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DWFTTW - Your Say

  1. Nov 26, 2008 #21
    That IS the critical question Uart. Thank you for asking it. Yes it matters greatly how the cart is configured. Mender, Spork et all go to great pains to emphasize that the reference frames are equivalent. Indeed, the frames themselves are equivalent. But the cart or boat or whatever is IN the frame must also be exactly the same in both frames. You can’t have a sail boat in one frame and a motor boat in the other, for example. ALL the evidence for DDWFTTW is based on a cart advancing on a treadmill. Then that evidence is used to prove that a cart can advance DDWFTTW. There is no evidence to support that claim other than the treadmill evidence. As I said, with a prop of a certain pitch, a cart can advance on a treadmill. In order for anyone to use that evidence that it can also go down wind, the prop pitch must be exactly the same as on the treadmill. If the pitch is changed to go down wind, the treadmill evidence means nothing as you are essentially comparing two different machines. Please consider that carefully.
  2. Nov 26, 2008 #22


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    The prop pitch on the mini-carts is fixed, 10 inches per revolution. The effective pitch will be less depending on the prop speed relative to the air speed.

    From a standing start at 0 mph in a 10 mph tailwind, the prop and cart itself have enough form drag to propel the cart forward. One of the videos clearly shows this, with the prop initially turning the wrong way and the tires sliding from a gust of wind. So the cart will be able to self start.

    At speeds well below the wind speed, form drag will keep the cart moving forwards. Form drag alone would probably allow the cart to reach 8 mph or so in a 10 mph wind (gearbox removed from cart). The thrust from the prop in effect uses the induced air wash itself as a bluff body that is moving backwards with respect to the cart opposing the tail wind, allowing the cart to go DDWFTTW, if the losses in the power conversions are low enough.

    For another treadmill analogy, imagine drafting a very large truck while riding a bicycle at 10 mph. To the bicycle rider the relative air speed is 0 mph, and the ground is moving backwards at 10 mph. To an observer on the side of the road, the truck, the bicycle rider, and the air surrounding the bicycler rider are all moving forwards at 10mph. Next, replace the truck with a 10 mph tailwind. The rider experiences the same relative speeds, 0 mph air speed, -10 mph ground speed. The roadside observer also sees the same conditions as before, bicycle and air moving at +10 mph.
  3. Nov 26, 2008 #23
    If schroder's argument between the frame of reference is that the pitch of the propeller has been changes between frames, he has no argument.

    The pitch of the prop is fixed and the same prop is used in both frames.

    Repeat: There are NO changes to the prop or the cart.

    Schroder still seems to be stuck in this mode that many people are stuck in that the little spinny pinwheel thing on the back of the cart acts as a "propeller" when on the treadmill and as a "turbine" when on the street.

    If he were correct on the above, his point (about pitch) would be valid. But he's just wrong so his point about pitch and his point about frames are both invalid.

    The spinny pinwheel thing on the back of the cart acts as a propeller at all times the cart is approaching, at or above TWS. The air flow through the spinny pinwheel device is from front to rear and the area in front is low while behind is high. That last sentence *defines* the difference between a prop and a turbine.

    Again, no change between treadmill and street.

  4. Nov 26, 2008 #24


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    I think he was getting at the situation where the cart is well below wind speed, in which case the prop acts as a bluff body (sail). Assuming the cart is geared properly for DDWFTTW, then the prop never acts as a turbine unless the driving wheels are sliding forwards.

    The effective gear ratio means that force at the prop is divided by the effective gear ratio when applied to the wheels which move forwards faster than the prop by the effective gear ratio. If the driving tires aren't sliding, then the forwards thrust force on the wheels is greater than the backwards force cause by reverse torque from the prop and differential at much slower than downwind speeds where the air flow through the prop is forwards (from back of cart to front of cart).

    Say the cart is geared 2:1 (wheel speed to effective prop pitch speed). At 0 mph, say a 10mph wind exerts 3 lbs of force on the prop, plus an addiional .5 lb of force on the cart itself, all due to form drag. Via the differential, the torque on the prop acting as a turbine goes through the differential and exerts the equivalent of 1.5 lbs of opposing force at the wheels. Since the forward force on the driving wheels is about 3 lbs, about double the opposing force due to "turbine" effect, the cart accelerates downwind and the wheels drive the prop even though the air is flowing the "wrong" way through the prop. The forward force of 3 lbs of the wheels is almost doubled via the effective gear ratio to about 6 lbs (minus losses) to opposed the 3 lbs of force from the air flowing the "wrong" way through the prop, so the prop also accelerates in the "right" direction. Since the forward forces are larger than the backward forces, the forces are in imbalance, and the cart and prop accelerate.
    Last edited: Nov 26, 2008
  5. Nov 26, 2008 #25
    Well, I can't say for sure what he thinks, except that for some reason he thinks that the pitch is changed between treadmill and street -- something that just isn't true.

    You are correct in your statement that when the cart is moving forward, the blades cannot act as a turbine unless the wheels are slipping and spinning backwards.

  6. Nov 26, 2008 #26


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