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Power Transportation Technology

Going Faster Than the Wind In a Wind-Powered Cart 315

Posted by Soulskill
from the any-way-the-wind-blows dept.
Shawnconna writes "Can a wind cart travel faster than the wind? A group of makers say, 'Yes!' Make: Online has published a story about the Blackbird wind cart that just set a record. This is a follow-up to an earlier story in which Charles Platt built a cart based on a viral video where a guy claimed he'd built a wind-powered vehicle that could travel downwind faster than the windspeed. Charles built one and said it didn't work. Heated debates broke out in forums, on BB, and elsewhere on the Net. In the ensuing time, a number of people have built carts and claimed success, most principally, Rick Cavallaro. He got funding from Google and JOBY to build and test a human-piloted cart. They claim success, with multiple sensor systems on board, impartial judges and experts in attendance."
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Going Faster Than the Wind In a Wind-Powered Cart

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  • Two words (Score:2, Funny)

    by Anonymous Coward

    TACO Bell

  • by BBrown (70466) on Saturday November 06, 2010 @05:06PM (#34149450)

    I probably am just thinking about this too simply, but can't something go faster than the wind if it stores some of that energy and uses it later?

    • Re: (Score:2, Funny)

      by Anonymous Coward

      That's not the point. They show that they can go straight downwind (i.e. in the same direction as the wind) faster than the wind with nothing but the wind at the same time for the energy source.

    • by Whammy666 (589169) on Saturday November 06, 2010 @05:12PM (#34149490) Homepage
      Not necessary to store energy to go faster than the wind.

      The reason this works is that the propeller is able to "push off" against the tail wind. Think of it like sitting on a skateboard and pushing off from a moving wall behind you with your arm. The difficulty in making it work is that you need very little drag and a very efficient propeller. But the energy equations for traveling faster than the wind do balance and there is no violation of energy conservation.
      • Re: (Score:2, Informative)

        by maxwell demon (590494)

        Actually, the opposite is the case: The propeller is used to take energy from the wind, which is then used to drive the wheels and move the vehicle forward. This is most easily seen if looking at it on its own frame of reference. At stationary speed the wind comes from the front (because it's moving faster than the wind), while the road underneath goes backwards. The propeller takes energy from that wind and uses that energy to drive the wheels, which then keep the vehicle in place, against the forces of th

        • by marcansoft (727665) <[moc.tfosnacram] [ta] [rotceh]> on Saturday November 06, 2010 @06:32PM (#34150030) Homepage

          Nope, you've got it backwards, the GP got it right, and this is absolutely the key to understanding how this works.

          The car isn't using the propeller as a turbine as a source of energy to power the wheels. That, indeed, would be impossible, because once you reach wind speed the force exerted on the propeller is zero.

          Instead, it works the other way around, as a fan to push air backwards and accelerate the car. The energy is transfered from the wheels to the fan.

          Assume that, to begin with, the car is moving at wind speed. The wheels are spinning (because the car is moving) and you can use that energy (i.e. brake the car) to push the propeller. The propeller blows air backwards, which propels the car forwards. If your mechanism is efficient enough, that push more than counteracts the braking action on the wheels and the car actually accelerates forwards. As it accelerates, the efficiency drops and it eventually stabilizes at some speed, faster than the wind.

          Now everyone is shouting "Perpetual motion! You're producing more energy with the fan than you're getting out of the wheels!". Nope. That's the final bit. Let's say that wind speed is 10km/h. If the car is moving at 11km/h (faster than the wind), then the motion on the wheels relative to the ground is 11km/h. However, the fan only has to push air backwards at 1km/h, as the wind is doing the rest and providing the base 10km/h of forward motion. This difference in velocity is what offsets the inevitable energy losses: the ground speed is whatever you're generating with the fan plus the velocity of the wind "for free". This "free velocity" goes down (as a fraction of total velocity) as you accelerate, until it matches the (in)efficiency of the system (energy loss), and this is the stable velocity that the car achieves, faster than the wind.

          This really isn't an issue with perpetual motion. It's easy to see that you could use a stationary turbine to generate (say, electric) power from the wind, and then use that power to accelerate a car (say, powered by a laser, so it is not tethered) in a different (windless) location faster than the original wind. Output velocity can be greater than input velocity. The difficulty lies in grasping the interesting mechanics and interactions of the downwind-faster-than-the-wind car uses to achieve this within the original wind itself. It's a mechanics puzzle, not an energy conservation puzzle. Another way to look at it is that the energy lies in the difference between the velocity of the wind and the ground, and the car always has access to both of these moving entities via friction (friction with the wind, and friction of the wheels with the ground), and thus can harness that power regardless of what its own velocity is.

          • by Bigjeff5 (1143585)

            Instead, it works the other way around, as a fan to push air backwards and accelerate the car.

            Like any good airfoil, it both pushes behind and pulls forward at the same time.

            One simple test the author did for a small rig was to use a treadmill to represent the forward motion from the tailwind. If the propeller is able to spin fast enough to pull the cart forward on the treadmill with no wind, then it can do the same when the treadmill is replaced by a tailwind.

            The treadmill test works, so the large-scale test should work as well. To prove it they built the cart and had the NALSA [nalsa.org] run the test.

          • by bwcbwc (601780)

            So the bottom line is that it's possible to capture enough energy from the wind to move faster than the wind, but you have to find a way to keep receiving that energy when you exceed windspeed. The mechanism satisfies this by capturing energy based on the ground speed rather than the wind speed.

            So here's a question for everyone: could you make it work in a boat? The analogue would be to have the water propel a screw that turns the propellor, which sounds possible, but the efficiency equations might not be w

            • Re: (Score:3, Interesting)

              by marcansoft (727665)

              Well said. I think it should be possible to gain some amount of velocity greater than wind speed on a sailboat with an added fan connected to an underwater turbine, but I'm not sure if the fractional speed gained will be useful. It would be a very cool demo to try, though, and even achieving 5% over wind speed would be very interesting.

              I think the main problem with a boat is that you have a massive amount of friction with the ground (the water), while on a car the axle friction of the wheels can be made ver

            • by Iron Condor (964856) on Saturday November 06, 2010 @11:40PM (#34151944)

              So here's a question for everyone: could you make it work in a boat?

              Yes. [boatdesign.net]

          • by Romancer (19668)

            Even confused a former science fiction writer and current Makezine author into making the following statement:

            "I have never denied that a vehicle may be designed that will move into a headwind if the propeller is geared appropriately. What I do not believe is that this vehicle can start from rest with the wind behind it, accelerate until it is moving at the same speed as the wind, and then continue to accelerate faster than the wind, i.e. into a net headwind, without any fluctuations in wind speed, and without any gear-shifting along the way. That is what the original video from Florida purported to demonstrate, and is where all the arguments began. I have always suspected that the Florida video was faked.

            I know very little about Rick Cavallaro's cart, and am not very interested, partly because Rick has been extremely abusive, obnoxious, and condescending to me, and partly because, as I say above, I am quite willing to believe that his vehicle can move into a headwind. Indeed, the very primitive cart that I built for my original MAKE article did succeed in edging forward into a strong blast from a large fan. Again, what I do not believe is that his vehicle or any other can start with a steady wind behind it, accelerate to a speed equal to that of the wind, and then continue to accelerate so that it is moving faster than the wind, in one uninterrupted process.

            I have repeated myself in an effort to make this clear. -- Charles Platt"

            He does not acknowledge the existence of this video:

            http://www.fasterthanthewind.org/2010/07/video-from-richard-jenkins-world-land.html [fasterthanthewind.org]

          • by SashaM (520334)

            Assume that, to begin with, the car is moving at wind speed. The wheels are spinning (because the car is moving) and you can use that energy (i.e. brake the car) to push the propeller

            No, you can't use that energy to move forward. The experiment boils down to two mediums moving relative to each other (we can choose to tie the coordinate system to the ground, to the wind, or at any constant speed). If you are stationary relative to one of the mediums, you can only use the other medium to change your velocity. You can only use it to change your velocity in the direction of the movement of that medium.

            Imagine two opposing winds with between them, moving at the speed of wind1 (in its direc

            • Re: (Score:3, Insightful)

              by Alsee (515537)

              First lets imagine zero wind, the car traveling 100 mph, and lets assume that all components of the car operate with 100% efficiency.

              We tie a perfect generator to the wheels to extract 100 watts of power. Conservation of energy says it will apply a force slowing the car down. We pipe the 100 watts running a perfect propeller. It applies a force speeding the car up. If all components are perfect, conservation of energy says the car will go at a constant 100 mph forever. Perfect balance.

              Now lets break that pe

      • More specifically, this works because the "propeller" (rotating sail, really (*)) goes slower than the wind, relative to it. And achieves this by exploiting the resistance of surface (the difference in speed between it and the wind) - just like sailboats do when tackling. (*)In their case the resistance allowing the sail to move sideways comes from the keel & water; here it comes from wheels and ground - and the sail also moves sideways! (relative to the wind, all that matters; don't let the propeller-l

        • Oh, this is a much nicer way of understanding what's going on.

          Though... you could also think of the sails in a tacking sailboat as being able to extract more energy from the air because they can bring the air to a full stop relative to the water underneath the boat. The keel is necessary to keep the air from pushing the boat sideways instead of coming to a stop.

          • by sznupi (719324)

            There's no "bringing the air to full stop" anywhere... (except when in a boat traveling directly downwind, sort of - but that's actually not the most efficient scenario, and one where keel isn't involved much)

      • by icebike (68054)

        Think of it like sitting on a skateboard and pushing off from a moving wall behind you with your arm.

        I'm sorry, but my Slash dot number is too low to understand this skateboard analogy thing...

        Could you rephrase that as a Car Analogy please?

        K, thanks, bye.

      • by l00sr (266426)

        Here is what bothers me about this whole thing.

        Although I believe it is theoretically possible, there is a certain whiff of woo about the experimenters. I'm not even saying they didn't achieve their objective--I'm just saying there are a couple of things about the experiment, especially with regard to the stored energy issue, that nearly broke my woo-meter.

        From the official rules [nalsa.org]:

        Energy shall not be accumulated and later used for propulsion of the yacht or to operate the controls of the yacht.

        It seems to me that this would preclude the use of massive windmills (i.e., flywheels), such as the one on the craft. Later, th

        • by marcansoft (727665) <[moc.tfosnacram] [ta] [rotceh]> on Saturday November 06, 2010 @07:04PM (#34150360) Homepage

          It's simple: the vehicle must be able to move forwards faster than the wind forever, as long as the wind keeps blowing. In other words, the energy stored in the moving parts must not decrease and eventually cause it to stop working. Or in yet other words: the system must achieve a steady state where energy is flowing in and out at a constant rate, while traveling faster than the wind.

          For a race where time matters, energy input initially into the system is relevant. However, for the purposes of proving that DWFTTW is possible, it isn't. Any amount of energy added initially will by necessity be dissipated in the friction losses of the system - you can't run a car forever on a fixed amount of energy. If it can, in fact, run forever on a steady wind, then you can discount any initially applied or stored energy, and conclude that it is being powered solely by the wind. If it does that while going faster than the wind, then you can conclude that DWFTTW is possible.

        • by KnightMB (823876)

          Here is what bothers me about this whole thing.

          Although I believe it is theoretically possible, there is a certain whiff of woo about the experimenters. I'm not even saying they didn't achieve their objective--I'm just saying there are a couple of things about the experiment, especially with regard to the stored energy issue, that nearly broke my woo-meter.

          From the official rules [nalsa.org]:

          Energy shall not be accumulated and later used for propulsion of the yacht or to operate the controls of the yacht.

          It seems to me that this would preclude the use of massive windmills (i.e., flywheels), such as the one on the craft. Later, the rules specifically prohibit flywheels:

          It is not permissible to use stored energy to propel the yacht or operate its controls. This might includes things like compressed gas, stressed springs, batteries, capacitors and flywheels. This includes energy stored before a run or during a run. No pumps, generators or mechanical devices that are intended in part or whole to provide energy to storage devices are permitted. Stored energy in the form of momentum of the yacht, its wheels or other **normally moving** or flexing parts of the yacht is allowed. These forms of stored energy are inherent in the operation of the yacht and either do not add energy useful for increasing the speed of the yacht or **do so in a trivial way**.

          (emphasis mine)

          What constitutes a "normally moving" part of the yacht? What constitutes a "trivial" use of stored energy to increase its speed?

          That's I thought to, but here is how it works with laymen terms for all (including myself). The vehicle is simply geared in a way that the propeller will move air from the front of the vehicle to the back faster than the air moves from the front to the back. So if the vehicle is moving 5 mph forward, the propeller is trying to force air in the same direction at 10 mph. What happens is, as the vehicle is being pushed by the wind, the vehicle moving forward is building kinetic energy. Eventually the vehicle

          • Re: (Score:3, Informative)

            by marcansoft (727665)

            So while the vehicle might be traveling faster than the wind in burst, it won't get you any place faster than the next wind powered vehicle.

            .
            The vehicle accelerates to a a speed faster than the wind, then stays at that speed forever (as long as the speed of the wind is constant) and does not oscillate. It really will get you to your destination faster than e.g. a balloon traveling at precisely the speed of the wind.

            There is a feedback loop, but it works like this: there is a wind velocity X, and a stable ve

        • by Bigjeff5 (1143585) on Saturday November 06, 2010 @07:14PM (#34150448)

          You do realize that NALSA certified it, right? As in they checked all these things?

          They installed a bracket on the shaft to ensure the propeller never drives the wheels, so all the momentum of the propeller is going to be able to do is allow the propeller to continue spinning. It never, ever, drives the wheels.

          Anyway, it's way beyond theoretical. The current land sailing speed record is 3.15 times wind speed 126mph in a 40mph crosswind, fast!), set with a traditional land sail in a crosswind. It was set the same day NALSA certified the first DDWFTTW record.

          Here's the explanation of the physics: http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind [wikipedia.org]

      • If these device will accelerate to and beyond the average velocity of the volume of gas it sits in, what prevents it from accelerating from a dead stop in a still volume of air?

        This really sounds like a perpetual motion scheme.

        • by wagnerrp (1305589)
          It will accelerate to some multiple of the average velocity of the volume of gas it sits in. Zero times any multiplier you want is still zero.
    • by u17 (1730558)
      IANAP but there may be a logical explanation in layman's terms. If there is no wind, the vehicle is stationary and its propeller is rotating, it will accelerate for a little while before the propeller slows down to zero and the vehicle gets stopped by friction and drag. Now consider what happens when the vehicle is stationary but the wind blows from behind. It will make the propeller rotate and also push the vehicle forward. Just the wind will be sufficient to push it up to the speed of the wind. When this
      • Re: (Score:2, Insightful)

        by whoever57 (658626)

        When this happens, the speed of the wind relative to the vehicle is zero (as if there were no wind) and the propeller is rotating, just like in the first scenario. So, as long as wheel friction and internal friction are small enough, the vehicle will accelerate to go faster than the wind.

        Congratulations, you just invented a perpetual motion machine!

        In the example described in the article, the author overlooks one huge fact -- the treadmill is a source of energy, so assuming that a treadmill in a room w

        • by emt377 (610337)

          In the example described in the article, the author overlooks one huge fact -- the treadmill is a source of energy, so assuming that a treadmill in a room with no wind is equivalent to traveling over a road with a wind from behind is fundamentally flawed.

          What he really overlooks is the fact that no matter how fast the vehicle moves on a treadmill, the wind will always be faster than the vehicle. This doesn't happen in reality, since eventually the vehicle will catch up with the wind.

          • by marcansoft (727665) <[moc.tfosnacram] [ta] [rotceh]> on Saturday November 06, 2010 @06:41PM (#34150136) Homepage

            On a treadmill, if the vehicle is moving forward (relative to the observer, not the treadmill belt), then it is moving faster than the wind (which is moving at velocity zero relative to the observer). It is simply a change of frame of reference. If you place the observer on the treadmill's belt, then the wind is blowing forwards as fast as the outside world is moving forwards, and the vehicle is moving forwards faster than that. On the flip side, if you take the real-world DWFTTW vehicle example, and place the observer on a balloon moving with the wind, then (just as in the treadmill scenario) the wind is moving at zero velocity relative to you, the ground is moving backwards (just like a giant treadmill), and the vehicle is moving forwards faster than you (just like in the treadmill example the car moves forwards relative to an outside observer, even though the treadmill moves backwards).

            To answer the GP, see my post above. Everyone (including myself at first) immediately assumes this is a turbine-powered car using a wind turbine to drive the wheels. That's backwards, it's a sailcar (simply pushed by the wind) which in addition to that uses the wheels as generators to drive a fan (not a turbine) to push air backwards and increase thrust, thus actually achieving faster than wind speed.

        • Re: (Score:3, Interesting)

          by leehwtsohg (618675)
          You should read the discover article [discovermagazine.com] on the thing. I was saying exactly the same as you before I did. The gist of it is this: Imagine the car going exactly at the speed of the wind. In the car there is no wind, except that the ground is moving. The ground moving turns the wheels powers the propellor, which rotates and gives the car a force forward. Since wind speed is 0, there is no resistance, no force to counter the propellors force, the car will now accelerate, i.e. start going faster than the wind. Onc
          • But the ground ISN'T moving, the car/wheels are. Someone draw me a free body diagram showing the forces acting on the vehicle allowing it to accelerate to a speed faster than the wind and I'll believe it when the forces don't sum to zero at Vcar = Vwind.

  • Duh? (Score:3, Interesting)

    by Annymouse Cowherd (1037080) on Saturday November 06, 2010 @05:07PM (#34149462) Homepage

    If sailboats can travel faster than the wind, of course wind-powered carts can.

    • Re:Duh? (Score:5, Funny)

      by camperdave (969942) on Saturday November 06, 2010 @05:11PM (#34149482) Journal
      It should be easier than in a sailboat. After all, all you need to do is find a steep enough hill.
    • Re: (Score:2, Informative)

      by Lonedar (897073)
      They can, but not directly downwind - which is what the article claims the cart can do.
      • Re:Duh? (Score:5, Informative)

        by gotpaint32 (728082) * on Saturday November 06, 2010 @05:20PM (#34149554) Journal
        I think they are overcoming that particular limitation with the propellor which is technically approaching the wind indirectly.
        • Re: (Score:2, Informative)

          by Lonedar (897073)
          Right, but I just cannot see what keeps the propeller turning once the cart hits windspeed, as at that point the apparent wind would be 0.
          • The mechanical connection to the wheels keeps the "propellers" turning ....

            It's not the wind which makes energy extraction (and locomotion) possible, it's the speed difference between the wind and the ground ... as long as there is a speed difference between the ground and the wind you can push against the ground to get energy out of the wind.

            How to do it is just a technicality.

          • by Lazy Jones (8403)

            I just cannot see what keeps the propeller turning once the cart hits windspeed, as at that point the apparent wind would be 0.

            The wheels keep the propeller turning, as they are connected to it.

    • Re:Duh? (Score:4, Informative)

      by beaker8000 (1815376) on Saturday November 06, 2010 @05:26PM (#34149590)
      In a sailboat or iceboat, to travel faster than the wind you head about 45 degrees off of the direction from which the wind is coming (called 'reaching'). The sails then work as airfoils, creating lower pressure on the outside of the sails, which in conjunction with the keel propel you forward damn fast if you choose (iceboats sometime 4-5 times the speed of the wind). However, when you are 'running' (heading directly downwind) the sails are not working as airfoils, but function merely as a wall the wind hits that propels you forward. You don't go faster than the wind in this case. The article specifically mentions heading directly downwind.
      • by Bigjeff5 (1143585)

        The article specifically mentions heading directly downwind.

        With a propeller, which is an airfoil and works on the same principle as the sailboat's 45 degree trick. The reason sailboats can't do it directly downwind is because they can't create an airfoil directly downwind with sails, not because it's impossible to create an airfoil that works directly downwind (it's actually more logical for an airfoil to work directly downwind, rather than crosswind).

    • Re: (Score:3, Informative)

      The achievement here is going faster than the wind in the direction of the wind. This is something sailboats cannot do. Sailboats can only travel faster then the wind when they are at an angle to the wind (usually going against the wind).

  • Couldnt you build something that oscillates a weight to speed this up?
    Have the propeller pull a weight up a 90* triangle as the weight hits the top fold the propeller for increased aero dynamics, then release the weight which adds torque to the wheels.
    Then have the triangle tilt to let the weight roll back to the initial position. When it hits the start position do it all over again?
    This could add extra turbo boost to the car.
    I'm pretty sure this can be done all without electric.

    • Re: (Score:3, Informative)

      by Omnifarious (11933) *

      You are just adding a complicated energy storage mechanism and then having the energy collection mechanism disable itself for part of the time. It would be slower.

      You could get the car up to speed faster by having a sail that folded itself as soon as the amount of energy it was extracting dropped off. Maybe a triangle sail with the base of the triangle along the bed of the vehicle and the tip at the propellor axis. Then have it spring loaded in such a way that when wind was pushing into the sail it also

  • The reason that I don't believe this claim, is because physical demonstrations can be rigged. I want to see the mathematics. Is it too much to ask? I mean, they build lots of models, including expensive ones, they wrote articles claiming they can do it, they posted numerous videos on youtube claiming they can do it.... Where is the fucking math? Why it so hard to post it?

    The main reason nobody believes these clowns, is because they're not good at explaining how it works. I don't even see an attempt at it.

    • Re: (Score:3, Informative)

      by Whammy666 (589169)
      The math is not too bad, but it does involve propeller theory which is where the magic happens. The goal is to make a propeller and cart that requires less energy than is provided by the wind pushing against the prop thrust. The energy supplied is:

      E = (wind speed * prop thrust) - (cart drag * ground speed).

      So if the energy required by the prop is less than E, the system works. You use the difference between cart speed - wind speed for the velocity of the air thru the prop.
    • by Eharley (214725) on Saturday November 06, 2010 @05:54PM (#34149802)

      Here's an analysis performed by Mark Drela of MIT (http://web.mit.edu/aeroastro/people/drela.html)

      http://www.boatdesign.net/forums/attachments/propulsion/28167d1231128492-ddwfttw-directly-downwind-faster-than-wind-ddw2.pdf [boatdesign.net]

    • by SmallFurryCreature (593017) on Saturday November 06, 2010 @06:04PM (#34149844) Journal

      It is not the job of engineers or gods to figure out the science. That is for the scientists. Apples fell from trees long before Newton thought about it.

      The scientists can be skeptical, they can demand reproducible tests, but once the tests have been done it is THEIR job to find an explanation, NOT that of the engineers.

      These guys build something, they opened themselves up to a lot of tests, so either you make some real accusations and not just "idiot slashdotter doesn't understand so it must be fake" or start to work out the math or just accept that you are an idiot along with everyone else and leave this to smarter people.

      But they do NOT have to explain to you how it works, they got far smarter people to convince, not some random kiddie on the net.

      • by dcollins (135727) on Saturday November 06, 2010 @11:49PM (#34151994) Homepage

        You know, if you read the "fasterthanthewind" website, the story is that the math actually came first, and the first one of these vehicles was built later on, because -- guess what -- there were skeptics who refused to believe the math. In the modern era I'd argue it's rare for something to get invented without the physics having been done before that.

        We learned today that Andrew Bauer passed on Sept 6. As our blog followers will recall, Andrew Bauer was not the original inventor of the concept, but did build the first successful DDWFTTW cart that anyone seems to know of. He did this to settle a friendly wager with colleague and notable aero engineer A.M.O. Smith in 1969. As we understand, the wager was based on a claim in a student's paper, written 20 years before, that DDWFTTW should in fact be possible. In some small way JB and I have tried to model ourselves after Andrew by doing the engineering and demonstrating the principle - rather than simply proving it on paper.

        http://www.fasterthanthewind.org/ [fasterthanthewind.org]

    • by catbutt (469582)
      This may not be so mathematical, but it explains it in a way so that you can wrap your head around why it can work. (as well as why people might be inclined to think it wouldn't work, even though it does) http://karmatics.com/dwfttw [karmatics.com]
    • Re: (Score:3, Insightful)

      by John Hasler (414242)

      How about if you post your math showing that it cannot work?

    • Re: (Score:3, Informative)

      by Bigjeff5 (1143585)

      So you're saying the North American Land Sailing Association is in the business of rigging official land speed record tests eh?

      For Christ's sake, land sails already go 2-3 times faster than the wind using the exact same principles used in these carts. This is not some kind of voodoo physics, it's simply maximizing the available energy.

      I'll break it down for you, since you obviously didn't bother to read the article where they already explained it and since I'm such a nice guy:

      At a dead stop, the propeller

    • by sznupi (719324)

      From what I saw, it's not unlikely that most of the people taken by this (possibly including at least some early explanations from the creators; I'm not sure, it's been a while) actually don't have very good understanding of what happens - which wouldn't be anything new. For thousands of years sailors and boatbuilders didn't have very precise understanding of how sails work, too (otherwise we would probably have at least gliders a lot sooner), most of them still don't.

      But it basically just reshuffles the co

    • by Jeremi (14640)

      . Until then, what am I supposed to believe? My gut instinct or my lying eyes?

      Which pair of eyes are you planning to read the mathematics with?

  • I have no familiarity with land-based wind vehicles, but sailing vessels have been able to travel faster than the wind for a long time. This is hardly something new.
  • Can a wind cart travel faster than the wind?

    If boats can go faster [unsw.edu.au] than [straightdope.com] the [sciencedaily.com] wind [wikipedia.org], why not a wind cart?

    So it should have been made much more clear that this is about down wind. OK. mentioned later, but who reads that far in a summery.

    • Directly downwind. No windsurfer/sailboat/whatever can do that.

    • So it should have been made much more clear that this is about down wind. OK. mentioned later, but who reads that far in a summery.

      Summery what? I also didn't read far enough in the inane post above, and provided a not-so-inane comment from my point of view. Punish me as you see fit.

  • Yachts (I guess they are cailled sailboats in the USA) have been 'sailing faster than the wind' for some time. They have even been sailing 'against the wind'. However to get the best speed, they need to zig-zag a bit depending on the wind direction (Upwind its called tacking, downwind its called jibing).

    In order to do this, they need to have a fair bit of room to manouver which they have at sea, but not so good for land navigation. You mostly see sail powered wind vessels on deserts or salt flats, they won'

  • L/D (Score:3, Informative)

    by florescent_beige (608235) on Saturday November 06, 2010 @07:06PM (#34150378) Journal

    At first blush you would say if the lift/drag ratio of the sail/wing/apparatus is > 1 (plus a bit for drag) then a wind vehicle can go faster then the absolute flow speed.

    The complication is that the range of possible angles of attack you can achieve gets dictated to you by trigonometry. Example, if you are on a beam reach (traveling 90 deg to the prevailing wind) and your speed is equal to the prevailing wind, the apparent flow is rotated 45 deg fwd of abeam. Now, a typical wing might give you an L/D of 20 at something like 10 degres AoA, so you would set your wing (sail) at 55 degrees from abeam. Your lift vector would be 55+90+atan(1/20) ~ 148 degrees from abeam, or 58 degrees off your bow.

    Well, that's forward of abeam (90 degrees off the bow), so you have a component of lift pushing forward. It's then just a matter of getting the drag of your superstructure and rolling components down low enough to make that component sufficient to accelerate you just a bit, whereupon you are going faster than the wind.

    For a boat, the "rolling components" are another wing in the water (the keel) which imposes more trigonometric limitations that make it tricky but not impossible to achieve this. Normally if it is possible it happens on a broad reach. With rolling vehicles it should be easier.

    I don't know why people argue about this.

  • I used four rubber stoppers and a six-inch metal rod. Two stoppers were #10's, and two were #5's. I pushed the #5's onto the rod first so they met in the middle of the rod with the big ends almost touching. I put the #10's on the ends of the rod with the big ends on the outside. The result was an axle with two big wheels at the ends and two smaller wheels near the middle. I set it on the table, then I slid a ruler under the small wheels and pressed upward lightly. The big wheels touch the tab
  • We all accept that this vehicle derives it's motion through the rotation of the propeller which drives the wheels.
    I think we all accept that the bigger a propeller being driven by the wind, the more energy you extract.
    The more energy you have the faster you can drive the wheels.

    Ok so far.. but, I just can't get my head around the fact that once the vehicle reaches (or even approaches) the same velocity as the wind, how there is any relative wind left to drive the propeller without reversing the blade pitch.

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