3 ms·

Right. The people mover is analogous to the wind. You can get to move at the windspeed through drag alone, at which point you no longer move relative to the air

by tofof 5y ago

Right. The people mover is analogous to the wind. You can get to move at the windspeed through drag alone, at which point you no longer move relative to the air. The air is being robbed of only the fraction of energy that you're losing to drag (rolling resistance, as there is no air resistance now). This is you riding along with the peoplemover on the skateboard. But then increase the rolling resistance - the generators on skateboard wheels -- but you still, by drag, move at the wind speed, you're just stealing more energy from the wind/peoplemover now. Then you use that energy (simply by a chain linkage) to spin the propeller, blowing air aft. The propeller pushes air backward, changing the speed of the air molecules in the region behind the vehicle from whatever they were moving at, e.g. 10 knots relative to the ground, to 0 relative to the ground. That energy pushes the propeller and with it the vehicle forward, by Newton's 3rd law. This is the second electric skateboard driving forward, in the frame of reference of the peoplemover. So now you're moving faster than wind speed, in the direction of the wind, stealing more power from the airstream (peoplemover) than you could steal by the (original) drag of your craft alone. You got there by increasing your rolling resistance, which won't slow your craft until the rolling resistance exceeds the available force from the wind. In the analogy, this would be equivalent to jamming your brakes so hard against the floor that you stop the peoplemover rail from moving at all. You use the extra energy -- the amount being spent by the wind to counteract your increased rolling resistance -- to push yourself faster forward.

thehappypm 5y ago
To add on — the power you’re stealing from the ground is the power you can use for propulsion. So before you switch on the handrail skateboard, you have some power, P, available from the generator. You can use that P to power an air conditioner or a light bulb, or to crawl down the rail. The fact that you’ll then use that power for more propulsion is not relevant — Newton’s laws are linear so you can just add up the two scenarios and they don’t have any non-linear interactions. The power input, P, is constant—you’re not going to magically get free energy from the skateboard system. With a constant power source, so your velocity increases but in a sqrt(x) fashion, with diminishing returns. With real-world constraints like friction you’d hit an equilibrium.
- rusk 5y ago
  It’s gearing. Albeit elaborate. Intuitively what’s difficult to get the head around is sailing directly into the wind. However the penny dropped when somebody says it’s like sailing into the wind and that basically the propellers act like triangular sails that are continuously reorienting. The only tricky part bow is the “faster than wind” bit. Presumably this “dynamic sail” is better at harnessing any divergent energy coherently. Kind of like how a LASER coaxes her photons out in phase to produce a coherent beam. The balancing coefficient is the (variable) pitch of the blades.
  - thehappypm 5y ago
    When you're going at exactly wind speed, there basically is no wind at all. If you had frictionless wheels, you could turn off your propellors and you'd just be coasting along, feeling no force whatsoever from the wind, as if there was no wind at all and you were not moving. The difference here -- you ARE moving, relative to the ground. You can, instead of coasting, reach down and grab some power from the huge mass rolling by below you. That'll try to slow you down -- it'll apply a force in the direction of the ground speed -- but if you harness some power and use it to drive a high efficiency propellor, you might be able to get a little extra speed, but as you speed up, you have more resistance both from the wind and the ground, so you can't do it forever.
    - rusk 5y ago
      What you suggest sounds analogous to a flywheel. You’re using the mass of the vehicle to stabilise your speed and drive the propellor when the energy stored in your momentum > the energy you get from the wind. However, this does not account for the full experience they are describing where you can attain speeds far greater than wind speed. If that’s the case there must be a different “angle” to it, pun intended. He’s not just harnessing the purely linear “push” from the wind. There must be other vectors superimposing. Could well be the lift provided by the Bernoulli effect on the propellor blades. As I mentioned elsewhere once you get to that level it’s all very domain specific.

by tofof 5y ago

thehappypm 5y ago

To add on — the power you’re stealing from the ground is the power you can use for propulsion. So before you switch on the handrail skateboard, you have some power, P, available from the generator. You can use that P to power an air conditioner or a light bulb, or to crawl down the rail. The fact that you’ll then use that power for more propulsion is not relevant — Newton’s laws are linear so you can just add up the two scenarios and they don’t have any non-linear interactions. The power input, P, is constant—you’re not going to magically get free energy from the skateboard system. With a constant power source, so your velocity increases but in a sqrt(x) fashion, with diminishing returns. With real-world constraints like friction you’d hit an equilibrium.

rusk 5y ago
It’s gearing. Albeit elaborate. Intuitively what’s difficult to get the head around is sailing directly into the wind. However the penny dropped when somebody says it’s like sailing into the wind and that basically the propellers act like triangular sails that are continuously reorienting. The only tricky part bow is the “faster than wind” bit. Presumably this “dynamic sail” is better at harnessing any divergent energy coherently. Kind of like how a LASER coaxes her photons out in phase to produce a coherent beam. The balancing coefficient is the (variable) pitch of the blades.
- thehappypm 5y ago
  When you're going at exactly wind speed, there basically is no wind at all. If you had frictionless wheels, you could turn off your propellors and you'd just be coasting along, feeling no force whatsoever from the wind, as if there was no wind at all and you were not moving. The difference here -- you ARE moving, relative to the ground. You can, instead of coasting, reach down and grab some power from the huge mass rolling by below you. That'll try to slow you down -- it'll apply a force in the direction of the ground speed -- but if you harness some power and use it to drive a high efficiency propellor, you might be able to get a little extra speed, but as you speed up, you have more resistance both from the wind and the ground, so you can't do it forever.
  - rusk 5y ago
    What you suggest sounds analogous to a flywheel. You’re using the mass of the vehicle to stabilise your speed and drive the propellor when the energy stored in your momentum > the energy you get from the wind. However, this does not account for the full experience they are describing where you can attain speeds far greater than wind speed. If that’s the case there must be a different “angle” to it, pun intended. He’s not just harnessing the purely linear “push” from the wind. There must be other vectors superimposing. Could well be the lift provided by the Bernoulli effect on the propellor blades. As I mentioned elsewhere once you get to that level it’s all very domain specific.