DDWFTTW - Directly Downwind Faster Than The Wind

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

  1. DCockey
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    DCockey Senior Member

    No where did I say anything about efficiency of the wheel to propeller connection.
    This claim is wrong. "Leverage", the force of the propeller on the air being greater than the force of the ground on the wheels, is what makes it work. An explanation of how it works while assuming the force of the propeller on the air is equal to or less than the force of the ground on the wheels requires an incorrect assumption such as the fanciful claim that a portion of the force of the wheels on the ground can be neglected from Newton's second law.

    RysiulPL - Do you understand that the force of the propeller on the air can be greater than the total force of the ground acting on the wheels? Perhaps you are confused by the difference between force and power/energy/work.
     
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  2. RysiulPL
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    RysiulPL Junior Member

    If the wheel is connected (rigid connection) to the propeller and they rotate equal. Without any leverage. You suggest it will not generate thrust force? Propeller can generate thrust force even with headwind (because it rotates fast enough). edit: i meant backwind.

    Have you gone through the analysis I did with Kerosene. Can you answer the questions I asked? Do you agree with the initial state of the experiment?
     
  3. kerosene
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    kerosene Senior Member

    have you looked at the videos I posted?

    did you look at my example with the powers and other numbers?

    BTW how do you define "no leverage"?

    what pitch of the prop and rpm would constitute "no leverage".

    I am out of this discussion soon, other commitments with better reward ratio.
     
  4. kerosene
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    kerosene Senior Member

    yes even though they have no props.

    But cars have wide cushioned tires and weigh from 1.5 tons up. Pushing a car on the road is pretty tough. I bet the wind cart is very easy to push around yet will have aerodynamic resistance in the same realm as a car.
    Rolling resistance on the wind cart is essentially meaningless.
     
  5. RysiulPL
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    RysiulPL Junior Member

    I will sit for it tonight or tomorrow, I have to do it in concentration. If you want I can relate to it after studying it.
    I refer to the definition proposed by Veritasium as an explanation of the phenomenon:
    “this car works like a lever or a pulley by applying a small force to the wheels over a larger distance, the propeller can apply a larger force over a smaller distance. This is just like when you're riding a bike going up hill, you move the pedals fast, but with smaller force, to make the wheels move slower over the ground, but with a bigger force."
    "No leverage" is a state in which we do not need to multiply the force by using a gear or a lever. And it works.
    I know that feeling, for me it is also only "hobby" but it is very interesting topic :) I hope you don't feel discussion with me was a waste of time. Best Regards!
     
  6. kerosene
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    kerosene Senior Member

    yeah I was pointing at the leverage definition as simple thing like gearing ratio from wheels to propeller or just changing to another propeller with a different pitch will be just that - changing leverage. There is no "wheels fixed to a propeller" standard or neutral setting that we could rely on. At least I cannot think of one.
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    Indeed there is not. In fact this thing would work much better with a multispeed transmission. But not critically necessary, obviously, since it has been proven to work without one.
     
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  8. gonzo
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    gonzo Senior Member

    I watched the video, and it has a fundamental flaw. It assumes the cart can move at the same speed as the wind. It does not take into account that the force applied goes to zero as the velocities become equal.
     
  9. Will Gilmore
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    Will Gilmore Senior Member

    Gonzo, you are still thinking the wind is driving the cart or the blades. It is not. The wind only starts the cart moving enough that the forces from the propeller, with the small addition of that true wind aid, allow the cart to accelerate beyond the True Wind Speed.

    Ultimately, the friction forces in the turning gears and shafts and bearings plus the skin friction of the apparent headwind on the cart are what keep it from accelerating forever.

    There may be a point at which the wheels turn the blades so fast that they cavitate or stall or simply fail to maintain a positive angle of attack, but the wind is not driving the cart like the wind drives a sail or a windmill.

    The TW is like the tiny force that keeps a driven harmonic system going with much bigger movement of the pendulum.

    -Will
     
  10. Will Gilmore
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    Will Gilmore Senior Member

    Another way to think about this problem is to imagine the cart is a blimp. It is completely immersed in the air, without any other means of propulsion, no propeller, no drive wheels, no rocket engine. Just floating in the air, she will move at wind speed, with the ground as the frame of reference.

    Now add a propeller that is capable of driving her forward but has no force to turn it. Attached to the propeller is a wheel that if turned, will turn the propeller. The cart is floating in the air over the ground moving at wind speed. Still, nothing but air and ground and the blimp/cart is moving with the air, zero velocity or moving relative to the ground at WS.

    Lower the wheel to the ground that is moving by at WS. If you don't account for friction, the wheel freely drives the propeller because it is being driven by the passing ground. The propeller adds its own driving force to the cart and she speeds up because now she has a driving force pushing/pulling her through the air. It may be a small force, she may not go much faster than the air, but she will be driven through the air because the propeller adds its force to the already surrounding force of the air. The air is only a force on the cart because we move the frame of reference from the air to the ground. Otherwise, freely floating in the air with the air as the frame of reference, the propeller becomes the only force acting on the cart.

    Add back in, friction. There should still be an overall net gain in forward movement unless the friction is enough to stop the cart completely and turn it into a wind mill standing still in the field with its blades frozen in place by the breaks of the wheels touching the ground or the driving force of the propeller is exactly matched by the friction in the bearings of the drive wheel.

    Perhaps the friction will only slow the cart down? But that would mean the propeller can't drive with more force than the friction that's slowing it. The drive wheel touches the ground and adds a negative force to the passing ground, but it also adds a positive drive force to the turning propeller blade.

    If the air were solid, that is, allowed for no slippage of the propeller blade in the air and so the blade moves through the air like a steel worm screw, the wheels moving against the ground would not be able to do anything but add to the force of the turning worm screw and forward motion would happen in the air. Friction be damned. The cart can then accelerate forever as she gains speed and the drive wheel goes faster and faster turning the worm screw faster in turn.

    But, the cart has friction and the propeller blade has slippage and it also has that added drive through the moving air. The cart just needs to get to that point, moved by the air, where the ground is going past fast enough to drive the propeller and add its driving force to help. The resistive friction of the bearings between the drive wheel and the propeller blade is much smaller than the driving force of propeller blade in the air. Propeller blades can drive airboats over 100 miles per hour. That's a lot of force.

    -Will
     
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  11. kerosene
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    kerosene Senior Member

    RysiulPL

    if you want a hint that maybe you should approach this from a learning point of view rather than lecturing:

    upload_2021-9-22_15-34-42.png

    FYI: Rick is the creator of the Blackbird cart.
     
  12. DCockey
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    DCockey Senior Member

    Without friction between the wheel and the ground the wheel will not turn.
     
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  13. kerosene
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    kerosene Senior Member

    That actually should ring some bells about the force and its direction. But I think there is assumption that operating the propeller doesn’t require work.
     
  14. Will Gilmore
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    Will Gilmore Senior Member

    So true. I was only considering the ideal pulley scenario we all had in our physics books. But yes. Friction is needed to turn the wheel against the ground. Friction over the blades of the propeller would also be required or the air would simply move aside without effort or effect.

    And also true. I was merely trying to change the paradigm for a different perspective on the same problem.
     

  15. kerosene
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    kerosene Senior Member

    I know you probably meant rolling resistance but in this thread even that has special meaning.
     
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