Mark Chu-Carroll is wrong, take two

I’m amazed that Mark has persisted in his argument for so long. Honestly, it’s astounding. He still can’t seem to admit that the video I wrote about yesterday demonstrates that downwind, faster-than-wind, wind-driven travel is possible. Let’s take another tack and systematically address the objections (Update: I’m updating this post as more objections come up) (Update2: In case you’re coming to this page directly from this site: Mark has now seen the light and admits that downwind-faster-than-wind is possible).

Once again, for reference, here’s the video demonstrating that the cart works:

Now let’s consider the objections:

1. The treadmill isn’t an authentic test. Mark and others say that the treadmill is adding energy to the equation, thus, this isn’t a perpetual motion machine.

They are right! It’s not a perpetual motion machine. Wind or some energy source is required! The treadmill is simply simulating the wind. The problem does not call for perpetual motion, just wind-aided travel that proceeds faster than the wind.

The treadmill gives us an easy way to see if the cart is moving faster or slower relative to the wind. Imagine an aircraft carrier moving 10MPH across a windless, flat sea. You’re on the deck. Your experience is a 10MPH wind. There is no difference between this and a treadmill — the treadmill just allows us to extend the length of the aircraft carrier indefinitely, and easily monitor the progress of our cart. Since the cart moves in the opposite direction from the treadmill, it goes faster than our simulated “wind.”

2. Since the wind is moving slower than the cart, there’s no way for the wind to push the cart forward

This conveniently ignores the mechanism of the cart and simply declares that the device is impossible. I’ve now explained how the device works several times, and no one has explained how my explanation doesn’t make sense, other than to make declarations like this. I think this is the simplest explanation:

Assume the prop is a simple one made of flaps angled at 45 degrees. As long as the rotational velocity is greater than the relative wind speed, the wind will be “pushing” against the prop.

So assume a 10 MPH wind. The force of the wind can move the cart 10 MPH without the assistance of the prop. Now assume at this speed the wheels can drive the prop at an average rotational velocity of 5 MPH. There is an effective additional wind force on the prop of 2.5 MPH. This makes the cart go 2.5 MPH faster. We’re now going 12.5 MPH, in a 10 MPH wind!

Obviously it will take things a while to equilibrate, and the final velocity computation is complicated, but the net result is a car moving downwind faster than the wind.

I gave a slightly more complicated version of this in a comment on Mark’s blog, but an illustration might make things clearer:

The wind isn’t pushing against the cart that’s moving faster than the wind. It’s pushing against the prop, which has a relative motion that means there’s still something to push against.

3. But the wind isn’t moving relative to the cart

See 2 above. The wind doesn’t have to move relative to the cart, just relative to the propeller.

4. The treadmill isn’t moving right-to-left. It’s a trick

No, the treadmill is moving right to left. Look at video, around 4:30. You can clearly see the stripes moving right to left. Any apparent motion of the wheels in the “wrong” direction is the wagon wheel effect. Watch the wheels closely as they start up and slow down to be sure.

5. The cart can’t get going on its own. It only works because it’s put on a running treadmill

This is debunked in the original video, where they take the cart outside and it starts on its own. Look at the video again, starting at 6:20.

6. BUT IT’S IMPOSSIBLE TO SAIL DOWNWIND FASTER THAN THE WIND!

No it’s not. Consider this example, which is explained by the cart’s creator here. I’ll summarize for you:

Sailboats can and do travel faster than the wind if they tack (zig-zag back and forth). Their downwind vector is faster than the wind. A fast sailboat can beat a hot-air-balloon in a steady wind. This is established fact — ice boats travel downwind in this way more than twice as fast as the wind. Attach two boats together with a telescoping rod and sit in the middle and you’re moving due downwind, faster than the wind.

7. You can’t draw energy from wind if the wind isn’t moving relative to you.

Yes, you can. See 2, 3, and 6 above. The trick is to have something else move relative to the wind for you and then harness that power — either the propeller or the two boats or some other ingenious device.

8. If this device worked as advertised, then it would also work in no wind at all, which is obviously impossible.

No, it wouldn’t. As explained in 2, 3, 6, and 7 above, it requires the wind to operate. The force to move the vehicle comes from the wind (or the motion of the treadmill, which simulates the wind). When they set the cart down in a still room, it doesn’t move. When they lift it off the treadmill, it slows to a stop.

The reason it goes faster than the wind is because the propeller captures more of the wind energy than is necessary to move the cart at a speed equal to the wind. The larger the prop, the more wind energy is captured (just as larger sails allow a boat to go faster). The trick is to move the propeller so that this extra energy can actually be used, as explained in 2 above.

9. Ice boats can’t really go faster than the wind.

Yes, they can. Here’s a document (PDF) that gives quite a bit of data demonstrating that fact. Ice boats routinely travel 70 MPH in 15-MPH winds. They can go more than twice as fast as the wind when tacking downwind. They can most definitely beat a hot air balloon to its destination due downwind.

10. If the wind isn’t driving the prop, then the prop isn’t driving the wheels.

That’s right. The wind is driving the wheels, and the wheels are driving the prop. Remember, there’s lots more energy in the wind than what is used to drive the car forward (assuming it had just an ordinary square sail). If you had an ordinary sail and were going due downwind, you wouldn’t be able to go faster than the wind. You could double, triple the size of the sail and it wouldn’t make a difference. But you’d have to use more braking power to slow the car with a larger sail. Now, imagine if you harvested that energy somehow. You could put on the brakes for five minutes and use that to generate energy (much like a Toyota Prius works). Then you could trim the sail to a smaller size and apply that energy by running a motor to drive the wheels. At this point, you’d be going faster the wind (but the sail would appear to be filling “backwards” from the relative headwind). This vehicle effectively does the same thing, but constantly and in a simpler fashion.

11. There is no way to make the transition from slower-than-wind to faster-than wind.

Yes, there is. The vehicle works the same way when it’s going slowly as it does when going fast. Look at the video at 6:20. It starts from a stop in the wind. The propeller rotates. The car accelerates. There is no transition. You could design a vehicle like this that would go at exactly wind speed. Just add enough friction to the works, and the car wouldn’t be able to go faster than the wind. This might only work at a single specified velocity. But you could design a vehicle with a wind detector on the front. It would automatically put on the brakes whenever the relative wind speed was zero.

In fact, the vehicle’s designers do exactly that when they run it on the treadmill and slow it with the spork to keep it from running off the end of the treadmill. The vehicle is going slower than “wind” when it goes backwards down the treadmill, and faster when it moves forwards up the treadmill. Each time it does this, it makes the “transition” from slower-than -wind to faster-than-wind.

12. If this is true, then why isn’t someone making a fortune off of this fantastic invention?

Because people don’t always want to go the same direction the wind is blowing. Also the wind is unreliable, and doesn’t always blow when we need it. That’s why powerboats and cars were invented. But maybe someone should consider attaching a generator to a windmill and selling that power to the electric company… I wonder if that’ll ever catch on.

This entry was posted in General. Bookmark the permalink.

36 Responses to Mark Chu-Carroll is wrong, take two

  1. Scott says:

    We haven’t seen a post from him in a while. I’m taking this as a sign that he’s in the depression stage of grief.

  2. jhn says:

    Another way to move downwind faster than the wind, using only the wind as your power source, is to use electricity.

    Imagine a sail-powered railcar, and a track that is lined up with a constant wind source. It is very efficient and friction-free, so if the wind is 10 knots it travels at 10 knots also. From the perspective of the railcar, there is no wind.

    Next to the rail is a series of windmills. Each generates electricity and powers the rail. The railcar draws power from the rail and adds to its speed. It travels above 10 knots, relative to the rail. The only power source is the wind. We’re just harnessing that same wind in a few ways at the same time.

    It is very easy to get power from wind into a vehicle that is not moving relative to the wind as long as, like you say, something somewhere is moving relative to the wind.

  3. MP says:

    There is an easy way to settle all the arguments. Take the vehicle outside and let it run. No electric fans, no treadmill, just the cart and the natural wind.

  4. Dave says:

    MP: They’ve done that. How do you prove it’s going faster than the wind, though?

  5. Scott says:

    MP,

    Outside is pretty much where this all started. The doubters said that a) it never going faster than the speed of “gusts” and b) it’s fake. The treadmill was intended to be a more controlled test so as to lay to rest the complaints about the uncontrolled outside version.

    Here’s the original video, from 2007

    http://www.boingboing.net/2007/02/06/video-can-a-vehicle-.html

  6. cm says:

    Hi, John. Can I ask some questions to clarify your points? Numbers correspond to your numbers…

    1. If I’m on the deck of an aircraft carrier at 10MPH on a windless day, I am moving:
    – 10 MPH relative to my support surface (ultimately, the sea).
    – 10 MPH relative to the still air (so I will feel wind in my face)

    That’s 10 and 10.

    But if I am on a treadmill at 10 MPH in a windless room and I am maintaining my position (say I am on roller skates), I am moving:
    – 10 MPH relative to my support surface (the belt of the treadmill…so this simulates movement of ship on the sea)
    – 0 MPH relative relative to the still air of the room (so no wind in my face hen I’m on a treadmill)

    That’s 10 and 0.

    Shouldn’t these numbers match if there is “no difference between [the ship scenario] and a treadmill”?

    2. In the cart-on-treadmill example, how can the wheels transfer energy to the prop so that the prop can do work–pushing off the air–without those wheels losing that same energy in the form of rotational speed? If it is doing work (pushing air), something has to lose energy. Under normal rolling, you have close to 100% of your energy going into your wheels pulling you along the ground (some loss to heat of friction I guess), so how could you then give some more energy to the prop without slowing the wheels’ rotation? And if the wheels rotation slowed, the cart would slow (is this akin to one’s car performing worse when one puts the car’s air conditioner on?). I don’t see why any thrust off the prop would necessarily compensate–and even surpass, as you state–the energy of the wheels rolling. I don’t know where the cart gets an extra 2MPH worth of energy, I just see energy being shuffled around.

    Thanks for the clarifications.

  7. Kurt says:

    Dave, this is a great post; you’ve explained things so clearly that I think only the most stubborn could remain unconvinced.

    As far as giving a ‘field demonstration’ of faster-than-wind travel: in Jack Goodman’s video there was a little ‘tail’ attached to the vehicle which indicated wind direction–initially it pointed forward, and after the cart was up to speed it pointed backward. Now in his case, the tail was located behind the prop, but it could just as easily be put on a high, thin pole away from the propeller. Then if it changes direction, it should be good evidence that the cart is going faster than the ambient wind speed.

  8. Kurt says:

    One after-thought: I suspect that a lot of people are leery of the treadmill demonstration because it is reminiscent of the airplane-on-a-treadmill controversy, and they think something similar must be going on. But of course the principles involved are totally different.

  9. cm says:

    Dave, I referred to you as John in my post above. My mistake, I’m sorry.

    So, Dave, then, could you help me out with understanding my confusions.

  10. not dave says:

    cm,

    My answers will not be as good as David’s will be, but I can try the first one.

    The treadmill simulates traveling at windspeed, downwind. It is meant to model a situation where there is no wind relative to the cart (only relative to the prop). A treadmill is how you model no windspeed. You’d use a wind tunnel to simulate the situation you describe.

    The claim isn’t that the treadmill is a universal way to model motion, just the case of going at exactly windspeed downwind– which is what we want to model here.

    Of course when/if the cart begins to move forward, there will be some headwind, just as there would be on a street.

  11. Scott says:

    CM,

    The point of the aircraft carrier example is not to get to 10 + 10, but accelerate the cart against the movement of the carrier and get to 0. At this point, the aircraft carrier is acting like a treadmill, except it runs out and you land in the water.

    In part two of your question, you want to know where the energy comes from to accelerate the vehicle. Well, if you have two objects of moving at different speeds, you can capture energy by bringing there speeds closer together. Imagine two balls heading towards each other, and a device in between which could capture the incoming momentum from each ball and feed that into a generator. The result is a charged battery and two balls now at rest to each other.

    The cart is gaining energy by breaking the treadmill relative to the still air of the room. The treadmill is slowed slightly, and the air of the room is no longer as still as it was. If the air of the room was ever accelerated to match the treadmill the device would not work.

  12. Mary says:

    So assume a 10 MPH wind. The force of the wind can move the cart 10 MPH without the assistance of the prop.

    Great, so now we are in the situation where the air is not moving relative to the center of the propeller, right?

    Now assume at this speed the wheels can drive the prop at an average rotational velocity of 5 MPH.

    So the wheels are forcing the propeller to turn though air which is motionless relative to its center of mass. This is like powering a fan. The air is speeding up after it hits the propeller (in the cart’s frame of reference.) The air is gaining energy from the prop, not giving energy to the prop, in this scenario. So where is that energy coming from? Right — the wheels. Which are now slowing down as they give up energy.

    There is an effective additional wind force on the prop of 2.5 MPH. This makes the cart go 2.5 MPH faster. We’re now going 12.5 MPH, in a 10 MPH wind!

    There’s an effective force on the car, but it’s a drag force. Just because the propeller is moving doesn’t mean it’s providing thrust. Analyze this in the frame of the prop’s center of mass. Does the air have more energy or less after interacting with the prop? If the answer is more, then the prop is a drag on your wheels, not a power source for it.

    Now assume at this speed the wheels can drive the prop at an average rotational velocity of 5 MPH.

    If the wheels are driving the prop, then the prop isn’t driving the wheels.

  13. cm says:

    If the wheels are driving the prop, then the prop isn’t driving the wheels.

    Mary, that is the succinct and elegant way to make the point I was making in #2 above. Thanks.

  14. notdave says:

    cm,

    Scott’s posts @ Chu’s blog (see #136) explain your objection #2.

    His sentence,

    The spinning prop is made of parts which are tacking!!! The prop is the two sailboats condensed into a simpler form.

    is the equivalent to me of the quote of Mary’s that you posted.

    If a sailboat can beat the wind to any given point by tacking port then starboard, explain why it is impossible to harness that same energy in another way.

  15. cm says:

    notdave,

    If a sailboat can beat the wind to any given point by tacking port then starboard, explain why it is impossible to harness that same energy in another way.

    Well, can I ask you what you believe first, too? Say you are standing (secured by magnetic boots, say) to the roof of a large truck and the driver starts driving in the same exact direction as a perfectly steady 30mph wind. He achieves 30mph with the truck and holds it, entirely matching the speed and course of the wind.

    What will you experience? What will it be like up there? Will there be wind? Wave your arms around, explore the space, look up, look down. What will it be like?

    Now you take out of your coat a child’s pinwheel toy. Can you find some angle or placement of the pinwheel such that it is spun by the air without your blowing on it? Do you believe you can?

    Now you take of your coat the cart in the video. Is there a place you could place it on the truck, or even a place you could hold it with your hand, at any angle, in which the prop would turn due to a wind?

  16. notdave says:

    Now you take out of your coat a child’s pinwheel toy. Can you find some angle or placement of the pinwheel such that it is spun by the air without your blowing on it? Do you believe you can?

    Yes, I can attach it to a stick and run it along the ground.

    It’s not an analogous system, though. The claim is not that the cart gets wind power from no wind; your example is a model that has no wind. It’s that it gets power from tacking, and the tacking is made possible by the spinning wheel turning the prop.

    The spinning cart wheel/prop alone doesn’t provide the extra force– you are right to point out that that would be impossible. Energy and speed would be lost if that was the only consideration.

    The claim is that the energy-costing system of wheel/prop moves the blades of the prop in such a way as the prop, and then the entire cart, gains energy by taking advantage of tacking. The energy lost by moving the prop blades is made up for by extra energy coming into the system from the relative movement of the wind against the prop blades.

  17. cm says:

    It’s not an analogous system, though. The claim is not that the cart gets wind power from no wind; your example is a model that has no wind.

    The claim is the cart can go faster than the wind that is pushing it. If that wind is at 10mph, when the cart gets up to 10mph the cart is the same as the truck in my story. At that point there is no wind. Please explain to me how, when the cart matches windspeed, it can “tack” to “take advantage” of the “wind” that clearly will not be present anywhere around the cart and then use that non-wind…to go even faster?

    It’s that it gets power from tacking, and the tacking is made possible by the spinning wheel turning the prop.

    Explain what you mean by “tacking”. When a sailboat tacks, its unpowered sail experiences a steady wind at an angle. When this cart “tacks”, as you call it, its wheel-powered prop cuts through still air. How are these two equivalent?

    Again, how do you get the cart to “tack” on top of the truck in my story? Because if your account is right, it can tack its way up the length of the truck even when the truck is at windspeed.

    The claim is that the energy-costing system of wheel/prop moves the blades of the prop in such a way as the prop, and then the entire cart, gains energy by taking advantage of tacking. The energy lost by moving the prop blades is made up for by extra energy coming into the system from the relative movement of the wind against the prop blades.

    But what wind at windspeed? There is no wind at windspeed. The prop blades are pushing into the air, yes, but, as Mary so nicely said, if the wheels are driving (powering) the prop, the prop can’t also be driving the wheels.

  18. notdave says:

    cm, I’m starting to doubt the “it’s like tacking” thought.

  19. notdave says:

    I still think that DWFTTW is possible (from the telescoping chair example), but not the way I thought it worked.

  20. cm says:

    I’m starting to doubt the “it’s like tacking” thought….I still think that DWFTTW is possible (from the telescoping chair example), but not the way I thought it worked.

    I will have to think further about the telescoping chair myself. There are multiple levels of things to understand in that fairly complex example.

    I have to go for the day, but will just add this thought to the mix…

    If a powered propeller on the back of a vehicle always added net energy to the system, shouldn’t all vehicles have them? I mean, wouldn’t a big prop on the back of my car, powered by the axles, cause my car to go faster for the same gas expenditure? Of course it doesn’t work that way: if you want to maintain speed and run the prop, you have to burn more fuel–use more energy.
    Thanks for the fun today.

  21. notdave says:

    Hell, I’m note even sure now that the ice boat examples are correct. I can only uncover layers of assertion.

  22. Kurt says:

    notdave, you need to get some sleep! You’re actually letting cm talk you into siding with the DWFTTW denialists!? You need to go back and reread Dave Munger’s explanation, and reread your own earlier comments on this thread!

  23. Stephen Wells says:

    I just put this analysis on Mark’s blog and thought I should crosspost here as Dave is collecting objections. I have no emotional stake in this but as a physicist I want to understand the issue.

    Let’s say the velocity of the ground relative to the cart is -v (the ground is moving left relative to the cart if I sketch this in the cart-stationary frame) and the wind has a velocity relative to the cart of u which could be positive, zero or negative. The air that travels through the rotor exits with a new velocity w. Optimum power from a turbine is obtained with w=u/3, from Betz’ law. Air has a density r and the rotor has area A. The air as it passes through the rotor has a velocity y which Betz’ analysis indicates can be taken as (u+w)/2.

    u positive means the cart has a tailwind. The force of the wind on the rotor (direct “sail” force on the cart) is F_rotor = r A y (u-w) which at optimum is (4/9) r A u^2. The rotor extracts power P= (8/27) r A u^3 at optimum. This power can go to provide a motive force F_motive where conservation of energy means (F_motive v) cannot exceed P. The cart experiences a frictional force which depends on V and which we’ll call q(v). Here both F_rotor and F_motive act to accelerate the craft and, if q is small enough, can get the craft close to windspeed. However as we approach windspeed u tends to 0 and so both forces tend to zero; the craft must therefore steady-state at some positive u.

    If u=0 then the turbine extracts no power, however much people want to protest, and if we extract some power from the wheels we must be exerting a force F_brake such that F_brake v = P, power extracted. The rotor can push on the air but cannot do more work than extracted from the wheel. Notice that if the rotor does push on the air, continuity requires that u become negative (we must be pulling air into the rotor). Which brings us to the third case.

    If the craft is beating the wind then it’s moving into a headwind given by a negative u, say u = -k where k is positive. Again we have F_rotor = r A y (u-w) which at optimum is (4/9) r A k^2. The rotor extracts power P= (8/27) r A k^3 which limits our motive force(F_motive v).
    Now, if the vehicle is to not slow down, the retarding forces F_rotor and friction q must balance against F_motive. The best you can do is near-zero friction so let’s set F_rotor against F_motive. In this case our F_motive, which is (8/27) r A k^2 (k/v), must equal our F_rotor drag force which is (4/9) r A k^2, leading to a requirement that k=3/2v, or in other words, the cart must be getting a headwind more than 3/2 of its ground speed. Sadly this contradicts the set up of the problem, in which k is the small amount by which the vehicle’s speed exceeds a wind speed which is (v-k) relative to the ground. Essentially the relatively large v kills the maximum F_motive you can exert without violating the energy-conservation requirement on the power.

    Let’s not while we’re at it that there would be no contradiction in a vehicle using a turbine to power its motion into a headwind at less than windspeed, that’s just reversing the sign of v here, and we conclude that the best you could possibly do with a turbine-powered upwind beater would be to reach a v of 2/3 the wind speed. Earlier in the thread I believe there were comments that such vehicles have attained up to 0.6 windspeed in practice, so the result is clearly rational.

    If anyone else wants to check my working then Betz’ law, there’s a clear derivation in the wikipedia entry if you want to check it, gives us a power extracted of (1/4) r A (u+w)(u^2 – w^2) and a force on the rotor of (1/2) r A (u+w)(u-w). My u and w are equivalent to the v2 and v1 of the wiki entry. I have not checked if the power/force requirements are better for us at other values of w, though I am investigating that at the moment.

    So at the moment I can’t see any way the device can be doing what you think it’s doing, and we’ve yet to see a sufficiently well-controlled experimental demonstration, hence my scepticism. Substantive responses to the calcs above would be welcome.

  24. dave says:

    The rotor can push on the air but cannot do more work than extracted from the wheel.

    Stephen, I’m not quite sure I understand your argument, but I think this is where you’re going wrong. See #10 above (in the post, not comment 10).

    You’re neglecting the fact that the rotor is acting both as a sail and a propeller. In its “sail” mode, it pushes the cart along at wind speed. The larger the sail, the more power is applied to the wheels, and the more energy is available to turn the rotor. If the sail is twice as big as it needs to be to propel the cart forward at wind speed, the rest of the energy can be devoted to turning the rotor. This in turn propels the cart faster than wind speed. The will be some equilibrating, but the net result is a vehicle powered by the wind, moving faster than the wind.

  25. rob says:

    objection 1: the treadmill is not a good model for the cart operating outside in a wind. the treadmill does not simulate the wind. the tread mill is providing power that turns the prop. the prop generates the wind. the wind is not turning the prop.

    objection 2: yes, the maximum speed in a 10 MPH wind for the cart will be at best 10 MHP if sources of dissapative forces are ignored. however, your assumption that the wheels can increase the velocity is faulty. the prop and wheels are connected, and cannot be considered as separate contributions to the final speed. if the cart goes a little faster, the prop will spin a little faster. this will provide a bit of extra acceleraton. whereas initially, the prop provided a force in the forward direction, when you try to accelerate the cart faster than the wind, the prop will actually provide a retarding force that counter acts the extra force from the wheel. the net force will be zero leaving acceleration zero, which means the cart stays at constant velocity, the velocity of the wind.

    objection 3: true. the wind isn’t moving relative to the cart. in objection 2 i showed you cannot get the cart to go faster than the wind, because any additional speed increase results in a retarding force.

    objection 4: i agree. the treadmill is moving in the standard direction. the cart is able to move on the treadmill exactly as shown in the video. however, the cart on the treadmill is not harnessing energy from a wind. there is no wind that input energy to the cart. the source of energy is the treadmill motor. thus, the treadmill experiments and outdoor wind experiments cannot be compared. they are different.

    objection 5: i agree. the cart can get going on its own outside. the video shows it. on the treadmill, it cannot get started on its own. a human has to hold it until the prop is spinning fast enough to keep the cart balanced against the force of gravity in the direction parallel to the plane of the treadmill belt. (you could probably get it to start on its own if the treadmill were long enough and you let the cart go at the top)

    objection 6: yes it is impossible. you cannot sail faster than the wind in the direction of the wind. as soon as your speed relative to the wind is zero, there is no way for the air to provide a net force to the sail. thus, the relative motion of the sail and the wind is what determines the top speed. this is different than the case of travelling at an angle to the wind. when you travel at an angle to the wind, there is always a component of the wind velocity that can provide accleration in the direction of motion. what limits the top speed in this case is friction. so, the two cases of sailing with the wind, and across the wind are different. one is limited by the top speed of the wind, the other is limited by friction.

    objection 7: not exactly true. if you are travelling in the direction of the wind at the wind speed, the net force of the air is zero. however, the wind is still providing the power to keep you moving at the same speed of the wind. the component of force in the direction of motion is the energy input. this is due to the wind. however, there is a component of force against the motion because the sail is acting like a parachute, retarding motion. this takes energy out of the system. the net change in energy is zero, so the speed doesn’t change. so, it is possible for the wind to keep adding energy, even though the net force is zero.

    objection 8: not true. this fallacy arises because people do not understand the work-energy theorem and vectorial nature of forces. in no wind at all, the work done by the wind is zero. the work done by the parachute effect is zero. net work is zero, so velocity does not change. it sits still.

    objection 9: ice boats can have a velocity greater than that of the wind. this is also due to the vectorial nature of velocity and force. when you travel in the direction of the wind, as you approach the speed of the wind, your net force goes to zero. this happens when the ice boat and wind have the same magnitude of velocity. when you travel across the wind, you will always have some component of the wind in the diretion of you motion. the other components may approach zero, but the one in the diretion of motion will not. in this case, it is the dissapative forces that limit the upper speed (air resistance, friction with ice).

    objection 10: i agree with the satement “If the wind isn’t driving the prop, then the prop isn’t driving the wheels” since the prop and wheels are mechanically linked.

    objection 11: not true. you can go faster than the wind if you move across it, like i wrote above. in the direction the wind it is not true. also, the treadmill experiment is not equivalent to the outdoor experiments. the reason why the kart goes up and down the tread mill is not evidence of FTW travel. it is because the guys who ran the treadmill experiment did not adjust the angle and the speed of the treadmil to keep the cart stationary relative to the room.

    objection 12: it isn’t true. if it were, that would mean that there is something wrong with the conservation of energy theorem. if you could go faster than the wind, then there would be some way for a clever person to extract the free energy and sell it and make a fortune.

  26. Dave Munger says:

    rob:

    1. See Newton, Einstein.

    2. You neglect the fact that the prop can collect more energy than is needed to power the cart.

    3. You grant this

    4. You grant this

    5. The video shows carts moving faster than wind both outside and on the treadmill. What more do you want?

    6. Now you’re not even paying attention to what I said.

    7. This is hand-waving. My point 7 merely refers readers back to 2, 3, and 6, which you haven’t refuted.

    8. What’s “not true”? I agree that it won’t work in no wind.

    9. I’m not quite sure what you’re saying. Are you questioning the empirical data demonstrating that sailboats can travel downwind faster than the wind?

    10. So you grant this too.

    11. You’re repeating yourself, and not addressing the point I make here.

    12. No, nothing is wrong with the conservation of energy. The energy comes from the wind. There is plenty of energy in the wind to make an object travel faster than the wind, as long as it is also in contact with another medium.

  27. Stephen Wells says:

    It’s incoherent to claim that the prop can be a sail, pushing the vehicle at wind speed, and also that it’s a powered prop producing thrust. Think about which way the air goes. My analysis starts with the assumption that the vehicle works exactly as claimed and finds that it’s not workable. If you want the prop to be powered by the wheels, explain how you can get more energy from prop thrust than you extract at the wheels (impossible); extracting energy at the wheels is braking. If you think otherwise show me a calculation of the forces.

    Incidentally, I’ve modelled the ice boat running at an angle downwind; this is equivalent to tacking into a slight headwind while travelling at a large ground speed; I see no reason why a low-drag vessel shouldn’t beat the wind as described, and will post the math if you want. There seems to be nothing wrong in principle with DWFTTW with a tacking model. The prop argument on this particular device, however, doesn’t hold water, which at the minimum means that whatever the device is doing it is not working the way you claim.

  28. TSK says:

    I think I have found the solution. I am posting it here because I don’t want to bury it in more than 300 comments on Mark’s site.
    After watching the video I was wondering why the cart was working definitely more effectively when moving on a steeper ramp. And then: *light*.

    The argumentation “Faster than wind speed is impossible” is flawed because the windspeed is *not* uniform. Air has viscosity, that means that the first layer of air molecules are practically sticking on the ground (v = 0). Every layer now adds wind speed until it approaches the final wind speed.
    In case of the treadmill it means that while we have still air above the cart the air molecules near the ground are moving *backwards*. In case of the cart outside air above the propeller axis is faster,
    below it slower. The trick is now that the velocity as function of height is not *linear* (and therefore cancels out), but almost quadratically, that means that (in case of outside) the speed above increases faster than decreasing below and therefore deliver additional power !
    Example for wind: (You can set feet for m)
    0 m -> 0 m/s
    1 m -> 1 m/s
    2 m -> 4 m/s

    So if the rotor axis is in 1m height and your propeller is 0.99 m long, you cannot drive faster than 4 m/s, but you *can* drive faster than 1 m/s. The opponents have failed to declare their “wind speed” because they erronously assumed that it is always the same round the car.

    If my explanation is right, following conditions will guarantee optimal performance:

    a) Use the highest possible difference between these forces: The bigger, the better
    b) Orientate the propeller perpendicular to the ground for maximum steepness of the increase
    c) Set the propeller down as near as possible to the ground

  29. ThinAirDesigns says:

    TSK:
    >After watching the video I was wondering why
    >the cart was working definitely more effectively
    >when moving on a steeper ramp. And then: *light*.

    Hi TSK:

    I’m JB and I’m the guy in the video. Just to be clear, the cart *does not* “work more effectively” when on a steeper ramp.

    Depending on the video you watch (we have a bunch on YouTube from which the MythBusters’s video was cut) you will see different wind speed and different slopes.

    The device acts just like you would expect a wind powered device to act — more wind, more speed … steeper hill, slows down.

    Hope this information helps with your clarity.

    JB

  30. TSK says:

    Hi JB,

    sorry, you are right. But I still think my idea will be correct.

    Could you test the following, please:
    Mount the propeller on a platform. On the four sides of the platform you mount a perpendicular bar and drill holes for the wheel axis in it; the cart will look like a table from the side.
    Now change the height between ground and platform/propeller by sticking the wheels in different holes.

    As you are using the same platform, the same propeller and the same wheels, there is no difference in air resistance or friction and there should be no difference in the efficiency. But I predict that there will be a difference: The higher the platform, the less efficent the cart.

  31. Dave says:

    TSK:

    Why don’t you try it? Here’s a parts list for making the cart. You can modify it to do your experiment and post your results.

  32. lh says:

    Hi Dave,

    i stumbled into these issues just now. Great explanations, although i’d had some difficulties; so i would like to explain what finally got it for me.

    2. Since the wind is moving slower than the cart, there’s no way for the wind to push the cart forward

    This conveniently ignores the mechanism of the cart and simply declares that the device is impossible. I’ve now explained how the device works several times, and no one has explained how my explanation doesn’t make sense, other than to make declarations like this.

    This is what i hope to do, explain why some parts didn’t make sense for me.

    Now assume at this speed the wheels can drive the prop at an average rotational velocity of 5 MPH. There is an effective additional wind force on the prop of 2.5 MPH. This makes the cart go 2.5 MPH faster.

    i still don’t get why the effective addional wind force on the prop is half of the average prop’s rotational velocity, or are they arbitrary sample numbers?

    7. You can’t draw energy from wind if the wind isn’t moving relative to you.
    Yes, you can.

    sounds like handwaving to me (sorry!) :-)
    I have the same conviction, then i realise perhaps the statement should be:
    You can’t draw additional energy from wind if the wind isn’t moving relative to you.
    i forgot that when the cart is moving at wind speed, it can’t draw additional energy from the wind, but then it is drawing energy from the wind since it is moving at wind speed, and some of it is being fed to the prop continuously. (in the case of treadmill, inertial frame of reference, the kinetic energy is of course being fed from the treadmill. The treadmill tends to make one forget about the kinetic energy at windspeed.)

    Thanks for your attention.

    Now, to find how ice-boats actually go faster than the downwind. :-)

    lh

  33. Austin says:

    Haha well being a sailor myself, and having done copious amounts of work trying to engineer efficient sailcars, I was skeptical at first, but like a good little scientist I gave it a chance and what Dave is saying makes perfect sense. If you are trying to shoot the man down with logic and reason, then you should probably go into the discussion without preconcieved notions. Pretend for a moment that you know nothing whatsoever about the principles that define wind, then reread his explaination and if it still sounds like b.s. to you then please feel free to lock yourself in a closet somewhere and stay away from the scientific community ;)

  34. As some may be aware, we have now built a 16′, 350 lb, manned DDWFTTW cart, and taken it to greater than 2.5X windspeed directly downwind on a dry lake bed in Ivanpah, NV.

  35. There is — I believe — effectively a reverse bow wave, that is the equivalent of focus a larger areal section of the area than the area of the propeller, which has the effect of focusing (integrating) up more force (due to the reverse bow wave caused by the spinning propeller).

    If you have a certain uniform air velocity over a large circle and funnel the flow down to a smaller circle the air will be compelled to speed up.

    But in this case it’s not really able to speed up due to the reverse bow wave pushing air backwards (which is what causes the reverse bow wave, which is what causes a dynamical compression where a larger area of the flow arriving at the propeller is actually pushing the propeller than the area circumscribed only by the spinning propeller due to the reverse bow wave effect).

    Pretty cool thing going on here. Fun videos. I am pretty sure some fluid or gas dynamics specialists could probably come up with some analytics about this if they don’t already exist … but a complete solution of this problem may well involve extraordinarily complex mathematics (but could possibly be simplified to a rough estimation of the area of the bow wave vs. the area the propeller traces out, with an estimate of effective higher wind speed — not really faster wind speed, but as if the wind were faster — by that ratio).

  36. Or ….

    … maybe if you think about a plane of constant pressure wind encountering such a device, and that the energy gets built up in the spinning propeller such that — PERHAPS — the effective resistance in the area of the propeller is effectively less than the ambient forward resistance felt by the wind, THEN a similar focusing effect would be manifest … as the wind rushes to that area of lesser resistance.

    Again this could be approximated as the ratio of some slightly larger effective area of wind actually being (effectively) funneled to the smaller area of the propeller.

    While I was first thinking it might be like a reverse bow wave, it seems like maybe what might really be happening is the above.

    Has anyone actually solved this problem anywhere. Surely someone has, no? (I’ve just happened across this here while my girlfriend is ironing before bedtime … so killing some time and haven’t “Googled” around yet on this.)

Comments are closed.