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Brain teaser: travelling faster than the wind.


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Is it possible to conceive of, design and construct a vehicle that, powered only by the wind, can sustain speed in excess of that wind while travelling in the direction of that wind?

I'm a new member so, if this teaser has been asked before, my apologies.

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2 minutes ago, Genady said:

Isn't it equivalent to travelling against the wind? Sailboats do it.

No. One thing sailboats can't do is sail directly upwind. They can travel upwind by tacking and high-performance yachts can exceed wind speed when doing it. The best they can do sailing directly downwind is to approach windspeed.

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Perhaps the puzzle also requires the vehicle to be self-contained or autonomous in some way. Otherwise, it could have two parts: one converts the wind into, say, electricity, the other uses it to go wherever it wants.

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39 minutes ago, Arthur Smith said:

Is it possible to conceive of, design and construct a vehicle that, powered only by the wind, can sustain speed in excess of that wind while travelling in the direction of that wind?

I'm a new member so, if this teaser has been asked before, my apologies.

Is this a genuine puzzle/brainteaser or is this a genuine physics question ?

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35 minutes ago, studiot said:

Is this a genuine puzzle/brainteaser or is this a genuine physics question ?

Both

45 minutes ago, Genady said:

Perhaps the puzzle also requires the vehicle to be self-contained or autonomous in some way. Otherwise, it could have two parts: one converts the wind into, say, electricity, the other uses it to go wherever it wants.

One vehicle.

17 minutes ago, Genady said:

One more question (for now):

Should it do it continuously as long as the wind is there? Or, it is OK if it moves in some "stop and go" manner?

Capable of sustaining above-wind speed indefinitely.

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1 hour ago, Arthur Smith said:

Is it possible to conceive of, design and construct a vehicle that, powered only by the wind, can sustain speed in excess of that wind while travelling in the direction of that wind?

I'm a new member so, if this teaser has been asked before, my apologies.

I recall a device claiming this perhaps 10-15 years ago. There was some controversy initially owing to some confusion about the description. It was a cart with a propeller that was driven by the wheels, but the propeller was sending air backward (so it was not the wind turning the propeller and  providing energy to the wheels), which means the propeller was acting somewhat like a sail in capturing its propulsive energy, and also providing thrust.

The propeller thrust depends on the ground speed, and as long as you capture enough energy from the wind it could work. So, big propeller, low mass.

edit:

link rot was thwarting me, but I finally tracked something down

https://boingboing.net/2007/02/06/video-can-a-vehicle-.html

https://www.youtube.com/watch?v=aJpdWHFqHm0

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Shucks. I was hoping for more controversy before the spoiler. 

Rick Cavallaro built a wind-powered vehicle "Blackbird" capable of exceeding windspeed while moving downwind in 2012. Last year he won a bet with a physics professor who claimed it was impossible.

https://www.businessinsider.com/youtuber-won-10000-bet-with-physicist-over-wind-powered-vehicle-2021-7?r=US&IR=T

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44 minutes ago, Genady said:

Quote: 

 

  • A UCLA professor bet $10,000 that the video was wrong, saying it broke the laws of physics.

 

I wonder, which laws of physics it supposedly broke?

Many people, including me to start with, think it flouts the law of conservation of energy. How, they ask, can the thrust generated by the propellor exceed the drag produced by the wheel on the road?

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46 minutes ago, Arthur Smith said:

Many people, including me to start with, think it flouts the law of conservation of energy. How, they ask, can the thrust generated by the propellor exceed the drag produced by the wheel on the road?

This is just a question, How. 

On the other hand, maybe it was a Law professor ... :)

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23 minutes ago, Genady said:

This is just a question, How. 

On the other hand, maybe it was a Law professor ... :)

No he's a bona fide physics prof.

https://en.m.wikipedia.org/wiki/Alexander_Kusenko

42 minutes ago, Externet said:

Still some find it hard to believe:

https://www.scienceforums.com/topic/38721-the-1969-blackbird-wind-turbine-sailing-craft-concept/

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28 minutes ago, Arthur Smith said:

Still some find it hard to believe:

Can't see why.

Swansont has already told you the essence of how to do it, collect enough energy from the wind.

As an observation he also said

3 hours ago, swansont said:

low mass.

 

So a 'lighter than air' balloon would make the collection burden easier.

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6 minutes ago, studiot said:

Can't see why.

Swansont has already told you the essence of how to do it, collect enough energy from the wind.

As an observation he also said

 

So a 'lighter than air' balloon would make the collection burden easier.

You may be misreading something I wrote earlier. I've known about Blackbird for a few years. I've no doubt it works and how it works. When I first came across that discussion, I was initially skeptical as at first glance I found it counterintuitive.  I'm referring to people in the link you didn't follow.

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I think the first reaction is thinking the propeller is driven by the wind, and powers the wheels. Such a device would not go to faster than the wind, because the thrust would drop to zero when the vehicle speed and wind speed are equal. I think that's the initial reaction of the people saying it's impossible.

 

 

2 hours ago, Arthur Smith said:

Many people, including me to start with, think it flouts the law of conservation of energy. How, they ask, can the thrust generated by the propellor exceed the drag produced by the wheel on the road?

These are losses and would initially be ignored. The first-order questions are what is the energy of the cart, and what is the energy extracted from the wind?

At the point where you are going downwind at the speed of the wind, the speeds are equal. So the mass of the air involved must exceed the mass of the vehicle, which suggest you need a big propeller, since you want to maximize that mass difference. A big propeller means the "column" of air has a large volume.

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Is it just me, or is this doing it the hard way.  Many sail configurations can give some advantage, as pulleys and other types of leverage do when they concentrate a force, and thus allow a sailed vehicle like a boat to exceed the windspeed.  Look up racing yachts.   If this can happen on water, it seems like it could happen on land with a large efficient sail and wheels with low rolling resistance and a smooth surface.  This method also skips over conversion problems where you convert a mechanical force into an electrical one, which is bound to be lossy.  

 

 

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5 minutes ago, TheVat said:

Is it just me, or is this doing it the hard way.  Many sail configurations can give some advantage, as pulleys and other types of leverage do when they concentrate a force, and thus allow a sailed vehicle like a boat to exceed the windspeed.  Look up racing yachts.   If this can happen on water, it seems like it could happen on land with a large efficient sail and wheels with low rolling resistance and a smooth surface.  This method also skips over conversion problems where you convert a mechanical force into an electrical one, which is bound to be lossy.  

 

 

There is the additional caveat of going directly downwind. A sail won't do this.

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24 minutes ago, swansont said:

I think the first reaction is thinking the propeller is driven by the wind, and powers the wheels.

Indeed. That was my initial reaction until I realised, watching a video of Blackbird perform, that the prop is turning against the wind on starting.

49 minutes ago, TheVat said:

Is it just me, or is this doing it the hard way.  Many sail configurations can give some advantage, as pulleys and other types of leverage do when they concentrate a force, and thus allow a sailed vehicle like a boat to exceed the windspeed.  Look up racing yachts.   If this can happen on water, it seems like it could happen on land with a large efficient sail and wheels with low rolling resistance and a smooth surface.  This method also skips over conversion problems where you convert a mechanical force into an electrical one, which is bound to be lossy.  

 

 

No fixed sail arrangement will enable you to sail directly downwind at greater than windspeed. Granted, the latest racing yachts (that lift hulls clear of the water on aerofoils) can achieve 3 times  true windspeed going cross wind and ice yachts 8 times or more. But not directly downwind. Blackbird has got to nearly 3 times windspeed. Of course there is no practical application. Rick Cavallaro just wanted to prove a point.

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Here's a short video of a model shown at the physics department of the Free University of Berlin, during the "Long Night of the Sciences" (open house).

 

Thanks to everyone who participated in the thread. I can't help being a little disappointed that nobody took the skeptical view that travel downwind faster than the wind is impossible because some fundamental law of the universe is being flouted.

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