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Here's a simple explanation: Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr dow

Here's a simple explanation: Let's assume that the windspeed is 10 km/hr. We all agree that the wind can push this vehicle so that it is now going 10 km/hr downwind, right? From the vehicle's point of view, the wind is now 0 km/hr. But its wheels are turning, which turns the propeller, which behaves like a fan, pushing the vehicle forward in the still air, so that it is now traveling faster than the wind. This isn't perpetual motion, though, because eventually the thrust generated by the propeller is insufficient to overcome the additional drag created by the vehicle's faster speed.

mumblemumble 5y ago
Your explanation that the propeller works like a fan implies a sort of perpetual motion, though, since it implies that the source of energy that pushes it past the speed of the wind is somehow coming from inside the car itself. The truth is that the car is always getting its energy from the wind, and not expending any of its own. It's just that it's been set up in a clever way that breaks our intuition. We tend to want to mentally simplify the propeller down to a disk whose motion vector is the same as that of the car. But here you've really got to think about the rotation of the propeller and the helical path its blades travel along. The wind is interacting with surfaces whose momentary motion relative to it at any given moment in time are very poorly modeled by the disk abstraction.
pkulak 5y ago
The problem with this explanation, at least to me, is that it doesn't require any wind at all. Why not just give it a push in still air and have to take off into the distance forever? EDIT: I think it's clicking. It's because the air and the ground are moving relative to the craft at different speeds. So if the ground and air were still, there would be no speed difference to draw energy from.
- tshaddox 5y ago
  Why would you consider those two scenarios analogous? In one scenario, the wind is “giving it a push” constantly, and in the other scenario, you’re giving it a push for only a brief moment.
  - DarmokJalad1701 5y ago
    When it is moving faster than the wind, is the wind still giving it a push?
    - tshaddox 5y ago
      Absolutely. If the wind stops, the vehicle will soon roll to a stop as well. It's not a perpetual motion machine.
- munchler 5y ago
  Right. This mechanism doesn’t actually require wind. You get the same effect by placing the vehicle on a treadmill in a windless room: It will move forward on the treadmill.
  - pkulak 5y ago
    Well, that's the same as having wind.
    - tunesmith 5y ago
      Yeah I think that if you put a fan in front of the treadmill, the vehicle would stop. Or is it behind the treadmill? In front, I think.
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ummonk 5y ago
There is no theoretical limit on the speed of the vehicle though - just a practical one based on limits to propeller efficiency and the fact that drag for the craft keeps going up.

mumblemumble 5y ago

Your explanation that the propeller works like a fan implies a sort of perpetual motion, though, since it implies that the source of energy that pushes it past the speed of the wind is somehow coming from inside the car itself. The truth is that the car is always getting its energy from the wind, and not expending any of its own. It's just that it's been set up in a clever way that breaks our intuition. We tend to want to mentally simplify the propeller down to a disk whose motion vector is the same as that of the car. But here you've really got to think about the rotation of the propeller and the helical path its blades travel along. The wind is interacting with surfaces whose momentary motion relative to it at any given moment in time are very poorly modeled by the disk abstraction.

pkulak 5y ago

The problem with this explanation, at least to me, is that it doesn't require any wind at all. Why not just give it a push in still air and have to take off into the distance forever? EDIT: I think it's clicking. It's because the air and the ground are moving relative to the craft at different speeds. So if the ground and air were still, there would be no speed difference to draw energy from.

tshaddox 5y ago
Why would you consider those two scenarios analogous? In one scenario, the wind is “giving it a push” constantly, and in the other scenario, you’re giving it a push for only a brief moment.
- DarmokJalad1701 5y ago
  When it is moving faster than the wind, is the wind still giving it a push?
  - tshaddox 5y ago
    Absolutely. If the wind stops, the vehicle will soon roll to a stop as well. It's not a perpetual motion machine.
munchler 5y ago
Right. This mechanism doesn’t actually require wind. You get the same effect by placing the vehicle on a treadmill in a windless room: It will move forward on the treadmill.
- pkulak 5y ago
  Well, that's the same as having wind.
  - tunesmith 5y ago
    Yeah I think that if you put a fan in front of the treadmill, the vehicle would stop. Or is it behind the treadmill? In front, I think.

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ummonk 5y ago

There is no theoretical limit on the speed of the vehicle though - just a practical one based on limits to propeller efficiency and the fact that drag for the craft keeps going up.