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Questions about DDWFTTW

  1. Dec 27, 2011 #1
    I know there have been several other threads about this, but I couldn't find answers to my specific questions, so as much as I hate to stir this up again, I'm hoping someone will take pity on a newbie and talk me through this. I would like to say that I have no doubts that these vehicles work, and I'm simply trying to understand the principles involved.

    First, what I think I understand so far:

    At first I couldn't understand how the propeller could deliver enough thrust to overcome the friction on the wheels, if the friction was providing the power. But then I realized that the propeller doesn't move as far through the air as the wheels travel across the ground since there is a wind. So for the same amount of power, the propeller can apply more force. So once at wind speed, thepropeller can accelerate the cart to faster-than-wind speeds. It can continue to accelerate until the headwind is strong enough to balance out the additional thrust, where it then reaches a constant, faster-than-wind speed.

    But what confuses me now is this:

    If you assume that the wind delivers its maximum amount of power to the cart when it travels at wind speed, as happens with a sailboat, you have a problem. At faster than wind speeds, the wheels have even more power, since they are moving faster. But at this point the wind should be applying negative power in the form of a headwind.

    I thought maybe the negative power from the headwind might simply balanced out the additional frictional power from the wheels, so that the cart as a whole has the same amount of power as it does at windspeed. But in order for this to be the case, the drag would have to be constant and equal to the frictional force. But drag changes with velocity, so this can't be.

    So my assumption that the wind is delivering max power to the cart at wind speed might be wrong, or else I am wrong about something else. How does the cart harvest additional power from the wind at faster-than-wind speeds?

    One other question:

    I read in one explanation that the propeller can act as a windmill to drive the wheels at slower-than-wind speeds, and then be turned by the wheels at faster-than-wind speeds. But after playing with a fan for a few seconds, I realized that if this were the case, the thrust provided by the prop in the second stage would be in the wrong direction. Could someone clarify how the prop acts at slower-than-wind speeds?

    Thanks in advance for your patience, and I apologize if I've made any particularly annoying mistakes in my first attempts.
  2. jcsd
  3. Dec 27, 2011 #2


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    That is the key aspect. There is effective gearing to increase force and decrease speed, enough so that inspite of power losses due to inefficiencies, there's still more force (but at a lower speed) at the prop than at the wheels.

    AT responded to this in the following post.

    Power is extracted from the wind when the wind is slowed down by the thrust from the propeller. This occurs as long as the thrust speed from the propeller is greater than the relative (wrt cart) headwind. If I recall correctly, the effecting gearing of prop thrust speed : ground speed is about 4:5 for the blackbird cart. Say the max speed is 3x wind speed (one test run was 2.8x), then thrust speed would be (4/5 x 3 =) 2.4 x wind speed, and relative headwind speed would be (3 - 1 =) 2 x wind speed, so thrust speed would be 1.2 x relative heawind speed, and the wind would still be slowed down.

    With a fixed pitch propeller which is what these carts use, at startup and at slow speeds, the prop acts as a bluff body, simply blocking (and slowing) the wind which pushes against the prop, pushing the cart forward. The initial acceleration is relatively slow. As speeds increase, the propeller starts to spin fast enough to start producing significant thrust, and then it's the thrust that slows down the wind as opposed to the propeller itself.
    Last edited: Dec 27, 2011
  4. Dec 27, 2011 #3


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    The best way to avoid confusion is to be precise:

    - Make clear which reference frame you are analyzing (power/kinetic energy are frame dependent quantities)
    - Distinguish between "air" with "wind" (movement of air relative to something)
    - Distinguish between true wind (relative to ground) with relative wind (relative to cart)
    - Distinguish between work done by the cart chassis on the air, with work done by the propeller on the air.

    Being precise in formulating the questions, often makes the answer obvious.

    That is not necessarily true. Depending on the propeller pitch the acceleration can be not maximal at WS but rather above it. So the increase in KE (seen from the ground frame) is maximal there. But the power transmitted though the vehicle always increases with speed. Here I posted some simulated values:


    From the ground frame: Some of the air is doing negative work on the cart chassis. But there is more positive work done on the propeller blades by the air.
    The propeller always slows down air relative to the ground. The faster you go, the more volume of air you encounter, that you can draw KE from. But that increase is linear, while chassis drag and transmission inefficiency increase non linearly with speed.

    Have a look at the table below that shows different settings for a variable blade pitch propeller, coupled to the ground via wheels. What you describe above is starting out in CASE A (that gives you maximal initial acceleration) and then at some point below 1WS switching to CASE C that allows you to go faster than wind.


    Note that the Blackbird didn't have that ability (even when it had variable pitch later). They didn't want the ability to turn the wheels with the prop, to avoid confusion about using stored energy. They used CASE C only.

    But Andrew Bauer was using his propeller as a turbine below windspeed. Here is video where you can see him starting in "windmill mode" and change the blade pitch later.
    See also the graphs on page 15 in Bauer's paper.
    Last edited: Dec 27, 2011
  5. Dec 27, 2011 #4
    Sorry for coming late to this. I don't know what DDTWFFTW is but as a sailor i can tell you that sailing faster than the wind is easily possible, both upwind and downwind, using only sails (and I can explain how to do it if needed). Can some person please give me a quick summary of what this thread is about?
  6. Dec 27, 2011 #5


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    DDWFTTW - directly downwind faster than the wind. This means a vehicle that doesn't tach like a sailcraft in order to achieve faster than wind speed. Instead, the propeller is linked to the wheels of the vehicle. The wind pushes the vehicle, while the wheels drive the propeller, which produces a thrust that opposes the wind. Since there are losses involved in the conversion, power output at the propeller is smaller than power consumed by the wheels, plus there is rolling resistance and aerodynamic drag to overcome when the vehicle is going directly downwind faster than the wind.

    There is effective gearing from the wheels to the propeller that multiplise torque and divides speed, which allows the propeller to generate more force (but at a lower speed) than the opposing force from the wheels, in spite of the reduction in power due to losses.

    This only works when there is a tailwind, because the ground speed (relative to the vehicle) is always greater than the (head) wind speed (relative to the vehicle), which allows for the effective gearing.

    Recent threads about this include links to videos of a full scale model. For a while, there was some debate about whether such devices would work.


  7. Dec 27, 2011 #6
    Directly Downwind Faster Than The Wind
  8. Dec 28, 2011 #7
    Interesting concept, never seen that before.

    We can exceed wind speed on a sailboat so long as the wind isn't directly ahead or directly behind.
  9. Dec 28, 2011 #8


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  10. Dec 28, 2011 #9


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    To visualize this comparison:


    Here the vectors for a sailcraft constrained to a fixed course:

    http://img253.imageshack.us/img253/6694/downwindvectorsen3.png [Broken]

    Here an example airfoil of a propeller blade, that is coupled to the wheels. The coupling constrains the airfoil to a helical path which also means a fixed angle to the true wind direction:

    http://img811.imageshack.us/img811/4922/propellervectors.png [Broken]
    Last edited by a moderator: May 5, 2017
  11. Dec 28, 2011 #10

    I do not understand this.

    I have never sailed an ice boat but in your link they are shown to have triangular rigs (sails) that look similar to sailing yacht rigs. I think they will handle in the same manner. The boat would be heading 0 degrees and the wind coming from around 150 degrees (each boat is different). This is downwind. In this position you can exceed the wind speed.

    Directly downwind means the boat is heading 0 degrees and wind coming from 180 degrees. A triangular rig cannot exceed the wind speed in that position using only power from the wind. The movie of the propeller car shows that it does it easily.

    PS: I must add this applies only to triangular rigs. Tall ships like the ones of old had square sails and are different.
    Last edited: Dec 28, 2011
  12. Dec 28, 2011 #11


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    The helical path isn't a requirement. It could be possible to construct a DDWFTTW vehicle using a tread mill with bluff bodies similar to a paddle wheel. The wheels would drive the treadmill and it's vanes upwind. The vanes would collapse when going downwind. A paddle wheel is not as efficient as a propeller, and the losses in the treadmill would be greater, and I don't know if an actual model could be made to work, but the basic concept would remain the same, the advance ratio would be set so that the treadmill speed would be less than the wheel speed.

    I didn't mean to imply the ice boat was heading directly downwind, only that it had a downwind component of velocity greater than the wind. In the pdf file from the second link in my previous post, there is a diagram showing 3 vectors, 18 mph wind speed, 70 mph boat speed, and 55 mph apparent wind speed (relative to boat). This translates into a situation similar to what you decribed, boat heading of 0° at 70 mph, true wind heading of 30° at 18 mph. VMG downwind for the boat would be cos(30) x 70 mph = 60.6 mph, about 3.37 x true wind speed. (That diagram shows beta as 8°, but it should read 9° (beta ~= 9.2°)).
    Last edited: Dec 28, 2011
  13. Dec 28, 2011 #12


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    Look at the "Sail to Prop" animation above. The propeller blades are not moving directly downwind even if the car does. This is equivalent to a sail-craft on a broad reach.
  14. Dec 28, 2011 #13
    Thank you to rcgldr and AT. This is understandable now.

    Now I ask about his propeller. If he is exceeding true wind speed, he has effective head wind. Putting the propeller on the back, or behind the supporting post for it, would cause the body of the craft to disrupt airflow reaching it? But I don't understand propellers! (They point forwards now on ships with pod drives)

    edit - Wait, I see now the other, newer DDWFTTW have forward pointing propellers. I'm OK now with that. I also have no idea how it steers but I do not think it's supposed to! :-)
  15. Dec 28, 2011 #14


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    Last edited: Dec 28, 2011
  16. Dec 29, 2011 #15


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    The one by Jack Goodman was radio controlled:


    That is the video that sparked the internet debate, and was initially declared a hoax by MAKE magazine.
  17. Dec 29, 2011 #16


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    I had forgotten about that video. I also think the more recent debate started at myth busters, but I'm not sure. There were a few threads here, some of the early ones were locked. The youtube link wasn't allowed to be embedded, so here is the url:

    Last edited by a moderator: Sep 25, 2014
  18. Dec 30, 2011 #17
    Having now looked at this I arrive at the conclusions that even though the machines work, they are not sailing ... the prop is linked the wheels (sometimes) and they have no sails. They should rename this.
  19. Dec 31, 2011 #18


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    Define "sailing".
    On a conventional sailing craft the sail is also connected to the keel, via the structure.
    Define "sail".
    The North American Land Sailing Association has no problem calling it an "unconventional sailing craft".
  20. Dec 31, 2011 #19
    Has anyone tried to build a boat version of this? A prop in the air geared to a prop in the water. Obviously the losses would be greater but It would be interesting to see a proof-of-concept prototype.
  21. Dec 31, 2011 #20
    This video above (which you can also view on its YouTube video page ) does *not* explain DDTWFTTW. In the video, Scenario 1 shows the sailboats clearly moving DTWFTTW, but not DDTWFTTW, and it shows in Scenario 2 the propeller moving DDTWATSSATW (directly down wind at the same speed as the wind).

    The correct explanation:

    We have terms for the net power of the vehicle (i.e. ΔKE / Δt)
    Code (Text):

    force    velocity  power    source
    +v^2     +v        +v^3     propeller thrust (v>0)
    +(w-v)^2 +(w-v)    +(w-v)^3 parachutic thrust (v<w) or parachutic drag (v>w)
    -(w-v)^2 -(w-v)    -(w-v)^3 wind2propeller2wheel drag (v<w) wind2propeller2wheel thrust (w>v)
    This can be represented as the sum of two cubic equations. Say for example:

    net power = x(v^3) + (y-z)(w-v)^3

    x, y and z are coefficients,
    v is the vehicle speed, and
    w is the wind speed.

    If y-z>x, then one can have a system of equations that looks like this:

    Attached Files:

    Last edited by a moderator: Sep 25, 2014
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