5 ms·

Then what is spinning the wheels?

by Valgrim 5y ago

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

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