 PM on October 11, 2011


Oh just read the damn article.
posted by mrzarquon at 12:36 PM on October 11, 2011 [2 favorites]


From the links, here's short video of the thing in motion.
posted by Nelson at 12:36 PM on October 11, 2011


Or this video, which shows the whole shebang: The vehicle starting from a stop, accelerates, and then encounters a relative headwind (meaning it's travelling faster than the wind).
posted by Popular Ethics at 12:40 PM on October 11, 2011 [1 favorite]


atbash: “You owe Mei's lost sandal 100 drachma. They're comparing it to a reference balloon, as the articles make perfectly clear.”

How on earth is asking a question the same as claiming you know why it won't work?

Oops, I guess I just lost 100 drachma too.
posted by koeselitz at 12:41 PM on October 11, 2011


See? We can work these things out.

Now, I'll get started on the conveyor belt if someone wants to wrangle up a 747.
posted by bicyclefish at 12:46 PM on October 11, 2011 [1 favorite]


"Either a thing is true or it isn’t. If it is true you should believe it and if it isn’t you shouldn’t. And if you cannot find out if it is true or isn’t you should suspend judgement." - B. Russell

So we have working mathematical models, working toy models, and working people moving versions. And people still don't believe.
posted by Chekhovian at 12:48 PM on October 11, 2011 [8 favorites]


koeselitz- OK, OK you got me, LD will get a refund.
posted by Mei's lost sandal at 12:49 PM on October 11, 2011


Discussing windspeed is really a red herring when considering using the wind to travel faster than the prevailing windspeed. Think of it instead as harnessing a given amount of energy, and then overcoming the drag associated with flying a sail.

Sailors will not be surprised. The fastest point of sail is a beam or broad reach (depending on the boat) or heading downwind between right agles to the wind to almost with the wind. Counterintuitively it's faster than a flat out run - sailing in the direction of the wind - as you're able to maximize the force of the wind while minimizing your wind resistance. Hell, you can sail upwind - a term known as 'beating'.
posted by jimmythefish at 12:49 PM on October 11, 2011 [1 favorite]


Popular Ethics, I get it now, thanks.

I still maintain that the author should have hired an editor. Or wired should have just used one of their regular people. We know they can hire competent writers, it's sad that they didn't in this case.

On preview, jimmythefish, you just answered my last questions. Thanks.
posted by Hactar at 12:50 PM on October 11, 2011 [1 favorite]


Popular Ethics: “The vehicle starting from a stop, accelerates, and then encounters a relative headwind (meaning it's travelling faster than the wind).”

Ah. So it's not so much a "brain teaser" or a "science problem" so much as a "deceptively-worded question designed to trick people so you can act like they're dumb." Hey, I've got another one – do you think I can throw a ball and have it come back to me? If you think I can't, you're wrong, because I threw it directly up in the air! HA!
posted by koeselitz at 12:51 PM on October 11, 2011


It's a pretty deep brain teaser, not just a poorly worded trick. Many many sailors and physicists vowed it was not possible.
posted by Mei's lost sandal at 12:53 PM on October 11, 2011 [1 favorite]


"Place a DDWFTTW vehicle on a garden-variety treadmill set at, say, 10 mph and if the vehicle moves forward against the treadmill it is moving faster than the wind."

This. Would be. The single greatest toy to purchase from a science museum. Ever.

Send me some drachma if you start production.
posted by pokermonk at 12:53 PM on October 11, 2011


What if the treadmill were aboard a 747?
posted by Mei's lost sandal at 12:54 PM on October 11, 2011 [4 favorites]


Was.
posted by Mei's lost sandal at 12:55 PM on October 11, 2011


"were" is correct.
posted by kenko at 12:57 PM on October 11, 2011 [2 favorites]


What if the treadmill were aboard a 747?

Surely the snakes would interfere.
posted by jimmythefish at 12:57 PM on October 11, 2011 [8 favorites]


Discussing windspeed

So, i'm far from a scientist, but couldn't a controlled environment like a huge hangar with fans on one end be used to make it so the 'wind' is constant, and then easily measured? Maybe i just watched too much mythbusters or something. ;)
posted by usagizero at 12:57 PM on October 11, 2011


"were" is okay there, I think. "Was" is conditional, "were" is subjunctive.

Mei's lost sandal: “It's a pretty deep brain teaser, not just a poorly worded trick. Many many sailors and physicists vowed it was not possible.”

There were actually sailors who claimed that the wind never blows on their faces when they're standing on the prow? I have a feeling those sailors just didn't understand the question.
posted by koeselitz at 12:58 PM on October 11, 2011


This. Would be. The single greatest toy to purchase from a science museum. Ever.

I very nearly went home the evening this was on MeFi to build one for myself, out of model airplane parts and coat hangars. I kind of wish I had because I would have found out very quickly that I had it backwards and what I believed was a windmill was in fact, a propeller.
posted by smoothvirus at 12:59 PM on October 11, 2011


Here is how I understand it:

They tow it up to speed. They have a generator on the wheels which turns the blades while slowing down the vehicle, which then drives the vehicle forward.

It seems to me that they are imparting kinetic energy to the vehicle from towing it which then gets converted to electrical energy, so it's not really entirely wind powered, is it?
posted by empath at 1:00 PM on October 11, 2011


No, the sailors just claimed the boat would never make that wind in their face if the boat was travelling exactly the same direction as the wind.
posted by Mei's lost sandal at 1:00 PM on October 11, 2011


empath- nope. read up.
posted by Mei's lost sandal at 1:01 PM on October 11, 2011


For the confused: imagine it at wind speed in the frame of reference of the wind.
The cart has velocity zero, there is no wind. The earth is moving under the cart at wind speed. There is a generator hooked to your wheels. The energy for the generator comes from slowing down the earth, and the two of you split the change in momentum. But, the entire change in energy goes entirely to powering your fan, so you can speed up. The key difference from free-energy at rest is to remember that the earth/treadmill is moving underneath you, and you extract energy from slowing it down.
posted by a robot made out of meat at 1:03 PM on October 11, 2011 [1 favorite]


Mei's lost sandal: “No, the sailors just claimed the boat would never make that wind in their face if the boat was travelling exactly the same direction as the wind.”

So, again – sorry, but I want this to be clear – you're saying these sailors claimed the wind never changes?
posted by koeselitz at 1:04 PM on October 11, 2011


They tow it up to speed.

Man, it's no wonder the drachma is so deeply devalued.
posted by mhoye at 1:05 PM on October 11, 2011


Here is how I understand it:

Did you read/watch ANYTHING other than the text of the FPP?
posted by ShutterBun at 1:05 PM on October 11, 2011


It's a thought experiment. We assume the wind never changes as a condition of the experiment.
posted by Mei's lost sandal at 1:05 PM on October 11, 2011


Hey, I've got another one – do you think I can throw a ball and have it come back to me?

To jump off this analogy... I think it's as if most people were thinking about curve balls while ignoring the thing that makes it curve, and even the people who were thinking about the Magnus effect figured there were hard limits to how much Magnusing it could do. But then some guy proved he could make a curve ball come back to himself by extending the effect until it did exactly that.

And then everyone was all like NO WAI.
posted by pokermonk at 1:06 PM on October 11, 2011


you're saying these sailors claimed the wind never changes?

Nobody has said that. "Goes fast in the wind and then the wind stops and it keeps going haha" is not what anyone is asserting here.
posted by mhoye at 1:06 PM on October 11, 2011


deceptively-worded question designed to trick people so you can act like they're dumb

No, it's really not. The vehicle starts from a stop (so, wind is coming from the rear), accelerates to the prevailing wind speed (no wind), and then will continue to accelerate, to the point where the wind actually seems like it's coming from the front (apparent wind from the front). So it's traveling faster than the prevailing wind speed, directly downwind.

That's not some sort of trick, it's actually a pretty interesting feat. It's not a sleight of hand or tricky question. A conventional sailboat cannot do that. Yes, you can develop apparent wind from the bow, but it doesn't come from exactly the direction you're moving in. (A whole lot of engineering goes into trying to minimize the angle between the wind and the direction of travel you can achieve.)

The vehicle works, once it has reached the wind speed, by exploiting the difference in wind speed between the air just above ground level and higher up. At least that is my understanding of the current design. Whenever you have a fluid moving at two different speeds, there is the potential to extract energy from them. It's a pretty neat implementation and frankly I'm stunned that, in the years that the whole argument has been going on, that so many intelligent people get so stubborn about believing that it's some sort of trick/joke/impossibility.
posted by Kadin2048 at 1:09 PM on October 11, 2011 [1 favorite]


It's a thought experiment.

For the love of God, People! It isn't a thought experiment! It's a real thing that real people built, attempted and demonstrated!" And the result as captured on video and observed by many people was that yes, with the right equipment you can drive directly downwind, in a steady wind, approximately three times faster than the wind.

If something demonstrably, repeatably happens, the fact that you don't understand how it happened does not retroactively make it unhappen!
posted by mhoye at 1:12 PM on October 11, 2011 [12 favorites]


Kadin2048- I'm pretty sure it's not the differing wind speed at elevation they are extracting energy from (though that can be huge on a tall rig). They are extracting energy from the difference in speed between earth and air, as a robot made out of meat notes above.
posted by Mei's lost sandal at 1:14 PM on October 11, 2011


As an aside, isn't this also the same basic principle that modern in-flight windspeed meters use to normalize against the rate of travel?
posted by atbash at 1:14 PM on October 11, 2011


But how does this relate to our DDWFTTW cart? Let’s not worry about how the cart gets up to speed for now, but instead see what happens if we tow it up to speed. For this purpose we’ll tow it to 55 feet/second (about 37.5 mph), and we’ll do that in wind moving 44 feet/second wind (about 30 mph). Of course with a tailwind of 44 feet/sec the cart only feels an 11 feet/sec (7.5 mph) headwind when it goes downwind at 55 feet/second.

Are they not towing it? It seems that they are. I asked because the article is confusing.
posted by empath at 1:15 PM on October 11, 2011


mhoye you are not paying attention if you think I am saying it is not possible.
posted by Mei's lost sandal at 1:16 PM on October 11, 2011


Kadin2048: “It's a pretty neat implementation and frankly I'm stunned that, in the years that the whole argument has been going on, that so many intelligent people get so stubborn about believing that it's some sort of trick/joke/impossibility.”

It probably doesn't help that the people advocating for this idea are like this guy:

“Rick, being an aficionado of brainteasers, realized this could be a great one. If this principle could be used to make a wind-powered vehicle that can go directly downwind faster than the wind without tacking, it would really bend some brains. So he gave it some thought.”

Note to all interested: science isn't what happens when you set out to "bend some brains" or play a "gotcha" trick on people. Science is what happens when you're good enough at understanding things to explain them to people so they will understand, too. Just going by that Wired article (which really is awful, and I'm working through the other links now) the problem here may be some stubbornness, but it's also a distinct inability to explain things clearly on all sides.
posted by koeselitz at 1:16 PM on October 11, 2011 [1 favorite]


What an elegant hack! I don't think it would work in water because of drag losses, but we know sailing craft can go faster than wind by getting the correct wind angle on the sailplane, so they just put up "sails" around an axle set to the correct angle. They spun, they tie that spinning axle to a wheel, and away it goes.

Needs a better transmission, I think -- the start is very slow, the classic answer to lack of power is to gear it down to trade speed for power.

I'd never encountered the problem before, and when I first read it, it was obvious that it could be done -- the energy to do so is clearly there, or no sail craft could ever travel faster than the wind, right? At this point, it's a simple matter of finding the best way to capture the power.

Now, they might need a different prop pitch, but I'll bet they could flip the chain and use that same vehicle to go *upwind*, and given they got 2.8x windspeed, I'm pretty sure with the right pitch (and possibly a geared transmission) they could go upwind faster than the wind in the same or a similar rig.

Fundamentally, it efficiently captures wind power and sends it to wheels. As long as they keep the prop pointed into the wind, it would provide power to the wheels. If they could put the prop on a pivot and get enough efficiency, they could go any direction.
posted by eriko at 1:21 PM on October 11, 2011


Yeah the Wired article is pretty bad. I think some of the confusion in the thread is because the Wired article contains a number of analogies that do not describe the actual experimental apparatus. E.g., the whole thing about a vehicle that is towed up to a certain speed and let go is not what actually was done. That's just a gedankenexperiment that the author uses to (poorly) illustrate how the craft works.

An actual description of one of the trial runs is here (PDF). If you look at the graphs, you can see that the craft's ground speed, as measured by GPS, begins at zero. The wind speed is not directly shown, but the ratio of craft speed (over ground) to wind speed is, and it peaks at 2.8. That means that at one point during the trial run, the craft was moving directly downwind (well, within 5-10 deg, which is considered "directly") at 2.8 times the wind speed. That's pretty impressive.
posted by Kadin2048 at 1:21 PM on October 11, 2011 [3 favorites]


A conventional sailboat cannot do that.

OK, this statement is contrubuting to the basic problem - that the problem description somehow leads people to make false assumptions about limitations to the solution.

The direction of the vehicle (whether boad, sled etc) is certainly part of the problem description, but only tangentially related to the solution to the problem. The solution to the problem is to harness a given amount of energy and use it to propel the vehicle at a speed that's faster than the prevailing windspeed.

As I said above, the current windspeed measure is a bit of a red herring in considering the solution. It has little to do with the potential that the wind has to propel something. If you think of the problem instead as harnessing a given amount of energy vs. overcoming air resistance. There is a tremendous amount of available energy in the wind.
posted by jimmythefish at 1:22 PM on October 11, 2011 [1 favorite]


Just to note- a robot made out of meat's explanation above is a perfect distillation of how the thing actually works (apart from there being no generator, just a direct link to the fan). Now I have to get back to work!
posted by Mei's lost sandal at 1:22 PM on October 11, 2011


The vehicle works, once it has reached the wind speed, by exploiting the difference in wind speed between the air just above ground level and higher up. At least that is my understanding of the current design.

Not quite. It's more like a feedback loop which uses an external energy source (in this case: moving air) and whose ability to accelerate approaches infinity as the efficiency of its components approaches 1. The prop blades act as both a sail and a propeller, and the wheels act as both forward momentum as well as propeller-spinners.

Here's a (previously linked) video of the same principle done on a treadmill, where it is actually climbing uphill. (warning: spork)
posted by ShutterBun at 1:22 PM on October 11, 2011


Note to all interested: science isn't what happens when you set out to "bend some brains" or play a "gotcha" trick on people. Science is what happens when you're good enough at understanding things to explain them to people so they will understand, too.

No. That's not science, that's teaching.

The other thing, where you take knowledge you've got and build something, that's engineering, which is what happened here. The principles involved were previously determined through science, and though plenty of people apparently don't understand them, they were already known ahead of time. The people involved took extant scientific theory and used it to build something really, really neat.

Science is the cycle of coming up with a hypothesis to explain some observation, formulating a way to test it (probably using some form of engineering), executing the test, and modifying the hypothesis. You do all of these things in a loop until the hypothesis matches the observations, or, more often, somebody else comes up with a better hypothesis and eats your lunch.
posted by atbash at 1:23 PM on October 11, 2011 [5 favorites]


OK, this statement is contrubuting to the basic problem - that the problem description somehow leads people to make false assumptions about limitations to the solution.

Exactly. If the problem doesn't specifically state "sailboat", then one is not required. One of the fundamental rules of solving problems is to ask "What problem are you trying to solve?"

Once you realize the claim isn't limited to sailboats, you've opened up many more ways to attack the problem.
posted by eriko at 1:24 PM on October 11, 2011


IT CANNOT BE DONE
posted by chavenet at 1:24 PM on October 11, 2011 [1 favorite]


Could you build a wind powered vehicle that could travel downwind, faster than the wind?

Sure. Larry Ellison's sailboat, which is taller than the Statue of Liberty and uses a wing instead of a sail, goes 2.5 times faster than the wind.

sailing craft can go faster than wind by getting the correct wind angle on the sailplane, so they just put up "sails" around an axle set to the correct angle. They spun, they tie that spinning axle to a wheel, and away it goes.

Exactly.
posted by The World Famous at 1:26 PM on October 11, 2011


The World Famous- I tried to get away, but no USA17 will not go faster than the wind directly downwind. That's why they tack downwind.
posted by Mei's lost sandal at 1:29 PM on October 11, 2011


Also, iceboats go a hell of a lot faster than the wind.
posted by The World Famous at 1:29 PM on October 11, 2011


but no USA17 will not go faster than the wind directly downwind.

It will until drag from the water slows it down. Put it on blades on a frozen lake, get it up to speed, and then turn directly downwind and it will take a good long time before it slows down to wind speed.

Putting the blades on a rotating axis effectively means that the craft is tacking all the time.
posted by The World Famous at 1:31 PM on October 11, 2011


We did do a good job designing the foils on USA17 I'll grant, but not that good ;)
posted by Mei's lost sandal at 1:33 PM on October 11, 2011 [4 favorites]


Also, iceboats go a hell of a lot faster than the wind.

But not *directly* downwind. That's where this vehicle is genius. By mounting the "sails" on an axle, they are always at a tack to the wind, as you've pointed out.

Some day, when I have a kid who's ready to start investigating the world for him / herself, I'm going to make that little treadmill toy and ask him to explain how it works. (Hopefully s/he'll come up with a clearer explanation than I have. More likely I'll get "I dunno").
posted by Popular Ethics at 1:44 PM on October 11, 2011


Whew! I was wracking my brain trying to recall, and if not recall, reconstruct, what I might have said on the subject in the past. I can get quite mouthy and opinionated, and sometimes wrong.

So I checked. I complained about the length of the video. Whew. And I am still right - seven minutes is too goddamn long to show what can be shown in twenty seconds. I am an early facilitator of the Wadsworth Constant.
posted by Xoebe at 1:44 PM on October 11, 2011


I don't think you can do the DDW thing in a sailboat without some sort of propeller under the water, which would be functionally equivalent to the wheels on the Blackbird. I'm pretty sure there's no way to do it with any sort of conventional sail/keel setup.

Although if someone demonstrates it I'll be happy to be proven wrong. :)
posted by Kadin2048 at 1:47 PM on October 11, 2011


I'm pretty sure there's no way to do it with any sort of conventional sail/keel setup.

Just off the top of my head, you could put paddle wheels off the sides of the boat. They wouldn't be as efficient as wheels, though.
posted by The World Famous at 1:59 PM on October 11, 2011


This machine also helps remind me that almost every mechanism can be thought of in terms of levers. In a sailboat, the keel acts as a fulcrum, leveraging the wind against the water to propel the boat forward. In the "Blackbird", the gearbox is the fulcrum with which the wind pushes against the ground. The catch is that the fulcrum itself is always rotating. It's kind of like how a planetary transmission works.

Kadin: Imagine you had sailboat making tacks at an angle to the wind downwind. We know that this boat can make distance downwind faster than the windspeed. So now build a large catamaran with a rail between hulls and point it directly downwind. Tie that first boat to the rail and have it continue to tack back and forth between the hulls. The sailboat will force the catamaran to travel directly downwind faster than the wind.

In fact, at that point the keel of the sailboat acts exactly like a propeller for the catamaran. Instead of traveling in a circle, the sail and keel reciprocate, but the principle is the same.
posted by Popular Ethics at 2:00 PM on October 11, 2011 [4 favorites]


Popular Ethics!

That helps me understand this.

Thanks!
posted by entropone at 2:21 PM on October 11, 2011 [1 favorite]


Where are the equations? I want to see some equations.
posted by Wemmick at 2:27 PM on October 11, 2011


Totally shopped. You can't go faster than the wind through the colours of my mind.
posted by clvrmnky at 2:32 PM on October 11, 2011


empath: Are they not towing it? It seems that they are. I asked because the article is confusing.

No, if you watch the video of the actual vehicle being tested, they aren't towing it. In their discussion of how the craft works, to simplify the explanation, they are assuming it's already going a bit faster than the wind (e.g. from towing), and showing that it will still accelerate. But that's because that's the tricky part.

At rest, the wind is pushing on the stationary propeller, so the vehicle moves forward just a bit. At slow speeds, the wind on the craft is still pushing it forward, even ignoring any motion of the propeller. But the linkage between the wheels and the propeller means that at the cost of some drag from the wheels, the propeller will turn making the wind push on it harder.

It's not surprising that a vehicle with a big propeller sticking up acting like a sail should accelerate downwind when it's not yet going as fast as the wind. So, Blackbird will (as is shown in the video) accelerate from a dead stop to faster than the wind in a constant speed wind.
posted by JiBB at 2:52 PM on October 11, 2011


I love this so much. I was so firmly in the NO camp for so long. And it was sort of easy since there were so many fallacies thrown around on both side. (even in this thread there are some pretty confused statements about boats and keels and what not)

But the way it felt when I read that Wired article and realized I was wrong was so delicious. A counter-intuitive concept finally explained and demonstrated in a way that I understood. That feeling of conceptual understanding of something despite so many prior attempts to dismiss it, was, for ole layman me, the feeling of how awesome science is. It's hard for me to overstate it, it felt soooo goood. Sorry if that's weird.
posted by Stonestock Relentless at 2:53 PM on October 11, 2011 [2 favorites]


We know that this boat can make distance downwind faster than the windspeed. So now build a large catamaran with a rail between hulls and point it directly downwind. Tie that first boat to the rail and have it continue to tack back and forth between the hulls. The sailboat will force the catamaran to travel directly downwind faster than the wind.

That depends on whether or not the sailboat can exceed the wind speed by enough to account for the extra distance its traveling.
posted by ShutterBun at 2:53 PM on October 11, 2011


aight so i'm still wrapping my head around this.

the propeller acts as a sail at below the speed of wind, and then, via power from the wheels, at and above the speed of wind, the propeller acts as... a propeller?

is that pretty much it?
posted by entropone at 2:54 PM on October 11, 2011 [1 favorite]


I still vote no
posted by 2bucksplus at 3:03 PM on October 11, 2011 [1 favorite]


That depends on whether or not the sailboat can exceed the wind speed by enough to account for the extra distance its traveling.

That's why I said "make distance downwind" rather than "go faster" than the wind. Many sailboats can perform this. Wikipedia has a good write-up.
posted by Popular Ethics at 3:07 PM on October 11, 2011


"...a student at UC-Berkeley wanted to invite us to speak there but was refused after the director of the Lawrence Berkeley National Laboratory insisted such a vehicle would violate the laws of physics."

This guy?
posted by warbaby at 3:17 PM on October 11, 2011


the propeller acts as a sail at below the speed of wind, and then, via power from the wheels, at and above the speed of wind, the propeller acts as... a propeller?

It acts as a self-tacking sail the entire time, repositioning itself as it rotates so that the sails (the blades of the propeller) are at an angle to the wind, even though the craft itself is aligned with the wind's direction.

(right?)
posted by The World Famous at 3:18 PM on October 11, 2011


the propeller acts as a sail at below the speed of wind, and then, via power from the wheels, at and above the speed of wind, the propeller acts as... a propeller?

Nope, it's always a sail. AFAIK, it's a pretty crappy sail below the wind speed, but a great one above it. Here's a visualization of how the wind is leveraged, if that helps. This page has a lot of other vector diagrams and visualizations which might help.

(The demo model using *does* use the propeller as a propeller, but that's because it's taking power from the "ground", rather than from the wind. It's equivalent, but confusing)
posted by Popular Ethics at 3:22 PM on October 11, 2011


Wait. Wait wait wait. How does the vehicle continue to extract power from the wind when the wind starts blowing the opposite direction (in the frame of reference of the vehicle)?
posted by The Tensor at 3:22 PM on October 11, 2011


Wait. Wait wait wait. How does the vehicle continue to extract power from the wind when the wind starts blowing the opposite direction (in the frame of reference of the vehicle)?

If I'm understanding it right, the rotation and angle of the sails (i.e. the blades of the propeller) and their airfoil shape make it so that that the wind is striking and acting on them at an angle other than the direction of the wind and, therefore, that the opposing airflow of the craft moving forward does not counteract the wind that is acting on the sails.
posted by The World Famous at 3:28 PM on October 11, 2011


The Tensor: At speeds lower than the windspeed, the turbine is pitched to extract power from the wind coming from behind. At speeds above the windspeed, the turbine pitch is reversed and, believe it or not, power is extracted from the apparent headwind. That means if you turned Blackbird around 180 degrees, it should drive right upwind! (which is what they were planning to do this year)

(From the point of view of the blade though, the wind is always hitting the blade at an angle to cause a tangential lift force which is transmitted to the wheels, as The World Famous said)

There seems to be an open question whether the Blackbird would work with a fixed pitch prop - would it get close enough to the wind speed with the blades angled the wrong way to start acting as an efficient turbine once the headwind starts? I'm inclined to say no, but I would love to be proven wrong again!
posted by Popular Ethics at 3:37 PM on October 11, 2011


I think next he needs to build a light-powered vehicle that can go faster than light.
posted by localroger at 3:40 PM on October 11, 2011 [4 favorites]


(somebody correct me if I'm wrong - the blog post I linked to talks about reversing the gearbox for the upwind runs which doesn't jibe with my analysis)
posted by Popular Ethics at 3:42 PM on October 11, 2011


Mei's lost sandal: We did do a good job designing the foils on USA17 I'll grant, but not that good ;)

We?? Do tell.
posted by zsazsa at 3:54 PM on October 11, 2011


It was a a big team. One of the many little cogs and all that. But none of our foils broke, the other guy broke a bunch. I am pretty proud of that.
posted by Mei's lost sandal at 3:57 PM on October 11, 2011 [2 favorites]


No one seems to have commented on the Google sponsorship of this contraption. I wish they would channel more of their money in to crazy cool gizmos...honestly they'd probably get more positive long term results than they do from all of their social media stuff anyway.
posted by Chekhovian at 4:05 PM on October 11, 2011


Also, Rick Cavallaro wrote this article about himself in the third person. WTF? Who does that? That's why the writing is so miserable.
posted by Chekhovian at 4:41 PM on October 11, 2011


The World Famous agrees with Chekhovian.
posted by The World Famous at 4:48 PM on October 11, 2011 [3 favorites]


So I could, like, ride a board on a wave faster than the wave?!
posted by onesidys at 5:08 PM on October 11, 2011


My understanding from the articles and watching the various videos is that the full-scale model is using the same mechanism as the treadmill model. That is, the power from the wheels is harnessed to spin the propeller and accelerate beyond windspeed, not the other way around.

Look at the angle of the propeller blades as it gets up to speed -- they're positioned to provide thrust. From what I can tell, the pitch of the prop does not change at any time. It would make sense that their small-scale model was built to show the same technique they used full-scale.
posted by Pantengliopoli at 5:14 PM on October 11, 2011


From the Wired article:

So let’s put a generator on the axle that creates 100 pounds of drag where the wheels meet the road. Thus the load of this generator is trying to slow the cart down – but it’s also producing exactly 10 horsepower to use however we want (55 feet/second x 100 lbs – 5500 feet-lbs/sec = 10 H.P.). If we want the cart to continue at the speed we towed it up to, we’d better provide 100 pounds of thrust to counter that 100 pounds of retarding force created by the generator. We’ll do that by turning the propeller with an electric motor powered by the generator. Remember the cart is feeling a relative headwind of only 11 feet/sec because it’s only going that much faster than the tailwind. If we want our propeller to put out 100 pounds of thrust we’ll have to give it 2 horsepower (11 feet/sec * 100 lbs = 1100 feet-lbs/sec = two h.p.).

Of course everyone knows that you can’t power a propeller from a generator that’s hooked to your wheels and hope to come out ahead. That’s as silly as pointing a fan into your sail and hoping it will propel you forward. And if not for the wind we couldn’t actually power our cart this way. The cart is basically a lever acting between the ground and the wind – where the long end of the lever pushes us downwind faster than the wind.

posted by Pantengliopoli at 5:18 PM on October 11, 2011


Ugh. All these engineer's descriptions are painful to read. This efficiency vs that that efficiency, ft-lbs there, etc. Just start with the ideal case with perfect efficiency and no fiction, draw your free body diagrams, and balance the forces. Add numbers in only at the very end. That's physics style problem solving. Later you worry about efficiencies when you're actually designing something.
posted by Chekhovian at 5:30 PM on October 11, 2011 [3 favorites]


Also read this: The Physics of Sailing, its from Physics Today, the magazine for physicists.
posted by Chekhovian at 5:31 PM on October 11, 2011


So I could, like, ride a board on a wave faster than the wave?!

Phase speed or group speed?
posted by nicwolff at 8:11 AM on October 12, 2011 [1 favorite]


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