DDWFTTW - Directly Downwind Faster Than The Wind

Discussion in 'Propulsion' started by Guest625101138, Jan 4, 2009.

  1. gonzo
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    gonzo Senior Member

    Tacking downwind is different from going directly downwind.
     
  2. DogCavalry
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    DogCavalry I aim to misbehave.

    There's a lot of going in circles, with irrelevant drivel thrown in, by folks with MA's. References to sailing downwind are equally meaningless. The premise is directly downwind, and nothing sails directly downwind even at wind speed, let alone faster. It's also an irrelevant digression to speak of lateral resistance. Directly downwind there's no lateral vector to be resisted.
    I'd like to see a vector analysis, at 3 states: slower than wind, at wind speed, faster than wind speed.

    It's a good point that the propeller is never a turbine extracting energy, because if it was, it would need a reverse gear when the test vehicle transitioned from below apparent wind speed to above wind speed.
     
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    If we are to treat no frame of reverence as privileged, then for this to work, it must be possible to drop this rig directly on a moving conveyor belt, and have it drive upstream, until it drives off the end against the motion of the belt.
     
  4. DCockey
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    DCockey Senior Member

    Gonzo, I assume that you are trying to understand how the device works.

    The energy comes from the velocity difference between the ground and the air; not from the air by itself or from the ground by itself.

    I assume no one is arguring that a propeller cannot create thrust the the velocity of the approaching air is zero or negative.

    When the frame of the device is moving relative to the ground at the same speed as the air (zero speed relative to the air) the flow of power in the device is from the wheels to the propeller. The net force of the propeller on the air is in the direction of the motion of the frame, and the net force of the wheels on the ground is in the direction opposite the motion of the frame. This is the opposite of what is usually assumed.
     
  5. gonzo
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    gonzo Senior Member

    That is incorrect. The energy comes from the difference in velocity between the wind and the propeller or turbine. Whether the energy is transferred to wheels, the hull of a sailboat or a grain mill it is not relevant. If you agree that a propeller can't absorb power, rather than create thrust, when the velocity of the air is zero, where is the energy coming from to move the cart? The velocity difference between the ground and the air is not relevant. If the cart was on a conveyor belt, and for the exercise of this discussion assume no losses due to friction, then the velocity of the cart over the belt will be the same in relation to the air, but change depending on the velocity of the conveyor belt. If it was as you say, the belt moving towards the wind would accelerate the cart, which is not true.
     
  6. DogCavalry
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    DogCavalry I aim to misbehave.

    Gonzo, you're still thinking about it backwards. When the cart is at wind speed, the wheels are turning very fast, unless they are dragging across the ground. Those very fast wheels are turning a prop, which sees very high apparent wind, because it's been turning the entire time, and it has fully developed the flow of air around itself.
    When the vehicle started moving, the prop was only a drag device, and from the first moment, was geared the opposite direction it would have had to be to function as a turbine.
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    I am quite certain this could not work as a boat. It would need a turbine in the water, at a crap energy recovery rate, driving a prop, with considerable gearing losses.
     
  8. gonzo
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    gonzo Senior Member

    What is making the wheels turn if there is no wind energy input on the turbine?
     
  9. DogCavalry
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    DogCavalry I aim to misbehave.

    Drag. The wind blows the object across the ground. The wheels can either turn, or be dragged.
     
  10. Will Gilmore
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    Will Gilmore Senior Member

    As I understand it. The wind velocity is only there to add to the power output of the propeller. The propeller is not driven by its relationship to the wind, but rather by its relationship to movement across the ground. Therefore, force does not drop to zero just because the cart's velocity matches wind velocity.

    Up until the velocity of the cart matches wind speed, the force of the propeller is pushing against a wind that is going faster than the cart, but the propeller's force is small because the cart's speed across the ground is slow. As the cart picks up speed, the propeller puts out more driving force until, at the point it is traveling equal in velocity to the wind, any acceleration after that is due to the additional force the propeller puts out beyond the force the wind is aiding in driving the cart. The wind will always be adding the power of its velocity to the cart.

    -Will
     
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  11. DogCavalry
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    DogCavalry I aim to misbehave.

    Will, you're getting close. Force would indeed drop to zero as the device reached wind speed. Except wind is also reacting against the air stream out of the prop, which by then is a meaningful value. So there is still large relative wind speed against the disc of the drop.
     
  12. Will Gilmore
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    Will Gilmore Senior Member

    That's what I was trying to say.
    The wind's force relative to the ground is adding to the propeller's force relative to the ground. When the friction forces in the system equal the sum forces of the wind plus the propeller, no acceleration is achieved, until then, the faster the wheels turn the more force the propeller is driving the cart with.
     
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  13. DCockey
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    DCockey Senior Member

    Let's start with the cart on a conveyor belt, held in place. It is in a steady state condition. The wheels are spinning, and the wheels are connect to the the propeller and turn the propeller. The wheels have a "drag" force which is trying to move the cart backwards. The propeller is spinning and creating thrust, trying to move the car forward. There is no aerodynamic drag because the cart is not moving relative to the air.

    The ratio of the propeller speed to wheel speed and the propeller size and pitch are selected so the the thrust of the propeller is greater than the drag of the wheels. When the cart is released the cart accelerates and travels forward.
     
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  14. Will Gilmore
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    Will Gilmore Senior Member

    I think we need to look at the wind speed of the true wind relative to the wind speed of the propeller driven wind.

    With wind moving in the direction of the cart at 10 knots, the force on the cart is from a 10 knot wind only when the cart is standing still.

    However, once the cart begins to move, the propeller generates a wind that pushes back against the true wind at some velocity n, where n is the speed of the propeller driven wind relative to the cart [Let's just say that n=2s: where s=cart speed]. That means, as the cart accelerates to 1 knot, the propeller may be putting out a backwards directed wind of 2 knots (for an off the cuff example). Now there is a relative force from only 9 knots of true wind pushing directly against the 2 knot propeller driven wind. This results in a total of 9 knots + 2 knots driving force. The sum total of the forces driving the cart are now 11 knots of wind speed.

    Accelerate the cart to 5 knots and the driving force from the wind will be 10 knots true wind - s + 2s, or 10-5+10=15 knots of wind speed to push the cart.

    Efficiency aside, once the cart has accelerated to 10 knots, it is still expriencing a growing net force from 10-10+20= 20 knots of wind speed.

    The 10 knots of true wind doesn't go away no matter how fast the cart goes. It is always added in a a positive value.

    -Will
     

  15. kerosene
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    kerosene Senior Member

    ground.

    did you watch the videos?

    this has been proven. many smart folk, like recently a UCLA professor who bet $10k against the principle, have come around from strong denial to understanding it. It is a brilliant brain teaser for being so unintuitive. I recommend for a while to accept (or pretend to agree) and try to understand it instead of putting the effort in proving it doesn't work. because it does

    Blackbird cart did 2.8x wind speed in a sanctioned run. it works. Even when it travels downwind faster than the air it is powered by the speed difference of air and ground.
     
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