5 ms·

I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the prop

I really would like a simple explanation of how that is possible. AIUI, when speed is stable, the wheels turning provide power to the propeller. But if the propeller pushes the car, which in turn makes the wheels turn faster making the car faster than the wind itself, what makes it different from perpetual motion machines? I would love a diagram showing how the power flows in this system.

eyko 5y ago
Veritasium did a video on the Blackbird (plus a bet to prove it does indeed work). https://www.youtube.com/watch?v=jyQwgBAaBag https://www.youtube.com/watch?v=jyQwgBAaBag
otherme123 5y ago
I felt this in a sail boat: when the boat is moving below wind speed, the wind pushes the sail and the boat. At some point, wind and boat equal speeds, so it feels like theres no wind at all except the sail is full. Some well built boats can generate pull (instead of push) in the sail, as if it was a plane wing. The air passing on the convex (forward) side travels faster than the air on the concave, making the pressure lower on the forward side and pulling the boat, and you feel again the wind but against your face (as if you were running forward). It's a bit counterintuitive, because what your eyes see is the sail pushing the boat. So it's not perpetual motion, because the wind needs to keep blowing or the boat slows again. What slows the boat is the friction boat-vs-water. In the case of the car above, the wind needs to reach the point of wind speed == car speed, and beyond that the wheels keeps rotating the "sails", and thus keeping the pull. If the wind stops, the car can keep going for some time, but as the speed of the wind through the sails is lower, the pull is also lower and at some point it stops. But in this case, the friction is wheels-vs-ground, much lower than the boat-vs-water above, so the car can keep going (same what happens with a bike when you stop pedaling in a flat surface, the bike can keep going for long, and even a faint pedaling or tail winds keep the thing going). This car is a clever construction to keep the wind at high speeds through the blades.
- ska 5y ago
  > as if it was a plane wing. This is how nearly all sailing works. Excepting the case of sailing directly downwind, the curve of the sail works like a wing, and the keel provides something to "push" against - the effective "lift" (if it were a wing) drives you forward. This is why you can sail into the wind at all...
  - abakker 5y ago
    Obligatory mentioned of the AC72 using rigid wing sails. - https://www.youtube.com/watch?v=hbPTnF6liF8 https://www.youtube.com/watch?v=hbPTnF6liF8
munchler 5y ago
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.
unanswered 5y ago
Where the power comes from is easy: the wind gets slowed down. Of course the nature of wind is such that you could never measure this, but the point is that when the propellor pushes against the air, that air's velocity relative to the ground is decreased while the vehicle's is increased. The vehicle leaves behind it an imperceptible region of less-energetic air and takes that energy with it.
- marcodiego 5y ago
  Now I think I'm beginning to understand... At first, wind propels the vehicle, moving it forward and turning its wheels. Its wheels turn the propeller which slows down the air, even the air in front of it. The energy lost by the air is now the energy that moves the vehicle! Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
  - unanswered 5y ago
    Ah, but not so fast. It actually depends on your frame of reference. Suppose the vehicle is facing the +x direction. Suppose the wind is travelling at +10m/s relative to the ground. Suppose the (experimentally determined) maximum velocity of the vehicle under these conditions is +12m/s. Now consider an inertial reference frame which, compared to the ground's reference frame, is moving at +11m/s. (I.e., pick the frame in which the ground moves at -11m/s.) Now let the vehicle run. There will come an instant when the vehicle is stationary in this reference frame (when the vehicle has accelerated to +11m/s relative to the ground, not yet having reached its maximum velocity). Consider the instantaneous change in kinetic energy at this instant. The air about to be pushed by the propeller has velocity -1m/s and will accelerate in the -x direction. The vehicle has velocity 0m/s and will accelerate in the +x direction. In both cases, the kinetic energy is actually increasing! So in this frame, where does the increasing kinetic energy of the air-vehicle system come from?! The answer is the ground. It has velocity -11m/s. When it pushes against the contact point of the wheels, it is therefore pushing the contact point in the -x direction. But this means the contact point is pushing against the ground in the +x direction. And therefore the ground is undergoing a minuscule +x acceleration. That acceleration is therefore decreasing the kinetic energy of the ground — by a lot, because of the mass factor. And this is why we say the energy is derived from the difference in velocity between the ground and the wind — because depending on your inertial reference frame, the kinetic energy might be coming from one or the other. In the frame of reference of the vehicle, it starts out coming from both, but when the (relative) direction of the wind shifts (so that relative to the ground the vehicle is travelling faster than the wind), the kinetic energy starts coming from the ground only, and the vehicle is forced to transmit some of it to the air to keep moving. But this might actually be the nicest explanation of how the vehicle outpaces the wind: in the faster-than-the-wind regime, the vehicle is transmitting energy from the ground into the air — which is exactly what you would expect to happen, given the mechanical linkage between the wheels and the propeller! — but some of this energy bleeds off into increasing the velocity of the vehicle itself, because it's on wheels and that's what happens when wheeled vehicles push against something behind them, no matter what the ground happens to be doing (such as moving backwards like a treadmill, in this frame of reference). Disclaimer: I did not do well in college-level Mechanics. But I think I have convinced myself of the above explanation, just barely.
lizardmancan 5y ago
imagine the thing has no wheels. the wind provides an energy source. now imagine a sail cart. you can put a dynamo on the wheel and extract power now imagine a sail boat sailing close to the wind. it can go much much faster than the wind speed now imagine a batter powerd propeller pushing a cart forwards. now imagine the battery powered prop is pushing against the wind. a+b now remove the battery and extract the power (a) from the wheels.
tshaddox 5y ago
It’s certainly not perpetual motion, because the vehicle would quickly come to a stop if the wind speed (relative to the ground) went to zero.
colechristensen 5y ago
You’re harvesting the energy from the difference in velocity of the ground and the wind. Sure it is easy to think about that being possible when you’re attached to the ground but it’s perfectly possible when you’re moving too. Regardless of how fast you are moving or in which direction there is always that difference been air and ground. To go faster than the wind you just have to be a bit clever in how you capture that available energy.
ummonk 5y ago
It’s not perpetual motion because it’s extracting energy from the fact that the wind is moving relative to the ground. Without wind moving relative to the ground it wouldn’t work.
cblconfederate 5y ago
The wind that pushes the cart can provide more push than can be harnessed by the cart passively (F=Pressure * area). To increase this force you can either increase the area of the cart facing the wind, or increase the pressure. The propeller harnesses part of the wind's energy to increase that pressure thus increasing the total force acting on the cart but the work done on the cart (F*displacement) is always limited by the energy that can be provided by the wind.

eyko 5y ago

Veritasium did a video on the Blackbird (plus a bet to prove it does indeed work). https://www.youtube.com/watch?v=jyQwgBAaBag https://www.youtube.com/watch?v=jyQwgBAaBag

otherme123 5y ago

I felt this in a sail boat: when the boat is moving below wind speed, the wind pushes the sail and the boat. At some point, wind and boat equal speeds, so it feels like theres no wind at all except the sail is full. Some well built boats can generate pull (instead of push) in the sail, as if it was a plane wing. The air passing on the convex (forward) side travels faster than the air on the concave, making the pressure lower on the forward side and pulling the boat, and you feel again the wind but against your face (as if you were running forward). It's a bit counterintuitive, because what your eyes see is the sail pushing the boat. So it's not perpetual motion, because the wind needs to keep blowing or the boat slows again. What slows the boat is the friction boat-vs-water. In the case of the car above, the wind needs to reach the point of wind speed == car speed, and beyond that the wheels keeps rotating the "sails", and thus keeping the pull. If the wind stops, the car can keep going for some time, but as the speed of the wind through the sails is lower, the pull is also lower and at some point it stops. But in this case, the friction is wheels-vs-ground, much lower than the boat-vs-water above, so the car can keep going (same what happens with a bike when you stop pedaling in a flat surface, the bike can keep going for long, and even a faint pedaling or tail winds keep the thing going). This car is a clever construction to keep the wind at high speeds through the blades.

ska 5y ago
> as if it was a plane wing. This is how nearly all sailing works. Excepting the case of sailing directly downwind, the curve of the sail works like a wing, and the keel provides something to "push" against - the effective "lift" (if it were a wing) drives you forward. This is why you can sail into the wind at all...
- abakker 5y ago
  Obligatory mentioned of the AC72 using rigid wing sails. - https://www.youtube.com/watch?v=hbPTnF6liF8 https://www.youtube.com/watch?v=hbPTnF6liF8

munchler 5y ago

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.

unanswered 5y ago

Where the power comes from is easy: the wind gets slowed down. Of course the nature of wind is such that you could never measure this, but the point is that when the propellor pushes against the air, that air's velocity relative to the ground is decreased while the vehicle's is increased. The vehicle leaves behind it an imperceptible region of less-energetic air and takes that energy with it.

marcodiego 5y ago
Now I think I'm beginning to understand... At first, wind propels the vehicle, moving it forward and turning its wheels. Its wheels turn the propeller which slows down the air, even the air in front of it. The energy lost by the air is now the energy that moves the vehicle! Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
- unanswered 5y ago
  Ah, but not so fast. It actually depends on your frame of reference. Suppose the vehicle is facing the +x direction. Suppose the wind is travelling at +10m/s relative to the ground. Suppose the (experimentally determined) maximum velocity of the vehicle under these conditions is +12m/s. Now consider an inertial reference frame which, compared to the ground's reference frame, is moving at +11m/s. (I.e., pick the frame in which the ground moves at -11m/s.) Now let the vehicle run. There will come an instant when the vehicle is stationary in this reference frame (when the vehicle has accelerated to +11m/s relative to the ground, not yet having reached its maximum velocity). Consider the instantaneous change in kinetic energy at this instant. The air about to be pushed by the propeller has velocity -1m/s and will accelerate in the -x direction. The vehicle has velocity 0m/s and will accelerate in the +x direction. In both cases, the kinetic energy is actually increasing! So in this frame, where does the increasing kinetic energy of the air-vehicle system come from?! The answer is the ground. It has velocity -11m/s. When it pushes against the contact point of the wheels, it is therefore pushing the contact point in the -x direction. But this means the contact point is pushing against the ground in the +x direction. And therefore the ground is undergoing a minuscule +x acceleration. That acceleration is therefore decreasing the kinetic energy of the ground — by a lot, because of the mass factor. And this is why we say the energy is derived from the difference in velocity between the ground and the wind — because depending on your inertial reference frame, the kinetic energy might be coming from one or the other. In the frame of reference of the vehicle, it starts out coming from both, but when the (relative) direction of the wind shifts (so that relative to the ground the vehicle is travelling faster than the wind), the kinetic energy starts coming from the ground only, and the vehicle is forced to transmit some of it to the air to keep moving. But this might actually be the nicest explanation of how the vehicle outpaces the wind: in the faster-than-the-wind regime, the vehicle is transmitting energy from the ground into the air — which is exactly what you would expect to happen, given the mechanical linkage between the wheels and the propeller! — but some of this energy bleeds off into increasing the velocity of the vehicle itself, because it's on wheels and that's what happens when wheeled vehicles push against something behind them, no matter what the ground happens to be doing (such as moving backwards like a treadmill, in this frame of reference). Disclaimer: I did not do well in college-level Mechanics. But I think I have convinced myself of the above explanation, just barely.

lizardmancan 5y ago

imagine the thing has no wheels. the wind provides an energy source. now imagine a sail cart. you can put a dynamo on the wheel and extract power now imagine a sail boat sailing close to the wind. it can go much much faster than the wind speed now imagine a batter powerd propeller pushing a cart forwards. now imagine the battery powered prop is pushing against the wind. a+b now remove the battery and extract the power (a) from the wheels.

tshaddox 5y ago

It’s certainly not perpetual motion, because the vehicle would quickly come to a stop if the wind speed (relative to the ground) went to zero.

colechristensen 5y ago

You’re harvesting the energy from the difference in velocity of the ground and the wind. Sure it is easy to think about that being possible when you’re attached to the ground but it’s perfectly possible when you’re moving too. Regardless of how fast you are moving or in which direction there is always that difference been air and ground. To go faster than the wind you just have to be a bit clever in how you capture that available energy.

ummonk 5y ago

It’s not perpetual motion because it’s extracting energy from the fact that the wind is moving relative to the ground. Without wind moving relative to the ground it wouldn’t work.

cblconfederate 5y ago

The wind that pushes the cart can provide more push than can be harnessed by the cart passively (F=Pressure * area). To increase this force you can either increase the area of the cart facing the wind, or increase the pressure. The propeller harnesses part of the wind's energy to increase that pressure thus increasing the total force acting on the cart but the work done on the cart (F*displacement) is always limited by the energy that can be provided by the wind.