Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

DWFTTW - Your Say

  1. Nov 25, 2008 #1
    Can a simple craft (consisting of a fan geared up to a skateboard) go “directly down-wind faster than the wind” (without any additional power source, tricks nor wind fluctuation)?
    Last edited: Nov 25, 2008
  2. jcsd
  3. Nov 25, 2008 #2
    The kinetic energy of the net movement of earth and of the atmosphere (in the centre of mass frame) is thermodynamically available and (neglecting friction and relativity) can accerate a sufficiently light mass arbitrarilly fast. Thus, DDWFTTW is obviously not physically impossible (and numerous conceptually-trivial schemes have been suggested).

    It is empirically known that yachts or sailing boats can continuously travel diagonally with (as well as against) the wind at speeds (such that even the component of their velocity in the direction of the wind is) significantly faster than the wind.
        In the boat's frame of reference (assuming steady nonaccelerating state, inviscid flow, etc) the sail can be oriented such that any (sufficiently) diagonal headwind (or tailwind) is redirected more into the sternward direction (with the same speed, by conservation of energy), such that the reaction force on the boat (while mostly perpendicular to the keel) has a positive component in the direction of the bow; (neglecting friction) a sailboat can accelerate forward as long as the wind relative to the boat is not arriving directly from the front.
        In the water's frame of reference (assuming the boat is already moving forward) this redirected breeze is always slower than the incoming wind (draw the trig'), losing energy and momentum to the boat (later frictioned to the water), independent of how fast this lets the boat accelerate compared to the windspeed. http://www.physclips.unsw.edu.au/jw/sailing.html" [Broken]

    The simple idea behind the fan and wheeled trolly contraption is that the belt (which couples their respective axels) performs exactly the role of the sailboat's keel (imagine sailing on a ringworld).
        The belt/gear ratio constrains the propellor-tip to move through space on a fixed helical trajectory of constant diagonalness (the ratio of forward to transverse motion, or pitch to circumference), ensuring that (as long as the atmosphere is moving forward relative to the ground) the propellor tips are never moving directly into the wind and thus (identifying the propellor blade with a yacht's sail) a forward thrust component can be obtained regardless of whether the velocity of the cart itself is less, equal or more than the wind velocity.
        The limiting factors are the aforementioned ratio, the fixed-angle pitch of the propellor-tip's blade-sail, the windspeed (relative to the ground), drag and friction. For any constant windspeed, the ratio and the propellor sail-pitch together determine a maximum cart velocity (downwind relative to the ground) at which forward thrust can be produced (this can be larger than the wind velocity but not infinite) but it is a tradeoff because the ratio simultaneously increases drag, and (with the ratio also fixed) tuning the propellor sail-pitch for higher cart-velocity decreases its efficiency at lower cart velocities.
    Last edited by a moderator: May 3, 2017
  4. Nov 25, 2008 #3
    Can you add one more category to that list? I'd like to add "yes but not limited to a design with a propeller".
  5. Nov 25, 2008 #4
    Prove it. ;-)

  6. Nov 25, 2008 #5
    Careful, buster, I just might do that!
  7. Nov 25, 2008 #6


    User Avatar
    Homework Helper

    Did you have to choose skateboard as the platform? I'm not so sure about the rolling friction in typical skateboard wheels, plus skate boards are relatively heavy, so you'd need a fairly large prop. If you change skateboard to cart on wheels with wheels driving a prop, then I'd agree with that.
  8. Nov 25, 2008 #7
    The reference frames consisting of 1) moving air with stationary water and 2) moving water with stationary air ar equivocal. Same is true for moving tread with stationary air vs moving air with stationary tread. The frames are equivocal. But a point everyone misses is the the craft in each frame must alo be exactly the same, with No modifications allowed. You cannot, for example use a sail in one frame and oars in the next. In the case of the cart, you cannot use one pitch on the propeller in one frame, and a different pitch in the other frame. If you do that, the frames are no longer equivocal and you cannot use what is happening in one frame to predict what is happening in the other. The entire case for DWFTTW is based on equating unequivocal reference frames because the prop pitch is changed. Therefore, the conclusion drawn from the treadmill is wrong. Directly DWFTTW powered by the wind is still impossible.

    By the way, while Russ has made it clear to me that he will only allow discussion in his thread only on this subject and has already hit me with an “infraction” because I circumvented that rule. So be afraid, be very afraid!:rofl:
  9. Nov 25, 2008 #8
    Here are my drawings which prove conclusively that that reference frames are being mixed and matched incorrectly:

    http://www.scribd.com/doc/8366040/Cart-Scam-1" [Broken]

    http://www.scribd.com/doc/8366038/CartScam-2" [Broken]

    http://www.scribd.com/doc/8366042/Cart-Scam-3" [Broken]

    Please examine them without any pre-conceived notions.
    Last edited by a moderator: May 3, 2017
  10. Nov 25, 2008 #9


    User Avatar
    Gold Member

    Get a bowling ball.

    Then get a really, REALLY, powerful leaf blower.

    Push the bowling ball down the street with the leaf blower.(might take half an hour or so....)

    When the camera is off, run in front of the bowling ball, throw leaves in front of the camera, turn on the leaf blower showing the bowling ball rolling down the street AGAINST the wind.(as indicated by the direction of the leaves)

    It is true. Trust me. Things can go faster than the wind.



    please don't ban me, please don't ban me, please don't ban me. ;)
  11. Nov 25, 2008 #10

    :rofl: But that is almost Exactly what is happening in the cart video! Why is it, at the very end of the video, when the cart slows down and we can finally get a chance to see which way the fan is turning, the cart goes OFF CAMERA? The ONLY place in the entire video where this happens! Hell of an accident or coincidence? That video is a hoax and a very poor one at that.

    Don't leave without voting. This battle is not yet started and physics will win out over superstitution this time!
  12. Nov 26, 2008 #11
    Give me a break, I'm a physicist not an engineer. :redface:

    His "My Way" isn't really a "discussion", is it? :wink:

    I think that counts as "tricks or wind fluctuations", are you changing your vote?
  13. Nov 26, 2008 #12
    No it certainly is Not a discussion! I have had remarks taken badly out of context and it is very clear Russ is too biased in the matter to fairly moderate a controlled thread. I did open a new thread with links to my drawings but it was deleted and I was given an infraction (right across the wrist) ooooh that hurts. Seriously, I am glad you started this new thread but I doubt it will last long.

    I think OmCheeto knows a vote will probably not settle this but he voted with his statement as far as I am concerned. Did you look at what I drew?
  14. Nov 26, 2008 #13
    Shroder, at the risk of being censored, I'll reply to you. I looked at your diagrams with as much of an unbiased eye as I could.

    I have a few questions.

    Cart Scam #1: why did you draw a mirror image of the treadmill test format that has been shown by JB and spork? Second, it has never been claimed that the cart will do 19m/s in a 10m/s wind, only that the treadmill test shows that it will do 10 mph in the equivalent of a 10 mph tailwind (impossible by any other means) when the cart is stationary in reference to the floor. And once again, the wheels are driving the prop - you have that backwards.

    Cart Scam #2: In all the treadmill tests, the treadmill surface is clearly moving from right to left. You again have a diagram showing the treadmill surface moving from left to right and labeled that way. Do you have a reason for that? In any case, it really doesn't matter because again the cart is able to remain stationary even when the front of the treadmill is tilted up by 4.4 degrees. It is able to climb a hill at 10 mph while running in a tailwind of 10 mph - at least that's how I interpret the test. You claim that it would move off the end of the treadmill - it didn't.

    Cart Scam #3: this is basically a conclusion which is dependent on your other two diagrams and explanations accurately representing what is going on in the treadmill tests. They don't, with the errors noted above. Your claims that the pitch of the prop is somehow being changed are based on - what?

    In reference to your claim that the prop has somehow been switched during an edit, have a good look at this video, where it is very plain to see that the prop starts in a particular direction and powers the cart against the treadmill. As the cart is lifted and the prop slows down again, you can see that the prop is still turning in the same direction, and you can see the direction of the blades of the prop and that the wheels are fixed to the prop and all are turning the right way.

    Even easier on this one that demonstrates a self start:

    You can clearly see the same orientation in this outdoor test using the same cart:


    And this one, different cart but still the same pitch and rotation:


    All of these show the same pitch and rotation. The small cart is shown in the treadmill test, a indoor self start test and an outdoor self start test with no changes.

    My question to you is did you return the favour and read my post with an unbiased mind and watch the videos with an unbiased eye? I looked at each of your diagrams and read each one at least three times to make sure I understood what you were saying, and referred back to them as I was replying.

    Was there anything in the videos that I linked that you didn't see before or didn't see correctly before?
    Last edited by a moderator: Sep 25, 2014
  15. Nov 26, 2008 #14


    User Avatar
    Homework Helper

    Sorry I misunderstood, I thought you meant modifying an actual skateboard.
  16. Nov 26, 2008 #15
    Mender, a cart on the treadmill with a properly pitched propeller will advance. I agree with that. A cart on the floor being pushed from behind by a wind with a properly pitched propeller will go down wind. I agree with that. I am saying the pitch is not the same in each instance. To prove your point that it is the same just do this: Run your treadmill test. With camera still rolling stop the tread and place the cart on the floor. Same cart, no changes! Put a fan behind the cart and turn it on and show the cart advancing at or near wind speed. All of this in one continuous unedited video. THat is very simple and can end all debate. Until you do that, you have proved nothing except the old bait and switch scam.
  17. Nov 26, 2008 #16
    I'm glad to hear that. I was really getting worried.

    Look very carefully at all the videos. It clearly is the same, which is why I went to all the trouble to find them for you.

    As I already asked, what are you basing that on? If it is because your logic dictates that because you have convinced yourself that this is impossible (which you have), you are presenting your case based on circular logic, not observation. You can prove anything with circular logic and a blind eye to what you don't want to see, and nothing will be proof enough for you.

    What would convince you that the cart is now doing the impossible and moving faster than the wind that is powering it? Seriously. How would you prove beyond a shadow of a doubt that the cart reached and exceeded the wind speed?

    I really hope Russ is able to clear up your problem with equivalent frames of reference.
    Last edited: Nov 26, 2008
  18. Nov 26, 2008 #17
    I was working with reference frames probably before Russ and you were born! If the prop pitch changes for any reason between frames the frames are not equivalent! You cannot fly a broad sail in one frame and a spinnaker in another! And since the frames are not equivalent, you cannot draw any conclusion about what will happen in one frame by makeing observations in the other. Are you willing to perform the demonstration as I delineated it or not?
  19. Nov 26, 2008 #18


    User Avatar
    Science Advisor

    Ok well this is a big change of heart. I thought this was the crux of the claim for DWFTTW. Are we now just discussing whether or not such a contraption can be self starting or not?
  20. Nov 26, 2008 #19
    There you go again! Why don't you answer my question for a change? What makes you think that the prop gets changed other than your circular logic?

    So I suppose that if it can be proven that the prop on the cart isn't changed, you'll come up with yet another reason why you can't be wrong.
  21. Nov 26, 2008 #20


    User Avatar
    Science Advisor

    Guys the argument seems to have shifted. Does it really matter if the device needs to have a variable pitch in order to self start and also to achieve DWFTTW (I'm not saying that it does need this, just that even if it does then it doesn't in any way invalidate that fact that DWFTTW is possible does it?)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook