7 ms·

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the pr

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

Valgrim 5y ago
Then what is spinning the wheels?
- wizzwizz4 5y ago
  The fact it's going forwards.
  - Vvector 5y ago
    And what's making it go forward?
    - Retric 5y ago
      The wind.
      - wizzwizz4 5y ago
        The propeller pushing backwards on the wind uses up less energy than it gets from the wind blowing it – when it spins backwards to “keep pace with” the wind, that means the wind is faster than it and it can get boosts from the wind.
- foerbert 5y ago
  The wind, I think. My current impression is that when it is starting, the whole thing is basically just acting as a big sail. You could lock the propeller and prevent it from spinning and it would still move forward. After that is where I start to get more fuzzy on the matter. On the one hand it makes sense that by using some of the speed from the wind to do other things, you can extract more total energy from the wind. And by doing propulsion as that "other thing" it makes sense you get up to a higher speed. But then it seems to me like there would be a problem once you go faster than the wind, because it's no longer pushing you. I'm not sure if some other effect takes over, or if I'm thinking about the wind in an incorrect way, or if my whole line of thought about this is wrong.
  - Joeri 5y ago
    I may be wrong, but this is how I understand it. The propellers push against the wind. Effectively you’re creating a faster wind by pushing back at it. The energy that drives the propellers still comes indirectly entirely from the wind, but wind across a larger volume behind the propeller so therefore carrying more energy.
    - jasonwatkinspdx 5y ago
      You've got it exactly right.
- mumblemumble 5y ago
  Asking what is spinning what is maybe framing it in a way that impedes understanding. A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else. I think that what this all adds up to is that, by introducing this mechanical linkage, the cart is extracting energy from the difference between the wind speed and the ground speed. This runs counter to our intuition, which assumes that it should be getting its energy from the difference between the wind's speed and its own speed. That's what allows it to go faster than the wind. If it were based off the difference between the wind speed and the cart speed, then the forward force would go to zero as the cart's speed approaches the wind speed. But the difference between the wind speed and the ground speed is not related to how fast the cart itself is moving. So the forward force on it doesn't disappear as it approaches the speed of the wind, and instead it will continue to accelerate until the forward force balances with drag and rolling friction. A kite might be a good starting intuition pump here? Kites generally don't do much of anything useful unless they're mechanically tethered to the ground. From there move on to sailboats. The reason why this cart can go downwind faster than the wind isn't all that far off from the reason a sailboat can sail into the wind (albeit not directly into the wind), but, critically, only if it has a keel or centerboard. (Edit: Another detail to point out is that the linkage causes the propeller to turn in the opposite direction it would if it were spinning freely. And remember that Newton's third law works in both directions at the same time.) (Edit again -- Another observation that might help change one's intuition is that, while the cart as a whole may be moving relative to the ground, the part of the cart that's currently touching the ground at any given moment is more-or-less stationary with respect to the ground. And is also mechanically linked to the propeller.)
  - deleted 5y ago
    [deleted]
  - jfoutz 5y ago
    Sailboats are one I struggled with, especially sailing into the wind. I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well. the closer you get to perpendicular to wind, the faster you can go. I still don't quite get sailing into the wind, but I'd imagine this wheel and propeller system works the same way. The vehicle motion restricts moving to just a line, the wind pushes along the x axis, so the vehicle can sort of zoom along in the y direction, much faster than moving along x. __edit__ oh, I see, it's the difference between ground and wind. a sailboat couldn't do this but something clever with propellers and linkages probably could
jrs235 5y ago
So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it? Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?
- Retric 5y ago
  No, it’s light enough for the wind to start pushing it.
  - jrs235 5y ago
    Even with 100+ pounds (50+ kg)?
    - Retric 5y ago
      Yes, at the extreme end you can find videos of 3,000+ lb cars flipped by the wind. It looks quite aerodynamic, but by comparison I can start pushing a 3,000lb car on flat level ground. So this thing probably needs less than 10 pounds of force to start moving.
- mumblemumble 5y ago
  Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough. The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.
  - jrs235 5y ago
    Understandable that it doesn't take much wind for a vehicle well under a pound. But what about one that weighs well over 100 pounds?
    - 5y ago
karmakaze 5y ago
A better way of looking at this is to not think of 'the propeller' as being in one plane for the wind to 'push'. Since the prop is rotating, consider the 'airspeed' of the surface of a blade. You can have zero airspeed when the vehicle is moving forward faster than the wind. Similarly the airspeed of the blade is slower (actually negative) than the vehicle's airspeed which allows the wind to push the blade surface when the vehicle is moving faster than the wind. If you think of a longitudinal vertical plane you can calculate the speed of a point on the cross-section of the blade moving backward relative to the vehicle as a function of the rotation speed. As the vehicle goes faster, so too does the prop surface 'move' backward.
cblconfederate 5y ago
Gravity does

You're right that it falls far short, but it corrects one of the fundamental misunderstandings many people have about the vehicle: the wind does not spin the propeller, the wheels do.

Valgrim 5y ago

Then what is spinning the wheels?

wizzwizz4 5y ago
The fact it's going forwards.
- Vvector 5y ago
  And what's making it go forward?
  - Retric 5y ago
    The wind.
    - wizzwizz4 5y ago
      The propeller pushing backwards on the wind uses up less energy than it gets from the wind blowing it – when it spins backwards to “keep pace with” the wind, that means the wind is faster than it and it can get boosts from the wind.
foerbert 5y ago
The wind, I think. My current impression is that when it is starting, the whole thing is basically just acting as a big sail. You could lock the propeller and prevent it from spinning and it would still move forward. After that is where I start to get more fuzzy on the matter. On the one hand it makes sense that by using some of the speed from the wind to do other things, you can extract more total energy from the wind. And by doing propulsion as that "other thing" it makes sense you get up to a higher speed. But then it seems to me like there would be a problem once you go faster than the wind, because it's no longer pushing you. I'm not sure if some other effect takes over, or if I'm thinking about the wind in an incorrect way, or if my whole line of thought about this is wrong.
- Joeri 5y ago
  I may be wrong, but this is how I understand it. The propellers push against the wind. Effectively you’re creating a faster wind by pushing back at it. The energy that drives the propellers still comes indirectly entirely from the wind, but wind across a larger volume behind the propeller so therefore carrying more energy.
  - jasonwatkinspdx 5y ago
    You've got it exactly right.
mumblemumble 5y ago
Asking what is spinning what is maybe framing it in a way that impedes understanding. A quick summary of the basic forces involved are: The wind pushes the propeller, the propeller pushes the wheels, the wheels push the ground, the ground pushes the wheels, the wheels push the propeller, and the propeller pushes the wind. Or perhaps it's better to say that everything is dynamically interacting with everything else. I think that what this all adds up to is that, by introducing this mechanical linkage, the cart is extracting energy from the difference between the wind speed and the ground speed. This runs counter to our intuition, which assumes that it should be getting its energy from the difference between the wind's speed and its own speed. That's what allows it to go faster than the wind. If it were based off the difference between the wind speed and the cart speed, then the forward force would go to zero as the cart's speed approaches the wind speed. But the difference between the wind speed and the ground speed is not related to how fast the cart itself is moving. So the forward force on it doesn't disappear as it approaches the speed of the wind, and instead it will continue to accelerate until the forward force balances with drag and rolling friction. A kite might be a good starting intuition pump here? Kites generally don't do much of anything useful unless they're mechanically tethered to the ground. From there move on to sailboats. The reason why this cart can go downwind faster than the wind isn't all that far off from the reason a sailboat can sail into the wind (albeit not directly into the wind), but, critically, only if it has a keel or centerboard. (Edit: Another detail to point out is that the linkage causes the propeller to turn in the opposite direction it would if it were spinning freely. And remember that Newton's third law works in both directions at the same time.) (Edit again -- Another observation that might help change one's intuition is that, while the cart as a whole may be moving relative to the ground, the part of the cart that's currently touching the ground at any given moment is more-or-less stationary with respect to the ground. And is also mechanically linked to the propeller.)
- deleted 5y ago
  [deleted]
- jfoutz 5y ago
  Sailboats are one I struggled with, especially sailing into the wind. I think my intuition there came from a boat going the same direction as the wind, say, north at 1 meter/sec. now the boat can't go faster than 1 m/sec, but if you turn 45 degrees to the east you can travel sqrt(2) m/sec, because you're still going north at 1/ms and the boat gets pushed along to the east as well. the closer you get to perpendicular to wind, the faster you can go. I still don't quite get sailing into the wind, but I'd imagine this wheel and propeller system works the same way. The vehicle motion restricts moving to just a line, the wind pushes along the x axis, so the vehicle can sort of zoom along in the y direction, much faster than moving along x. __edit__ oh, I see, it's the difference between ground and wind. a sailboat couldn't do this but something clever with propellers and linkages probably could

jrs235 5y ago

So, did/does the vehicle in the desert start with a push (from a force other than the wind) or did it really get started moving by the wind pushing it? Edit: FYI in the video he does sort of cover his claim even if it got started by a non wind force by stating the vehicle can maintain a speed faster than the wind pushing it. But I'm still curious, was a non wind force needed in the desert to get it started?

Retric 5y ago
No, it’s light enough for the wind to start pushing it.
- jrs235 5y ago
  Even with 100+ pounds (50+ kg)?
  - Retric 5y ago
    Yes, at the extreme end you can find videos of 3,000+ lb cars flipped by the wind. It looks quite aerodynamic, but by comparison I can start pushing a 3,000lb car on flat level ground. So this thing probably needs less than 10 pounds of force to start moving.
mumblemumble 5y ago
Any wind that's strong enough to overcome static friction would do the trick, and I can't see why 10mph wouldn't be enough. The small models that you can build at home (sorry, can't find the plans anymore; this was 10 years ago now) do not require a push to get started.
- jrs235 5y ago
  Understandable that it doesn't take much wind for a vehicle well under a pound. But what about one that weighs well over 100 pounds?
  - 5y ago

karmakaze 5y ago

A better way of looking at this is to not think of 'the propeller' as being in one plane for the wind to 'push'. Since the prop is rotating, consider the 'airspeed' of the surface of a blade. You can have zero airspeed when the vehicle is moving forward faster than the wind. Similarly the airspeed of the blade is slower (actually negative) than the vehicle's airspeed which allows the wind to push the blade surface when the vehicle is moving faster than the wind. If you think of a longitudinal vertical plane you can calculate the speed of a point on the cross-section of the blade moving backward relative to the vehicle as a function of the rotation speed. As the vehicle goes faster, so too does the prop surface 'move' backward.

cblconfederate 5y ago

Gravity does