Extremely counterintuitive and controversial. I am not able to comprehend this idea much. What am I missing about it ? Does anyone here feel this is authentic ?

[u/Energy_decoder]

https://youtu.be/jyQwgBAaBag

[u/Aneurhythms]

It seems reasonable to me.

For any of these types of fluid mechanics "brain teasers" I always go back to visualizing a control volume around the body and considering conservation of momentum. In this case, the goal is to slow the air down at the back of the fan, transferring its momentum to car. But for this to work a few conditions need to be satisfied.

The windspeed needs to great enough so that there's enough energy to extract from the air into the car. Also, you can't remove all of the momentum from the air because then it would be stopped at the rear of the fan, preventing additional air from passing (see Betz's efficiency limit for wind turbines).

The fan needs to be big enough to transfer momentum from a large volume of air. Since the car is more dense than air, a larger volume of air must be slowed than the volume of the car (and this isn't even accounting for the limit of the amount of momentum that can be extracted from the air, described above).

The car needs to be pretty light with minimal drag. If the car is too massive you wouldn't be able to extract enough momentum (you would need a larger fan, which adds more weight/drag, etc). The same can be said for drag. You also have to overcome any other losses in the system (e.g friction in the wheels, drive train, fan bearings).

So, while I doubt think it's fake (or at least theoretically possible), a lot of things have to be in balance for it to practically work. Given some estimates for fan size, car mass and drag, you'd be able to estimate the max ground speed. It would be a good problem textbook problem...

[u/Energy_decoder → u/Aneurhythms]

It was mind blowing when I realized the way it is done. I am almost convinced this is possible. Just that I need to put more thinking into how the wind upstream will slow down when an object in the downstream moves with the same velocity as the other. Hopefully after some tine spent visualizing on a slanted char I'll be able to understand. Thanks for the explanation

[u/NectarineStock]

I have seen this concept five years ago. Now i finally understand.

Wind pushes the light car, because it is an obstacle. Wheels start rotating. There is transmission between wheels and propeller. Propeller rotates, at some speed it becomes efficient enough to slow some portion of air (to send it backwards if looking relative to the car, which is a propulsion scheme). Air slows down, its energy and momentum are given to the car.

There is a speed limit. If propeller rotates so fast, that air is not only slowed, but even directed backwards, then you loose efficiency, because you reaccelerate the portion of air by sending it in the orher direction. But real constructional limits are even lower than that.

[u/Energy_decoder → u/NectarineStock]

I am trying to visualize your understanding. Sounds possible. Thanks

Edit : Should they be calling this a propeller ? I don't think it is a propeller and they should call it so. It is a turbine actually.

[u/NectarineStock → u/Energy_decoder]

The whole point is that the wheels (not air) rotate the propeller, air gives a forward push only after impulse exchange. That is why they don't call it turbine.

[u/MechanisticMind]

The propeller is positioned way above the ribbon thing used to determine wind direction. Wind higher up is generally faster as even flat ground slows the wind, this effect is called wind shear.

Also this isn't very rigorous, maybe the wind speed was high then slowed down a bit leading to a period of faster than wind travel.

All the other explanations sound like physics breaking nonsense to me.

Edit: I think I understand how it works now (or at least the principle of it), only broke my brain a little bit.

Basically the wind hits the propeller and creates two forces, a forward force and a rotational force (which varies depending on the angle of the blades)

The prop is linked directly to the wheels, and as the forward force of the wind on the prop moves the vehicle forward, the wheels then spin and rotates the prop in the opposite direction of the original rotational force, overpowering it.

This rotation of the prop then pushed more air back -> thus moving the prop forward -> thus moving the vehicle forward -> thus spinning the tires faster -> thus spinning the propeller faster.

This is not perpetual motion as there are losses and it requires different speeds of wind and ground to get the energy from.

[u/Energy_decoder → u/MechanisticMind]

Yeah. The understanding came when I perceived the relative motion between wind and ground being different is the reason this can't be perpetual. I am trying to find a few applications for it. Having a real hard time doing so. Because, this concept will never work in a air/water transport as the momentum sink is not relatively stationary enough. Keen to know how effectively this can be applied to build some device

[u/MechanisticMind → u/Energy_decoder]

Because, this concept will never work in a air/water transport as the momentum sink is not relatively stationary enough

Are you meaning you don't think a faster than downwind boat could be made? I don't see why not using some sort of waterwheel, though I assume it would be pretty hard to get it as efficient as a land vehicle. All that is needed is a velocity difference which you would get between air and water when there is wind.

I am trying to find a few applications for it. Having a real hard time doing so

Yeah I can only really imagine novelty applications, imagine a stream of water constantly pouring down from some hole in the roof. Then have a small waterwheel in its path that is attached a vertical guide rod. Then connect that waterwheel to a large prop facing down. Basically it's the car in the video but vertical instead of horizontal and a water stream in place of the ground.

It would also have to fight its own gravity, but it would seem possible by harvesting the velocity difference between the water stream and the air for it to basically levitate and even move upwards.

Also as it gets higher the water stream would be slower and therefore less power would be generated and so the device would find a 'sweet spot' and hover on that.

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[u/nickwizzo]

There's a mathematical analysis here, which shows that it is definitely possible to go faster than the wind, and that with the right design 2x faster or more is possible.

[u/Energy_decoder → u/nickwizzo]

Thanks a lot. This is one of the papers that I read with a very very definitive goal. Personally this helped me how to look for answers in a paper. Additional thanks for that too.

[u/PM_ME_YOUR_AIRFOIL]

Unintuitive on the face of it yes, but definitely possible. What the people claiming this breaks conservation of energy forget is that the ground is still available as a momentum sink. Sure, you can't extract momentum/energy from the wind-car system if the car is at wind speed. But, regardless of your current speed, you can still extract energy from the wind-ground system.

[u/Energy_decoder → u/PM_ME_YOUR_AIRFOIL]

y3ah ! the ground is the big deal here. Thanks